I have a confession to make: I am not a scientist. By inclination, education, and profession I am an engineer. Engineering and the sciences have an intimate connection; however they are not one and the same. Scientists are more concerned with absolute facts. They love being technically correct—the “best” kind of correct. We engineers don’t care so much about technical correctness. We only care if something works; if it behaves the way we expect it to. Engineers construct models and abstractions. We abstract away the physical world’s complexity to provide a simplified model that can be readily utilized to do actual work in the real world. We do this because the real world is too complicated and fraught with contradictions, yet engineers and our technically incorrect models make the world run.
Scientists often like to play at being engineers and engineers often like to play at being scientists. With rare exceptions, this generally results in academic disaster. Because of this I think it’s high time I embraced my engineering background and use its robust and powerful toolset to analyze physiology. That is the aim of this article. I promised in the last installment that I would discuss the adipostat in relation to obesity and I plan to do just that.
However, before we can venture into that discussion I am going to present you with my working model of the human metabolism. I must caution you that like all models this one is not technically correct and is not free from bias. Engineering does not demand technical correctness or objectivity. The merit of an engineering model is instead judged solely on its utility. From this standpoint I believe you will find this model to be a complete success.
The basics of metabolic thyroid regulation were presented in Leptin: The Next Big Thing IV. However, we will review and expand on the evidence presented in that installment to develop a fully functional model of thyroid hormone regulation.
Thyrotropin Releasing hormone (TRH) secretions by the PVN are regulated directly by Neuropeptide Y (NPY), Melanocyte Stimulating Hormone (MSH), and GABA, with NPY playing the most important role. As a consequence TRH secretions are regulated indirectly by leptin, glucose, and insulin. Because the metabolic triumvirate of leptin, glucose, and insulin ultimately control GABA, NPY, and MSH, one may be tempted to simplify this model by relating thyroid output directly to leptin, glucose and insulin. However, NPY must remain present in the model for reasons that will become clear when we examine the physiological abnormalities of the obese.
Both VMH/Hepatic derived GABA and Lateral Hypothalamus (LH) derived NPY suppress the release of TRH in the PVN (1). ARC derived MSH potently promotes TRH release. This makes a lot of sense when you think about it. GABA and NPY are going to be inversely related in normal people. When leptin levels are normal NPY will be suppressed and alpha-MSH will increase in the ARC. This will serve to increase TRH production. Leptin also decreases VMH/Hepatic-derived GABA delivery to the PVN, thus further increasing TRH production. So in this state TRH levels stay in the normal range. In this state leptin is suppressing TSH secretion from the pituitary and strongly stimulating Thyroxin secretion from the thyroid gland (2). Thus we end up with normal thyroid hormone levels.
The regulation of Thyroid Stimulating Hormone (TSH) secretion is depicted in figure 7.1. TRH, leptin, and Thyroid Hormone (T4/T3) directly regulate the secretion of TSH. The synchronicity of plasma TSH and leptin levels demonstrates leptin’s position as TSH’s most potent regulator (4).
Thyroxin secretion is modulated directly by leptin and TSH in the thyroid gland. Either may be the dominant metabolic regulator depending primarily on plasma leptin levels. The TRH, TSH, Thyroxin loop is a redundancy in case metabolic regulation by leptin is not present. Thus when plasma leptin levels are in the normal range, leptin is the prime thyroid regulator, however when leptin levels are very low or very high TSH becomes the dominant signal.
Corticotrophin Releasing Factor (CRF) secretions for the PVN are controlled directly by CRF, NPY, GABA, Nor-Epinephrine (NE), Arginine-Vasopressin (AVP) and leptin.
CRF promotes its own release (5,6). Injection of either CRF or a beta-adrenoeceptor (B-AR) agonist in to the PVN of rats promotes CRF secretion by altering DBH protein in the neural bundles (6). Sympathetic Nervous System (SNS) projections run from the spinal column and the basal sympathetic nodule directly to the PVN. These neural circuits sense immunological stress, physiological stress, or stimulants.
In vivo, most regulation is accomplished by the alpha2-adrenoreceptor (A2-AR) and not the B-AR. NE binding to A2-AR sites in the PVN dramatically increase CRF production (7). The effects of NE on the PVN are not temporary either. By altering DBH protein in the neural bundles the PVN is sensitized to activation of the Hypothalamus – Pituitary – Adrenal (HPA) Axis. The effects of direct injection into the PVN of rats lasted 3 weeks. The effects may have lasted even longer, however at this time they had terminated the rats to examine their brains (6).
NE is also delivered to the PVN by afferent projections from the Locus Coeruleus (LC) (8). The LC is an extremely complicated neural structure. It is extensively studied as abnormalities in the LC often result in psychological disorders. For our purposes we may consider the LC to be the psychological stress response center.
The LC is one of the brain regions that is strongly correlated with brain wave patterns. This is one reason that even small amounts of sleep depravation result in highly elevated levels of corticosterone and cortisol. When we enter slow wave sleep patterns, NE delivery to the PVN is reduced. It is also reduced during times when we do not have to pay very close attention to things. NE release from the LC is strongly correlated with attention, vigilance, and psychological stress. Thus we can conclude that brain wave patterns are a pretty good indicator of NE activity in the PVN.
