A frequent criticism of the popular low-carbohydrate style of dieting is that not taking in enough carbs severely inhibits your exercise capacity. Even the most vocal proponent of low-carb dieting, Robert Atkins, M.D., warned in his first book, Diet Revolution, that athletes may have problems training hard under extreme low-carb conditions because the body can’t convert fat into the glucose, a sugar, rapidly enough to support the requirements of intense training. Anaerobic training, such as typical bodybuilding workouts, is fueled primarily by glycogen stored in muscles and liver, as well as glucose circulating in the blood. Since carbohydrates are the prime fuel for anaerobic training, not eating enough carbs would impede high-intensity training.
Several studies have found that athletes who severely restrict carb intake suffer a loss of training intensity. On the other hand, the body is capable of using other forms of fuel to power high-intensity workouts, among them ketones, which are acidic by-products of fat metabolism. Ketones are produced in greater amounts when the body doesn’t have enough carb on hand or when a person suffers from uncontrolled diabetes. Ketones can fuel muscle training, although the adaptation to switching from a glucose base to ketones for energy purposes can take two to four weeks. In the interim, a sense of fatigue is apparent during hard training.
Another possible fatigue factor: The brain and central nervous system prefer to run on glucose. The brain alone is thought to use more than 100 grams of carbohydrates daily in its normal functions. Not feeding it enough carbs is akin to putting cheap gas in your car. With any fuel other than carb, the brain can “ping” a bit, which is evident as a loss of concentration and focus—as well as a sense of fatigue. Recent studies using more sophisticated tools to monitor brain activity, however, show that the brain is capable of fully adapting to other fuel sources, including ketones and lactate. Some go so far as to suggest that the brain may actually prefer ketones to glucose, although that point is a matter of debate.
Low-carb diets almost always feature high protein. Numerous studies show that getting more protein when you’re on a low-carb regimen confers a number of benefits. One example is the satiety factor you get when you eat a lot of protein. A higher-protein diet helps release appetite-suppressing substances and hormones that make it easier to stick with the diet. A major problem of other diets, especially those that restrict protein, is the ravenous appetite that often results, which not only spells doom for the diet but also guarantees a rapid return of any lost weight. For those engaged in bodybuilding, a high-protein intake is a necessity. Certain amino acids, particularly the branched-chain amino acid leucine, have the ability to preserve muscle during low energy intake.
Contrary to the belief of many bodybuilders and athletes, the body has a finite capacity for using protein. The extraneous protein is oxidized in the liver. With a low-carb diet, however, the excess protein offers an advantage, since 57 percent of the protein you eat is converted in the liver to glucose in a process called gluconeogenesis. The liver can also convert lactate and 10 percent of dietary fat into glucose—glucose that can also be used as a fuel by the brain and central nervous system.
Because carbs are the most efficient fuel for high-intensity training, the issue still remains: Will a drastic reduction in carb intake interfere with training intensity? One recommendation is to eat more carbohydrates every few days. The rationale is that the carbs will replenish depleted glycogen stores and may even have an anabolic effect because of an upgraded insulin release. For insulin to have an anabolic effect in muscle, however, you must also have a lot of amino acids in your blood, which means you must also take in a high-protein source, such as whey.
Other studies have found that it takes a certain amount of time for the body to switch from a primary sugar-burning machine into a fat-burning machine, which is the goal of low-carb diets. They suggest that the metabolic conversion occurs in two to four weeks, during which there will be some increased training fatigue. That would show itself as less muscle endurance and possibly less strength after the first set of an exercise. Once the ability to use ketones and fat fully kicks in, though, subjective fatigue abates.
A recent study compared the effects of a low-carb, high-fat diet with a high-carb, lowfat diet.1 Sixty men and women, average age 49, all of whom were obese and sedentary, were randomly assigned to either a low-carb, high-fat diet or a high-carb, lowfat diet. Both diets contained the same number of calories and differed only in carb and fat contents. Both diets were designed to provide a 30 percent calorie deficit. The low-carb diet contained 35 percent protein, 61 percent fat and 4 percent carb. The high-carb diet contained 24 percent protein, 30 percent fat and 46 percent carb. The study lasted eight weeks, and at the start and completion the subjects were tested for aerobic capacity and muscle strength.
As was shown in several previous studies, those in the low-carb group lost more weight after eight weeks, and body composition tests revealed that the weight loss was mostly bodyfat. The low-carb subjects also experienced greater fat oxidation during aerobic exercise than the high-carb group, who had none. Tests of hand-grip strength showed decreases in both groups, but anaerobic leg strength didn’t change. The conclusion of the study was that during low-carb dieting, fuel use shifted toward fat but didn’t have any detrimental effects on exercise. The male subjects were more efficient at fat burning than the female subjects.
This study involved fat, out-of-shape, middle-aged adults, and some may point out that the results may not apply to younger bodybuilders. Indeed, in one study subjects engaged in intermittent, high-intensity exercise did experience deficits in training intensity during low-carb conditions. The increase in fat oxidation, however, appears to compensate for the lack of carbs during submaximal exercise. In this study none of the subjects in the low-carb group showed an increase in perceived effort or greater training fatigue, even when doing the anaerobic part of the exercise tests.
The final factor to consider is the volume and frequency of training. If you train with a higher exercise volume, your chances of losing training intensity when you’re on a low-carb diet are greater. The same holds true for training more frequently, such as twice daily or every day. On the other hand, taking in more protein should enable most bodybuilders to still train efficiently. Perhaps the best way to deal with that is to target carb intake: Eat and drink your carbs prior to and just after training. The rest of the time, limit carbs. That gives you all the metabolic advantages of carbohydrates—while experiencing minimal inhibition of the fat-mobilizing effects of low-carb diets.
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Brinkworth, G.D., et al. (2009). Effects of a low-carbohydrate weight-loss diet on exercise capacity and tolerance in obese subjects. Obesity. In press.