Gary Taubes' Good Calories, Bad Calories provided a nice and readable description of the current understanding of fat metabolism, in particular the major mechanism of how dietary calories wind up in fat cells, and how stored fat is made available for energy. The mechanism is fairly simple, and is a scientific "fact" as much as there ever can be one (lots of supporting evidence, no alternative hypotheses). Dietary fats, as well as those created in the liver from carbohydrates, are transported around the body in large molecules called lipoproteins. We've all been inundated with propaganda about lipoproteins, e.g. low-density lipoprotein (LDL) is "bad cholesterol", high-density lipoprotein is "good cholesterol", very low-density lipoprotein (VLDL) is "triglycerides", which are also "bad". The popular nomenclature is terrible and confusing.
Lipids are substances like fat and cholesterol which are not water soluble. To be carried in the blood (which is mostly water), lipids are carried inside of large lipoprotein molecules, which basically wrap up a droplet of lipids in a protein coat. Protein is water soluble, problem solved. The specific proteins on the surface of the lipoprotein allow it to bind to various receptors, so different lipoproteins can perform different functions, depending on receptor binding. Thus, cellular LDL receptors grab LDL from the blood so the cells can extract cholesterol, while HDL bind to receptors that allow it to take away "used" cholesterol for recycling in the liver, e.g. when cells die.
Most of the fat transported by lipoproteins is in the form of triglycerides (more technically known as triacylglycerol), a largish molecule consisting of three fatty acids attached to a "backbone" molecule of glycerol. Two kinds of lipoproteins carry most of the triglycerides: chylomicrons and VLDL. Chylomicrons are manufactured in the intestinal lining, packaging up digested fatty acids and cholesterol. The chylomicrons are (for reasons unknown to me) transported through the lymphatic system and dumped into the blood via the thoracic duct. Cells then have the opportunity to grab fat or cholesterol from the chylomicron, and some other changes happen to the surface proteins which rather quickly render it a chylomicron remnant. The liver vacuums up chylomicron remnants and repackages any lipids as VLDL (which also carries fat created by the liver from excess glucose). The VLDL then returns to the blood, and again cells can grab fats as necessary.
The triglyceride molecules carried by chylomicrons and VLDL are too large to pass across the cell membrane. In order to get some fat into a cell, the individual fatty acids must be released from the tryglyceride; fatty acid molecules can cross the cell membrane. The primary enzyme which performs this tasks is lipoprotein lipase, or LPL.
So that (long-winded) explanation gets us through part one of how fat is stored: LPL frees fatty acids from triglycerides in lipoproteins so they can get inside of the fat cells. Now, fat cells don't store fatty acids directly, but instead create their own triglycerides. However, the glycerol molecule itself also cannot cross the cell membrane. Instead, the fat cells ultimately make their own glycerol (actually a substance known as alpha glycerol phosphate) from glucose, which in turn must be supplied by the blood. Fat storage thus requires two crucial ingredients: action of LPL on chylomicrons or VLDL to free fatty acids, and availability of glucose including the ability to transport that glucose from the blood into the fat cell, which requires some specialized molecules called glucose transporters, or GLUTs.
Now Taubes points out that the primary control mechanism for both LPL activity and glucose transport is the hormone insulin. More insulin means more LPL and more glucose transport, thus more fat storage. Additionally, inside the fat cell lives an enzyme called hormone sensitive lipase, or HSL. HSL performs the same essential task as LPL, but from inside the fat cell: it frees fatty acids from stored triglycerides, so they can be made available to the blood (being carried away bound to the blood protein albumin). HSL response to insulin is opposite of LPL: less insulin means more HSL activity. So when insulin is high, fat tends to be stored, and when it is low, fat tends to be released. It's a nice tidy story, and gives a biochemical basis for the hypothesis that overconsumption of carbohydrates is what drives most obesity. Eating carbs not only raises insulin, it also makes available lots of glucose, thus supplying both of the critical ingredients for fat storage, while simultaneously suppressing the release of fat from fat cells.