Leptin directly increases corticosterone and epinephrine production through multiple pathways. First by lowering VMH derived GABA delivery to the PVN it increases firing rate in the PVN, resulting in increased CRF secretion. Leptin also enhances secretion of AVP (9). It further upregulates the V1 AVP receptor, promoting additional CRF release from the PVN (10).
AVP and CRF act at the pituitary to increase adrenocorticotropin (ACTH). However their effect is not additive, but is in fact synergistic. AVP strongly potentiates CRF-induced release of ACTH. Thus leptin is a potent activator of the HPA.
NPY is very important for the regulation of CRF and TRH (12). The PVN can operate in either pro-TRH mode or pro-CRF mode. In pro-TRH mode, activation of the PVN, by reduction in GABA levels, causes more TRH to be released. However in pro-CRF mode activation of the PVN, again through GABA, results in increased CRF production. This behavior is controlled by NPY. The main regulator of TRH production from the neuron’s perspective is alpha-MSH. Alpha-MSH enhances the phosphorylation of CREB (PCREB), which promotes TRH secretion. NPY determines the specific neurons in which PCREB accumulates. So alpha-MSH does not really promote TRH secretion; its role is to restrict the enhancement of PCREB levels. When injected in the PVN of rats, NPY altered the cellular CREB levels of the pro-TRH neurons versus those in the pro-CRF neurons. In the pro-TRH neurons it reduced CREB levels, while in the pro-CRF neurons it elevated CREB levels. So when alpha-MSH was applied it produced an increase in PCREB in the CRF neurons, leading to enhanced CRF secretion.
Sex Hormone Regulation:
Sex hormones are primarily regulated by estrogen and metabolic state. Both excess estrogens and the fasted hypoglycemic state can drastically reduce sex hormone levels. Emerging evidence calls into question the significance of the Hypothalamic – Pituitary – Testicular Axis (HPTA). In my opinion the HPTA is only of real significance when one is utilizing supraphysiological amounts of testosterone.
Estrogen modulates the secretion of Leutinizing Hormone Releasing Hormone (LHRH or GnRH). It does this by binding the beta-estrogen receptor (B-ER). Unlike the singular androgen receptor, there are multiple estrogen receptors. The B-ER is mostly expressed in the dorsal region of the PVN. By binding to the B-ER in the dorsal region of the PVN, estrogen can lower LHRH secretion. The true significance of estrogen-induced suppression of LHRH is suspect, however.
This is because metabolic state also plays a large part in determining sex hormone levels. Increased CRF or NE delivery to the PVN directly reduces the amount of LHRH produced by the neurons in the PVN. As a result, psychological stress, physiological stress, or stimulants possess the ability to reduce LHRH secretion. Both CRF and NE accomplish this feat by increasing B-ER density and sensitivity in the dorsal region of the PVN (13).
We all know stress can reduce natural testosterone production, but it is not often discussed that fasting also decreases testosterone. The fasting induced decrease is mediated by several mechanisms (14). In the short term when the stomach is empty the afferent vagal nerves are innervated. These nerves connect directly to the medulla oblongata. The medulla oblongata has nor-adrenergic nerve projections running to the PVN. Activation of the medulla oblongata results in increased NE delivery to the PVN, which in turn upregulates B-ER receptors and leads to suppressed LHRH.
If caloric restriction is continued the SNS is eventually activated, leading to even more NE release, further exacerbating the problem. Caloric restriction also increases NPY; an increase in NPY promotes CRF secretion, further lowering the setpoint for estrogens. This ultimately leads to a decrease in LHRH, and hence a reduction in LH and FSH, causing reduced testosterone secretion from the testes.
What really calls into question the HPTA’s very existence is that a reduction in LHRH production is not even necessary for metabolic state to reduce testosterone production in the testicles (15). In a very interesting experiment, the PVN of rats were injected with CRF or a beta-adrenergic agonist. They then administered human chronic gonadotropin (hCG). What they discovered was that injection of those compounds into the PVN attenuated the testicles’ response to hCG.
To determine how the PVN was directly manipulating the testicles, the scientists employed a pseudo rabies viral tracer and found that there exist direct efferent nerves running from the PVN down to the testicles. To ensure they were correct, the scientists lesioned the PVN of some of the rats. They found that by damaging the receptor surface of the PVN, they ameliorated CRF or beta-adrenergic attenuation on hCG-induced testosterone secretion.
What this means is that the pituitary and hence blood levels of LH and FSH can be taken out of the loop. The PVN can directly control the leydig cells in the testes.
Sympathetic Nervous System:
The Sympathetic Nervous System (SNS) is actually quite complicated. As evidenced above there are numerous nor-adrenergic projections from various brain regions used for compartmental communication. However if we restrict our interpretation of the SNS as those neurons that deliver catecholamines to the body then its analysis is simplified. The resulting SNS can be divided into two distinct sections.
