Low-Carb, High-Fat Diets for Female Endurance Athletes

By Michael Raynor, MS, CISSN and Leonela Connolly, MS, CISSN

Michael and Leonela have been working their butts off researching this topic over the past year for their thesis. It paid off! They are proud recipients of their Master of Science in Nutrition from Meredith College. Congratulations!

The Ketogenic Diet

You may have heard of a ketogenic diet. Ketogenic diets are very-low carbohydrate diets (typically less than 40-50g of carbohydrate per day) that lead to a metabolic state called ketosis. There are really only two substrates that the brain can use for energy: glucose and ketones. Ketones (hence the name ketosis) are a byproduct of the oxidation of fatty acids for fuel. By drastically reducing carbohydrate intake, you’re decreasing the availability of glucose as a fuel source and simultaneously increasing the availability of fat for fuel. As a result, the body begins to burn the more available fuel source: fat.

Can ketogenic diets improve endurance?

Research regarding ketogenic diets for athletes have increased drastically since the 1980s. The theory behind the interest of a ketogenic diet for endurance athletes is that a diet that optimizes the body to burn fat and spare glycogen could lead to huge increases in endurance.

If you’ve been in endurance sports for long, you’ve likely experienced the dreaded “bonk” that results from depleted glycogen stores. Bonking can have huge detrimental effects on performance and endurance. Theoretically, the longer you can avoid depleting glycogen, the longer you can exercise. For reference, even after carb-loading, the body can only store about 2,500 calories in glycogen, whereas even very lean athletes have 40,000+ calories stored as fat!

But does the research suggest that a ketogenic diet affects endurance that drastically?

Well, in short, no, but there is support for it. Research suggests that a ketogenic diet leads to increases in the rate of fat oxidation. However, not much research has explored if this effect really translates into improved endurance. (check out our YouTube video on the benefits of the Ketogenic diet here).

Regardless, a ketogenic diet is extremely strict and is, for many people, unsustainably low in carbohydrates (and very high in fat). However, a less strict, more sustainable alternative has been proposed: a low-carbohydrate, high-fat (LCHF) diet. Leonela and I just spent the last year or so researching this concept for our thesis at Meredith College.

Low-carb, high-fat diets for female endurance athletes

Leonela and I wanted to explore if a LCHF diet could elicit similar favorable metabolic effects in female endurance athletes as a ketogenic diet does. Without getting into too much detail about our study, we found that there’s no definitive answer. Some studies suggest that a LCHF diet does produce similar metabolic adaptations and body composition changes to a ketogenic diet, but other studies fail to reproduce this finding. However, this variation may be a result of drastic variation in the designs of the studies completed. For our study, we had participants consume 55-60% of calories from fat, 25-30% of calories from carbohydrate, and 15-20% of calories from protein.

As for the results of our research, Leonela and I found a large degree of individual variation. In other words, no two people responded to the diet the same way. Furthermore, even the degree to which people responded varied. Our study was small, so we can’t draw any general conclusions. However, our final take-away was that there may be something to the diet, but there also may not be credence to it. Some of our participants experienced increased rates of fat oxidation while on the diet, yet others saw no change to rates of fat oxidation. When looking at the study group as a whole, there were no persistent changes as a result of the diet.

More research is necessary on low-carb diets and endurance exercise

While there is some data to suggest the LCHF diet may have an effect, our overall data suggest that it results in no persistent metabolic or body composition changes in female endurance athletes. However, our study was small, and alcohol consumption may have complicated things by inhibiting adaptation to a LCHF diet by interfering with lipid metabolism. While the literature contains mixed results, there are some results out there to suggest that a LCHF diet may increase rates of fat oxidation.

Want to give it a try? We can help you navigate the process using our metabolic testing and nutrition expertise (just book your FREE Discovery Session to get started!), so go for it and let us know what you think! In fact, Michael did his own little experiment last year on a low-carb, high-fat diet, called Fat for Fuel. Check it out here.