The Effects of a Ketogenic Diet on Exercise Metabolism and Physical Performance in Off-Road Cyclists

Abstract: 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.

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Alex’s Notes: All forms of exercise are an intimate dance between your body’s energy systems and substrate use, and the demands of the skeletal muscle. In endurance sports that last an hour or greater, mitochondrial respiration is the main energy system, yet for the past few decades most athletes enjoy carbohydrate loading and carbohydrate-rich diets that increase glycogen stores to improve performance. However, this also increases the use of carbohydrates as fuel during exercise.

Ketogenic diets have been used for years in the battle against obesity and rare medical conditions such as epilepsy. Recently, new concepts with regard to performance enhancement have arisen on the premise that a ketogenic diet will increase the use of fatty acids for fuel and spare glycogen for the more intense portions of the endurance event. This holds promise when we consider that the major adaptation to endurance training is increased fat oxidation, greater mitochondrial and capillary density, and reduced lactate production. However, many events are not purely aerobic and often have more strenuous portions that require higher-intensity effort. This makes the need for ATP out-pace the ability of the body to breakdown fat, leading to an increase in glycolysis and suppression of fat oxidation (carbohydrates can synthesize ATP 2.5-5 times fast than fatty acids). Increased lactate production further suppresses the use of fat for energy.

So overall, both carbohydrate-dominated and ketogenic diets have their ups and downs, and the main objective of the study at hand was to determine the effects of a long-term ketogenic diet on aerobic performance, body mass & composition, lipoprotein profiles, hormones, and exercise metabolites. This study was conducted on eight male off-road cyclists with at least five years of experience. The experiment was two phases separated by a one week recovery Macrocycle, with subjects randomly assigned to one of two dietary interventions for phase one and a switch to the other during phase two. Each phase included four weeks of a mixed diet or ketogenic diet with identical training that was very high-volume and moderate intensity, and the testing portion of each phase was conducted during the last three days after the four weeks of training.

The diets used were a ketogenic diet (KD) and a standard Western diet (WD), both supplying about 3865 kcal daily. Accordingly, I broke down the diets in the table below.



















One of the first things you should notice is the carbohydrate intake on the ketogenic diet. This seems far too high for a low-carbohydrate diet, and for most it is. However, we must keep in mind the study population is athletes that are undergoing very high-volume training that, despite its moderate intensity, will slowly diminish glycogen over the course of four weeks. Considering that the nervous system and red blood cells use about 120 grams of glucose daily, that only leaves a negligible amount to become glycogen and will thus still promote an increased usage of fat and ketone bodies to supply energy during training and at rest. Another point of interest is the protein. A high-protein diet can put some out of ketosis because of gluconeogenesis from the excess, but why these diets were not matched for protein I have no idea.

Regardless, the ketogenic diet worked. There was a significant increase in ketone bodies, greater fat oxidation at rest and during exercise, and a reduced rate of lactate production and carbohydrate usage compared to the WD. However, the testing session ended with a 15 minute maximal intensity protocol that demonstrated the necessity of some carbohydrates, as evidenced by the significantly more pronounced suppression of fat oxidation in the KD group, as well as a lower power output, likely because of lower muscle glycogen stores and reduced activation of glycotic enzymes. Nonetheless, the KD demonstrated a significant improvement in aerobic capacity that the authors speculate may be due to the greater oxygen uptake necessary to facilitate the increased fat oxidation or by enhanced sympathetic activation.

Interestingly, for body composition, only the KD group reduced their body-fat percentage and body weight. The weight was likely due to lower amounts of glycogen and water, but the reduced body-fat percentage was an interesting find, especially since the groups were eating an identical amount of calories. Not surprisingly, insulin levels plummeted at rest but were similar during exercise, and cortisol was also lower at rest but greater during exercise. Testosterone in KD group also dropped, which is contrary to what would be expected on a high-fat diet. Finally, the KD diet also had significant benefits on the athletes’ lipoprotein profile. Specifically, there was no difference in LDL-C, but triglycerides decreased by 23%, total cholesterol increased by 14%, and HDL-C increased by 21%.

Overall, the results seem pretty clear. A ketogenic diet may be beneficial in competitive endurance athletes at least during the preparatory period when training volume is high and low- to moderate-intensity. However, they impair performance at higher intensities and would thus not be suitable for any events that require significant anaerobic exercise.

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