One I will call the metabolic branch. This branch of the SNS delivers NE to the pancreas, kidney, liver, adipose tissue, and muscle tissue. The second segment is fight/flight branch. This branch is not innervated by the metabolic state of the PVN. Instead it’s innervated by the LC, which is the brains stress response center. Stressors like fight/flight stimuli, heat shock, or ephedrine activate both branches, whereas metabolic regulation by the PVN only activates the metabolic branch.
Activation of these branches is accomplished through low-level dopamine stimulation (16). Dopamine is technically an excitatory neurotransmitter. However in the body it can end up being a relaxant in some cases, as high levels of dopamine reduce NE’s stimulatory effect on neurons. Additionally high levels of dopamine suppress vasopressin and glutamate activation of SNS neurons (17,18). The PVN or the LC delivers low-level dopamine to the various branches of the SNS, which in turn promotes NE release. Leptin increases SNS activity by increasing nerve firing in the PVN, the result of which is enhanced dopamine delivery to the spinal cord.
Because of this strange phenomenon, different segments of the SNS can be regulated quite differently based on metabolic state. This effect will become more important when we discuss obesity later on.
The pancreas is regulated directly by NE, leptin, glucose, insulin, and GABA. NE and glucose however play the most important role. Both leptin and NE prevent calcium influx into the beta cells and thus attenuate insulin release. Glucose, by closing potassium sensitive ion channels, promotes depolarization and calcium influx, leading to insulin and GABA co-secretion (19).
The pancreatic beta cells posses both the insulin-independent GLUT2 and the insulin-dependent GLUT4 glucose transporters. When glucose is transported in via glut it promotes insulin release. Insulin then binds to the neighboring beta cells and further increases beta cell glucose uptake. Because of this feed forward amplification pathway, insulin promotes its own release. This amplification is partially responsible for the different phases of insulin release; it also allows for a sudden and dramatic increase in insulin output as seen during phase one of insulin release.
This rapid amplification would go on indefinitely if it weren’t for two things. First, if the dramatic insulin release in phase one is sufficient to drive down blood sugar, then there is no stimulus to continue insulin secretion. In subjects with hyperglycemia this does not happen. In this case the beta cells are being overdriven. When this happens the beta cells start to secrete there own leptin. They use this leptin to become insulin resistant. This prevents serious damage and cell death of the beta cells and islets.
Thus at this point we enter phase two of insulin release. Phase two tends to be much lower but has a longer duration. The pancreas co-secretes the peptide GABA whenever it secretes insulin. It has been hypothesized that the secretion of GABA is responsible for the reduction in glucagon release seen during times of elevated insulin (20).
By binding to any of the adrenoreceptors, NE reduces insulin secretion from the beta cells by preventing calcium influx.
Renal metabolic Control:
The kidneys play an important role in regulating blood glucose, blood pressure, and plasma leptin levels. In the hypoglycemic state glucose is transported across a concentration gradient in the proximal tubular cells, via sodium-linked glucose transporters. This results in filtered glucose being completely reabsorbed. Renal glucogenesis also plays an important role in preventing hypoglycemia by contributing to blood glucose levels (21).
Elevated blood pressure can affect metabolic regulation in several ways. By restricting blood flow, elevated blood pressure can reduce glucose disposal in adipose and skeletal muscle tissue. Thus an important feature of the hypoglycemic state is often elevated blood pressure. There does also appear to be marked sexual dimorphism in this aspect of metabolic regulation. Leptin, through interactions in the VMH and PVN, increases SNS outflow to the kidneys. In men this often results in elevated blood pressure by causing increased activity of the Rennin-Angiotensin System (RAS) as well as decreased sodium excretion. The sexual dimorphism has been attributed to altered alpha-adrenergic receptor iso-form distribution (22). Men often express the alpha iso-form of the A2-AR. This iso-form of the receptor, when activated by said SNS segments, results in elevated blood pressure.
It has been proposed that in women this phenomenon is less common because elevated blood pressure could be damaging to a developing fetus. Leptin normally balances out this effect via direct interaction with the JAK2/STAT3 pathway in the kidneys. This activation elicits a diuretic result (23). Thus, directly leptin acts as a diuretic, however its effects though the SNS increase blood pressure and increase renal glucose re-absorbtion.
The kidneys are also the prime sites for leptin disposal (25). It has been proposed that the kidneys remove up to 80% of leptin—both free and bound—from the blood stream. This appears to be a saturable phenomenon, restricted primarily by blood flow to the kidneys. This is yet another reason men exhibit lower leptin levels than women, as blood delivery to the kidneys in men is roughly 50% greater than in women. The effects of the kidneys in maintaining plasma leptin levels can be seen in patients with late stage renal failure. Such individuals exhibit hyperleptimania in spite of the fact that they generally lose weight and reduce caloric intake.
Saturation in the kidneys may also play an important role in the elevated leptin levels seen in the obese. Plasma leptin levels increase exponentially with relative body fat increases. Yet obese people do not posses an exponentially greater number of fat cells. In addition, the adipose cell hypertrophy that accompanies obesity is known to slightly reduce leptin secretion. Thus it is possible that reduced renal leptin degradation may play a role in the elevated plasma leptin levels seen in the obese.
