The low-carb high-fat (LCHF) diet has long been a topic of debate in the endurance community. Many top runners, especially those participating in ultra-endurance events, embrace this style of eating, but a new study from the Journal of Physiology suggests that this diet can not only impair distance performance, but it can also be difficult to quickly reverse its effects.
Researchers looked at a group of world-class race walkers who walked two 10,000m races: one after consuming a high-carb diet and another after following a low-carb high-fat diet for only five days (the control group continued to eat as normal). After this short-term change in diet, the LCHF group saw a decline in results and had difficulty returning to a carbohydrate oxidizing state quickly (burning primarily carbs as fuel).
Scientists are no longer debating that the diet is effective – an LCHF diet will cause the body to burn fat as fuel. The question now is, is this helpful to endurance performance? And what are the short- and long-term effects?
Fat oxidation happens faster than researchers have previously thought
In previous studies, researchers have had endurance athletes follow the LCHF diet for weeks or months while observing changes in their body’s efficiency. But for this study, athletes only followed the LCHF diet for five days and, surprisingly, achieved similar rates of adaptation. Basically, the body might adapt to this diet much faster than researchers had previously thought.
Athletes don’t switch between systems automatically
Once an athlete has adapted to the LCHF diet, researchers found that switching back to primarily burning carbs didn’t happen immediately. In fact, they found it could take a few days to reverse the effects, due to the suppression of carbohydrate oxidation caused by the diet change.
Six out of seven walkers didn’t see any improvement in performance
Finally, the diet reduced performance levels in all but one walker. The LCHF group saw a 3.4 per cent decrease in performance in the 10,000m race compared to the high-carb group, which saw a 5.6 per cent improvement in their results.
While the LCHF diet is undeniably effective at encouraging the body to burn fat as fuel, it’s not necessarily linked to performance gains in endurance sport. While more research needs to be done in this area over longer events, be cautious when experimenting with this diet, as reversing its effects might take longer than expected.