The Research Behind LCHF
In 2003, researchers at the University of Connecticut compared the effects of a LCHF diet (61 percent fat) and a normal diet (25 percent fat) on cycling performance at three different intensities. A standard Wingate test (a 30-second all-out sprint) was used to measure anaerobic capacity. Six weeks of LCHF eating reduced peak power in this test by 10 percent. A standard VO2 max test was used to measure aerobic capacity. Six weeks of LCHF eating reduced VO2 max by 6.5 percent. (However, the subjects on this diet also lost weight, and there was no change in VO2 max relative to body mass.) Finally, a 45-minute cycling time trial was used to measure intensive endurance performance. Six weeks of LCHF eating reduced total work output by a massive 18 percent in this test.
More: 10 Bitter Foods That Cleanse the Body and Boost Performance
Subsequent research explained why LCHF diets impair anaerobic and high-intensity aerobic performance. Although these diets increase the fat-burning capacity of the muscles, they also reduce the muscles' carbohydrate-burning capacity. At higher intensities the muscles must use carbs to supply energy as quickly as it is needed. Athletes who eat too much fat and not enough carbs essentially lose their top two gears because the cellular machinery that functions to release energy from glycogen and glucose molecules is degraded.
These are not the only problems with a LCHF diet for runners. Other research has shown that it reduces overall training tolerance. For example, in a 2004 study English researchers placed runners on either a high-carbohydrate (65 percent of total calories) or a low-carbohydrate (41 percent of total calories) before and during an 11-day period of intensified training. Before and after this period all of the runners ran 16-km time trials. The runners on the high-carb diet performed just as well in the second time trial as they did in the first, indicating their bodies were able to fully absorb the increased training load. But the runners on the moderate-carbohydrate diet performed 8.2 percent worse in the second time trial, most likely because inadequate carb intake compromised their recovery.
More: 3 Foods for Fast Muscle Recovery
Should You Go LCHF?
Is there no place at all for LCHF diets in the life of a runner? I wouldn't go quite that far. They do seem to be effective for weight loss, so you could go LCHF during brief periods when shedding excess body fat (not improving your fitness level) is your top priority. However, I recommend a low-carb, high-protein diet during these "quick starts," as I like to call them.
Also, research has shown that a LCHF diet can boost fat-burning capacity in as little as five days. Therefore, it's not a bad idea to switch to such a diet when tapering for a long race such as a marathon, when your hard training is already behind you. This will give you a quick boost in fat-burning capacity without interfering with your preparation. Just be sure to carbo-load during the last three days before your event because carbs are good for running performance.
More: Are You Eating Enough Carbs?
Perfect your nutrition to boost your performance. Sign up for a race near you.