VMH Leptin and Glucose Sensor:
The VMH is the brain’s main leptin and glucose sensor. As discussed in Leptin: The Next Big Thing V, the KIR neurons in the VMH sense brain glucose concentrations. The VMH is specifically suited to this task because of its location—it is extremely close to the endothelial cells. The VMH can literally keep tabs on blood glucose directly, whereas the rest of the brain relies on brain glucose transport across the blood brain barrier (BBB). Additionally, the VMH is comprised of basically the only neurons in the brain that posses GLUT4 glucose transporters. As a result, insulin can further drive glucose signals in the VMH.
Hepatic Glucose Sensor:
It has long been known that modulating the liver’s glucose supply alters firing rates in different regions of the brain, yet only recently has the mechanism been identified (26,27). Using viral tracers, scientists were able to determine that there are afferent nerve connections running from the liver directly to the PVN. These nerves serve to deliver GABA to the PVN and thus slow its firing rate, just as the VMH does. This makes sense, as the liver is a great barometer of blood glucose levels. It can relay this information directly to the PVN and reduce hypoglycemic response if it feels it’s not warranted. The liver possesses GLUT2, GLUT4, and the fructose-specific GLUT5 transporters. Because of this fructose can activate the hepatic glucose sensor, whereas it cannot activate the glucose sensors in the rest of the body.
Portal Vein Glucose Sensor:
There is also the portal vein glucose sensor to contend with. Many studies often mix up the effects of the portal vein and the hepatic glucose sensors, as they are very close to one another. Recently it has been shown that they are indeed separate systems, though because of location activation of one generally entails the activation of the other. Fructose however highlights why the two cannot be lumped together.
The portal vein glucose sensor is responsible for non insulin-stimulated glucose disposal seen in the post absorptive state. If you infuse the portal vein with glucose you can induce hypoglycemia in rats—an effect specific to the portal vein. It is known that this response depends on GLUT4 and AMPK, however it does not depend on insulin, as insulin receptor knockout mice still display this behavior. It is unknown at this time what the mechanism of action is, though a likely candidate is glucagon, as glucagon strongly stimulates AMPK and can lead to insulin-independent glucose disposal.
The portal vein glucose sensor only expresses the non insulin-dependent GLUT2. It is known that there are afferent nerves running from the portal vein glucose sensor to the adrenal medulla. These nerves serve to deliver GABA to this brain region and reduce the secretion of corticosterone (27).
Applying the Model to Obesity
It is my opinion that there are as many roads to obesity as there are obese people. However they all start with excessive eating and decreased physical activity. I am of the opinion that several additional factors accelerate or exacerbate the road from lean to obese. Namely:
1. Over consumption of refined carbohydrates.
2. Over consumption of fructose.
3. Over consumption of grains at the expense of vegetables.
4. Imbalanced omega-6 to omega-3 fatty acid intake.
5. Over consumption of all fatty acids.
6. Overuse of stimulants.
7. Over consumption of sodium.
These special topics will be given precedence as we apply the above model to an obese person’s physiology.
As I am sure you are aware the majority of leptin is produced in adipose tissue. However, numerous other cytokines are produced in adipose tissue as well, the signal for production of almost all of which is glucose influx. However, not all adipose cells are created equal. Adipose tissue can be subdivided in to three disparate categories: Subcutaneous Adipose Tissue (SAT), deep-layer SAT, and Visceral Adipose Tissue (VAT). For our purposes we will consider deep-layer SAT and SAT to be the same thing. It’s worth noting that men tend to store more fat in deep-layer SAT and women tend to store more fat in SAT. For the most part these two types of tissue behave identically. The only major difference between the two is that deep-layer SAT tends to be slightly more responsive to NE-induced lipolysis. For the rest of this article I will treat them as though identical.
VAT and SAT however are vastly different. Their respective physiology is summarized in table 7.1.
Effecter VAT vs. SAT Effect
Catecholamine induced lipolysis VAT > SAT Increased fatty acid turnover in VAT
Anti-lipolytic effect of insulin VAT < SAT Increased fatty acid turnover in VAT
Glucocorticoid receptors VAT > SAT Elevated lipoprotein lipase activity
11-Beta-HSD-1 Activity VAT >> SAT Local conversion of corticosterone to cortisol
Leptin secretion VAT << SAT Lowered SNS regulation of VAT implies reduced insulin sensitivity
Adiponectin secretion VAT < SAT Lowered whole body fatty acid oxidation and increased insulin resistance
Angiotensinogen secretion VAT >> SAT Enhanced pre-adiopcyte recruitment
IL-6 secretion VAT > SAT Inflammation, Cardiovascular risk, and reduced IGF-1 signaling
Plasminogen Activator Inhibitor – 1 (PAI-1) VAT > SAT Cardiovascular risk
PPAR-gamma induced pre-adipocyte recruitment VAT < SAT PPAR-gamma mediates SAT cell number.
