Why Are We So Fat?

Obesity rates in America are disturbingly high. The CDC estimates that around 37% of adults in the United States are obese and about 70% are either overweight or obese (NIDDK). While these statistics may be surprising to some, it is unlikely that most readers are completely taken aback by the premise that the majority of the adult population in the United States does not maintain a healthy body weight. After all, over the past decade, the issues of nutrition and body composition have been increasingly reported on, becoming a part of the national zeitgeist. Thus, the awareness of just how fat we really are is currently at the forefront of many American minds. Despite this, due to a circulation of false—or more frequently, misleading—information, the overwhelming body of information relating to the issue of obesity has done little to quell the grave health problems we face.

The primary bit of misleading information regarding body weight, one that has been circulated to an almost comically large degree, is this: in order to lose weight, one must burn more calories than one consumes. While this statement is bonafide scientific fact, backed by our fundamental understanding of the thermodynamic principles of energy conservation, it is the logical equivalent of simply telling an obese person to stop being fat. Although it is true that weight gain, fundamentally, is caused by consuming a surplus of calories, telling those with weight problems to simply burn more calories than they consume is unhelpful, as it does not provide a clear solution that can be followed in practice. Instead, the question we must consider is this: how does one burn more calories than one consumes?

The most common answer to this question, one that expands upon the thermodynamically informed statement discussed previously, is to simply eat less. If you eat considerably less and your metabolic rate remains constant, then surely you will lose weight, right? The problem with this solution is simple: metabolic rates are anything but constant; they change according to a variety of factors, including caloric intake. Therefore, an obese person who eats less but does nothing to change the macronutrient composition (the proportions of protein, fat, and carbohydrates) of their diet will be unlikely to lose a significant amount of weight, as their metabolic rate will drop in accordance to their caloric deficit.

Another purveying answer to the question of burning more calories than one consumes is to exercise more—which, I might add, is logically very similar to the last “solution” to the problem of obesity. The problem with this solution is equally simple: if you exercise more, your appetite increases, and you eat more. Think about it; if you plan on having a nice meal and want to work up an appetite, you are very likely to participate in some form of physical activity beforehand to enable yourself to eat more. Thus, just exercising more without any dietary changes will result in a complementary increase in caloric intake. And while it is true that caloric intake can be controlled, keeping caloric intake constant while increasing physical activity is likely to create a sort of burnout effect—insufficient energy will make it difficult to continue to exercise sufficiently so as to burn a significant amount of calories.

That being said, it is entirely possible to temporarily lose weight either by eating less or exercising more; in other words, while it is difficult to create a caloric deficit through these two strategies, it is not impossible. The problem is that, fundamentally, this kind of caloric deficit is unsustainable, as the human body eventually demands a higher caloric intake. And while it is almost impossible to consume and burn and exactly equivalent amount of calories (think about the precision that would require), it is likely that ending a caloric deficit will result in a caloric surplus. Thus, we have returned to the very same conditions whence we came, as a surplus of calories is the very condition which caused any weight gain in the first place; after all, this initial diet must have favored weight gain or no attempt at weight loss would have ever been made! Therefore, while it is possible—albeit difficult—to lose weight through the two previously discussed strategies, without any change in macronutrient composition, it is likely that any weight lost from either strategy will be gained back when the period of caloric deficit ends.

So if simply eating less and exercising more do little to help us lose weight, then is weight loss impossible? Well, obviously not. The problem is that logically, both solutions, which seem intuitive, arise from the thermodynamic principle that a human will lose weight if they expend more calories than they consume. As the solutions of exercising more and eating less respectively attempt to increase the expenditure of energy and to limit the intake of energy, it is quite obvious that they have directly descended from this misleading—albeit true—principle of thermodynamics. The problem with the thermodynamic approach and its two descendant solutions of eating less and exercising more is one of gross oversimplification. Fundamentally, they consider humans as energy driven machines rather than the complex biological systems that we are. Therefore, in attempting to answer the question of how one loses weight, rather than making considerations in terms of energy transfer, it is far more helpful to examine the biological science behind the human metabolism.

The human metabolism, along with a large portion of the biological operations of the system which we refer to as the body, is primarily controlled by hormones—the endocrine system. Hormones are like your body’s messengers, using the circulatory system to disseminate important information throughout the body. These messengers serve a wide range of functions, from preparing you to face a deadly threat by raising your heart rate and blood glucose levels and dilating your pupils (epinephrine) to maintaining the proper content of water and dissolved solids within your body by mediating the water retention of your kidneys (vasopressin). For answering the question of how one loses weight, the primary hormone that should be considered is insulin, which is involved in the mediation of blood glucose levels and, in accordance to our inquiry, the metabolism of fat.

