Male athletes on keto diet burned more fat, but effect muted in women, study finds

Researchers from the University of Poznan in western Poland reported on the effects in a study published in the Journal of the International Society of Sports Nutrition​​​. The study was published online last week.

A ketogenic diet is one in which the majority of calories come from fat, with a lesser fraction of protein and very little in the way of carbohydrates.

Keto diets associated with metabolic, cognitive effects

The researchers noted that the ketogenic diet was first researched in the 1930s as a potential treatment for epilepsy, and more recently has been associated with positive effects on weight loss, insulin resistance, diabetes and high blood pressure.

In addition, one of the major components of many ketogenic diets and products—coconut oil—has been associated with neuroprotective effects via its ketone content. Dr Mary Newport, MD, has extensively documented the effects of coconut oil supplementation on the progression of her late husband’s early onset Alzheimer’s disease​​. His cognitive scores increased significantly with the intervention, which she attributed to the ketones in the coconut oil delivered to the brain as alternative fuel to glucose, building on research that shows impaired glucose utilization in the brains of Alzheimer’s sufferers.

The theory behind the diet is that, despite the high exogenous fat intake, because of the paucity of glucose circulating in the blood, the body is forced to convert to mainly burning fat, and thus there is less endogenous fat buildup. The diet has also been postulated to have performance enhancing effects, because of the way the diet shifts the molecular mechanisms of cellular signaling.

“Activation of these signaling pathways may lead to a significant increase in the physical and exercise capacity, by stimulating, e.g., mitochondrial biogenesis, capillarization, regeneration processes, and especially, effective fat energy substrate utilization,​” the researchers from Poznan wrote.

To test this hypothesis the researchers recruited 30 adult CrossFit participants who were actively training at two clubs in Poznan. The criteria were ages 18 to 40, and training at least four times per week. CrossFit is a training regimen which is gaining increasing popularity that features rapid, short busts of activity at high power outputs and includes weight lifting and plyometric exercises.

Energy utilization tested at range of oxygen uptakes

After baseline measurements, the participants shifted to the ketogenic style of eating for four weeks. The ketogenic diets provided about the same caloric intakes as what the subjects had been consuming. The diets consisted of 15% protein in the form of meats, fish and dairy, and 75% fat in the form of coconut, rapeseed and olive oils, butter and nuts. The remaining 5% consisted of carbohydrates.

At baseline testing the subjects performed an incremental cycling test, in which the resistance was increased at intervals to exhaustion to measure changes in maximum oxygen uptake (VO2 max). Blood was also drawn and urine samples taken to determine if and how the subjects were shifting their energy utilization. The subjects did another cycling test while on the ketogenic diet.

The researchers found that about 20% of the participants did not adapt well to the diets and dropped out for reasons of drowsiness and irritability, rather than because of gastrointestinal discomfort. Four men and two women dropped out for these reasons, and one man and one woman dropped out due to injury.

Among the 22 remaining participants, the researchers found that they all shifted their energy utilization to some degree toward burning more fat. But only the men showed statistically significant shifts in this direction at all ranges of VO2 max, whereas for the women this change was significant only at around 65% of VO2 max, which equates to a moderately hard, but sustainable effort.

Support of keto diet for short burst sports questioned

Some trainers have postulated that increased fat utilization could be beneficial to athletes in endurance sports, like long distance cycling or running events, in which muscle glycogen stores might become fully depleted. This is the source of the somewhat passé notion of ‘hitting the wall,’ where the body shifts toward burning fat reserves for fuel.

There is less support for the notion that fat utilization might boost performance in short burst sports, like sprinting, American football or CrossFit, something the authors noted in their conclusion.

“The presented data on metabolic adaptation to KD [ketogenic diet] seems to indicate that CrossFit athletes may effectively adapt to training at wide, but gender-dependent range of exercise intensities. However, these observations should be taken with caution, because in practice the metabolic adaptation may not support the exercise performance, especially in high-intensity disciplines like CrossFit,”​ they noted.

Source:​ Journal of the International Society of Sports Nutrition​​
Effect of a four-week ketogenic diet on exercise metabolism in CrossFit-trained athletes​
2019 Apr 5;16(1):16. doi: 10.1186/s12970-019-0284-9
Authors: Durkalec-Michalski K, Nowaczyk PM, Siedzik K