The date of the first publication: October 25, 2020.
The date of the last review and update: November 7, 2023.
A short summary.
Volek et al., 2016 (2), published a very interesting study that compared fat oxydation and a number of biomarkers in elite endurance athletes on a low-carb ketogenic diet. The authors recorded considerably higher fat oxydation rates in the low-carb group. However, there was a serious error in the study design. Indeed, the high carbohydrate group had a meal with a high carbohydrate content 90 minutes before the test run started. This spiked insulin levels in the high-carb group and made their fat less accessible as fuel. The low carbohydrate group had a meal with only a small amount of carbs. By the time the run started, the insulin level was a lot lower in the low carb group. If both groups had the same meal, fat oxydation could have been the same in both groups.
A serious error in the way Volek et al., 2016 (2), designed their study.
Below, there is a figure from the study by Volek et al. (2). We can see again that the athletes on a high carb diet burn fat right from the start.
The low-carb athletes consumed a low carb meal before the run and their insulin levels were not affected to the same degree as that of the athletes on a high carb diet.
There is, by the way, a serious error in the study (2) design. Indeed, Volek and colleagues (2) gave a high carb meal to the athletes on a high carb diet one hour before the race started. The insulin levels in the high carb group spiked and made fat less available at the start of the run. The low-carb athletes consumed a low carb meal before the run and their insulin levels were not affected to the same degree as that of the athletes on a high carb diet. Hence, higher fat oxidation right from the start.
If both high-carb and low-carb group started the run fasted, the fuel utilization in the two groups may have been almost the same.
If we look at another graph from the study by Volek et al., 2016 (2), we can see that there was a drop in free fatty acids in high-carb athletes right before the treadmill run started. If both groups consumed that same meal, or, better, if both high-carb and low-carb group started the run fasted, the fuel utilization in the two groups may have been almost the same.
Is keto-adaptation in athletes a myth?
Keto-adaptation in athletes is not necessarily a myth. The difference in serum fatty acids between the high-carb and the low-carb groups was statistically significant only at the time point “0 minutes”, that is, at the start of the run. Yet, the fat oxydation was significantly higher in low-carb athletes at all time points. However, a high-fat meal for the low-carb group before the start of the run remains a confounding factor.
Conclusions.
Even good researchers make errors.
Jeff Volek is a good researcher. But even good researchers make errors.
There still appears to be a higher rate of fat oxydation in athletes on low-carb diet. However, high-carb athletes may burn fat at almost the same rate as low-carb athletes if they do not consume carbohydrates before their training session.
Our Collaborative Working Group on performance, hormonal and metabolic health of endurance athletes.
Optimal management of carbohydrate intakes and glycogen stores, optimal management of hormones and of the amount of carbohydrates in endurance athletes is a complex task. We invite practitioners and athletes to participate in our Collaborative Working Group on performance, hormonal and metabolic health of endurance athletes.
Our comment on Jeff Volek’s presentations from December 23, 2018:
Volek, however, never talks about elevated levels of cortisol, low levels of thyroid, abysmally low testosterone levels in “ketogenic” athletes. There are many questionable or detrimental “adaptations” that accompany higher levels of ketones…
Your critique is not invalid, but it turns the question on its head. Why give everyone a high-carb shake before the experiment??? The question Volek is interested in asking is not “does insulin suppress fat burning?” (which seems already a well-established fact) but rather something like “Have fat-adapted athletes really induced a differential metabolic capacity in their system?” Having all the participants start the day with a low-carb shake and low insulin would be more consistent with that question. Or have them simply eat nothing and do the experiment after the 10-hour fast.
I assume he DIDN’T design the experiment this way because he wanted to do something more like what racers would want to do in the “real world.” The way he did it is reassuring to low-carb athletes, but maybe not as good for the science of physiology.
Thank you for your comment.
Actually, this article did not suggest to give everyone a high-carb shake. The same low-carb meal or starting fasted would answer Volek’s question better, indeed, and this is what this article suggested: “If both groups consumed that same meal, or, better, if both high-carb and low-carb group started the run fasted, the fuel utilization in the two groups may have been almost the same.”
In this study, Volek studied ultra-runners during 180 minutes at 70% VO2max. 3 hours is too short for ultra-running competition. 70% VO2max is too low for marathon. So, the value of Volek’s findings has limited relevance for competition.
The findings of Volek are not necessarily reassuring. What this article shows is that high-carb athletes burn a lot of fat if they start their training session in a fasting state or after a low carb meal. Hormonal profile of many low-carb athletes is deleterious. In another study, Volek described hormones and bone health of the same group of athletes, omitting, however, a detailed description of the diet (high-, low-carb or both). Mean total testosterone was low. Some had low bone mineral density.
Both “high-carb” and “low-carb” athletes need to monitor their health biomarkers closely.