Everyone knows Aesop’s fable about the fox and the grapes. A fox is walking along in the forest when he spots a juicy bunch of grapes hanging from a branch. He leaps upward and tries to snatch the fruit between his teeth but he misses. He tries a second time and misses again. After a third miss he gives up and slinks away, muttering, “They’re probably sour anyway.”

Psychologists have a term for what the fox was experiencing: cognitive dissonance. The term refers to any situation where reality contradicts one’s preferred self-conception. The natural response to such situations is to change one’s beliefs about reality in such a way as to preserve one’s self-conception. In Aesop’s fable, the fox decided he never wanted the grapes in the first place so that he would not have to admit he couldn’t jump high enough to get them.

There is an epidemic of “sour grapes syndrome” (as I call it) in endurance sports today. Its victims are endurance athletes who cannot cope psychologically with being slower than they would like to be and who resolve this cognitive dissonance by replacing the goal of doing their sport well with that of doing it “right.” The syndrome is being spread by various movements that promote alternative methods that are contrary to those practiced by the most successful athletes. Examples include barefoot running, interval-based training, and high-fat diets.

By latching onto one of these methods, athletes can claim a kind of victory over superior competitors. “You may have finished the race ahead of me, but you were heel striking whereas I was forefoot striking, and since the real objective of racing is not to finish as quickly as possible but to exhibit the best form, I actually beat you.” That sort of thing.

It sounds infantile—and it is—but it’s a widespread mentality. I hasten to add, though, that the vast majority of slower endurance athletes are perfectly okay with being slow. They are focused on improving their own best times and couldn’t care less how many people finish in front of them. It’s only a small minority that is eaten alive by its slowness to such a degree that it feels compelled to change the rules of the game. Interestingly, but not surprisingly, most endurance athletes who suffer from sour grapes syndrome are male.

Elite athletes never go in for the alternative methods, for a couple of reasons. The first is obvious: As the fastest athletes, the elites do not experience the gnawing envy of faster athletes that is at the heart of sour grapes syndrome. The second reason is that the alternative methods don’t work as well as the dominant best practices. Athletes whose livelihood depends on being as fast as possible can’t afford to fool around with inferior methods the way slower athletes with alternative goals can.

I try not to judge athletes with sour grapes syndrome, but I do feel compelled to speak up when a particular movement becomes visible enough that it begins to win converts among athletes who are not, in fact, wracked with jealousy of faster athletes but who simply don’t know any better. There are signs that the high-fat/low-carb diet trend has passed this tipping point. I’m hearing from more and more athletes whose training has been ruined by a switch to a HFLC diet that was precipitated by an earnest wish do get faster and not by a desire to have something to hold over faster athletes who eat lots of carbs.

So I’m standing on my soapbox and putting a bullhorn to my lips to spread the message that HFLC diets are not good for most endurance athletes. They reduce tolerance for high-intensity training and impair performance in all races except perhaps ultra-endurance events such as 100 km trail runs.

The latest scientific proof of this already well-proven fact comes from a new study published in the journal Nutrients. Polish researchers placed eight mountain bikers on each of two diets for four weeks in random order. One diet was high in fat and low in carbohydrate, consisting of 15 percent carbohydrate, 70 percent fat, and 15 percent protein. The other diet was balanced, consisting of 50 percent carbohydrate, 30 percent fat, and 20 percent protein. At the end of each dietary intervention, the subjects underwent three days of physiological testing that culminated in a 90-minute stationary bike ride at 85 percent of lactate threshold power followed by a 15-minute time trial.

Before I share the results, let me pause to note that the founding principle of HFLC diets for athletes is that carbohydrate does nothing good for the body either at rest or in motion. Proponents of these diets believe not only that eating carbohydrate is fattening and otherwise unhealthy, but also that burning carbohydrate during exercise hurts performance by hastening exhaustion. The rationale for prescribing HFLC diets for endurance athletes is that they increase the reliance of the muscles of fat and decrease their reliance on carbohydrate, thereby enhancing performance.

The Polish researchers who conducted the study in question found that the HFLC diet did everything it was supposed to do physiologically. For starters, it made the subjects leaner. After four weeks on the balanced diet, the subjects’ average body fat percentage was 14.88. After four weeks on the HFLC diet it was 11.02. The HFLC diet also increased fat burning during exercise. Scientists use a measure called respiratory exchange ratio (RER) to quantify the relative contributions of fat burning and carbohydrate burning to muscle work. The higher the number, the more carbs are being burned relative to fat. On average, the subjects’ RER during exercise was 6 percent lower on the HFLC diet than on the balanced diet.

There was only one problem. All of these “favorable” physiological changes were associated with a significant loss of performance. On the balanced diet, the subjects generated 257 watts at lactate threshold intensity and 362 watts during the 15-minute maximal effort. On the HFLC diet, these numbers dropped to 246 watts and 350 watts. According to the study authors, the cause of the loss of performance capacity on the HFLC diet was impairment of the muscles’ ability to burn carbs. Oops!

So that’s the science. But what about all those social media testimonials you see from athletes who have made the jump to HFLC diets? Let me answer this question by asking another: How many times have you seen testimonials for weight-loss pills and muscle-building supplements that don’t really work? I’m not one to dismiss all anecdotal evidence out of hand, but when anecdotal evidence contradicts the results of formal scientific inquiry, it should be given very little weight.

When the barefoot running fad was raging a few years back, many runners who ditched their Brooks Beasts for Vibram Five Fingers reported that going natural had made them better runners. But I never encountered a single barefoot runner who, when pressed, could point to a big new PR that was set soon after the switch. It became clear to me that what these runners really meant when they said that barefoot running made them better was that it hadn’t made them worse.

I do believe there is a minority of endurance athletes who don’t get worse on HFLC diets. I don’t believe there are any who get better. And I believe that most runners who try HFLC diets don’t really do it for the sake of improving, although they convince themselves otherwise. If you really care about your performance, don’t go on a high-fat/low-carb diet… or run barefoot, or switch to an interval-based training approach, or do anything else that race winners don’t do.