One example: To maintain normal testosterone production, you need to get at least 20 percent of your daily calories from fat. And not just any fat. Studies show that only two types of dietary fat support testosterone synthesis, monounsaturated fat, such as is found in olive oil, and saturated fat. Polyunsaturated fat, such as is found in fish oil, has no link to testosterone, although recent studies show that it does appear to have anabolic effects in muscle.
Of the three big macronutrients, carbohydrate has the worst reputation. Indeed, one of the most effective fat-loss methods is ingesting a low carbohydrate diet, which is based on the fact that carbohydrates—particularly, “simple” carbs, or those registering high on the “glycemic index”—are absorbed rapidly in the body, triggering the greatest release of insulin of all nutrients.
Insulin, in turn, encourages bodyfat production and blocks the release of fat for energy. So the basis of all low-carb diets involves controlling insulin. It works for most people, especially those who are insulin insensitive, an estimated 75 percent of the population. Basically, the cells become less sensitive to insulin, and as a result more insulin is secreted when they’re exposed to carbs, leading ultimately to more calories being stored as fat.
Bodybuilders and other athletes are often confused by the admonishments of many so-called sports nutritionists that they should take in copious amounts of carbohydrate. It’s based on the fact that carbs are used to replenish glycogen, which is the stored form of carbohydrate in the body and the primary fuel for anaerobic exercise, such as bodybuilding training. The confusion lies in the fact that many diet experts who extol the virtues of high carbs for athletes don’t differentiate between endurance athletes and strength athletes. Endurance athletes use stored glycogen to a greater degree than strength athletes and do require more carbohydrates to replenish depleted glycogen stores in liver and muscle.
Is there a nutritional requirement for carbohydrate? Contrary to popular belief, there is no actual requirement for carbs in human nutrition. While it’s true that certain tissues and organs of the body function better on carbs, such as the brain (this is debatable) and red blood cells, they can also use other fuel sources, including ketones, which are the result of incomplete metabolism of fat, as well as lactate and glycerol from fat. The latter two substances can be converted in the liver into glucose, the only form of sugar that circulates in the blood.
Glycogen itself is a polysaccharide, a fancy term for complex carbohydrate. It has a branched-chain structure that allows it to pack a large amount of glucose into a relatively small space, enabling more effective storage. The primary sites of glycogen storage in the body are the liver—glycogen can constitute up to 8 percent of liver weight—and muscle.While muscle does store less glycogen, about 1 percent of weight, since there is more muscle in the body, muscle glycogen reserves are greater. An important point is that liver glycogen is available systemically; that is, it can be broken down into glucose and then transported into the blood. It takes about 12 to 14 hours of no eating to exhaust liver glycogen stores, although you can do it in about two hours with moderate exercise on an empty stomach.
Unlike liver glycogen, the glycogen stored in muscle can only be used by the muscle it’s stored in. That’s because muscle lacks an enzyme that would enable its glycogen to be broken down and enter the blood for systemic use.
Even so, the body has a way to get by that. In the course of anaerobic metabolism, which involves energy production without oxygen, lactate is produced from the breakdown of carbs. The lactate can leave the muscle and then travel in the blood to the liver, where it’s converted back into glucose and sent back into the blood for use as an energy source, a chain of events known as the Cori cycle. What isn’t generally known is that the same lactate that is produced during intense training can be used to replenish depleted muscle glycogen stores, even in the absence of any food intake. That’s an important point, since it explains how people who are on very low-carb diets can continue to train hard, even though they’re not getting the primary energy fuel, carbs.
Consider that a single resistance-exercise session can reduce muscle glycogen stores by 24 to 40 percent, depending on the duration, intensity and volume of exercise. Obviously, the more you work out, the greater the depletion of muscle glycogen will be. Training with higher reps using moderate weights leads to the greatest depletion of muscle glycogen, and this effect is most pronounced in the type-2 muscle fibers that are most prone to growth. The big debate is whether training with low muscle glycogen, as would occur with a very low-carb diet of, say, 60 grams or fewer a day, would hamper training intensity.
