A few caveats first. Metabolic regulation is a complex and evolving subject, and much of the knowledge is very recent (if you want to give yourself a headache, check out this spreadsheet I made trying to illustrate the various parts and their relationships). Even if you were to consider all of the available science I doubt the picture is anywhere near complete, and of course I've probably only been exposed to some smallish subset of what is known. If anybody out there finds gaps in this presentation, please fill them in via the comments. Additionally, much of the research on metabolic regulation is done on animals and extrapolated to humans. Nobody is going to do experiments where, say, they directly infuse oleic acid into the brains of people. Some of the published reviews are unfortunately vague as to whether the mechanisms discussed have been studied in humans.
One final issue is that the reviews are very focused on dietary fat, and to a lesser extent carbohydrates. But protein is essential for life, so there must be appetite and metabolic controls regulating protein intake, but this is largely not discussed. For instance, I'm guessing somewhere in the body there's something that detects amino acids and influences appetite, particularly preference for protein-rich food.
With that in mind, we'll start at the beginning. Animals eat when they're hungry. In a healthy organism, hunger is a signal that available and/or stored energy is getting low and need to be replenished. Humans have three primary energy stores: the stomach, glycogen (starch) in muscle and liver tissue, and fat (triglycerides) in adipose tissue. Now, if this system is working right, low available energy should be equivalent to low stored energy. But we're going to see it's quite plausible that conditions can arise where the body thinks available energy is low, yet excess energy is in storage.
The brain acts as a central controller, receiving various signals from the body and adjusting many different "knobs" to maintain a healthy state. Peripheral tissues also exercise some independent controls as well, e.g. the pancreas will secrete insulin in response to rising blood glucose without nervous system control. This combination of central and peripheral controls provides for both robustness and responsiveness.
The major nervous system player in metabolic regulation is the hypothalamus, an area at the base of the brain, roughly the size of an almond. The hypothalamus is the main connection between the rest of the brain and the various hormone systems of the body, sharing a private circulatory system with the pituitary gland, and projecting nerve connections to various other endocrine organs as well. The hypothalamus is also well situated to sample various chemical concentrations in the blood. Most of the brain is protected by the "blood-brain barrier" (BBB), closely-packed cells which tightly control what substances pass from the blood to the brain. But the hypothalamus is located near a region where the BBB is incomplete. It's leaky, in a sense, so the hypothalamus gets a taste of much of what's in the blood. The hypothalamus can be further divided into "nuclei", which have different sensory and control functions. Of particular interest here are the arcuate nucleus (ARC), the ventromedial nucleus (VMN), and the dorsomedial hypothalamus (DMH).
The brainstem is a close neighbor of the hypothalamus, sharing lots of neural connections. The particular region called the nucleus of the solitary tract (NTS) is the termination of the afferent fibers of the vagus nerve (afferent nerves cause signals to arrive at the brain; efferent nerves allow signals to exit the brain). The vagus nerve connects to many different organs, including those of the digestive system. The NTS appears to integrate different signals (both hormonal and nervous) and send them along to the hypothalamus. The hypothalamus does some additional integration, and projects to other brain areas involved in behaviors like finding food and eating it.
I've used the term "integrate" a couple of times. What does that mean? The neurons in the brainstem and hypothalamus receive many different signals: from other nerves, from hormones like insulin, and can directly sense nutrients like glucose. The "decision" of whether the neuron fires or expresses certain proteins must factor in all of these signals. For example, the brain requires a certain blood glucose concentration to function properly. If glucose falls, regardless of the level of insulin, the brain should take some action (like stimulating appetite), because otherwise you'll die.
The ARC in particular contains two populations of special neurons. One of these expresses cocaine- and amphetamine-related transcript (CART) along with pro-opiomelanocortin (POMC). These neurons seem to be associated with appetite suppression. For instance, POMC can be chopped up to yield alpha-melanocyte-stimulating hormone (alpha-MSH), which in turn binds to the melanocortin-4 (MC-4) receptor. Genetic problems causing defects in the MC-4 receptor result in obesity characterized by overeating. The other population expresses agouti-related protein (AgRP) and neuropeptide-Y (NPY), both of which increase appetite. If NPY is infused into rat brains, they respond with a several-fold increase in food intake that lasts 6-8 hours, similar to rats that have been fasted for 36-48 hours.
