1 November 2013
A few well-known sports scientists have recently challenged the conventional high carb diet dogma and begun promoting instead the idea that athletes can perform better on a low carb diet. What’s the evidence and should we all be cutting out carbs? Here’s my take on the low carb vs. high carb debate.
There have been no properly controlled trials comparing low carb and high carb diets on athletic performance WHILE KEEPING PROTEIN THE SAME. Yes, there was a 1983 study by University of Connecticut researchers that suggested a low carb diet did not compromise endurance performance (time to exhaustion) but, as Alan Aragon points out, this study involved just 5 subjects (!) , two of whom in fact experienced a massive drop in endurance after the low carb diet (the others experienced either an increase or nothing, producing a mean result of ‘no change’). Well, a positive result in just 3 cyclists is hardly a newsworthy bit of science. Speaking of which, there is little else to support the low carb case for athletic performance apart from a large number of anecdotes.
What about the idea that a low carb diet ‘trains’ your muscles to burn fat instead of carbs thus allowing you to make use of your vast fat store? Indeed, Australian researchers have demonstrated that the muscles adapt by making more mitochondria (that burn fat) and more fat-burning enzymes …a theoretical advantage for which there has, so far, been no proof that it actually produces better performance in competition. In any case, its hard to see how a low carb diet can improve high intensity endurance performance. You need carbohydrate to fuel activities above 65% VO2max. Decades of research tells us that without carbs, you will fatigue quickly and performance will drop. And you feel pretty terrible, to boot!
When it comes to weight loss, body composition and heart health, low carbs may work better than low fat diets for some people. But the evidence from large meta-analyses here and here suggests low carb diets are overall no worse or no better than low fat diets. Now here’s the crucial point: the reason low carb diets appear to ‘work’ is that they provide more protein and fat, which increase satiety, so you spontaneously eat fewer calories and end up losing weight (and improve your cardiovascular risk profile).
The truth is there is no one diet that fits all. It is not possible to preach particular level of carbohydrate intake that’s best for everyone. The world’s top endurance athletes from Kenya and Ethiopia attribute their success to a high carb diet containing 9 – 10g/ kg, well within current ISSN and ACSM recommendations. While some individuals appear to perform better on a lower carb intake, this may be explained by the ‘insulin resistance’ theory (if you are insulin resistant, it means your cells are less sensitive to the actions of insulin so your body is not able to process carbs into fuel efficiently). Generally, athletes are more insulin sensitive than non-athletes (so will perform better on a higher carb diet). Those who are more inslulin resistant will perform better on a lower carb diet. Anecdotally, older male athletes tend to feature higher up on the insulin sensitivity continuum. If this is you, a lower carb intake may help you lose weight, increase CVD health and perform better.
At the moment, in the absence of decent (randomized controlled) trials, it’s not possible to make a convincing case for low carb diets for athletes. Moderation is a better message right now.