Are Low-Carb Diets and Weight Training Good or Bad?

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There is a debate between experts in low-carb diets and those who are experts in exercise and weight training about the role of carbohydrate restriction in building muscle and strength. Here are points made by exercise expert Paul Rogers and responses from a low-carb perspective.

Carbohydrates as Fuel for Exercise

Rogers says that carbohydrates are the main fuel for exercise, particularly "fast and intense exercise". As such, they are important for "athletes, weight trainers, and heavy exercisers". He points to a couple of studies to prove his point, saying that neither fat nor protein are good energy sources for high-performance exercisers. He also asserts that low-carb, high-protein diets may adversely affect bone density.​

However, many people on a low-carb diet are, at most, moderate recreational exercisers. They have not dedicated athletes, bodybuilders, or the like. You may also separate those who exercise heavily into groups athletes who do primarily endurance activities which are mainly aerobic (e.g. running, cycling) and weightlifters and others who do very short bursts of anaerobic muscle activity.

Prior to the advent of agriculture, hunter-gatherer tribes in temperate climates ate diets naturally low in carbohydrate (in winter, very low in carbs). These diets were generally high in fat, which was prized. Just to stay alive, they led a very active lifestyle on a low-carb diet. By most measures, our health began to decline once we began cultivating grains, although this probably also enabled "civilization" as we know it.

Low-Carb Diets and Exercise

This is an area that has not had a ton of research, but these are some things that are suggested by the studies that have been done. Heavy exercisers who do endurance-type exercise tend to have a drop in efficiency in the early weeks of a low-carb diet, but their bodies usually recover within two to four weeks. One New Zealand study showed a typical pattern for endurance athletes, who have reduced energy initially but then improved wellbeing. Some athletes report a surge in exercise efficiency and improvements in performance once they are used to the diet.

This process has been referred to as "keto-adaptation" or "fat adaptation" as the body becomes better able to use fat for energy under exercise conditions. There is a debate about how low in carbohydrates the diet must be to trigger keto-adaptation. Some say below 20 percent of calories, but there is a lack of studies.

Keto-adaptation is probably of more limited use in short-burst anaerobic exercise, although there is also debate on this point. On the other hand, this does not necessarily mean that a low-carb diet is precluded for weightlifters. A college professor of athletic training says that members of the powerlifting team at his college all eat a diet of 20 percent or less carbohydrate. A nutritionist who is a bodybuilder says what is required is a modest amount of additional carbohydrate prior to lifting. "Approximately 5 grams of carbs every two sets is enough to replace glycogen lost during training. So for example, for 15 sets, around 35 grams of carbs would do the trick." This is the amount of carbohydrate in, for example, 1 1/2 cups of grapes—not inconsistent with a diet which is much lower in carbohydrate than is generally recommended.

During weight loss, low-carb diets have repeatedly been shown to preserve lean body mass as compared to higher-carb diets.

Points of Agreement

Rogers says that protein is not a good energy fuel at all, although the body routinely makes some glucose from it. Physical training regimes are likely to suffer in the early weeks of a low-carb diet. However, studies done in this time frame must be viewed through that lens. Some extra carb before a workout can be a good idea, although this can still be done in the context of a low-carb diet.

Points of Disagreement

Some opinions are formed on studies of subjects after five days on a low-carb diet, which is during the three to five day period post-diet change. It is arguably the very worst time to do a study like this, as glycogen stores are depleted without keto-adaptation being fully underway to compensate. Nevertheless, significant keto-adaptation can be seen even in that short time frame.

The other point Rogers makes is about low-carb and/or high-protein diets adversely affecting bone mineral density. Low-carb diets are often assumed to be very high in protein, but they are usually not.   Studies show that there is no negative effect on bones from a high-protein diet and that there may be a small positive effect. However, it is an issue on which you can find studies with results on either side.

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