Performance from the Inside Out

Nutrition Education and Coaching

Forging Fitness Fueled by Fat

You’ve heard of the ketogenic diet. You’ve heard anecdotal stories of prolonged endurance, of rapid or sustained weight loss. Maybe you’re in a weight class sport looking to cut weight without losing strength, or maybe you’re looking to maximize endurance. Could switching to a ketogenic diet take your performance to the next level? Let’s take a look at the current research on the impact of low carb ketogenic diets on body composition, metabolism, and athletic performance.


“Low-carb” diets have been around for at least 100 years. This type of deliberate dietary modification is used with the intent of manipulating body composition, body chemistry, and/or metabolic response to exercise (1). Interest in low carbohydrate ketogenic diets (LCKD) for sport intensified in the 1980s, and has seen a resurgence in recent years. This interest stems from the idea that dietary modification that causes the body to preferentially use lipids and ketones for fuel could be advantageous, particularly in endurance sports, as the body’s ability to store lipids greatly exceeds its ability to store fuel in the muscles in the form of glycogen, even in lean individuals (2). This would reduce the need to provide carbohydrates during exercise to meet energy demands and maximize fat loss in response to exercise. A LCKD typically refers to a diet that includes less than 50 grams, or less than 10% of calories, from carbohydrates, about 20% protein, and at least 70% of calories from fat.


          Endurance sports rely on a steady supply of energy to match energy expenditure. The amount of glycogen the body can store in the liver and muscles is limited, so conventional practice has been to consume adequate carbohydrates before, during, and after exercise to ensure adequate glucose is available in the bloodstream and to replenish glycogen stores after. In theory, manipulating metabolism to prioritize use of lipids as fuel would take advantage of fat stores. One concern is that metabolism of fat is slower than that of carbohydrate, and metabolic adaptation to prioritize fat could decrease metabolic flexibility and reduce the ability to use carbohydrates as fuel. This reduced flexibility could limit the intensity an athlete can exert or sustain (3)

Recreational Endurance Athletes

          In a ten week study of five recreational endurance athletes, transitioning from their usual diet (greater than 250 grams carbohydrates per day) to a LCKD (less than 50 grams) led to a reduction in body fat, an improvement in sense of well-being, and an unexpected improvement in other health conditions, such as skin conditions. Overall aerobic capacity was reduced, as was the ability to engage in intense bouts of energy expenditure. This was likely due to an impaired ability to metabolize glycogen. (4)

Ultra-Endurance Running

          A study of ultra-endurance runners compared ten athletes on a high carbohydrate diet (greater than 250 grams of carbohydrate per day) and ten on a LCKD (less than 50 grams) (5). The keto-adapted athletes exhibited a unique lipid profile. Athletes consuming a LCKD had significantly higher levels of total cholesterol, LDL, and HDL (65%, 83%, and 60% respectively). These levels are much higher than those observed in non-athletes on a LCKD. The concentration of large LDL and HDL particles was also much higher in the LCKD group, which is typically associated with a lower risk of cardiovascular disease. (6) These high levels of circulating lipids may serve a functional purpose, acting to transport lipids in high demand during intense, prolonged exercise. The long-term health effects of this phenomenon have not been studied. (5)

Explosive Exercises

          What about sports requiring explosive bouts of energy expenditure? Glycogen stores and glucose in the bloodstream provide a pool of readily available fuel. Several recent studies on sports requiring explosive energy expenditure looked at the impact of a LCKD on body composition and on performance. These studies consistently illustrate that LCKD can reduce body fat without harming performance. (7, 8, 9, 10, 11)


High Intensity Interval Training (HIIT) describes short workouts that are composed of short bursts of intense exercise meant to increase heart rate and maximize fat loss in a short amount of time. In one study, eighteen moderately trained male recreational athletes were recruited to follow a ketogenic diet for four weeks. Both control and LCKD groups saw a reduction in body weight, likely due to a reduction in calories. The LCKD group had an increase in lipid oxidation and blood lactate concentrations, but there was no difference in HIIT performance between the two groups with total time to exhaustion remaining the same. (7)

Powerlifting/Olympic Weightlifting

Low carbohydrate diets have been used for cutting weight for years, making them of interest to athletes in weight-class sports. Competing at the top end of a weight class is ideal, and rapid weight loss prior to competition is a common strategy. A reduction in body mass without a loss of strength increases an athlete’s strength to power ratio. Fourteen intermediate to elite competitive powerlifters and Olympic weightlifters followed either their usual diet (>250g carbohydrates) or a LCKD (<50 g carbohydrates) for 3 months, and then crossed over. Calories were unrestricted and remained comparable, as tracked using online/mobile tools (MyFitnessPal and Qualtrics). Body weight was significantly lower after 3 months following a ketogenic diet and lifting performance remained comparable. (8)

In a study of 24 men engaged in eight weeks of resistance training, the group on a LCKD had a reduction in body weight and body fat and no change in muscle mass, whereas the control group had no loss in body fat but had an increase in muscle mass. (9)