Studies that have examined the issue are paradoxical, with some showing definite drops in exercise intensity and others showing little or no effect. Much depends on the time span of the study, since it takes two to three weeks for the body to adjust to using fat as its primary fuel source rather than carbs. If the study was short-term, ending before the body has enough time for adjustment, the findings usually show negative effects on training intensity related to low carb intake and lack of sufficient muscle glycogen. After about three weeks of low-carb dieting, however, the body can adjust to using ketones, lactate and, to a lesser extent, fat as major fuels for training. Still, the “cleanest” and most effective fuel for intense training is muscle glycogen, since it takes more time for the body to metabolize other fuels. One supplement that would aid the use of alternative fuels is medium-chain triglycerides. Usually made from coconut oil, MCTs favor greater ketone production during low-carb diets, which both provides muscle energy and spares muscle from the catabolic effects of dieting.
The greater use of fat as a fuel source is considered one of the main advantages of a lower-carb diet. Without sufficient glycogen in liver and muscle, the body turns to using more fat for energy. There are, however, dangers associated with the technique. For one thing, training with depleted glycogen tends to promote the release of AMPK, an energy sensor in muscle that encourages the use of fat as an energy source during exercise, with the majority of the fat coming from intramuscular stores. Unfortunately, it also interferes with the action of another protein, mTOR, that promotes muscle protein synthesis. So in people with low bodyfat stores, training under glycogen-depleted conditions may trigger the increased breakdown of muscle into amino acids, which are sent to the liver, converted into glucose and used for energy.
Can training with depleted glycogen interfere with the muscle-protein-synthesis process that underlies muscle growth? A recent study examined that issue.1 Sixteen young, healthy men were assigned to either a nutrient or placebo group. They engaged in one-legged cycling with the purpose of depleting glycogen in one leg without working the other leg. Then they underwent an overnight fast and an amino acid isotope infusion to measure rates of muscle protein synthesis.
The next step involved training, doing eight sets of one-legged leg presses using 80 percent of their one-rep-max weight. Immediately after the workout and two hours later the men drank a 500-milliliter formula containing 20 grams of whey protein and 40 grams of maltodextrin, a carbohydrate, or a placebo drink. Muscle biopsies were extracted from the men’s front-thigh muscles, at rest and at one and four hours after the workout.
The results showed that muscle glycogen levels were higher in the leg that hadn’t been exercised in both the nutrient and placebo groups. The major finding was that rates of protein synthesis didn’t differ between the groups, although those getting the nutrient drink did show an enhanced anabolic effect after the exercise.
The authors suggest that even in the depleted-glycogen legs, there was still enough left in the muscle to support muscle protein synthesis reactions. On the other hand, the glycogen-depleted legs showed greater amounts of proteins linked to muscle catabolism four hours postexercise. Also, drinking the protein and carbs didn’t make any difference in the rates of muscle protein synthesis between the low-glycogen legs and the normal-glycogen legs. The nutrient drink did elevate levels of mTOR more in the normal-glycogen legs, but the four-fold boost in mTOR from the exercise alone was enough to equalize the muscle protein synthesis between the low- and normal-glycogen legs.
The bottom line is that training when you have low-glycogen stores will not interfere with muscle-protein-synthesis reactions to exercise. On the other hand, other studies show that some carbs must be present to activate intramuscular IGF-1, which is important for activating satellite cells, muscle stem cells that repair damaged muscle and are needed for growth to occur. In addition, performing intense eccentric muscle contractions, or lengthening of muscle during contractions, results in muscle damage leading to delays in complete muscle glycogen synthesis and complete muscle recovery following intense exercise due to a blunting of the activity of GLUT-4, the major glucose carrier into muscle tissue.
So it’s probably not a good idea to curtail all carb intake if your goal is to increase muscle mass. In addition, muscle glycogen increases by about 20 percent following a five-day creatine-loading protocol of 20 grams a day, and recent studies show that taking in caffeine with carbs also boosts muscle glycogen repletion.
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1 Camera, D, et al. (2012). Low muscle glycogen concentration does not suppress the anabolic response to resistance exercise. J Appl Physiol. In press.
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