Having two opposing systems (as opposed to just one that gets turned up or down) allows for rapid fine-tuning of metabolism; this idea of opposing systems which maintain balance is found elsewhere, e.g. the sympathetic and parasympathetic endocrine systems. Both classes of neurons appear to "detect" both available energy in the blood as well as hormonal levels and probably nerve signals, with opposing results. Energy nutrients go through part of the same cycle used to actually generate energy, and the resultant metabolic products appear to trigger opening/closing of ion channels on the cell membrane. Hormones like leptin and insulin have similar effects, hence the "integration" of these signals. Changing the balance of ions inside and outside the neuron affect the "action potential", make it more or less susceptible to firing, expressing proteins, etc.
NPY neurons, for example, are glucose inhibited (GI), meaning the more glucose is around, the less active they beome. If blood sugar falls, the NPY neurons become more active, and as we saw above, NPY appears to strongly stimulate appetite. So blood sugar falls, and you get hungry. Similarly if insulin or leptin falls, these neurons are activated, and again you get hungry. But what if insulin is high AND glucose is low? Well, the brain needs a certain glucose level to operate, so I would guess that the low glucose wins, because the alternative is a hypoglycemic coma and death. Have you ever had a major blood-sugar crash a few hours after a large carbohydrate-laden meal? It's the "Chinese food makes you hungry an hour later" thing (see e.g. Teriyaki Stix Beef Bowl: 102g of carbohydrate, probably all highly refined). I would bet the extreme feelings of hunger (kind of like you were starved for 36-48 hours) is the result of increasing NPY concentrations in the hypothalamus, in turn triggered by low blood glucose, even though your insulin is still elevated. Rats show a preference for high-carbohydrate meals when stimulated with NPY. If the same is true for humans, then we shouldn't be surprised that a blood-sugar crash sends us scurrying for the vending machine to fearlessly slay and consume a candy bar, regardless of how much energy is stored in the stomach or fat. So we begin to see how the system can be broken to store excess energy, mainly fat.
The scenarios described above relate more to the instantaneous availability of energy in the blood as opposed to the amount stored. The major energy store (in terms of calories) is white adipose tissue (WAT). Fat cells, or adipocytes, are not passive buckets, but rather metabolically active both in the storage/release of fatty acids as well as the secretion of hormonal signals relating to appetite and metabolic regulation. The best-known of these is leptin, a hormone whose secretion is proportional to the amount of stored fat. Leptin suppresses appetite, probably via multiple actions. Leptin inhibits the NPY/AgRP neurons (which stimulate appetite) and excites POMC/CART neurons (which decrease appetite). Leptin also slows gastric emptying, the rate at which food leaves the stomach and enters the small intestine. So more leptin (everything else being constant) should keep the stomach fuller for a longer time, and the stomach of course sends it's own signals relating to appetite and satiety. Leptin may also increase base metabolic rate via diet-induced thermogenesis, a topic we'll explore later. There is a genetic defect that causes people to secrete little or no leptin. Individuals with this genetic problem tend to overeat considerably, and extrapolating from rats may additionally have a lower metabolic rate, with the result of extreme obesity. Administration of leptin to these individuals substantially aids this condition.
Fat cells secrete other hormones as well. Adiponectin secretion is inversely correlated with stored fat: more fat, less adiponectin, and vice versa. Adiponectin has potentially influences many things, including appetite, insulin sensitivity, inflammation, and vascular function. Interleukin-6 (IL-6) causes insulin resistance in fat cells, which tends to make them release fat instead of store fat. The hypothalamus also expresses and contains receptors for IL-6, particularly in areas controlling body composition. Fat cells also express tumor necrosis factor alpha (TNF-alpha), which inhibits lipoprotein lipase (the enzyme required to get fat out of lipoproteins and into fat cells), stimulates breakdown and release of triglycerides in fat cells, and may also induce insulin resistance.
So we see mechanisms in place to control fat storage through appetite. As more fat is stored, more leptin is secreted, which should blunt the appetite. As fat is lost, leptin levels drop, which should promote appetite. Leptin (and other hormones from fat cells) may additionally modulate metabolic rate, to encourage fat burning when there is an excess, and discourage it during a deficit. So again there's a lot of knobs to turn, all aimed at maintaining fat storage in a particular range.
