Exploring low-carb diets for high-intensity exercise performance Original paper

Athletes are typically advised to consume high-carbohydrate diets to maintain muscle glycogen stores and support vigorous-intensity exercise, which is fueled almost exclusively by carbohydrates (glucose). However, recent evidence suggests that low-carbohydrate/high-fat (LCHF) diets may promote greater fat oxidation during exercise,^[1] perhaps making them a suitable choice for athletes, despite some studies noting performance impairments during an LCHF diet.^[2]

Because LCHF diets also improve blood glucose control and cardiometabolic health, it’s worth exploring whether this dietary pattern offers health and performance benefits compared to high-carbohydrate/low-fat (HCLF) diets.

In this randomized crossover study, 10 highly trained male athletes (average age of 40) completed two 31-day dietary interventions separated by a 2-week washout period. The dietary interventions had the following prescribed macronutrient targets:

• LCHF: less than 50 grams of carbohydrates, 75%–80% fat, 15%–20% protein
• HCLF: 60%–65% carbohydrate, 20% fat, 15%–20% protein

The LCHF diet was also supplemented with 1–2 grams of sodium per day from bouillon cubes or homemade broth and was designed to promote continuous nutritional ketosis throughout the 31-day intervention period. Nutritional ketosis was verified by measures of blood ketones on days 3, 7, 14, 21, and 28.

The primary study outcomes included running performance (1-mile time trial and 6 x 800-meter repeated sprint performance), carbohydrate and fat oxidation during exercise, body composition, continuous glucose, and cardiometabolic biomarkers (HbA1C, total cholesterol, LDL cholesterol, very-low-density lipoprotein cholesterol, HDL cholesterol, triglycerides, insulin, and high-sensitivity C-reactive protein (CRP)). Outcomes were measured before and after each 31-day intervention.

After the LCHF diet, average fat oxidation increased (+190%), and average carbohydrate oxidation decreased (−20%) during the 1-mile time trial. During the repeated sprint test, average fat oxidation increased (+92%) and average carbohydrate oxidation decreased (−54%). Performance on the 1-mile time trial and repeated sprint test was no different before and after the LCHF diet.

There were no changes in carbohydrate oxidation, fat oxidation, or performance during the 1-mile time trial or repeated sprint test after HCLF.

Average blood glucose was lower during LCHF than during HCLF on days 8, 13, 15–20, and 22 of the diet. Total cholesterol, LDL cholesterol, and HDL cholesterol were higher after LCHF compared to HCLF. Body weight and BMI decreased after both the LCHF and HCLF diets.

While both diets were isocaloric (contained a similar amount of calories), they differed in their fat, carbohydrate, protein, cholesterol, fiber, and sugar content. With the exception of protein intake, which was 31 grams higher on LCHF, all other nutrient differences were an expected outcome of the dietary prescriptions.

Dietary carbohydrates are used to maintain blood glucose and glycogen (glucose stored in the muscles and liver), which can be used for energy during exercise. Muscle glycogen is the body’s preferred fuel source during high-intensity and long-duration exercise. Indeed, consuming carbs during exercise delays and reverses muscle fatigue, and though the practice is questionable, endurance athletes have long practiced “carbohydrate loading” before big races to saturate their glycogen stores in hopes of improving performance.

Why are carbs so essential for athletes? At rest and during low-intensity exercise, the body mainly burns fat for energy. But at a certain intensity of exercise, the body begins to derive a larger percentage of its energy from carbohydrates rather than fat. This is known as the “crossover point.”^[3] As exercise intensity increases above the crossover point, more glucose and less fat is used, until finally, at around 85% of maximal aerobic capacity, fat’s contribution to energy is negligible, and the body is getting almost all of its energy from glucose (carbs).

The “crossover concept” of exercise metabolism

At rest and during exercise below 60% of maximal oxygen uptake, fat is the main fuel source used to generate ATP. Above 75% of maximal oxygen uptake, glucose and muscle glycogen become the dominant fuel sources. The crossover point refers to the intersection of carbohydrate and fat metabolism, beyond which more energy is derived from carbohydrate (glucose) and less is derived from fat.

The crossover effect explains why high-carb diets are promoted for performance and the general hesitancy to adopt LCHF diets. However, there has lately been an increase in the interest in LCHF diets among athletes.

The FASTER study was published in 2015. In that study, researchers characterized the metabolic profiles of “keto-adapted” endurance athletes — those who had been eating a LCHF/ketogenic diet for several years.^[4] The study questioned some long-held beliefs about exercise metabolism.

The low-carb athletes had rates of peak fat oxidation that were more than two-fold higher than the high-carb athletes, and they hit their peak fat oxidation rate at around 70% of VO₂ max, compared to 55% in the high-carb group. Despite consuming less than 50 grams of carbohydrates per day, the low-carb athletes also had similar levels of muscle glycogen at rest and after a 3-hour endurance run when compared to the high-carb athletes. These results were some of the first to suggest that habituation to a low-carb diet can shift the crossover point during exercise, allowing athletes to use more fat at a higher exercise intensity and increase the exercise intensity where the crossover point occurs.

The results of the current study support these findings. Peak fat oxidation rates occurred at 85% of the athletes’ VO₂ max after just a month on a ketogenic diet. Interestingly, peak fat oxidation occurred at 80% after the high-carb diet, though it was still nearly half of that observed after the low-carb diet. In addition, some of the highest fat oxidation rates ever recorded were observed in these middle-aged athletes following the LCHF diet, in excess of 1.85 grams/minute.

Altering diet composition definitely shifts energy metabolism during exercise. But athletes care about performance. In this regard, previous studies haven’t been so supportive of LCHF diets.

Numerous studies published in the last few years have shown that keto adaptation could be costly. Athletes who adopted a LCHF diet for 5–6 days^[2], 3 weeks^[5], and 25 days^[6] displayed worse exercise economy and impaired performance during simulated competition compared to when they consumed a high-carbohydrate diet. Six days of a LCHF diet also impaired high-intensity sprint performance but did not affect 100-kilometer cycling performance in a group of cyclists.^[1]

This study was designed to further test the hypothesis that high-intensity exercise performance is impaired following a LCHF diet. This hypothesis that was not supported by the results. The athletes in this study performed a 1-mile time trial and a series of 6 high-intensity 800-meter sprints — both of which are thought to be primarily carbohydrate fueled. Nonetheless, performance on each test was equivalent following the LCHF and HCLF diets — 367 seconds and 374 seconds (1-mile time trial) and 1,236 and 1,254 seconds (total time for the 6 x 800-meter sprints) after the LCHF and HCLF diets, respectively.

Low-carb vs. high-carb for exercise performance and metabolism

It’s important to underscore the metabolic health benefits experienced by the participants during the LCHF diet. Despite being healthy and fit athletes, 30% of the participants in this study had average glucose levels during the HCLF diet that were greater than 100 mg/dL, putting them in the “prediabetic” range. Average glucose and glycemic variability significantly improved in all participants during the LCHF intervention and to a greater extent in the few participants with elevated glucose levels.

LCHF provided therapeutic benefits without compromising performance, suggesting that this dietary pattern may be suitable for athletes who want to optimize their health and exercise routines. Though the sample size was small (10 participants) and included only middle-aged men, which limits the generalizability of the findings, the results of this study will certainly ignite some discussion.