Crossfit is not a weight class sport, but as with many sports, an increased strength to weight ratio will lead to an increase in performance. Crossfit is designed to improve all aspects of fitness, incorporating lifting, sprinting, gymnastics, plyometrics, and flexibility into constantly changing daily workouts. 31 male and female athletes, age 19-56, were recruited from Crossfit gyms. 13 followed their usual diet and 14 followed a LCKD (less than 50 grams carbohydrates) for 6 weeks. Each participated in four Crossfit training sessions per week, participated in no other intense workouts, and took no supplements starting 7 days prior to study. Compliance was measured through urinary ketones and bi-weekly food logs. Calories were consistent between groups. The LCKD group had a significant reduction in total body mass with no reduction in lean body mass. Participants in both groups has an improvement in speed, but there was no difference in performance between the two groups. (10)


          In a study of eight elite male artistic gymnasts who followed a ketogenic diet for 30 days (22g carbohydrates daily, 40% protein, 55% fat), there was a significant loss in total body weight and in body fat with a non-significant increase in lean body mass. No difference was noted in performance. (11)

Long term health

          Long term effects of following a very low carbohydrate diet have not been studied in athletic or non-athletic human populations. Most research shows that adherence to a ketogenic diet results in a reduction of plasma triglyceride levels, an increase in total cholesterol, a reduction in glucose and insulin levels, and an increase in insulin sensitivity.

So, is this the diet for you?

Keep in mind there is no one ideal diet for any sport, and most people can do well on more than one type of diet. One question to ask yourself is:

  • What is your goal?
    • If your goal is weight loss, there is significant evidence that LCKDs can lead to a decrease in total body weight, including a decrease in fat mass. (4, 7, 8, 9, 10, 11)
    • Changes in lean body mass are typically not significant, so if you are trying to increase muscle mass this is probably not the right diet for you. (7, 8, 9, 10, 11)
    • If your goal is performance improvement, the research to date does not suggest that a LCKD is likely to have much impact on performance. (3, 4, 7, 8, 9, 10, 11)
  • Beyond weight and performance consider how you might feel on a LCKD.
    • Some athletes have a higher perceived energy expenditure during exercise, requiring more encouragement to train. (1)
    • Others noted an increased sense of well-being. (4)
    • Being in ketosis can also suppress hunger. (1)

There are limitations to the current body of research. The majority of the research has been done on male athletes. The limited research done comparing male and female athletes shows that female athletes proportionally metabolize a lower percentage of carbohydrates and a higher percentage of fat during exercise than comparably trained male athletes. (1) The long-term effects of LCKDs haven’t been established, but if you don’t have any medical conditions, experimenting with a ketogenic diet is low risk. Metabolic adaptations can occur in as little as five days, however whether or not this is beneficial to performance is unclear. A LCKD is restrictive, and any diet that cuts out entire food groups has the potential to lead to nutrient deficiencies. Restrictive diets can also be challenging to maintain long term. A ketogenic diet may be effective in leading to weight loss, but its impact it has on performance is mixed, most often having little to no impact. Should you decide to experiment with a ketogenic diet and still have questions, a registered dietitian can help you develop a plan to optimize your nutrition while adapting to a different way of fueling.


  1. Karpinski C, Rosenbloom C. Sports Nutrition A Handbook for Professionals. 6th Edition. Sports, Cardiovascular, and Wellness Nutrition Dietetic Practice Group. 2018.
  2. Volek J, Noakes T, Phinney S. Rethinking fat as a fuel for endurance exercise. Eur J Sport Sci. 2015;15(1):13-20. doi:10.1080/17461391.2014.959564
  3. Hawley J, Leckey J. Carbohydrate Dependence During Prolonged, Intense Endurance Exercise. Sport Med. 2015;45(1):5-12. doi:10.1007/s40279-015-0400-1
  4. Zinn C, Wood M, Williden M, Chatterton S, Maunder E. Ketogenic diet benefits body composition and well-being but not performance in a pilot case study of New Zealand endurance athletes. J Int Soc Sports Nutr. 2017;14(1):1-9. doi:10.1186/s12970-017-0180-0
  5. Creighton B, Hyde P, Maresh C, et al. Paradox of hypercholesterolaemia in highly trained, keto-adapted athletes. BMJ Open Sport Exerc Med. 2018;4(1):1-10. doi:10.1136/bmjsem-2018-000429
  6. Kontush A. HDL particle number and size as predictors of cardiovascular disease. Front Pharmacol. 2015;6(OCT):1-6. doi:10.3389/fphar.2015.00218
  7. Cipryan L, Plews D, Ferretti A, et al. Effects of a 4-week very low-carbohydrate diet on high-intensity interval training responses. J Sport Sci Med. 2018;17(2):259-268.
  8. Greene D, Varley B, Hartwig T, et al. A Low-Carbohydrate Ketogenic Diet Reduces Body Mass Without Compromising Performance in Powerlifting and Olympic Weightlifting Athletes. J Strength Cond Res. 2018;32(12):3373-3382.
  9. Vargas S, Romance R, Petro JL, et al. Efficacy of ketogenic diet on body composition during resistance training in trained men: A randomized controlled trial. J Int Soc Sports Nutr. 2018;15(1):1-9. doi:10.1186/s12970-018-0236-9
  10. Gregory R. A Low-Carbohydrate Ketogenic Diet Combined with 6-Weeks of Crossfit Training Improves Body Composition and Performance. Int J Sport Exerc Med. 2017;3(2). doi:10.23937/2469-5718/1510054
  11. Paoli A, Bianco A, Grimaldi K. The Ketogenic Diet and Sport. Exerc Sport Sci Rev. 2015;43(3):153-162.

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