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The more you exercise, the fitter you get – but only up to a certain point.

Elite athletes have long known that pushing too hard for weeks or months without adequate recovery can lead to a mysterious and poorly understood condition called overtraining syndrome.

Scientists haven’t been able to reliably pinpoint when athletes are in danger of crossing the line, or exactly what goes wrong in an overtrained body. But in some cases, according to a recent scientific paper by researchers in Canada, Europe and Australia, the mystery may have a relatively simply explanation: The athletes aren’t necessarily training too much – they’re simply not eating enough to fuel their training.

Fatigue is, of course, a non-negotiable part of athletic life, particularly in endurance sports. Athletes in heavy training often get temporarily slower or weaker, and this “overreaching” stage is normal if it lasts for a few days. But if it settles in for weeks or months, even after the training load is lightened, that’s overtraining.

Scientists have tried many different ways of testing for overtraining, including tracking hormone levels, immune function and psychological questionnaires, but none of them work reliably. For now, overtraining syndrome remains a diagnosis of exclusion: Your performance is reduced for a long period of time, and you’ve eliminated other potential explanations such as iron deficiency, thyroid dysfunction or mononucleosis.

One of the key exclusions is caloric deficit, which in the sports context is referred to as Relative Energy Deficiency in Sport, or RED-S. But the new paper, published in Sports Medicine by a team led by Trent Stellingwerff of the Canadian Sport Institute Pacific, argues that overtraining and RED-S are frequently conflated – not just by coaches and athletes, but also by scientists.

That’s partly because the typical symptoms of overtraining and RED-S are similar. In addition to performance loss, athletes suffering from either condition may see a drop in sex hormone levels, changes in basal metabolism and heart rate, immune suppression, psychological disturbance and other changes in the body.

But there’s also a more direct link: If you’re training hard, your appetite doesn’t necessarily keep up with your caloric needs. That means athletes may be running a caloric deficit without even realizing it, and attributing the resulting fatigue to their training.

Stellingwerff and his colleagues reviewed 21 previous studies in which athletes were deliberately overloaded with heavy training in order to induce overreaching or overtraining, and in which information about the subjects’ food intake was available. In 18 of those 21 studies, the overtraining intervention also triggered a significant deficit of either calories or carbohydrates, the macronutrient most crucial for exercise.

As a Globe and Mail investigation found last year, eating disorders are a serious hazard for elite athletes in weight-dependent sports, sometimes fuelled by “target competition weights” based on shoddy or non-existent science. The new results suggest that caloric deficits can also develop without the athletes intending or even realizing it.

This insight doesn’t just apply to elite athletes, Stellingwerff notes. “The threshold for overtraining syndrome and RED-S might be lower in full-time, high-performance business people, who work 60-plus hours per week, travel like crazy, have a family, and want to train and compete in endurance races.” Amid the constant stress of busy lives, it’s easy to miss snacks or settle for inadequate meals.

Accurately measuring how many calories you eat and how many you burn remains a huge challenge, even in science labs. As an alternative, Stellingwerff suggests watching for the following warning signals that you may not be eating enough to fuel your workout routine: inconsistent training, more hunger throughout the day, altered sleep patterns, mood changes, inability to focus and decreased libido.

Of course, there is such a thing as true overtraining, even if you’re eating enough. Adequate recovery between workouts is crucial. But if you’re feeling some of those warning signs and can’t figure out why, try filling your plate a little higher before you conclude that you’re overtraining.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.