Cortisol, Androgen, and Angiotensin II induced pre-adipocyte recruitment VAT > SAT Androgens, Cortisol, and AngII determine VAT tissue distribution.
Apoptosis VAT > SAT VAT cells tend to go into cell death
Return to pre-adipocyte physiology VAT < SAT SAT cells tend to return to being pre-adipocytes instead of undergoing programmed cell death
Visceral adipose tissue is located in the abdomen behind the abdominal muscles, near your organs. Because of its location and altered cell physiology, VAT is tightly linked to Syndrome X/Metabolic Syndrome/Polycystic Ovarian Syndrome (PCOS)/Childhood Adrenal Hyperplasia (CAH). The collection of syndromes and diseases above all have one thing in common: elevated corticosteroid production and often elevated androgen levels in women.
Visceral adipose tissue is often associated with morbidity. This is attributed to the so- called portal vein theory. The portal vein theory states that because of VAT cell location, it dumps a lot of free fatty acids and adipokines into the portal vein. As stated above, the portal vein is responsible for non insulin-dependent glucose disposal. The excess free fatty acids in the portal vein reduce glucose uptake and prevent this glucose disposal mechanism from operating properly. This results in hyperglycemia and elevated basal insulin levels throughout the day.
Furthermore, interaction at the liver with all of these cytokines alters the entire body’s physiology, leading to a host of unhealthy conditions such as high blood pressure, cardiovascular disease, cholesterol abnormalities, reduced insulin sensitivity, increased plasma glucose levels, hyperleptimania, elevated plasma insulin, increased reactive oxygen species production, pre-mature death, etc..
Just as fat cell location is important, so too is adipose cell size. Normally, as adipose cells grow in size, they increase cytokine production and secretion linearly with respect to their volume. However at a certain point, adipose cells seem to undergo a metabolic shift: the amount and type of cytokines they secrete changes. This so called Hypertrophied Adipose Tissue (HAT) does not behave as expected. Not only does it alter its secretion patterns, it also becomes insulin-resistant. Until recently it was not well understood how these changes were taking place, however a very recent study sheds a lot of light on the issue.
For a long time it was just assumed that more fat cells interacted with each other to alter adipose-derived hormone levels. Researchers believed that this interaction led to the alterations in adipose tissue physiology seen in the obese. However a study by Le Lay et al. (28) seriously disputes this endocrine-centric perspective.
The mechanism these doctors proposed was one of altered cholesterol regulation. It has been identified that in HAT cells, cholesterol starts to appear in the perilipin encased lipid droplets. This is quite abnormal. Normally your body is not so keen on storing cholesterol. The authors proposed that as fat cell size increases, the amount of cholesterol present in the cell membrane relative to surface area decreases. The authors further suggest that because cell-derived cholesterol was not present in the membrane, it was free to interact in the fat cell, and that these interactions were regulating the HAT cell’s odd physiology.
To examine this behavior they utilized unpopulated cyclodextrins. Cyclodextrins are a doughnut shaped molecule; the outside ring is hydrophilic yet the inside ring is lipophylic. This is why they are often used to deliver lipid-based molecules like pro-hormones across mucus membranes. By using unpopulated cyclodextrins the scientists were able to literally rip the cholesterol out of the cell membrane to see what kind of effect it would have on adipose cell physiology.
What they found is summarized in Table 7.2 below:
Protein Cholesterol depleted vs. Standard adipose cells.
Genes positively regulated by cholesterol depletion –
Genes negatively regulated by cholesterol depletion –
Mildly unaffected genes –
Caveolin 1 1.8
VLDL receptor 1.1
Insulin receptor 1.1
Fatty Acid Transolcase (FAT) 1.1
The mechanism behind this altered physiology was identified as a reversal of the serum responsible element binding protein (SREBP) ratio, or the SREBP2/SREBP1a ratio. Normally SREBP1a is the dominant isoform of the SREBP family; it activates numerous gene promoters. SREBP’s are how foodstuffs bring about gene transcription.
The SREBP family is normally restrained in a holding cell in the endoplasmic reticulum. Insulin however frees SREBP’s to move about the cell and activate gene promoters through hetrodimerization. SREBP1a is particularly interesting in that it promotes both the lipogenic hormone FAS and the lipolytic hormone leptin. What is fascinating is that what activates SREBP1a determines which of the two genes it promotes. If E-Box binding is promoted then FAS is transcribed but if SREBP is promoted then leptin is transcribed.
By reducing membrane cholesterol in adipocytes the SREBP ratio became inverted. As explained, SREBP1a is typically dominant. However, in the cholesterol-depleted cells SREBP1a decreased, SREBP2 nearly doubled, and SREBP1c was unaltered.
These results taken together with those demonstrating increased VAT tissue provide a nice working example of what the adipose derived endocrine profile of the obese looks like.