I like this story, but have long had the nagging suspicion it is not complete. Consider, for example, the Inuit, whose traditional diet consists almost entirely of protein and fat. Protein does raise insulin. Insulin is the sort of the "key" for opening cells the macronutrients (protein, fats, and carbohydrates). Even if you don't eat any carbs, you need insulin to go up in response to protein consumption so your cells can take up the constituent amino acids and use them for building tissue, making functional proteins like hormones, etc. Protein consumption also triggers the pancreas to secrete another hormone called glucagon, which amongst other things blocks the entry of glucose into cells.
So, naively, a meal containing only fat and protein is somewhat blocked from having the fat stored, because glucagon inhibits the fat cells from taking in the glucose required to build triglycerides. But you do need to store some fat. Fat cells are a sort of energy reservoir, providing a steady source of energy between meals, so even if you eat zero carbohydrates, there should be a mechanism for storing a bit of fat. My guess was that this was accomplished through a precise balance of insulin, glucagon, and blood glucose. And it has to be precise, because too little storage and you run out of gas, but too much and you get fat and slow, making it more likely that you become polar bear food. But biological systems are rarely precise, rather achieving balance through robustness rather than precision. It also seemed like there should be some dose dependent mechanism for fat storage, e.g. eat more fat, store more fat. We certainly evolved that mechanism for storing away energy from carbohydrate-rich meals, and it seemed that something similar should be in place to take advantage of fat-rich meals, like bone marrow.
So this post at the Emotions for Engineers blog caught my attention, because at one point it mentions an alternative metabolic pathway fat storage. Sounded juicy, so I dropped a comment asking for elaboration, and was directed to information on acylation stimulation protein, or ASP. There seems to be a fair amount of confusion both in the scientific literature and on the Internet as to exactly how/why ASP did it's thing, and the implications for obesity. I did a big of digging, and though I certainly haven't solved the mystery, I did uncover some clues. This paper, in particular, provides a lot of useful information.
Fat tissue is increasingly recognized as an endocrine organ, generating several hormones related metabolism. You've probably heard of leptin. When fat cells expand from storing fat, they release leptin. Leptin does several things, most notably sensitizing other parts of the body such as the hypothalamus to the effects of hormones affecting satiety and gastrointestinal activity (see this excellent review for more). In short, when fat cells store more fat, they release more leptin, which makes you less hungry, until the fat cells shrink causing them to release less leptin, allowing you to get hungry again. There are many different such mechanisms regulating energy storage, metabolism, and hunger, forming a robustly controlled system, one that works well across a wide variety of input conditions.
ASP is another hormone secreted by fat cells, with several effects. First, ASP can increase LPL activity, making fatty acids available for transport into the fat cells. Second, ASP increases the expression of glucose transporters in fat cells, allowing them to bring in the glucose required to store fat. So ASP plays roughly the same role as insulin in fat storage, but rather than being generated by the pancreas in response to carbohydrates, is generated by the fat cells themselves. Better yet, ASP stimulates the production of triglycerides inside the fat cells. But what causes ASP to be secreted?
The answer, at least in test-tubes, is chylomicrons. When fat cells are exposed to chylomicrons they generate lots of ASP. By contrast, exposing the same cells to glucose, fatty acids, VLDL, HDL, or LDL elicits little ASP response. Further, the ASP response exhibits both a time and concentration dependence on chylomicron concentration.
This is an important clue. As discussed above, chylomicrons are the first step in transporting dietary fats into the body. When you eat a lot of fat, you make more chylomicrons, which causes the fat cells to make more ASP, which stimulates greater fat storage. But the chylomicrons only hang around for a relatively short time, being converted in the liver to VLDL. The receptor for VLDL (VLDL-R), when activated, does increase LPL activity, but to my knowledge does not stimulate glucose transport into fat cells. Thus the fat in VLDL is available to be used for energy, because the LPL frees the fatty acids for transport across cell membranes; but without some other hormonal signal (e.g. insulin), rather little of this fat can be stored in adipose tissue.