Basically, insulin works like this: when your blood glucose levels rise—usually caused by eating some form of carbohydrate—your pancreas responds by releasing insulin, which facilitates the uptake of glucose primarily into muscles, adipose (fat) tissue, and the liver. So how does insulin relate to fat metabolism? Well, in addition to insulin’s effects on blood glucose levels, the hormone also inhibits the breakdown of triglycerides (fat molecules) in adipose tissue. This inhibition, which works by preventing an enzyme called lipase from breaking down stored fat—the triglycerides—within adipose tissue into smaller, more usable subunits called fatty acids, effectively prevents any significant fat loss. As fat loss occurs when the body breaks down triglycerides found in adipose tissue and uses them for energy, the release of insulin—which prevents the catabolic metabolism (breakdown) of fat—effectively stops you from losing any significant amount of weight.

Insulin has one other significant fat-sparing effect on the body’s metabolism. When blood glucose levels rise, insulin stimulates the uptake of glucose into the liver so your body can replenish its glycogen, a more complex carbohydrate found in liver and muscle cells as a way to store energy in the short term. However, when the liver becomes saturated with glycogen, additional glucose taken up by liver cells are used to synthesize fatty acids, which exit these cells as lipoproteins. These lipoproteins provide a means of transportation for the fatty acids synthesized in the liver and are transported to a variety of different cells. Significantly, adipocytes (fat cells) are involved in the uptake of lipoproteins. Adipocytes use the fatty acids they contain, along with glucose from the bloodstream—the presence of which is due to the effects of insulin—to synthesize triglycerides, accumulating them for storage. Therefore, not only does insulin prohibit the breakdown of fat, but it also indirectly stimulates a metabolic pathway that leads to an accumulation of fat within adipose tissue.

By examining metabolism from an endocrinological perspective, the answer to our question, of how one should go about losing weight, becomes simple. In order to lose body fat, or to limit fat gain, one must limit the body’s secretion of insulin, as insulin directly prevents catabolic metabolism of the body’s fat stores. Dietarily, this is fairly simple to accomplish: just limit your consumption of foods that cause large releases of insulin: carbohydrates, especially high glycemic, refined carbohydrates like white bread and sugar. A study published in the European Journal of Nutrition found that a high carbohydrate diet stimulated lipogenesis in rats, while a high fat diet strongly inhibited this anabolic pathway, which, given our understanding of endocrinology and fat metabolism, should be expected. (Ferramosca, et. al.). Furthermore, a recent study with human participants found that a ketogenic diet—that is, one with high fat and low carbohydrate intake—caused “marked reductions in whole body adiposity,” suggesting that limiting intake of carbohydrates facilitates fat loss (Kephart, et. al.).

That being said, there are a few notable exceptions to this rule of maintaining a healthy body weight. Athletes can afford to eat—and should eat—more carbohydrates than those not involved in sport, as training and performing depletes the body’s glycogen stores. The liver prioritizes the use of glucose for the synthesis of glycogen over the synthesis of fatty acids—which, if you recall, are necessary to the production of triglycerides in adipose tissue in the process discussed previously. Therefore, high performance athletes can eat substantially more carbohydrates than the average person without worrying about any lipogenic effects, as a large portion of their consumed carbohydrates will contribute to the replenishment of their glycogen stores in their liver and muscles cells rather than producing more triglycerides.

Moreover, athletes participating in anaerobic training can afford to eat more high glycemic carbohydrates before and during their training sessions. The metabolic pathway of anaerobic respiration is fueled primarily by blood glucose, so athletes performing anaerobic activities will burn off much of the sugar, converting it to ATP rather than having it be taken up by adipocytes that will use it to produce triglycerides.

Finally, it is important to note that dietary suggestions, such as these given to limit lipogenesis and promote fat loss, are anything but hard and fast rules. Everyone’s body is different, and the dietary needs of the individual cannot be sufficiently met by any generalized dietary plan. Furthermore, while it is recommendable to limit the consumption of high glycemic, refined, or processed carbohydrates, it is advisable to include healthier carbohydrates, such as fruits, vegetables, and whole grains, into one’s diet. While all three of these types of foods contain a large amount of carbohydrates, they also contain dietary fiber, digesting much slower. Thus, they do not produce a significant spike in blood glucose levels, not warranting a large release of pancreatic insulin that causes the adverse effects previously discussed. And while it is true that a high fat diet will not stimulate insulin secretion, questions have been raised about other health effects of such diets—especially those high in saturated fats—on the cardiovascular system. (AHHA).

Therefore, when considering what to eat, you must consider your lifestyle, dietary needs, and individual characteristics rather than strictly adhering to someone else’s diet plan. Proper nutrition requires adaptability, as your dietary needs fluctuate from day to day, especially if you adhere to a varied training schedule. With more understanding of the biological mechanisms behind metabolism, it less necessary to rigidly follow a diet plan, as you are afforded the ability to properly adapt your nutrition to your current needs to enhance your training and/or your lifestyle.

References: https://docs.google.com/document/d/1WM1NSLfLVNSCB_tfY5UT3JGFTc6_LpG7HXZpt6Rv7rU/edit?usp=sharing