Of no surprise to you I am sure, as a person slowly overeats their fat cells grow. This causes linear elevations in adipose derived cytokines to be present in the blood. This ideally serves to induce the rest of the body to burn off this excess stored energy, though it is ill equipped to handle this task when people continually and chronically overeat. Eventually, the cell reaches a point where it no longer populates the membrane with cholesterol. Because of this we end up with the nasty endocrine profile summarized above. The large increase in angiotensinogen—a strong regulator of VAT cell growth—is particularly disturbing. Along with this VAT growth, hypertrophied fat cells also promote high blood pressure. Taken together this hormonal profile induces insulin resistance, hyperglycemia, and hyperinsulinemia, all of which promote a slow metabolism and metabolic deregulation.
Skeletal muscle serves an important role in metabolic regulation: it is the primary site for glucose disposal in the body. If skeletal muscle fails to dispose of blood glucose the body will spend less time each day in the hypoglycemic state, and will as a result have a lower metabolism.
Skeletal muscle glucose control is strongly linked to fatty acid metabolism, since fats and carbohydrates serve as the primary fuel source for muscle. Fuel selection operates in a seesaw or pendulum fashion. That is to say, if one oxidizes a large amount of glucose, later the pendulum will swing the other way resulting in the oxidation of a large amount of fatty acids. This back and forth regulation can be seen at the cellular level.
When excess amounts of fatty acids are oxidized, large amounts of acytal-CoA are produced. Acytal-CoA inhibits phosphofurctokinase-1 (PFK1), which is the rate-limiting step in the utilization of glucose-6-phosphate (G6P). It’s important to note that transport into the cell, not PFK, is the rate-limiting step in glucose utilization. Once inside the cell however, glucose is converted to G6P, at which point it is added to the pool. PFK-1 does limit the utilization of G6P. So, fatty acid oxidation can inhibit glucose utilization so long as excess glucose is not entering the cell (30).
Glucose oxidation also inhibits fat oxidation. Excess glucose oxidation results in the production of citrates. Citrates inhibit AMPK and thus promote the action of ACC. ACC takes the acytal-CoA that was antagonizing PFK-1 and turns it in to malonyl-CoA, which then inhibits CPT, preventing fat oxidation. It is the acytal/malonyl ratio that determines nutrient usage at rest (31). This ratio is largely mediated by concentration, with glucose being generally favored (thought as we will see this is not always the case).
Obese subjects often begin to store fatty acids in their muscle cells. Known as intra-myocellular lipids (IML), these muscle resident fatty acids are the bane of an obese person’s physiology. Through increased oxidation these fats lead to insulin resistance. What’s more, because of the elongated shape of these fatty acids, storage in the muscle cell often gets in the way of enzymatic reactions. Additionally, increased IML peroxidation by peroxisomes produces cellular hydrogen peroxide and reactive oxygen species that attenuate insulin signaling even further. Coupled with the fact that excessive leptin levels tend to cause insulin resistance, and we are left with a truly frightful scenario.
Excess glucose influx into muscle cells inhibits further uptake by two distinct mechanisms. First, it results in glucose being directed into the hexosamine pathway. Hexosamines then inhibit GLUT4 translocation to the cell membrane and prevent further glucose uptake (32). Second, excess glucose influx sends large amounts of glucose through the glycolytic pathways. This results in a spike in reactive oxygen species—specifically hydrogen peroxide (34). Excessive cellular hydrogen peroxide then inhibits insulin’s signaling cascade, and further prevents glucose uptake. This attenuation remains until hydrogen peroxide is buffered by gluthoine, cystine or dihydrolipoate (the metabolite of alpha-lipoic acid) (35).
Muscle contraction, NE, and glucagon serve to activate AMPK. This results in non-insulin dependent glucose uptake in the muscle cells. However, activation of AMPK actually slows cellular metabolism. So in the short term it mediates glucose disposal, however in the long term it can decrease glucose disposal by reducing cellular energy requirements.
Lets take a moment to step back and summarize. Obviously the system is meant to operate like a pendulum, vacillating throughout the day between hypoglycemia and hyperglycemia. Problems start to arise when we eat too much food, and in particular when we eat too much fat. As fat cells fill they start to become cholesterol depleted. This promotes VAT growth, and causes high blood pressure and poor cortisol metabolism. This results in blood sugar control issues. This subject (even if not yet obese) is spending less and less time each day in the hypoglycemic state, resulting in a suppressed metabolism. However this situation is still reconcilable. The situation does not go totally awry until leptin resistance sets in. With the onset of leptin resistance, the subject in many ways crosses a point of no return.
Leptin resistance is very interesting; its very existence has in the past been hotly debated, though now it is generally accepted that it does indeed occur. Why was this ever debated ?—because though some of leptin’s effects disappear with obesity (hence resistance), some are enhanced and/or never completely disappear. This apparent disparity can basically be divided into effects that are caused by transcription (exhibit resistance) and those that are induced by ion channel modulation (don’t exhibit resistance). This should not be surprising. As we discussed in Leptin: The Next Big Thing V, there is a negative feedback path in the STAT signaling cascade (though leptin’s effects on ion channels are not necessarily mediated though the JAK/STAT pathway).
