Ketogenic Diet and Sports Performance


We are full of fat, as an interesting review article published last year in The European Journal of Sport Sciences points out. The article, “Rethinking Fat as a Fuel for Endurance Exercise,” notes that even the leanest marathon runner has more than 30,000 kilocalories of fatty tissue in reserve — enough fat to fuel multiple marathons.

Carbohydrates, stored in the muscles as glycogen, is generally what athletes focus on as fuel because dietary fat is not as readily available as glycogen, which is easy to liberate and burn from the muscles. Before it can be used as fuel by the muscles, fat must first be broken down into fatty acids and other component. It’s this step that makes dietary fat less accessible and less efficient as a fuel, especially during intense exercise.

So, theoretically, being able to burn fat as fuel even in the most prolonged and strenuous exercise may aid in recovery from that exercise by reducing inflammation and muscle damage and improve performance.

According to Dr. Burke, the head of sports nutrition at the Australian Sports Commission, there has been no study that has shown ketogenic diets to be effective at “enhance sports performance,” only that they increase fat burning rates in elite endurance athletes. And the same studies generally show that high-fat diets inhibit performance during high-intensity sprints, which demand fast-burning glycogen stores.

Should you decide to try fat-loading your diet, bear in mind that the switch is likely to disrupt your training in the beginning. Performance actually declines dramatically during the first several weeks. The body runs low on glycogen before it becomes well-adapted to using fat. Low glycogen may leave you feeling fatigued, heavy-legged, nauseated and even ill for up to a month.

What the research shows:

In the first study to profile elite athletes eating very low-carbohydrate diets, researchers studied 20 ultra-endurance runners age 21-45 who were top competitors in running events of 50 kilometers or more. Ten athletes ate a low-carb diet consisting of 10 percent carbs, 19 percent protein and 70 percent fat. The other group of athletes consumed a high-carb diet, getting more than half their calories from carbs. The athletes were similar: in other aspects, such as elite status, age, performance, training history and maximum oxygen capacity.

During an endurance run, the two groups showed similar ratings in oxygen consumption, perceived exertion and calorie expenditure. However, fat-burning rates were about twice as high in the low-carb athletes. Scientists reviewed the athletes’ maximum oxygen intake to gauge carb- and fat-burning rates.

The high-carb runners were very healthy, and were great at burning fat by conventional standards, yet their peak fat burning is less than half that of endurance athletes eating low-carb diets.

Another key finding: Despite their low intake of carbs, these fat-burning athletes had normal muscle glycogen levels — the storage form of carbohydrates — at rest. That means they broke down roughly the same level of glycogenas the high-carb runners during the long run, and made the same amount of glycogen in their muscles during recovery as the high-carb athletes.