Endurance athletes prepare better for races with a low-carb diet

Endurance athletes who are preparing for competitions may achieve better results if they cut their carbohydrate consumption right down, and replace the carbs with unsaturated fatty acids. Polish sports scientists drew this conclusion after performing a study in which 8 experienced male mountain bikers followed a low-carb ketogenic diet for four weeks.

Endurance sports paradox

There’s something strange about the diet that most endurance athletes follow. While performing endurance sports the body burns mainly fat, but many endurance athletes eat huge amounts of carbohydrates (and often the kind that would give anyone with a bit of nutritional knowledge the heebie jeebies).

You’d expect that endurance athletes would be better off following a diet that provides most kcals in the form of fat – which would encourage their body to learn how to oxidise fats.


That was the reasoning behind the Polish study, which was recently published in Nutrients. The researchers gave their subjects a standard diet for four weeks, and then put them on a ketogenic diet for another four weeks. The emphasis in the ketogenic diet was on unsaturated fatty acids. During both periods the subjects also followed a demanding training routine.



For a start the ketogenic diet was good for the participants’ heart and blood vessels. After four weeks on the low-carb diet the mountain bikers had significantly more ‘good cholesterol’ HDL in their blood. The effect on the ‘bad cholesterol’ LDL was not significant.



The low-carb diet also made the subjects slimmer. They lost no muscle mass whatsoever, but they did lose three kg body fat.


Muscle damage

After each diet period the researchers got the subjects to cycle for 90 minutes at moderate intensity, and at the end of that time to cycle to the point of exhaustion while the level of exertion was increased every minute.

The researchers monitored the concentration of the enzymes LDH and CK in the athletes’ blood. The higher the concentration, the more muscle damage there is. From their measurements the Poles could see that the low-carb diet reduced muscle damage incurred during exertion.


Performance capacity

The low-carb diet boosted the body’s oxygen uptake. After the ketogenic period the subjects’ VO2max had risen.


On the other hand the subjects’ maximum workload had decreased – at least, after the low-carb diet the athletes generated fewer Watts. From this the Poles concluded that a ketogenic diet is probably ideal preparation for competition, but that just before and during races endurance athletes are probably better off eating carbohydrates.


“High fat diets may be favorable for aerobic endurance athletes, during the preparatory season, when a high volume and low to moderate intensity of training loads predominate in the training process”, the Poles write.

“High volume training on a ketogenic diet increases fat metabolism during exercise, reduces body mass and fat content and decreases post exercise muscle damage. Low carbohydrate ketogenic diets decrease the ability to perform high intensity work, due to decreased glycogen muscle stores and the lower activity of glycolytic enzymes, which is evidenced by a lower maximal work load during the last 15 min of the high intensity stage of the exercise protocol.”

The effects of a ketogenic diet on exercise metabolism and physical performance in off-road cyclists.


The main objective of this research was to determine the effects of a long-term ketogenic diet, rich in polyunsaturated fatty acids, on aerobic performance and exercise metabolism in off-road cyclists. Additionally, the effects of this diet on body mass and body composition were evaluated, as well as those that occurred in the lipid and lipoprotein profiles due to the dietary intervention. The research material included eight male subjects, aged 28.3 ± 3.9 years, with at least five years of training experience that competed in off-road cycling. Each cyclist performed a continuous exercise protocol on a cycloergometer with varied intensity, after a mixed and ketogenic diet in a crossover design. The ketogenic diet stimulated favorable changes in body mass and body composition, as well as in the lipid and lipoprotein profiles. Important findings of the present study include a significant increase in the relative values of maximal oxygen uptake (VO2max) and oxygen uptake at lactate threshold (VO2 LT) after the ketogenic diet, which can be explained by reductions in body mass and fat mass and/or the greater oxygen uptake necessary to obtain the same energy yield as on a mixed diet, due to increased fat oxidation or by enhanced sympathetic activation. The max work load and the work load at lactate threshold were significantly higher after the mixed diet. The values of the respiratory exchange ratio (RER) were significantly lower at rest and during particular stages of the exercise protocol following the ketogenic diet. The heart rate (HR) and oxygen uptake were significantly higher at rest and during the first three stages of exercise after the ketogenic diet, while the reverse was true during the last stage of the exercise protocol conducted with maximal intensity. Creatine kinase (CK) and lactate dehydrogenase (LDH) activity were significantly lower at rest and during particular stages of the 105-min exercise protocol following the low carbohydrate ketogenic diet. The alterations in insulin and cortisol concentrations due to the dietary intervention confirm the concept that the glucostatic mechanism controls the hormonal and metabolic responses to exercise.

PMID: 24979615 [PubMed – in process] PMCID: PMC4113752

Source: http://www.ncbi.nlm.nih.gov/pubmed/24979615

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