Lets use the above model to walk though what happens when leptin resistance manifests itself. Leptin’s signaling in the VMH is not dependent on the JAK pathway. In fact, even in the obese, VMH signaling is still reduced by leptin. However in obesity there exists so much leptin, even after accounting for reduced BBB transport, that it constantly suppresses firing in the VMH. This totally removes the brakes on the PVN.
Essentially, the VMH-PVN link construes blood glucose to be constantly low. Even though obese people often exhibit hyperglycemia, their brain can’t decipher what’s going on because high leptin concentrations are totally overpowering the glucose signal in the VMH. This is why leptin injections do not do anything for the obese.
So at this point we have a hyperactive PVN that is just itching to activate the SNS and HPA. Now the PVN can operate in either TRH or CRF mode, an outcome that (as stated before) is controlled by NPY. Now you will recall that NPY creation in the LH is not mediated by leptin directly, as the LH does not possess leptin receptors. Leptin and insulin work together through the ARC to make alpha-MSH, which itself acts to reduce NPY. As you will recall the creation of alpha-MSH is not performed though ion channel modulation, but is instead regulated by PMOC protein transcription. Because of this, it is subject to leptin and insulin resistance. Bingo!
NPY is downregulated, and again the body thinks your starving. NPY interacts with the PVN and switches it into CRF mode. The fact that alpha-MSH is lowered also reduces TRH production, resulting in lowered thyroid hormone. This time around the TRH-TSH systems are indeed the prime determiners of thyroid function, as leptin resistance in the thyroid gland has stopped its regulation at that site. At the same time the PVN is primed to overactivate the HPA. This results in massive increases in cortisol, epinephrine, and in women, elevated androgen levels—symptoms that resemble PCOS or adrenal hyperplasia.
Both cortisol and elevated androgen levels promote VAT growth—vicious cycle number one. Elevated epinephrine results in increased blood pressure, which again alters vasculature and ensures that the subject spends more time each day with elevated blood glucose. Epinephrine also reduces insulin-simulated glucose disposal, again keeping blood glucose elevated—vicious cycle number two. Excess cortisol increases muscle protein breakdown, and further slows metabolism—vicious cycle number three.
With the PVN firing constantly the SNS is innervated, releasing NE into the blood stream. Eventually the SNS is over innervated and it receives too much dopamine. This results in the slowing of NE outflow from the SNS so commonly seen in the obese. However, some segments in the SNS that are rarely activated normally, are now getting just the right amount of dopamine to activate—specifically the segments feeding the kidneys. This further promotes high blood pressure, especially in men.
The near constant elevation of CRF brought about by increased NPY causes reduced sex hormone secretion as evidenced above. What makes this state so sad is that ironically all of these changes actually promote more leptin synthesis. Beta-adrenoreceptors (B-AR) are primarily mediated by androgens and thyroid hormone. Androgens determine B-AR density, and thyroid hormone determines their activity at the cellular membrane. Thus the reduction in thyroid hormone and androgens lowers basal lipolysis. This reduction in lipolysis, accompanied with elevated plasma glucose levels, leads to even more leptin synthesis. So as you can now see, the ridiculously leptin levels are actually sustaining an obese person’s weight. In order to step out of this vicious cycle, the obese subject needs desperately to reduce plasma leptin levels.
The Disease of the Industrialized World
As I stated before there are numerous ways to become overweight, though it all generally starts with fat cell hypertrophy or elevated blood pressure. I stated previously that several factors are most likely to blame for America’s current obesity epidemic in addition to the obvious culprit, excessive caloric consumption. These factors are:
Over consumption of refined carbohydrate sources:
In the modern world we eat far too much refined starch and sugar. Ideally what type of carbohydrates we consume should not matter. Refined carbohydrates elicit a large increase in plasma insulin levels, however. This drives blood glucose too low and serves to increase hunger shortly after eating. For our ancestors this was not a problem, as they could not simply open the cupboard when they became hungry. For you and me this is a serious issue. It results in excessive caloric consumption with a reduced sense of fullness. Refined carbohydrates are also digested much more quickly. This increases firing of the vagal nerve to the medulla oblongata, which ultimately leads to a lowered estrogen setpoint, reduced testosterone levels, as well as elevated cortisol release.
Over consumption of fructose:
Fructose is an odd carbohydrate. It is really only usable by your liver as it requires GLUT5 for transport into cells. Fructose is essentially the opposite of refined starches as described above. Because of the special way in which fructose is metabolized it skips the rate-limiting step of PFK-1. Because of this it oxidizes in the liver incredibly quickly and tricks your liver in to thinking you have plenty of glucose even if you don’t. This affects the hepatic glucose sensor described previously. It causes increased GABA delivery to the PVN, thus slowing metabolism. It can induce hyperglycemia (our arch nemesis) by interactions with several systems. As stated, it reduces PVN firing so that means less thyroid, cortisol, and NE release.
