Low Carbs and Metabolic Momentum

Proponents of ultralow-carbohydrate diets often claim that they’re superior to other popular diets in terms of fat loss for a number of reasons. One is that high-protein, low-carb diets provide more satiety value than high-carb, lowfat diet plans. Since an increased appetite is a major barrier to successful fat loss, compensating for that by making you feel fuller can prove to be a major advantage of a low-carb plan.

Ultralow-carb proponents also say that your body uses calories differently when you follow a low-carb diet. That’s a topic of major controversy among researchers. Some scientists say that a calorie is a calorie, and all diets, including low-carb plans, lead to fat loss because they involve eating fewer calories. They believe that you cannot change the laws of physics—unless you reduce calories sufficiently in relation to physical activity or exercise, you won’t lose bodyfat.

Low-carb proponents suggest that low-carb diets provide a significant thermogenic effect, mainly due to the combination of a higher protein intake and fewer carbs. That suggests that your body uses calories both differently and more efficiently when you’re on a low-carb diet.

In actuality, there are two types of low-carb diets. In one carbs are lowered to 40 percent or less of total calorie intake. The other can be termed “ultralow carb” and usually involves eating no more than 20 grams of carbs daily, a.k.a. a ketogenic diet.

Critics say that unless you eat sufficient carbs, your body will turn to other energy sources, including protein and fat. The problem here involves protein. Some say that not getting enough carbs brings on the conversion of muscle protein into glucose in the liver, a process called gluconeogenesis. When that happens, you lose muscle mass. Obviously, that isn’t ideal if you’re seeking added muscle size and strength.

Gluconeogenesis, however, involves substances besides amino acids. Its fuel includes lactate and glycerol, which makes up 10 percent of the triglyceride, or fat, structure. Ketones are another source of fuel during carb restriction. They are metabolic acids, specifically incomplete products of fatty acid metabolism. They rise when increased amounts of fat are used for fuel, as during low-carb diets, and in a pathological state, during uncontrolled diabetes. That latter production of ketones, however, bears no resemblance to what occurs under low-carb-diet conditions.

Ketones can be used as a fuel source by both the muscles and brain in the absence of carb intake. While the preferred fuel of the brain and central nervous system is indeed glucose, some emerging studies make the heretical hypothesis that the brain uses ketones even more efficiently than it does glucose. Those alternate fuel sources, including amino acids from protein, explain why, contrary to popular belief, carbohydrates are not considered an essential dietary component.

Gluconeogenesis also explains another effect: Why higher-than-normal protein intake is absolutely essential during ultralow-carb diets. It turns out that as much as 57 percent of protein is converted in the liver into glucose, which is used to maintain the function of tissues and organs, including the brain and red blood cells, that show a preference for glucose as fuel. This also explains why it isn’t necessary for the body to tap into muscle for amino acids during low-carb regimens. Not eating enough protein while completely eliminating all carbs is another story. In that instance, you can indeed lose muscle, especially if your calorie intake is also low. The combination of low protein and no carbs blunts insulin release, which adds to the problem because insulin provides an anticatabolic effect in muscle.

What about the increased resting energy expenditure linked to ultralow-carb diets? That, too, is related to gluconeogenesis, which is energy intensive. Simply put, gluconeogenesis is expensive in terms of calorie use. A recent study featured 10 men placed on high-protein, zero-carb diets or normal-protein diets. The zero-carb diets consisted of 30 percent protein, 0 percent carb and 70 percent fat. The normal-protein diet consisted of 12 percent protein, 55 percent carb and 33 percent fat. That diet is similar to what diet experts often recommend for those seeking weight loss. The goal of the study was to determine whether a high-protein, zero-carb diet increased gluconeogenesis, and whether the increase explained the increased energy expenditure typically seen during ultralow-carb diets.

At the start, all the study subjects depleted existing glycogen stores through intensive cycling exercise. The two diets contained the same number of calories. The subjects followed them for only 1 1/2 days. That’s interesting because the metabolic changes induced by low-carb diets, such as a switch to using fat as fuel in place of glucose or sugar, usually takes about two weeks to begin working in earnest. Various tests measured energy expenditure and level of gluconeogenesis on the diets.

Glucose production in the body was lower in the zero-carb group, while gluconeogenesis was higher. The resting metabolic rate was also higher in the zero group, which turned out to be a function of increased gluconeogenesis. Specifically, the contribution of gluconeogenesis to resting energy expenditure was 42 percent, while the energy cost of gluconeogenesis itself was an expensive 33 percent of produced glucose. Although insulin influences gluconeogenesis, it played no role during the study. One key point involved the rapidity of increased gluconeogenesis under zero-carb conditions. While it would take several days for that to occur under low-carb conditions, it began within hours.

Another interesting finding was that the 12 percent protein intake in the normal low-carb group wasn’t enough to maintain protein balance. That’s related to the fact that both groups started the study with depleted glycogen, and it led to increased protein breakdown and oxidation of branched-chain amino acids, resulting in a negative protein balance. The 30 percent protein intake in the zero-carb group was enough to offset the problem, which highlights the importance of increasing protein intake whenever you severely restrict carb intake. While the zero-carb group also ate 70 percent of calories as fat, which is quite high, the lack of carbs, coupled with the existing glycogen-depleted state, resulted in a heightened use of fat as fuel; in other words, the fat was rapidly oxidized, a situation that does not normally occur when you also eat carbs. Since dietary fat does not provide a thermogenic effect, it didn’t play any role in the increased energy expenditure seen in the zero-carb group.

The authors note that the glucose produced during gluconeogenesis can be used to slowly replenish depleted glycogen reserves, while other fuel sources, including lactate and ketones, can act as primary energy sources for muscle work. That explains how bodybuilders adapt to ultralow-carb diets and why such diets do not lead to muscle loss, contrary to some people’s opinions.

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Veldhorst, M., et al. (2009). Gluconeogenesis and energy expenditure after a high-protein, carbohydrate-free diet. Am J Clin Nutr. 90:519-526.