Advice for runners about performance nutrition can be confusing and often contradictory. Should you eat more fat and restrict carbs to increase fat burning? Or should you increase your carbohydrate intake so you always have a quick source of fuel? New research suggests that for most runners, the best diet is one that contains a balance of fat and carbohydrates for optimal performance.
Fat oxidation and performance
When you exercise, your body primarily uses carbohydrates (glucose) and fat for energy. Your body will always use the easiest energy source first, which is usually blood glucose. From there, it will draw upon your glycogen stores to use for fuel, but this process takes a bit longer because you have to convert glycogen to glucose before you can use it.
The problem with glucose and glycogen, however, is that your body only has so much of it, so it needs to be replaced by taking in exogenous carbohydrates (think gels, sports drinks, etc.). Fat, on the other hand, is stored in the body in much greater quantities, and provides more energy per gram than carbohydrates. The problem with fat, unfortunately, is that your body needs oxygen to burn fat for fuel (a process called fat oxidation), which means you have to slow down enough so that you can get the oxygen you need for this process to happen.
This isn’t helpful for short-distance runners like sprinters, or runners competing in races like the 800m or the mile, but fat is a valuable fuel source for long-distance athletes. As long as you can get enough oxygen to use fat, you could presumably run for hours while burning fat for fuel.
Over the years, researchers, coaches and athletes have experimented with high-fat, low-carbohydrate diets during training to “teach” your body to burn fat more efficiently for fuel, but is this really necessary, or effective? That is the question researchers recently attempted to answer.
The authors of the study, published in the journal Frontiers in Physiology, had 28 competitively-trained male distance runners (who ran half-marathons, marathons and ultras) complete an incremental exercise test to exhaustion, as well as three three-hour runs, each with a different carbohydrate-feeding protocol. Throughout the experiment, participants were consuming a balanced macronutrient diet of approximately 57 per cent carbohydrates, 21 per cent protein and 22 per cent fat.
After the endurance tests, the researchers found that the participants were able to achieve a high level of fat oxidation despite consuming a mixed macronutrient diet and consuming carbohydrates during their run. They also found that runners who consumed the highest amounts of carbohydrates during their run were the most likely to experience GI distress, highlighting the importance of moderating your mid-run carb intake to maximize performance while minimizing stomach troubles.
The bottom line
If you’re wondering whether you should consider using a low-carbohydrate, high-fat diet to maximize your body’s ability to burn fat for fuel, this research says it’s likely not necessary. Runners who consume a diet that has a balance of carbohydrates, fats and protein are able to use fat for fuel efficiently during a long run.
Of course, the runners in this study were well-trained athletes, which means their bodies are able to use all types of fuel very efficiently. So how can less-experienced runners improve their rate of fat oxidation? Simply by training. By training consistently, your body will become more and more efficient at using both carbs and fats for fuel, which will allow you to run faster and longer.