Taking Control of Your Dietary Composition Part 10

In recent years, ketogenic diets have become popular in large part because of the belief that this way of eating promotes fat loss. Aketogenic diet requires restricting the amount of carbohydrates you ingest to increase the degree to which your body relies on stored fat for fuel. On the surface, it seems reasonable to think that this would make you lose more fat because if you’re using more, you should be losing more. But if you’ve read previous installments, you should realize that this might not be the case. Simply stated, body fat is increased, decreased or maintained solely as a function of energy balance; that is, how many kilocalories you ingest each day compared to the amount you expend. So, even if your energetic requirement is satisfied exclusively by using the energy contained in your fat stores, you could wind up not losing any of those stores (or even adding to them) if you’re not taking in less energy than you use (i.e., if a caloric deficit is not present). And the energy in foods is contained in all three macronutrients, which means it is provided by protein and dietary fat in addition to carbohydrates. Consequently, banishing carbs in favor of one or both of the other macronutrients in and of itself will not ensure that the caloric deficit you require to reduce fat stores is present.   

Consider, for example, brothers with identical caloric maintenance levels (i.e., energy intakes required to maintain their current fat stores; in this case, let’s say 2,000 kilocalories per day) who embark on fat-loss diets. Carl decides to ingest 1,500 kilocalories per day and chooses to obtain most of this energy from carbohydrates. Specifically, he eats 225 grams each day, which means carbs provide 60 percent of his caloric intake (one gram of carbohydrates contributes four kilocalories of energy, so 225 grams would provide 900, which is 60 percent of 1,500). He is also aware that the USDA recommends that 45-65 percent of energy intake should be devoted to this macronutrient, so he’s confident that this is the best strategy. On the other hand, Keith believes that carbs are the culprit, so he decides to go the ketogenic route. Specifically, he cuts carb intake to 75 grams per day and compensates by ingesting more of the other two macronutrients. The end result is that, like his brother, he winds up taking in 1,500 kilocalories per day; however, only 20 percent of that energy is provided by carbs.

Carl is operating at a caloric deficit of 500 kilocalories per day and there are 3,500 kilocalories contained in a pound of fat, so he will lose one pound of fat each week. However, other than fat stores, everything else in his body will remain constant, which is not the case for Keith. Carbohydrates are stored in the body as both glycogen (in the liver and muscle) and glucose (in the blood) and during the initial stage of Keith’s diet, his glycogen stores will be reduced and he will lose this weight. What is more, for every gram of glycogen stored, ~2.7 grams of water are packed with it, so he will experience additional weight loss as this water also leaves his body. However, once glycogen stores bottom out (they will not be totally eradicated, but they will be present in unusually low levels), Keith will begin satisfying his 500-kilocalorie-per-day deficit just like his brother was from the beginning, which is by accessing existing fat stores. And from this point forward, the brothers will reduce body fat at the same rate.

Being that the loss of stored fat (as opposed to glycogen or water) is the goal, the brothers will achieve similar results (i.e., loss of one pound of fat per week). However, their disparate strategies will result in marked differences in the way their bodies operate. Carl will satisfy energetic requirements by using either stored fat or glycogen, which is beneficial because the latter is the required fuel when rate of energy utilization is moderate/high. This is why endurance athletes carb load prior to events to maximize stores and, conversely, why marathon runners “hit the wall” and reduce their pace when stores run low and they must rely more on fat. But it is important to note that even though Carl is using glycogen instead of fat in these circumstances, his energy deficit will ultimately be satisfied exclusively by fat breakdown because glycogen stores will be used/replenished at the same rate. Conversely, Keith must rely on fat to satisfy a much greater percentage of his energetic needs, which limits the rate at which he can expend energy (for example, perform aerobic exercise to help set up his deficit) and also alters how his fat is broken down. It is this latter change that is responsible for the effects of the ketogenic diet, which range from potentially fatal if ketosis is severe (e.g., diabetic ketosis) to relatively benign if it is mild (dietary ketosis; for example, like that which I have described above). It is this latter form of ketosis that might be beneficial for fat loss according to findings by Johnstone et al. although this wouldn’t be the case for Keith given the circumstances I describe.





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