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Metabolic Characteristics of Fat-Adapted Runners

There’s been a lot of anecdotal evidence that adopting a low carb high fat (LCHF) diet might help improve athletic performance and general health. But up until now, there has been very little in the way of solid scientific evidence to validate these claims.

This is about to change with the eminent publication of a study in the journal Metabolism1. The study was the brain child of Drs’ Jeff Volek & Stephen Phinney; two prominent scientific figures in the LCHF world who have co-authored several books on a LCHF diet for both health and performance. jeff volek stephen phinney

To undertake their study Volek & Phinney successfully recruited two groups of well-matched elite ultra-endurance runners, as evidenced in the table below. 


One group contained runners who habitually consumed a high carb diet (i.e. 59% carb, 14% protein and 25% fat of total energy intake), while the other group was low carb (i.e. 10.4% carbs, 19.4% protein and 69.5% fat of total energy intake). The specific breakdown of each diet is shown in the table below. 


Importantly, athletes in the low carb group had been on the diet for an average of 20 months. This is in contrast to other studies that have put athletes on a LCHF diet for between 2-4 weeks. Volek & Phinney assert that full adaptation to the diet may in fact take several months and therefore it might be premature to make any major conclusions from studies that have only had athletes on a LCHF diet for a matter of weeks.

For the most part, the study was essentially exploratory: to define and measure how metabolism during steady state aerobic exercise might vary between the two different groups of runners. It’s important to highlight that each of the subjects were highly conditioned; being in the top 10% of finalists competing in sanctioned running events ≥50 km and/or triathlons of at least half iron-man distance (113 km). Several athletes had sponsors (55%), course records (30%), national records (10%), international records (10%), and national/international-level appearance for Team USA (25%).

Testing Protocol & Pre-Exercise Meal

Each subject underwent a gruelling testing protocol that involved measuring VO2max on the first day and then a 3-hr run at 64% of their pre-determined VO2max the following day. 90 minutes before the run, all subjects received a shake. The macronutrient profile (%carbohydrate:fat:protein) of the shake for the low carb group was 5:81:14, while for the high carb group it was 50:36:14. In terms of total calories, this equated to 343 kcal for the low carbers (i.e. 4.3g carbohydrate, 31.3g fat, and 12.6g protein) and 332 kcal for the high carbers (i.e. 42.7g carbohydrate, 13.7g fat and 12.4g protein).

Proportions varied, but both the low carb and high carb shakes were made from heavy cream, olive oil, whey protein, walnut oil, and strawberries. The high carb shake also contained banana and agave syrup.

Before, during and after the 3-hr run, blood, urine, DNA and muscle biopsy samples were taken to help define each groups metabolism. No performance measures were included in the study. The researchers initially just wanted to see how metabolism and fuel usage during exercise differed.


The key findings to emerge were low carb runners had on average 2.3x higher peak fat oxidation than the high carb group (i.e. 1.54 vs 0.67 g/min). Moreover, the peak fat oxidation value for every runner in the low carb group (range 1.15 to 1.74 g/min) exceeded the highest value in the high carb group (range 0.40 to 0.87 g/min). This is seen clearly in the graph below.


These findings are significant from the standpoint that ~1.0g/min was traditionally thought to be the upper limit of fat oxidation in highly conditioned athletes2. The fact that the average fat oxidation rate was over 2x this level is remarkable.

These differences in peak fat oxidation rates were further reflected in the substrate usage during the 3-hr run. The graphs below show how much fat and carbohydrate was used in each group of runners. Once again, the differences in fuel usage are clearly evident with the low carb athletes burning substantially more fat and less carbohydrate and vice versa for the high carb group.


