Restricting carbohydrates with a keto diet or fasting will tire you earlier when you exercise (Sports Medicine, January 21, 2020;11:1-28). Many studies show that low-carbohydrate diets impair performance in sports that require speed (The J of Sports Med and Phys Fit, April 4, 2018; J of Physiol, December 23, 2016). On a low-carbohydrate diet, you can’t train very fast and you can’t move as fast in races. The limiting factor to how fast you can move when you exercise is the time it takes to move oxygen from your bloodstream into your muscles. When you start to run low on oxygen, your muscles burn and hurt, you gasp for breath and you have to slow down.

Muscles burn primarily carbohydrates and fats (and a small amount of protein) for energy during exercise, and carbohydrates require less oxygen than fats. The faster you move, the greater the percentage of carbohydrates your muscles burn, and when you exercise at lower intensity, your muscles burn a greater percentage of fat. You can exercise equally fast at low intensity on low or high-carbohydrate diets, but when you pick up the pace, you can’t exercise as fast on a low carbohydrate diet because you need more oxygen (J Physiol, May 1, 2017;595(9):2785–2807).

Flawed Studies to Support Keto Diets for Athletes
With ketogenic diets (also called Low Carb, High Fat or LCHF diets), you try to get your body to use fat as the prime energy source for your muscles. To do this, you must restrict both carbohydrates and protein. Carbohydrates are just sugars in singles and chains, and they provide sugar to power your muscles. When your body is forced to use mostly fat for energy, the fat is converted to ketones that can also be used to fuel your muscles. If you eat a lot of protein, your liver uses gluconeogenesis to convert protein to sugar, and thus you are not on a low-sugar ketogenic diet.

Anything that increases a person’s maximal ability to take in and use oxygen (VO2max) will also help them to move faster and with more force over distance. One report appears to show increases in off-road cyclists’ maximal ability to take in and use oxygen with a LCHF diet (Nutrients, 2014; 6(7)). That would have made them faster, but VO2max depends on a person’s weight and a low-carbohydrate diet can cause you to lose weight. When the study is corrected for the diet-induced weight loss, all of the oxygen capacity gains appear to be from the loss of weight rather than from taking in more oxygen or going faster.

Some studies do show that low-carbohydrate diets can help athletes lose weight (Br J Nutr, 2013; 110: 1178–87), and possibly have greater endurance in sports that are done at low intensity and below the lactate threshold (in which you do not get short of breath), such as multi-day running races (Exercise & Sport Sciences Reviews, July 2015;43(3):153–162). Athletes should never go on keto or LCHF diets unless they are getting a lot of protein to protect themselves from shrinking muscles and losing strength (J Int Soc Sports Nutr, 2012; 9 (1): 34). You are not truly on a LCHF diet if you eat a lot of protein because your liver, muscles, kidneys and intestines can convert proteins into sugar.

Why Do Some Athletes Use Keto or LCHF Diets?
Some athletes and sports nutritionists believe that if you could teach your muscles to burn more fat and less sugar, you could keep the sugar in your muscles and liver longer, and have extra sugar for the last sprint at the end of a race (Metabolism, 2016;65(3):100-10). That is correct, but burning more fat and less sugar for energy means that you have to slow down during training and in races (Nutrients, 2014;6(7)). You have enough fat stored in your body to supply you with energy for weeks, but you only have enough sugar stored in muscles and your liver to last for about 70 minutes of all-out exercise. Runners start to run out of sugar after an hour of racing, which is why all competitive racers take sources of sugar during races that last more than an hour. When your muscles run out of sugar, they hurt and you find it difficult to move them, which runners call “hitting the wall.” When your liver starts to run low on sugar, your blood sugar drops, your brain runs out of its main source of energy and you feel dizzy and can pass out.

Severely restricting carbohydrates causes your body to use fat as its primary source of energy and produce ketones, but nobody has ever shown that the extra ketones help an athlete to move faster or with greater strength (Journal of Applied Physiology, 2006;100(1):7-8). When your liver converts fat to energy, it produces ketones that your brain can use for energy. Your brain gets almost all of its energy from sugar that passes through your bloodstream into your brain. Fat cannot pass into the brain from your bloodstream, but ketones can pass into the brain, so having extra ketones gives your brain a secondary source of energy if your blood sugar levels should ever drop. Again, no good data show that restricting carbohydrates improves athletic performance (PloS One, June 4, 2020;15(6):e0234027).

There is evidence that a keto or LCHF diet can help athletes lose weight, but it has not helped athletes to race faster, even in very long races such as in 100k (62-mile) time trials, even though their muscles burned more fat (Metabolism, Nov 3, 2017).

Anything that helps your liver store more sugar helps you to exercise more intensely for longer periods of time. The major way that weight loss helps you to have greater endurance and exercise longer is that losing body fat takes fat out of your liver, which allows your liver to store more sugar, thus giving you greater speed and endurance.

Don’t Eat Large Amounts of Refined Carbohydrates
The worst way to prepare for a race is “carbohydrate loading,” eating lots of spaghetti, bread or any other refined carbohydrates the night before the event. Almost 50 years ago, I showed that taking in huge amounts of refined carbohydrates can harm marathon runners by causing heart attacks (J Am Med Assoc, March 26, 1973;223(13):1511-1512). The extra carbohydrates are converted to fat that increases risk for forming plaques in your arteries. The extra fat also ends up in your liver. Extra fat in your liver reduces the amount of sugar that your liver can store, so you tire earlier. This is why “carbohydrate loading” has been abandoned by all knowledgeable athletes.

My Recommendations
For both athletes and non-athletes, I recommend a diet that is high in the “good carbohydrates:” vegetables, fruits, beans, whole grains and other seeds, with plant sources of fats such as nuts, avocados and oils. I think the most healthful diet is low in red meat, processed meats and fried foods. Sugar-added foods and drinks should be avoided except during prolonged, intense exercise. Not only will this diet help to protect your heart and blood vessels from arteriosclerosis, it should help you avoid excess weight which harms exercise performance and health. Storing extra fat in your muscles and liver reduces the amount of sugar that can be stored there and therefore harms performance. Loss of excess weight can improve performance by helping you store extra sugar in your muscles and liver.

I do recommend using intermittent fasting, which stimulates your body to switch to using ketones for short periods; see Why Intermittent Fasting Works. Various types of interval training for sports also have a similar effect; see Interval Training for Sports