A study from St. Louis University showed that a low-carbohydrate, high-fat (LCHF) ketogenic diet impairs performance in sports that require speed (The J of Sports Med and Phys Fit, April 4, 2018). Most other studies come to the same conclusion (The Journal of Physiology, December 23, 2016).
On a low-carbohydrate diet, you can't train very fast and you can't move as fast in races. Your body uses primarily carbohydrates and fats, and a small amount of protein, to supply you with energy when you exercise. A ketogenic diet means that you try to get your body to use fat as the prime energy source for your muscles. To do this, you must restrict carbohydrates and protein. Carbohydrates are just sugars in singles and chains, and they provide sugar to power 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. 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.
Why a Keto Diet Slows You Down When You Exercise The limiting factor to how fast you can move 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. Carbohydrates require less oxygen than fats do to power your muscles. The faster you move, the greater 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 work at near-maximum speed and intensity, 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).
The Flaw in Studies Used to Support LCHF for Athletes Anything that increases a person's maximal ability to take in and use oxygen (VO2max) will also help them to move faster over distance. One report appears to show increases in off-road cyclists' maximal ability to take in and use oxygen (VO2max) with a LCHF diet (Nutrients, 2014; 6 (7): 2493–508). 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 show that high protein-LCHF diets can help athletes lose weight (Br. J. Nutr, 2013; 110: 1178–87), and possibly to have greater endurance in sports that require competing over many consecutive days with little rest, 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 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 branched-chain amino acid proteins to sugar (gluconeogenesis).
Why Do Some Athletes Use LCHF? 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;6.7:2493-508). 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 12 hours when you sleep (1600-2000 calories) and far less than that when you 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 is called "hitting the wall." When your blood sugar starts to drop, your brain runs out of its main source of energy and you feel dizzy and can pass out. There is evidence that a keto diet can help athletes to 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).
LCHF Diets and Ultramarathon Runners In the 1960s, I rode up to the start of the Boston Marathon in a bus with a young man named Tim Noakes. Today he is a highly-respected professor of sports medicine in South Africa. He was one of the first researchers to show that marathoners do indeed suffer heart attacks and one of the first to prove that hyponatremia (low blood sodium) in athletes is caused by excess fluid intake. He proved that carbohydrate loading can harm performance in marathon runners. I had already shown that excessive carbohydrate loading can cause heart attacks in marathon runners (J Am Med Assoc, March 26, 1973;223(13):1511-1512).
Dr. Noakes believes that ultra-marathon runners may benefit from a LCHF diet (Eur J Sport Sci, 2015;15(1):13-20). Ultra-marathoners usually run in races that exceed the conventional marathon distance of 26 miles and 385 yards. He says that nine of 11 papers on LCHF diets in athletes showed benefits (British Journal of Sports Medicine, May 26, 2014). Athletes on the LCHF diet got much less of their energy from sugar (carbohydrates) and up to four times as much energy from fats. This should allow them to exercise longer, since humans have almost an unlimited amount of fat and a very limited amount of sugar stored in their bodies. However, the authors in these studies did not show that the athletes were able to exercise faster or further when they moved fast enough to exceed their lactic acid threshold (gasping for breath).
My Recommendations For 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. Storing extra fat in your muscles and liver reduces the amount of sugar that can be stored and therefore harms performance. Loss of excess weight improves 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 my report above on Why Intermittent Fasting Works. Various types of interval training for sports also have a similar effect; see Interval Training for Sports
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