Contributed by Dr. Steven T. Devor – Director of Performance Physiology for MIT and OhioHealth, and Associate Professor of Exercise Physiology, Department of Human Sciences, and Department of Physiology and Cell Biology, The Ohio State University
We often get questions about potentially adopting a low carbohydrate, high fat, high protein diet to fuel your running. There is little question this sort of diet has gained popularity in recent years, even with endurance athletes. However, as I have said many times, exercise activities like running, cycling, and swimming, and even high volume or high intensity resistance training, are very dependent on a steady supply of carbohydrate to replace the muscle glycogen that is utilized to generate ATP, which is the fuel your contracting skeletal muscles use for movement. As a vast amount of research has repeatedly shown time and time again, when it comes to improving athletic performance, and maintaining optimal glycogen levels, a diet that is rich in fresh fruits and vegetables, beans and legumes, and complex and 100% whole grain carbohydrates, is vastly superior to a low carbohydrate, high fat, high protein diet.
Often times when individuals want to adopt a low carbohydrate diet they tend to lump all carbohydrates together, and label them all as bad. This is not critical thinking and is not advised. Remember that foods such as broccoli, spinach, kale, beans, legumes, strawberries, and oranges (among countless other healthy vegetable, bean, and fruit choices) are all carbohydrate. Carbohydrate is not just 100% whole grain pasta and 100% whole grain breads. However, sugar frosted flakes are also carbohydrate, so is cake, so is frosting, so is white bread. I think you know there is difference. Yes, it is wise to eliminate what I will call "junk" carbohydrate. But do not throw out all carbohydrate or consider it all a poor nutritional choice.
So, can your body obtain the needed fuel, ATP, for skeletal muscle contraction from fat and protein? Of course. But, again, as I have said many times, the RATE that your body obtains the ATP from fat and protein is only half as fast as your body can get the ATP from carbohydrate. Said another way, carbohydrate supplies the ATP needed for skeletal muscle contraction twice as fast as the ATP is supplied from fat or protein. This is the key. The faster your body is able to obtain ATP from a foodstuff (carbohydrate, fat, or protein), the faster your skeletal muscles can contract. In turn, the faster you run. "Hitting the wall" or "bonking" are alternative ways of saying you are glycogen depleted. Can you still run at that point? Again, of course. But you do not feel very good, and your pace will slow dramatically. Running pace, or intensity of exercise, is determined in large part by how fast the ATP fuel that you need is obtained from foodstuffs.
I believe many endurance athletes are convinced to try this sort of high fat and high protein diet due to an unscientific theory known generally as the “fat adaptation” theory. According to this hypothesis, your performance and general mood may suffer during the first week or so of a low carbohydrate diet, but following this adaptation phase, your body will become extremely adept at running on fat. Subsequently, your performance in your chosen sport will improve vastly. You will also not have to worry about glycogen depletion or “hitting the wall”, because your newly fat-adapted body will just keep drawing on body fat to fuel your workouts and races. There is an element of truth here. Yes, your body will make and burn ATP out of what it is fed. If you eat a high fat and high protein diet, that is what you will use as fuel. However, what is left out is that your body will never perform optimally. You will never be able to supply ATP at the necessary rate to perform at your peak. Finally, keep in mind where the funding comes from for much of the research that supports the adaptation of a high fat, high protein diet to fuel athletic performance. There are several high profile organizations that function to promote a high fat, high protein diet. If those organizations are funding that research, even if it is done at a major university, it is my opinion you need to interpret those results with caution.
Best wishes for your continued training success.