Our final stop is the gastrointestinal (GI) tract along with the closely related pancreas. When we eat, food hits the stomach, which does a nominal amount of digestion both mechanical and chemical. The stomach represents short-term energy storage, more or less the "gas tank" for the body, and so it's no surprise that the stomach is involved in appetite as well. Indeed, most people think of appetite in terms of "my stomach is full/empty", but we've seen above that many other factors come in to play as well. The stomach signals the full/empty state both through nerves and hormones. Stretch receptors on the stomach wall send signals via the vagus nerve indicating fullness. The stomach also secretes the hormone ghrelin, which strongly stimulates appetite. Empty stomach means more ghrelin, full stomach means less. Increasing ghrelin increases brain concentrations of NPY. So an empty stomach definitely tends to increase your appetite, but gastric signals must be integrated with the variety of other signals to actually determine the degree of appetite stimulation.
Most of the hormonal action occurs in the small intestine and pancreas, and indeed there is some interplay between these organs. The pancreas is not only an endocrine organ (which sends hormones into the blood), it is also exocrine, emitting various substances like enzymes and bile salts require to break down food so it can be absorbed through the small intestine. The small intestine itself secretes a several hormones in various quantities, depending on the total caloric content as well as the individual levels of carbohydrate, protein (really amino acids), and fat. These hormones have a wide variety of effects, including stimulation/inhibition of pancreas endocrine and exocrine functions, modification of the rate at which food passes through the GI tract, metabolic control, and of course appetite. I'm not going to cover nearly all of these hormones or their effects. Check out the spreadsheet, or this paper and this paper for details.
A major hormone secreted by the small intestine is cholecystokinin (CCK, and no, I don't know how to pronounce it). Dietary fat and protein more potently stimulate of CCK release than does carbohydrate, and long-chain fatty acids seem to have a greater effect than short-chain. CCK affects a number of systems, e.g. inducing gallbladder contraction (to release the bile needed to digest the fat which stimulated CCK release in the first place). CCK also slows gastric emptying. Again this makes sense from a regulatory standpoint. Once the small intestine has received some energy nutrients, CCK signals the stomach to stop sending more until the present batch is done processing.
CCK also strongly suppresses appetite. In rats, administering CCK reduced food intake in a dose-dependent manner: more CCK, less food eaten. The exact mechanism is unclear, but it seems to be a combination of reduction in gastric emptying (stomach stays full) and detection by the nervous system. In both monkeys and humans, the fullness of the stomach seems to modulate the appetite suppression of CCK. The afferent fibers of the vagus nerve as well as the brainstem express CCK receptors. The Otsuka-Long-Evans-Tokushima fatty rat (try saying that 3 times fast) is a genetic variant which lacks the CCK-1 receptor, and both overeats and becomes obese.
A few notes on other GI hormones. PYY-36 is released in proportion to calories and meal composition, with fat resulting in higher concentrations than protein or carbohydrate, and may inhibit food intake. Glucagon-like peptides GLP-1 and GLP-2 are cleavage products of preproglucagon. GLP-1 increases insulin secretion and suppresses glucagon release. It also slows gastric emptying and inhibits food intake. Key areas of the brain such as the ARC express GLP-1 receptors. GLP-2 release is potently stimulated by fat and carbohydrates, and may enhance the digestive and absorptive capabilities of the small intestine. Oxyntomodulin (OXM) is released in proportion to calories ingested. It suppresses appetite and gastric motility, enhances insulin secretion, decreases food intake, and possibly increases metabolic rate.
So the takeaway here is that the GI tract sends numerous hormonal signals indicating energy is present and being absorbed, please don't send any more. One interesting side-note is that the levels of some of these hormones, notably PYY-3-36, GLP-1, and OXM, all increase after gastric bypass surgery. The effect of this should be to suppress appetite, and possibly increase metabolic rate, which would explain the success of such surgeries to reduce obesity. I find this interesting, because by itself I would guess reduction in stomach size should probably have little effect on overall food intake because of the other mechanisms regulating appetite based on stored and available energy. But diddle the relevant hormones, and voila, sustained appetite reduction and weight-loss. Hopefully the increasing understanding of these regulatory mechanisms will give rise to better treatments, since surgery seems an extreme way of accomplishing the desired effect.
Finally, we come to the pancreas. The best-known pancreatic hormone is insulin, arguably the Big Mama of metabolic regulation. It is interesting to note that the protein structures of both insulin and NPY are remarkably conserved across evolution. If you look at a primitive animal like a hagfish, it's insulin and insulin receptors are fairly similar to that of humans, so much so that hagfish insulin significantly stimulates human insulin receptors. The implication is that the role of insulin is central in metabolism and development, and fairly successful as relatively drastic changes across species required little mutation of insulin. We're most familiar with insulin's role in controlling blood sugar, both by increasing tissue uptake of glucose and by regulating glucose output from the liver. Insulin also regulates many other aspects of metabolism, like fat storage and cell division. Subsequent posts will visit these in greater detail.