Two questions then arise in the context of a low-carbohydrate/high-fat diet. The most obvious one is "can I get fat by eating too much fat?" Taubes lays out the case that overconsumption of carbohydrates drives fat storage through the action of insulin, but can overconsumption of fat do the same via the action of ASP? When viewed with the most narrow lens, the answer is clearly "yes". While insulin's effects on LPL and glucose transport are considerably stronger than ASP, ASP does ultimately trigger the same conditions leading to fat storage. So if you eat enough fat for a long enough time, in principle you will become obese.
But if we take a step back, things are not so simple. The body has many feedback mechanisms for regulating energy content, such as leptin secretion by large fat cells, leading to suppression of appetite. These mechanisms regulate feelings of hunger, metabolic rate, how fast the stomach empties, etc. The system has presumably evolved to be robust over a wide range of environmental and nutritional conditions, allowing us to have enough energy to make it through times between meals while not having to carry so much that physical performance and other health aspects are compromised. The whole chain of events described above provides a nice example. Eat lots of fat, intestines create lots of chylomicrons. Chylomicrons stimulate fat cells to make ASP, which in turn increases fat storage. As fat cells store fat, they release leptin, which suppresses appetite and sensitizes the body to other satiety signals. But chylomicrons are fairly quickly turned into VLDL, which do not stimulate fat storage, but do make fat available for energy. The brain can detect VLDL levels, and regulate gastric emptying, appetite, etc. until the fat in the VLDL is used up. And that's just one of a complex web of interactions between hormones, the nervous system, metabolism, and digestion.
To become obese (at least without trying really hard), some key regulatory mechanism needs to be broken. For instance, there is a genetic defect which causes the fat cells to not produce leptin. People (or mice) with this defect have an unstoppable appetite, and become extremely obese. Treating them with leptin can reverse this condition. Another example is Cushing's disease, which is a small tumor on the pituitary. The net effect of Cushing's disease is that it causes the body to have high levels of the hormone cortisol. I had a friend with Cushing's disease. He ran five miles every day, and by any measure ate a healthy diet, yet continued to gain weight. Why? Increased cortisol (from the sympathetic endocrine system) can cause compensatory secretion of insulin (from the opposing parasympathetic endocrine system). Chronically high insulin will make you fat no matter how much you exercise or how little you eat. Keep insulin high, and you can literally starve to death while remaining obese.
But it appears the big hitter is carbohydrate consumption, particularly refined carbohydrates. These cause both drastic increases in insulin levels and make available lots of glucose for triglyceride storage. Though insulin nominally acts to suppress appetite and GI motility, high levels drive energy nutrients out of the blood and into the cells, ultimately leading the brain to "override" other mechanisms such as leptin, because low levels of energy nutrients in the blood basically signal imminent starvation; indeed, the brain itself needs a certain level of blood sugar to be maintained for proper operation. So eating carbs not only causes you to efficiently store fat, it also drives you to eat more food, and that food is typically more carbs to stabilize your blood sugar, leading to a vicious cycle.
I don't see a similar issue when eating a high-fat/low-carb diet. Fat ingestion does not cause hormonal derangement. Energy levels in the blood are maintained, allowing the various appetite regulation mechanisms to operate normally without getting an emergency override to eat more food despite available energy in the body. ASP production is stimulated only by chylomicrons, which are relatively short-lived, allowing a limited amount of dietary fat to be stored, while the rest is made available as energy. In principle, you could get fat by eating enough fat, but in practice it would probably be very difficult. You would have to force yourself to eat even though you felt extremely full, and continue to do so over a long time period. Not impossible, but definitely an uphill battle against a whole host of hormonal and nervous control systems, very much the analog of trying to lose weight on a low-fat/high-carbohydrate diet.
While it may be hard to gain fat through a high-fat diet, it is likely possible to keep on a certain level of body-fat. Low-carbohydrate diets are known to "stall", where the last 20 or so pounds just won't come off, regardless of carbohydrate restriction. I suspect our friend ASP plays a crucial role here. The low insulin levels on a low-carb diet will allow the fat cells to free fatty acids, but if you are consuming enough fat, at some point this effect will be balanced by that of ASP, and voila, no more fat loss.