I have been doing some lecturing this week, and part of the information I imparted concerned the potential advantages of eating a diet relatively low in carbohydrate. It’s not uncommon for individuals to ask about the potential effect of low-carb eating on sporting performance. This question is usually rooted in the notion that those engaging in regular sporting activity or exercise need to maintain their stores of a starch-like substance called glycogen (found mainly in the muscles and liver), which provides relatively ready fuel during exercise. Glycogen is a carbohydrate, and so exercisers are often encouraged to ‘carb-load’ in an effort to fill up the glycogen stores so that plenty of ready energy will be available during exercise. People downing plates of pasta the day before a marathon is borne out of this theory.
However, is carb-loading really necessary for most mere mortals? Let’s work through some figures. A 30-minute jog will burn about 250 calories in addition to those that would be burned sat still. A significant proportion of the fuel for this exercise will come from fat. Let’s imagine, that during the jog, 150 of the calories come from carb. Each gram of carb contains 4 calories, so in theory to replenish the glycogen lost during exercise is going to need the consumption of about 40 grams of carb. That’s about the same amount of carb found in a couple of apples.
In other words, for most individuals engaged in recreational exercise that doesn’t go on for hours, glycogen depletion is unlikely to be an issue unless carb consumption is cut to very low levels.
For endurance sports, low-carb eating is potentially more of an issue. If you’re training for a marathon, for instance, and racking up 50 miles or more a week, then glycogen depletion is a real risk. However, one thing that needs to be borne in mind is that when carbohydrate is restricted, the body automatically turns to other fuels (principally fat) to make up the difference. This adaptation can take time, so adopting a low-carb diet is not the thing to do a week before a marathon, particularly if your goal in not just to ‘get round’ but crack your personal best.
At lower intensities of exercise, however, adopting a low-carbohydrate diet does not appear to be an impediment to activity. A major review on the subject concluded that “endurance performance can be sustained despite the virtual exclusion of carbohydrate from the human diet.”.
There is no doubt in my mind that the very physically active can tolerate more carb in their diet than those who are not. If this applies to you, then aim to get your additional carbohydrate into the system within an hour or so after a sporting event or training session (this help replenishment of glycogen in the muscles for future use).
If you do feel the need to do this, I would counsel against using bread, pasta and white rice as your carbohydrate sources. Not only are these foods generally disruptive to blood sugar, they also offer precious little from a nutritional perspective. Generally slower sugar-releasing and more nutritious forms of carb include fruit, vegetables and legumes (beans and lentils).
1. Phinney SD. Ketogenic diets and physical performance. Nutr Metab (Lond). 2004;1:2