Insulin is manufactured by the pancreatic beta-islet cells (B-cells). When glucose enters the B-cell, it is metabolized to ATP, the primary short-term "energy currency" of the body. But rather than using that ATP for energy, some of it closes potassium ion channels. This depolarizes the cell membrane, allowing calcium ions to enter the cell and causing stored insulin to be released. The presence of glucose in the cell additionally signals the cell to manufacture more insulin. Amino acids also trigger insulin release to varying degrees, depending on the particular flavor, as do ketone bodies. The effect of fatty acids is complex and not well understood. It appears that fatty acids are necessary for normal glucose-stimulated insulin secretion. Increasing fatty acid concentrations in the short term (1-2 hours) will cause more insulin to be released for a given glucose concentration. But long term, elevated fatty acids impair insulin secretion. Both the nervous system and other hormones also affect the amount of insulin released.
Insulin affects appetite, both directly and indirectly. The indirect path involves sensitization of the body to other satiety signals like CCK (a role shared with leptin). Insulin also appears to directly signal the hypothalamus, increasing activity of POMC/CART neurons and decreasing activity of NPY/AgRP neurons. We all know that the pancreas secretes insulin in response to blood glucose, but insulin secretion is also modulated by a number of other factors. We saw above how some GI hormones potentiate greater insulin release (the so-called incretin effect). Insulin levels are also a function of body-fat: the more fat that is stored, the higher insulin is in all states (fed, fasting, etc.) So insulin signals both energy availability and energy storage, but the primary effects indicate that over the long term insulin (along with leptin) signal how much fat is stored.
If insulin is infused directly to the brain (of a rat, presumably), the result is a decrease in food intake and loss of body weight in a dose-dependent manner. If insulin receptors are blocked, food intake and body weight increase. When insulin levels in the brain are held constant over long time periods via slow infusions, animals modify their diet and body composition until a certain body weight is achieved, and that weight is subsequently defended at a level determined by the insulin concentration.
We saw an example above where high insulin could be overridden by low blood glucose to cause hunger. Insulin suppresses appetite only when blood glucose is maintained at a proper level. Insulin-induced hypoglycemia (whether from a high-carbohydrate meal or administration of insulin) triggers an override mechanism in the brain, inducing hunger and eating to avoid going into a coma. Type 1 diabetics have the opposite problem: high blood glucose and low insulin. Type 1 diabetics are typically ravenously hungry despite high glucose, again showing the integrative capacity of the brain; yet they will fail to gain weight regardless of how much they eat, as the lack of insulin disrupts other metabolic functions.
The pancreatic B-cells also co-secrete another hormone called amylin. Insulin and amylin a secreted in a fixed molecular ratio of about 10 to 100 to one. Various disease states (including obesity) and pharmacological interventions increase the amount of amylin relative to insulin. While insulin appears to primarily signal stored fat levels, amylin signals both the amount of stored fat and energy availability from food intake. Amylin is secreted in proportion to body fat and meal size. Giving rats a does of amylin prior to a meal reduces meal size. Blocking amylin receptors produces a long-lasting increase in food intake and fat storage. Amylin appears to act in the area postrema (AP) of the hindbrain. AP neurons activated by amylin are also activated by glucose, CCK, and GLP-1, and the AP projects to the NTS, which in turn projects to areas of the hypothalamus regulating appetite and metabolism.
Type I diabetes occurs due to destruction of the pancreatic B-cells, so Type I diabetics also lack amylin, which is thought to contribute to their large appetites. Type II diabetics treated with insulin often gain more weight (duh), but this can be mitigated be treatment with an amylin analog. Some doctors are prescribing amylin analogs in obese patients who are not being treated with insulin. Since amylin serves both as a satiety an adiposity signal, this works as expected: these people both eat less and lose fat. But just as most obese people are insulin resistant, they're also amylin resistant. Administration of amylin to an already overtaxed system is, I think, a questionable long-term strategy. Additionally we know the body wants to keep the insulin/amylin ratio fixed, probably for a good reason. Adding exogenous amylin to the mix perturbs this balance even more than it already is, rather than helping to restore it to a healthy state.