But perhaps the most interesting and contentious findings of the study concerned the glycogen usage and replenishment rates in both groups of runners. The LCHF diet for athletes has traditionally drawn criticism because it is thought it would diminish glycogen levels and associated high intensity exercise capacity. While such findings have been uncovered in earlier studies that have used low carb diets in athletes, the studies have only run for 1-4 weeks3-5. It remained to be seen if this would be different in athletes who have been on a LCHF diet for over a year. Such athletes are commonly referred to as being ‘fat-adapted’ or ‘keto-adapted’.

The graph below shows the time course of muscle glycogen levels in both groups of runners immediately before, after and 2 hours following the 3-hr run. It is evident that both groups had similar rates of glycogen depletion and resynthesis. However, the low carb group had less variability (i.e. more uniform) in their rates of glycogen resynthesis when compared to the high carb group. 


More uniform replenishment of glycogen levels in low carb athletes who only consumed 4g of carbohydrate after their 3-hr run is perhaps the most intriguing finding of this novel study. It suggests that athletes on a LCHF diet for over 6 months may develop adaptations that allow them to resynthesise muscle glycogen from substrates other than glucose. Volek & Phinney suggest these substrates may be glycerol and/or lactate as their levels were two-fold higher at the end of exercise in low carb athletes. In their study Volek & Phinney site research on Alaskan sled dogs running over 100km a day, several days in a row, who show no depletion of glycogen despite eating a 15% carbohydrate diet high in fat6-8.

Future studies will need to explore more closely the possible mechanisms responsible for glycogen resynthesis in highly conditioned athletes on a low carb high fat diet.

Nonetheless, this landmark study provides the first documentation of the metabolic adaptations associated with long-term consumption of a very low-carbohydrate/high-fat diet in highly trained fat-adapted ultra-endurance athletes. No doubt the increasing number of practising low carb athletes including the Paleo and CrossFit community will feel vindicated with the release of this study. It certainly spells exciting times ahead for the sports nutrition industry.

  1. Volek JS, et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism (2015), doi: 10.1016/j.metabol.2015.10.028
  2. Venables MC, Achten J, Jeukendrup AE: Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study. Journal of Applied Physiology. 2005;98(1):160-167.
  3. Helge JW, Watt PW, Richter EA, Rennie MJ, Kiens B. Fat utilization during exercise: adaptation to a fat-rich diet increases utilization of plasma fatty acids and very low density lipoprotein-triacylglycerol in humans. Journal of Physiology. 2001 Dec 15;537(Pt 3):1009-20.
  4. Lambert EV, Goedecke JH, van Zyl C, Murphy K, Hawley JA, Dennis SC, Noakes TD. High-fat versus habitual diet prior to carbohydrate loading. Effects on exercise metabolism and cycling performance. International Journal of Sports Nutrition and Exercise Metabolism. 2001; 11: 209-225.
  5. Burke LM, Hawley JA, Angus DJ, Cox GR, Clark SA, Cummings NK... Adaptations to short-term high-fat diet persist during exercise despite high carbohydrate availability. Medicine and Science in Sports and Exercise. 2002 Jan;34(1):83-91.
  6. McKenzie E, Holbrook T, Williamson K, Royer C, Valberg S, Hinchcliff K, Jose-Cunilleras E, Nelson S, Willard M, Davis M. Recovery of muscle glycogen concentrations in sled dogs during prolonged exercise. Med Sci Sports Exerc. 2005 Aug;37(8):1307-12.
  7. McKenzie EC, Hinchcliff KW, Valberg SJ, Williamson KK, Payton ME, Davis MS. Assessment of alterations in triglyceride and glycogen concentrations in muscle tissue of Alaskan sled dogs during repetitive prolonged exercise. Am J Vet Res. 2008 Aug;69(8):1097-103.
  8. Miller BF, Drake JC, Peelor FF 3rd, Biela LM, Geor R, Hinchcliff K, Davis M, Hamilton KL. Participation in a 1,000-mile race increases the oxidation of carbohydrate in Alaskan sled dogs. J Appl Physiol (1985). 2015 Jun 15;118(12):1502-9.
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