A study of elite race walkers shows that a low-carbohydrate, high-fat diet will slow their race times and training (J of Physiology, February 14, 2017).
Thirty world-class race walkers participated in an intense, high-speed training program and took in the same total calories. They were divided into three groups:
- High-carbohydrate
- Alternating days of high- and low-carbohydrate intake
- Low-carbohydrate, high-fat (LCHF) diet
They trained intensely for three weeks. Results showed that:
- The high-carbohydrate and the alternate-day high-carbohydrate groups improved their race times. The LCHF group did not improve.
- The LCHF group burned a greater percentage of fat during intense exercise.
- The LCHF group required more oxygen to exercise intensely at 80 percent of their maximum speed for two hours. The LCHF diet forced the athletes to burn more fat and less carbohydrates so they required more oxygen, and therefore had to slow down.
Why LCHF Diets Have Incorrectly Been Promoted to Improve Athletic Performance
Your muscles burn primarily fat and carbohydrates for energy. You have enough fat stored in your body to exercise for many days. However, you can store only 1,600-2,000 calories’ worth of sugar (carbohydrate) in your muscles and liver, and will start to run out of your meager supply of sugar after 70 minutes of intense exercise.
Some people believe that if you restrict carbohydrates, you will teach your muscles to burn more fat and preserve their small store of sugar (Metabolism, 2016;65(3):100-10). That is correct, but burning more fat and less sugar for energy slows you down (Nutrients, 2014;6.7:2493-508).
The limiting factor to how fast you can move over distance is the time it takes to move oxygen into muscles. Since sugar requires about 20 percent less oxygen than fat does to fuel your muscles, you need to slow down and have less power when your muscles burn fat rather than sugar.
The faster you move, the greater percentage of sugar that your muscles use for energy, so restricting carbohydrates (your source of sugar) and forcing your muscles to burn more fat slows you down in both training and racing. The key to improving your speed over distance is to move very fast in training. Anything that makes you train at a slower pace limits your improvement, and of course, anything that slows you down in a race can make you a loser.
Low-Carbohydrate Diets Can Slow You Down
In another study, competitive bicycle racers ate a high-fat or high-carbohydrate diet for six days, followed by a high-carbohydrate diet for one day and then completed time trials on their bikes. Then they ate the opposite diet for six days followed by a high carbohydrate diet for one day and repeated their time trial. The diets did not affect their times or power output for 100 kilometers (62 miles), but the high-fat diet slowed their ability to sprint for 0.6 miles (J Appl Physiol, 2006, 100(1):194-202).
Many other studies show that it doesn’t make any difference what a trained endurance athlete eats on the week before competition because the muscles of trained athletes store the most sugar as glycogen when they reduce training for several days, regardless of what they eat, provided that they do not restrict all carbohydrates.
Any sprint that takes less than 50 seconds is not affected by diet, because you can work up to 50 seconds anaerobically, without requiring additional oxygen. However, a low-carbohydrate diet before extended sprinting hurts performance. Restricting carbohydrates before a sprint taking more than 50 seconds increases oxygen needs, which slows you down.
Gabe Mirkin, M.D., is a sports medicine doctor and fitness guru. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin has run more than 40 marathons and is now a serious tandem bike rider with his wife, Diana. His website is http://drmirkin.com/. Click to read Gabe’s full bio.
The only thing that would convince the high fat dieters would be a study like the one quoted but that lasted 6 months. Anecdotally you will read that somebody felt crappy for several weeks on a high fat diet and then suddenly they felt great, faster than before and able to ride long and fast without food. All the research says no, but the stories are out there and it would take long term studies to decide the issue in some minds.
I suspect part of the problem is that everyone is different. For some, the high-fat diet (done long term) may work better, but some others may not be able to adapt to it, no matter how long they stick with it.
Read Dr. Mirkin’s column carefully. He’s talking about competitive racers doing time trials, etc. He says that a high-fat diet limits their performance. The American College of Sports Medicine makes the same point: a high fat diet limits high-end performance. However, a high-fat diet doesn’t inhibit riding at an endurance pace.
This advice is fine for a typical young athlete who has been raised on a high carb junk food diet and has no well-established (6-12 months) ability to metabolize fat quickly during exercise. In the very short test durations quoted above, removing carbohydrates is devastating to performance. Been there, done that. That is normal. A fat adapted athlete will burn fewer carbs and much more adipose tissue during an event leading to less gastric distress (fewer gels and sugary drinks) and more available glycogen for high-intensity efforts. A fat adapted athlete will also consume carbs during an event, just fewer. Recreational fat-adapted athletes don’t need any carbs for their low-intensity efforts. Recovery times are quicker for low carb athletes. I notice that the post-ride naps are shorter or non-existent with low carb high fat.
For those who are older, recreational athletes, and for those who are trying to avoid diabetes, a low carbohydrate (<50 grams/day) diet is preferable as it leads to weight loss, reductions in vascular inflammation, and improved mental sharpness. As a side effect, you are typically not a slave to mealtime or snacks off the bike also. What I continue to enjoy is the ability to hop on the bike and do a 70-mile ride with the 23 mph group and take in only water. If you ride at an aerobic pace with limited sprints then water is sufficient and if you are like me at 60 years old your cardiologist will tell you that your EKG is that of a healthy teenager.