A Review by Alyssa Bialowas

The ketogenic diet has been making waves due to its effective weight loss and health benefits, such as reducing blood sugar levels, increasing good HDL cholesterol and improving heart health. If you haven’t heard of the keto diet before, it’s a high fat, low carb diet plan that depletes your body of its store of sugar, breaking down protein and fat for energy and causing ketosis.

On the keto diet, you’re supposed to restrict your daily carb intake to 20 to 50 grams per day, replacing those reduced calories with fats (Cipryan et al, 2018). The keto diet has been shown to lower basal glucose and insulin levels, increase fat oxidation rates and upregulate an alternative energy source in the form of ketones (Cipryan et al, 2018). This triggers the body to use fat and ketones as their primary fuel sources, which could be advantageous for prolonged exercise.

Research out of the Czech Republic aimed to determine whether the keto diet offers a sufficient energy source for athletes and if it has any effect on the physiological variables during high-intensity interval training (Cipryan et al, 2018). They compare the effects of shifting from a habitual mixed Western diet to the keto diet on the physiological responses to a graded exercise test and a HIIT session. Since aerobic and anaerobic exercise performance has previously been found to be highly reliant on the availability of carbohydrates in the body, do limited carbohydrates affect athletic training and performance?

Related Article: How HIIT Changes Our Body

The Study

This study was comprised of eighteen moderately trained males between the ages of 18 and 30, who were divided into two groups: (1) A very low-carb, high-fat diet group, and (2) A habitual mixed Western diet group. Participants attended the exercise physiology laboratory at baseline, after two weeks (mid measurement) and at four weeks (post measurement) of the controlled experiment. A maximal incremental treadmill test was performed at baseline and four weeks, and a HIIT session was performed at all three times. Tests were separated by 48 hours.

Training sessions were conducted in the morning, three hours after participants’ last meal in a thermally controlled lab room. Participants were not to participate in vigorous activity 24 hours before lab testing. They were asked to perform 3 to 5 sessions per week of non-supervised training, to record their heart rate using a heart rate monitor, and to keep an exercise diary. Heart rate (HR), oxygen uptake (V̇O2), respiratory exchange ratio (RER), maximal fat oxidation rates (Fatmax) and blood lactate were measured.

Related Article: 6 Tips to Fuel Your HIIT Nutrition Plan

The Results

High-intensity performance wasn’t found to be compromised during the final stages of the graded exercise test or the HIIT sessions. However, the researchers did find increased fat oxidation rates in the low-carb, high-fat diet group after four weeks. Between-group differences in maximal fat oxidation rates were substantial, revealing greater increases in the low-carb, high-fat group versus the Western diet group. Mean RER decreased in both groups but was even more pronounced in the group on the low-carb, high-fat diet. A substantial decrease of the peak RER values from baseline to mid measurement to post measurement was apparent in the low-carb, high-fat group. Blood lactate levels after the last two high-intensity repetitions during mid-and post-measurement increased compared with pre-measurement in low carb group.

The Takeaway

Performance and cardiorespiratory responses during a graded exercise test and HIIT session were not impaired after consuming a keto diet relative to a mixed Western-based diet. A four-week adaptation period to a keto diet preserved high-intensity exercise performance. The results of this study challenge the necessity of a carbohydrate-rich diet for high-intensity exercise performance.

Related Article: Which Type of Protein Provides the Best Workout Recovery?


Cipryan, L., Plews, D.J., Ferretti, A., Maffetone, P.B., and Laursen, P.B. 2018. “Effects of a 4-Week Very Low-Carbohydrate Diet on High-Intensity Interval Training Processes.” Journal of Sports Science and Medicine. 17, 259-268.

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