Fat Metabolism – why humans get chronically fat

  •         Both dietary fat and a considerable portion of the carbohydrates we consume are stored as fat—or, technically, triglycerides—in the adipose tissue before being used for fuel by the cells.  Fat in the cells of the adipose tissue is in a continual state of flux.
  •         Insulin plays a preeminent role in the control of fat metabolism. Just as diabetes has traditionally been perceived as a disorder of carbohydrate metabolism—even though fat metabolism is also dysfunctional—insulin has always been perceived as a hormone that primarily functions to regulate blood sugar, though, as we’ve discussed, it regulates the storage and use of fat and protein in the body as well. Because blood sugar could be measured easily through the first half of the twentieth century, but not yet the fats in the blood, the focus of research rested firmly on blood sugar.
  •         obesity is caused by a defect in the regulation of fat metabolism. At the risk of repetition, it is important to say this is, by definition, a disorder of fat accumulation, not a disorder of overeating. For whatever reason, the release of fat or its combustion is impeded, or the deposition or synthesis of fat is promoted, as Astwood said, and the result is obesity. That in turn will cause a deficit of calories elsewhere in the body—Astwood’s “internal starvation”—and thus a compensatory hunger and sedentary behaviour.
  •         Man alone in the last ca. 40 years (and our pets) grows chronically obese, not other species of animals.  The likely explanation is the effect of diet on this regulation of fat metabolism and energy balance. Since insulin is the hormone responsible for promoting the incorporation of fat into our adipose tissue and the conversion of carbohydrates into fat, the obvious suspects are refined carbohydrates and easily digestible starches, which have well-documented effects on insulin. This is what Peter Cleave argued, albeit without understanding the underlying hormonal mechanisms at work, and what the geneticist James Neel, father of the thrifty-gene hypothesis, came to believe as well.
  •         It’s the effect of these carbohydrates on insulin that would explain the dietary observations—the futility of calorie restriction, the relative ease of weight loss when carbohydrates are restricted, and perhaps two centuries of anecdotal observations that sweets, starches, bread, and beer are uniquely fattening.
  •         People / mice / mammals are overeating because they’re socking all the calories away into adipose tissue and they can’t get to those calories. They’re not getting fat because they’re overeating; they’re overeating because they’re getting fat. It’s not a trivial difference. The causality is quite different.”
  •         When you don’t have carbs in your diet, you will mostly be burning fat. But if your intake more calories (fat or protein or carb) than required, then the excess fat will remain in the fat stores.
  •         Any macronutrient (macros in food are carb, fat & protein) will make you gain fat if you overeat them.
  •         It’s as though our cells have the option of using fatty acids or glucose for fuel, but when surplus glucose is available, as signaled by rising insulin or blood-sugar levels, the fatty acids are swept into the fat tissue for later use.
  •       First, the body will burn carbohydrates for fuel, as long as blood sugar is elevated and the reserve supply of carbohydrates stored as glycogen in the liver and muscles is not being depleted. As these carbohydrate reserves begin to be tapped, however, or if there’s a sudden demand for more energy, then the flow of fatty acids from the fat tissue into the circulation accelerates to take up the slack.  Meanwhile, a significant portion of the carbohydrates we consume and all of the fat will be stored as fat in our fat cells before being used for fuel. It’s this stored fat, in the form of fatty acids, that will then provide from 50 to 70 percent of all the energy we expend over the course of a day.
  •         Some of the triglycerides in our fat tissue come from fat in our diet. The rest come from carbohydrates, from a process known as de novo lipogenesis, which is Latin for “the new creation of fat,” a process that takes place both in the liver and, to a lesser extent, in the fat tissue itself.
  •         A single molecule plays the pivotal role in the system. It goes by a number of names, the simplest being glycerol phosphate. This glycerol-phosphate molecule is produced from glucose when it is used for fuel in the fat cells and the liver, and it, too, can be burned as fuel in the cells. But glycerol phosphate is also an essential component of the process that binds three fatty acids into a triglyceride. It provides the glycerol molecule that links the fatty acids together.
  •         The glycerol phosphate molecule that links the fatty acids together is a product of carbohydrate metabolism—i.e., burning glucose for fuel—is an essential component in the regulation of fat metabolism: storing fat in the fat tissue. In fact, the rate at which fatty acids are assembled into triglycerides, and so the rate at which fat accumulates in the fat tissue, depend primarily on the availability of glycerol phosphate. The more glucose that is transported into the fat cells and used to generate energy, the more glycerol phosphate will be produced. And the more glycerol phosphate produced, the more fatty acids will be assembled into triglycerides. Thus, anything that works to transport more glucose into the fat cells—insulin, for example, or rising blood sugar—will lead to the conversion of more fatty acids into triglycerides, and the storage of more calories as fat.
  •         Dietary glucose is the primary source of glycerol phosphate. The more carbohydrates consumed, the more glycerol phosphate available, and so the more fat can accumulate.
  •       The bottom line: as the blood-sugar level decreases, fatty-acid levels rise to compensate.
  •         There are several steps before protein can be converted to body fat, ends up with an efficiency / availability of only 35% of the original calorie content.
  •         As for fat, to convert to body fat, the fatty acids must combine with glycerol-3-phosphate (triglyceride) to make triglycerol; but to produce triglyeride, you must consume carbohydrates. Without carbohydrate consumption, storage of fatty acids is difficult.  This is why foods containing the combination of carbohydrates and fat are very fattening combination. This combination is found nowhere in nature, but it is found in much western food: cakes, muffins, donuts, deep fried foods, snack foods, etc (The Calorie Myth).