In a recent article I talked about the ketogenic diet, its benefits, and its possible down-sides. In this article, I’d like to go into a lot more detail on how, or if, a ketogenic diet can benefit athletic performance.
Low-carbohydrate and high-fat (LCHF) diets are a current nutritional trend, especially for endurance runners. The idea is that eating mostly fat triggers adaptations that make the body better at burning fat for energy. Transitioning to running on fat can effectively solve the fueling problem for endurance athletes, eliminating the risk of hitting the wall and the need to refuel during long races.
On the other hand, very-low-carbohydrate diet (VLCD) with less than 50g carbs per day, frequently referred to as “ketogenic”, result in a metabolic state of ketosis. An increasing number of scientific studies show improved effects on fat loss and type 2 diabetes 1.
The science of sports nutrition states that carbohydrates are essential, and that they are the preferred fuel for athletes — before, during, and after exercise. It’s suggested the endurance athlete train 3 to 4 hours a day and consume 10g/kg carbs per day. I personally believe that this opens a big door for the supplement and sports beverage industry. Endurance athletes in ketosis can perform for many hours while consuming far fewer calories than carb-dependent competitors. As a result, ketosis may be a good solution for athletes who consistently struggle with gastric distress, especially during ultra-distance events 2.
How the body chooses the proportion of carbohydrate and fat to use for fuel is complex, the more carbs that are available, the more the body burns, while at the same time shutting down access to its much larger fuel reserve — fat. Glucose can be metabolized directly to make adenosine triphosphate (ATP), and can also be stored in modest amounts as glycogen in skeletal muscle, and to a lesser extent in the liver. On average, our maximum glycogen store is 400–500g. And since 1g carbohydrate equals 4kcal, we max out at about 1600–2000kcal in our carbohydrate fuel tank. Each 1g glycogen is also stored with 2g of water. If we carry more muscle mass, are well trained, and eat a high carbohydrate diet, our glycogen stores could be increased by up to another 50%, but the total amount of carbohydrate available in the body is still relatively low compared to available fat stores. Body stores of fat fuel are about 40,000 kcal, and carbohydrate fuel about 2,000 kcal.
Ketones are about 25% more efficient at producing ATP than glucose or fatty acid 3. Fueling tactics that emphasize carbohydrate-based diets and sugar-based supplements bias the metabolism towards carbohydrate while simultaneously inhibiting fat mobilization and utilization. This suppression of fat oxidation lasts for days after consuming carbohydrates, not just the few hours following digestion when insulin levels are high. In order to sustain a high level of performance under conditions of glycogen depletion and decreased glucose availability, cells must adapt to using fat as a fuel. This process, referred to as keto-adaptation, has the potential to improve human performance and recovery. Unlike our limited storage of glycogen, fat cells have a vast capacity to store fat. Since fat contains 9 kcal per gram and is stored with minimal water, they are an efficient storage form of energy that can be mobilized quickly when blood insulin levels are low.
Even in a very lean athlete, the total amount of energy stored as fat will typically be more than 20 times the maximum level of carbohydrate stored in the body. Thus, whereas vigorous exercise can deplete glycogen reserves in just a few hours, when adapted to burning primarily fat, this thin athlete has enough fat to fuel several days of exercise. During prolonged exercise, when body stores of carbohydrate as glycogen are depleted, there is increasing dependence on the liver to maintain blood glucose levels. This is not just to provide the exercising muscles with glucose, but also to support normal body functions, especially the central nervous system. Decreased carbohydrate availability, particularly for the brain, results in a sharp decline in physical and mental performance. The current practice of carbohydrate loading increases glycogen stores and is often accompanied by ingestion of sugar-based sport drinks and gels during exercise. These carb-based fueling tactics may delay the onset of hitting the wall, but for most athletes extending beyond marathon-duration events, they will still crash and burn despite having many thousand calories worth of fuel tucked away in their fat cells.
Keto-adaptation allows rapid mobilization and utilization of “non-carbohydrate” lipid fuel sources. This process involves the conversion of fat to ketones in the liver, and these ketones help supply the brain with energy when glucose levels fall. This affords even a very lean (10% body fat) athlete access to more than 40,000 kcal from body fat, rather than starting a prolonged event depending primarily on ~2,000 kcal of glycogen. Avoiding wide excursions in blood sugar and insulin by burning predominately fatty acids and ketones can lessen the “stress” response to exercise. Less metabolic stress, improved fuel flow, and healthier membranes translate into faster recovery from exercise and less exercise-induced inflammation, immunosuppression, gastro-intestinal distress, insulin resistance, muscle damage, and soreness.
Low carbohydrate diets are anti-inflammatory, producing less oxidative stress during exercise and more rapid recovery between exercise sessions. At the practical level, effective training for both endurance and strength/power sports can be done by individuals adapted to carbohydrate restricted diets, with desirable changes in body composition and power-to-weight ratios.
In order for the body to shift from glucose to fat for fuel, there need to be alterations in inter-organ fuel exchange — the process that partitions lipid fuels to specific sites for oxidation. Although skeletal muscle has the capacity to take up and oxidize ketones, over time muscles switch to using fatty acids provided from blood and probably muscle triglycerides. This process, a key element of keto-adaptation, allows ketones to achieve levels in the blood that allow them to meet most of the brain’s fuel needs. Thus there is a reciprocal relationship between blood ketones and uptake in muscle, such that when blood ketone levels are low, muscle uptake is high 4.
However, all individuals are different and need a basic understanding of how low carbohydrate diets function to support human physiology. The ketogenic diet is a great tool for athletes wanting to achieve their goal. Many ultra-runners, after switching their diet, see improved performance 5. Also, the ketogenic diet can affect some individual’s digestion, as a result of gut bacteria regulating the expression of genes involved in glucose and fat absorption.
If the athlete is completely content with their body, health, and performance on a high carbohydrate diet then there is probably no reason to consider a low carbohydrate diet. However, most athletes do not have the same level of carbohydrate intolerance as someone who is overweight with metabolic syndrome or diabetes. Yes, I believe more research is needed, but I also believe that we are all individually very different, so we need to do our own research on our own bodies. Professional athletes, of course, need to work with a sports nutritionist so that they can adjust their diet with the correct macronutrients ratio, and maximize their performance.