Each runner has an optimal racing weight. I define this as the weight at which a runner performs best in races. Because runners are more often above their optimal racing weight than right at it, they tend to associate losing weight with gaining performance. This association leads some runners to make weight loss their primary focus and to push performance into the background.

The problem here is that it is possible to lose weight in a way that unnecessarily limits or even sabotages performance. Indeed, the dietary and training methods that produce the most weight loss are entirely different from those that maximize performance. If you practice the methods that are most effective for weight loss, the best possible outcome is that you will improve more slowly and to a lesser degree than you would otherwise. And in the worst-case scenario, you will end up below your optimal racing weight and slower than you were when you started the process.

In other words, if your goal is maximum performance, you need to stay focused on performance. Eat and train to get faster and trust that your weight will take care of itself. Do not eat and train to get lighter and count on your performance to improve as the number on the bathroom scale comes down.

Let’s get specific. The most effective way to lose weight is to maintain a low-carb diet and follow a mostly high-intensity training program. The most effective way to gain fitness is to maintain a moderate- to high-carb diet and follow a mostly low-intensity training program. But don’t take my word for it—let’s look at the research.

Running on Empty

A number of studies have shown that well-trained endurance athletes who switch from a moderate- to high-carb diet to a low-carb diet get even leaner than they were to begin with. The most recent study of this kind was conducted by a team of Polish researchers led by Adam Zajac. Eight competitive mountain bikers were placed 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.

The low-carb diet made the athletes significantly leaner. After four weeks on the low-carb diet, their body fat percentage was 11.02. Compare that to an average body fat percentage of 14.88 after four weeks on the balanced diet. But the fat loss that the athletes experienced on the low-carb diet was associated with a significant decline in performance. On the balanced diet, the subjects generated 257 watts at lactate threshold intensity and 362 watts during a 15-minute maximal effort. On the low-carb diet, these numbers dropped to 246 watts and 350 watts. According to the study authors, the cause of the decline on the low-carb diet was impairment of the muscles’ ability to burn carbs, which is critical to performance at higher intensities.

Train Slower to Get Faster

For reasons that remain a mystery to me, many experts who unwisely advocate low-carb diets for runners also advocate a training approach that is based on high-intensity intervals. This recommendation is equally misguided, because training programs that are biased toward speed work fail to improve running performance as much as traditional programs biased toward low intensity even though they stimulate greater fat loss.

This was shown most recently in comprehensive study done at the University of Salzburg. Forty-eight endurance athletes, a plurality of them runners, were assigned to four different training groups: a high-volume group that did lots of training at low intensity and none at high intensity, a threshold group that did a balance of work at low and moderate intensities, a high-intensity group that did 57 percent of its training at high intensity and the rest at low intensity, and a “polarized” group that did 68 percent of its training at low intensity and 24 percent at high intensity. All of the athletes completed a battery of performance tests both before and after nine weeks of training.

The high-intensity protocol had the strongest effect on body weight, by a long shot. In fact, it was the only training program that had any effect on body weight. On average, members of the high-intensity group lost 3.7 percent of their starting weight while members of the other three groups stayed right where they were at the beginning.

But performance was another story. The athletes in the more traditional, polarized training group experienced an average 17.4 percent improvement in time to exhaustion in a graded exercise test, or exactly double the improvement seen in the high-intensity group. Improvements in aerobic capacity and in peak speed or power in the same graded exercise test were also significantly greater in the polarized training group.

It’s Your Choice

Research comparing the effects of different diets and training programs on runners present a clear choice. If you want to look fast, eat a low-carb diet and follow a high-intensity training program. No other approach will make you leaner. But if you’d rather be fast (while still looking the part), follow a moderate- to high-carb diet and do most of your training at low intensity.