Paul's PointsPaul 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.
My ResponseFirst of all, I think we have to make a couple of distinctions, especially for my readers, most of whom are not as dedicated exercisers as Paul's readership. The first is that we aren't talking about moderate recreational exercisers, but dedicated athletes, body builders, and the like. The second is that we also must separate "heavy exercisers" and athletes who do primarily endurance activities which are mainly aerobic (e.g. running, cycling) from weight lifters and others who do very short bursts of anaerobic muscle activity.
Some PerspectivePrior 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 ExerciseThis 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 2-4 weeks. For example, I have spoken to a few triatheletes who were discouraged when first cutting carbs, as their training tended to suffer, but each of them found that over time they had a surge in exercise efficiency and then had gratifying improvements in their times, leading to a burst of personal bests. (Check out this article about Paleo eating for athletes in Runner's World - the description in the paragraph that begins "Still, Joe Friel was skeptical..." is very typical.) 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 debate about how low in carbohydrates the diet must be to trigger keto-adaptation. Some say below 20% of calories, but I haven't found good hard data on this. I have the impression that this is not known with certainty.
- 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 weight lifters. I recently talked to a college professor of athletic training who said that members of the power lifting team at his college all eat a diet of 20% or less carbohydrate. I also talked to a nutritionist who is a body builder - he said that what is required is a modest amount of additional carbohydrate prior to lifting - he says, "Approximately 5g of carbs every 2 sets is enough to replace glycogen lost during training. So for example, for 15 sets, around 35g of carbs would do the trick." This is the amount of carbohydrate in, for example, 1½ 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 AgreementI agree with some of the points in Paul's article. Protein is not a good energy fuel at all, although the body routinely makes some glucose from it. As I think I've made clear by now, physical training regimes are likely to suffer in the early weeks of a low-carb diet, so I have no problem with pointing this out. However, studies done in this time frame must be viewed through that lens. I also agree that a 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 DisagreementTwo of the studies Paul cited were performed after only 5 days on a low-carb diet - the 3-5 day period post-diet change 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, in the Stellingwerff study significant keto-adaptation had already occurred. Note, though, that these were endurance athletes, and not those who engage in significant anaerobic exercise (except for the short sprints). A another study on cyclists in a metabolic ward showed that after 4 weeks on a very low-carb diet, their endurance did not suffer (though sprinting was not mentioned).
The other point Paul 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 usually they are not.) I've written about this issue in the past, pointing to studies which showed no problem with low-carb or high protein diets. However, it is an issue on which you can find studies with results on either side. So what is the preponderance of the evidence? Research was published in the American Journal of Clinical Nutrition last year which analyzed 30 years of research on the matter, concluding that there is no negative effect on bones from high-protein diet, and that there may be a small positive effect.
Whew - that got longer than I intended! I now turn this over to Paul - and I'm very grateful that he's agreed to discuss this with me.
Selected Additional Sources:
- Dipla, et al An isoenergetic high-protein, moderate-fat diet does not compromise strength and fatigue during resistance exercise in women. Br J Nutr. 2008 Aug;100(2):283-6.
- Lambert EV et al Enhanced endurance in trained cyclists during moderate intensity exercise following 2 weeks adaptation to a high fat diet. Eu Jo Appl Phys and Occ Phys. 69 (4): 287-293. 1994
- Phinney, SD Ketogenic diets and physical performance. Nutr Metab (Lond). 2004 Aug 17;1(1):2.
- Phinney, SD et al The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism. 1983 Aug;32(8):769-76.
- Volek, JS - Low-Carbohydrate Diets Promote a More Favorable Body Composition than Low-Fat Diets - Strength and Conditioning Journal, 2010 Feb; 32:1) (Jeff Volek's work as a whole is worth looking at.)
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