Last, but not least, is glucagon, manufactured and secreted by pancreatic alpha-islet cells (A-cells). Metabolically, glucagon tends to counter the effects of insulin, e.g. increasing glucose output from the liver. Glucagon secretion is stimulated mainly by protein, possibly by fat, and not at all by carbohydrate; indeed, glucose inhibits A-cell glucagon secretion. Pancreatic hormones are dumped into the portal vein, so the liver gets first shot at them. Apparently the liver removes most of the glucagon, and rather little makes it into systemic circulation. Even so, glucagon acts as a satiety signal. Rather than acting directly in the brain, glucagon's action probably occurs in the liver, which then sends a signal to the brain via the vagus nerve. Animals whose afferent vagal nerves have been blocked do not have their feeding inhibited by glucagon.
So let's see how some different meals may affect appetite. We'll revisit these later, after we've gone through the other aspects of metabolic regulation and can look at the big picture; but the isolated effects on appetite are still interesting. These are my guesses, not proven by any scientific research. Feel free to add your own scenarios to the comments.
We discussed above what may happen after a high-carbohydrate low-fat meal, like the Teriyaki Stix Beef Bowl (102g carbohydrate, 33g protein, 7g fat). The stomach fills, reducing ghrelin secretion and sending the "full" signal to the brain. The large amount of refined carbohydrates should elicit a large insulin and amylin response, further potentiated by the release of hormones like GLP-1 and OXM. The protein in particular stimulates CCK release, which along with insulin, amylin, and other GI hormones suppress appetite. But the major insulin release induces hypoglycemia. The initial effects probably are an increase in gastric emptying via nervous system control to try and balance out the blood sugar, but of course this tends to raise insulin even more. Sooner or later the depressed blood glucose causes an increase in brain NPY, and powerful hunger, despite the fact that rather little of the meal may actually have been used for energy.
How about a "healthy" low-calorie meal, maybe a really big salad, lots of veggies and fat-free dressing. The conventional wisdom is that the large fiber load (and amount of water) fills up the stomach, contributing to satiety. That's true, to a certain extent, as filling the stomach triggers both the stretch receptors and reduces ghrelin. But the relative lack of any energy nutrients implies correspondingly low secretion of appetite control hormones like CCK. The brain also will not detect much rise in blood sugar or fatty acids. In turn, gastric emptying and intestinal motility is not inhibited and may in fact be accelerated, so the stomach empties faster than it would in a high-calorie meal. Appetite suppression from stomach distension rapidly fades, and you're hungry again.
How about a "cardiac arrest" meal of a big steak smothered in mushrooms and butter? This is a calorically dense meal, probably occupies considerably less stomach volume than the big salad, so maybe we don't get as much from stretching the stomach. But once this hits the small intestine, we get should get lots of hormones like CCK and glucagon to suppress appetite and gastric emptying. Some insulin and amylin are secreted as well. The glucagon helps keep blood sugar stable, and the additional protein from the steak may temporarily bump up blood sugar as well. The fat makes it into the circulation more slowly, and should help to both suppress appetite and gastric emptying over the longer term. Additionally, fat sensing by the hypothalamus also help the liver regulate blood sugar, so we don't get the "low glucose" override.
So to summarize:
- The high-carbohydrate high-calorie fast-food meal makes you get hungry faster due to insulin-induced hypoglycemia. This happens despite consumption of plenty of energy.
- The low-calorie low-glycemic salad fills you up in the short term, but you get hungry again quickly simply due to lack of available energy.
- The high-fat high-calorie meal suppresses appetite for a longer time, both by avoiding adverse conditions like hypoglycemia, as well as providing a measured release of energy into the blood via the small intestine.
These examples are interesting (so I think), but again must be considered in the larger context of metabolic regulation. Obesity is not a simple result of overeating, fat-loss not the simple result of undereating. Both are a combined effect of different regulatory mechanisms. Appetite is just one piece of the puzzle. Genetically-modified rats, for example, illustrate different behavioral and physical outcomes depending on the nature of the mutation. Some overeat and maintain normal body weight. Some eat normally and get fat, and some both overeat and get fat. Conversely, to lose fat almost certainly requires restoration of the proper regulatory balance. By itself, the recommendation to "eat less and move more" is meaningless. We need to consider the effect on the hormonal and nervous system mechanisms, which require detailed thinking about the effects of food and exercise on human biochemistry.