More importantly, fructose is non-insuligenic. Because of this it does not stimulate leptin or alpha-MSH production. So the LH in your brain never deciphers that the body is being fed. The third problem with fructose is that it fails to activate the portal vein glucose sensor, and thus it does not activate this essential glucose disposal mechanism. So in summation, fructose: slows metabolism, lowers leptin, fails to decrease hunger, and causes hyperglycemia. All in all too much fructose is just plain bad. Keep in mind a little fructose can be good, especially if you are an athlete. However for someone that is almost never glycogen depleted, fructose is detrimental and only does harm (36).
Over consumption of grains at the expense of vegetables:
Over consumption of grains at the expense of fibrous vegetables causes two problems. First, to a lesser extent, the arguments given for refined carbohydrates apply to grains as well—at least when compared to vegetables. High yield grain agriculture is a relatively recent advancement in evolutionary terms. Put rather simply we were never meant to eat this much grain. Secondly, grain consumption at the expense of vegetables can result in low-grade metabolic acidosis (37). This has a bad effect on GH and insulin sensitivity as well as bone and muscle anabolism. Finally it can exacerbate the problems associated with excess fat consumption. The rate-limiting enzyme in fat oxidation is CPT. CPT is extremely sensitive to pH (38). The small reduction in pH seen during exercise completely deactivates CPT. This is why you tend to burn glucose exclusively during high intensity exercise. So, chronic sub-clinical metabolic acidosis can result in attenuated fatty acid oxidation. The result: more dietary fat is stored instead of utilized as fuel.
Omega-3 fatty acid deficiency:
Omega-3 fatty acids have so many diverse effects that I can’t hope to touch on them all in this article. However I will attempt to highlight the key points that are not often discussed. First and foremost omega-3 fatty acids need to be a constant part of one’s diet. This is because omega-3 fatty acids are preferentially released by adipose tissue during lipolysis. Thus over time most of the fat stored in your fat cells tends to be of the saturated, monounsaturated, and omega-6/9 polyunsaturated fatty acids. In our modern diets we tend to pick and choose which parts of animals we eat and we tend not to eat the same thing everyday. Our ancestors did not have such luxury. So even when we eat the occasional meal rich in omega-3 fatty acids it is not of tremendous benefit. Unless consumption is chronic, as in everyday usage, it can be difficult to maintain omega-3 fatty acid stores.
Secondly, DHA deficiency reduces BBB transport of glucose and also lowers KIR neuronal sensing of glucose. The last thing we want is deregulated brain glucose control, as it just makes all the systems so unstable. Rats that were purposefully given DHA deficient diets showed lower levels of GLUT3 protein in neurons. GLUT3 is the transporter that moves glucose in to neurons. Thus DHA deficiency can make the brain look resemble that of an obese person’s (39).
Overuse of stimulants:
Overuse of caffeine and other stimulants has negative long-term ramifications for body composition. Caffeine elevates cAMP, which activates AMPK in skeletal muscle. This is one way it aids in fat burning. However, AMPK also lowers cellular metabolism in the long run, reducing your energy requirements and slowing your metabolism. AMPK is basically a cellular brake; it is activated by the endocrine system during the hypoglycemic state. This serves to switch the cell to fat usage to preserve glucose for the brain. It also reduces the cell’s metabolism however, thus saving any blood glucose for the brain as well.
Secondly, caffeine exhibits some nasty effects brought about by being an adenosine antagonist. Caffeine reduces cell volume by acting as a cellular diuretic. Cell volume is intimately tied to anabolism. In fact it has been proposed that the majority of insulin’s anabolic action is exerted through increases in cell volume. Adenosine antagonist’s also induce insulin resistance, and cause one to spend more time each day in the hyperglycemic state.
Finally, caffeine interferes with the conversion of omega-3 fatty acids into EPA and DHA, thus emulating some of the negative aspects of omega-3 deficiency.
Overuse of ephedrine and other NE promoters is also counterproductive. As discussed above, NE activation in the PVN leads to long-term changes in the PVN that favor CRF over TRH. This both reduces metabolism and lowers sex hormones, resulting in decreased anabolism and increased muscle loss.
Over consumption of sodium:
Too much sodium has been implicated in accelerating obesity in those who are already prone to it. It is likely this occurs through several distinct mechanisms, the primary ones being increased blood pressure and increased renal re-absorption of glucose. Both of these conditions lead to elevated blood glucose levels.
Sodium’s effect in this regard can be seen in an interesting study on Sprague-Dawley rats. These rats are basically normal. If overfed, about half of the rats get fat and about half don’t—much like people. The authors decided to see what effect increased salt intake would have on those that are prone to obesity. Salt increased the size and reduced the number of adipocytes; in other words, it created hypertrophied fat cells as described earlier. The salt fed rats also had double the leptin levels of their non salt-fed counterparts. Thus sodium seems to accelerate the endocrine system’s adaptations to obesity (40).
‘Till Next Time
I hope you enjoy this culminating piece in the leptin series. In the future I plan to apply the above model to the different human phenotypes so that I can offer more practical advice for use in the real world. After all, this science stuff is fun, but in the end we’re all just interested in putting on some muscle and losing some fat. As always I will be available online at the Avant Labs forum to answer any questions you might have.
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