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This study compares the characteristics of 20 elite endurance athletes, 10 of them having been on a long-term low carbohydrate diet<ref name="VolekFreidenreich2016"/>. The study is sometimes referred to by the backronym FASTER (Fat Adapted Substrate use in Trained Elite Runners.) The study has some significant flaws and it's hard to find any useful conclusions. The athletes on the low carbohydrate diet had higher levels of fat burning than their high carbohydrate comparisons, but did not seem to have any of the benefits that high fat burning would normally convey.
* The subjects were 20 elite ultrarunners or triathletes, finishing in the top 10%, some of them with course records, or national/international records. The athletes had broadly similar average characteristics.
* While the title of the study suggests that the athletes are on a [[Ketogenic Diet]], this does not appear to be the case. The low carbohydrate athletes had a resting [[Ketone LevelLevels]] of ~0.7 mmol/L, which I feel is borderline for Ketosis. Their diet indicates their [[Ketogenic Ratio]] was only 1:1, which is far too low for Ketosis and there is no information provided to indicate they are [[Ketoadaptation| Keto-adapted]].
* The study consists of two tests on consecutive days. The first day involved a [[VO2max|V̇O<sub>2</sub>max]] test and the second day a sub-maximal three-hour treadmill endurance run. The [[VO2max|V̇O<sub>2</sub>max]] test was after a 4 hour fast, and the endurance run was 90 minutes after a nutrient shake that was different for the high and low carbohydrate athletes. This difference in nutrient intake makes the results endurance run hard to interpret as it's impossible to know if the differences in results are due to the prior nutrition and training, or due to the different nutrient intake.
* The [[VO2max|V̇O<sub>2</sub>max]] test resulted in the low carbohydrate athletes had a 2.3x higher peak fat burning capability than the low carbohydrate athletes, both from an absolute and as a percentage of [[VO2max|V̇O<sub>2</sub>max]] where the peak fat burning occurred. This might suggest that athletes on a low carbohydrate diet adapt to being able to burn fat at a higher rate, or it might suggest that athletes with naturally higher fat burning capability are more likely to choose a low carb diet.