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As a track runner at McGill University, I got my big break thanks to a performance-boosting supplement.

One of my teammates was taking prerace hits of sodium bicarbonate, but he overdid it and had to withdraw from the 4 x 800-metre relay thanks to a bout of baking soda-induced explosive diarrhea. I was added to the relay as his replacement, ran a new personal best and earned my first trip to the national championships.

That happened more than two decades ago, but such unintended consequences are among the pitfalls discussed in a new consensus statement about sports supplements issued by the International Olympic Committee, which appears in this month’s issue of the British Journal of Sports Medicine. While supplements are widely used by athletes and non-athletes alike, their benefits are often backed by flimsy or non-existent evidence, and their downsides are ignored until it’s too late. But, as the new guidelines point out, there are some rare exceptions – if you know where to look.

The report was produced by a 25-member panel of scientific and medical experts who gathered at the IOC headquarters in Lausanne, Switzerland last year, including Stuart Phillips of McMaster University, Lawrence Spriet and Margo Mountjoy from the University of Guelph and Alan Vernec of the World Anti-Doping Agency in Montreal.

Athletes turn to supplements for a wide variety of reasons, but the panel chose to focus on three main goals of supplementation: correcting deficiencies in specific micronutrients; supplying convenient sources of energy and macronutrients such as protein; and providing a direct enhancement of performance or training.

In the micronutrient category, vitamin D, iron and calcium are common choices, and there’s reasonable evidence for their use – but only under certain specific conditions. While many people take supplements as an “insurance policy” just in case they have a deficiency, the new guidelines suggest turning to supplements only after confirming that you really have a deficiency, and then getting further testing to check whether the supplement really reverses that deficiency.

The second category is more about convenience. For the most part, there’s nothing magical about sports bars and protein powders except that they’re easy to transport and digest. A recent meta-analysis by Phillips and his colleagues, for example, found that protein powders and shakes do improve gains in strength and muscle mass from resistance training. But the effect is subtle and may disappear if (like many Canadians) you’re already getting more than 1.6 grams of protein per kilogram of body weight from your diet anyway.

The third category, supplements that promise a direct improvement in your performance, is the one that generates the most hype. Of the “many thousands” of such products on the market, the panel identified four with an “adequate” level of evidence supporting their use:

  • Caffeine: Unique in its versatility, caffeine boosts long endurance tasks, short all-out efforts and repeated sprints like those required in field sports, by altering your perception of effort.
  • Creatine: This widely used weight-room supplement provides cellular fuel for short, all-out efforts, which translates into greater gains from strength training over the course of weeks or months.
  • Nitrate: In the past decade, nitrate-rich beet juice has emerged as a popular endurance-booster. Studies show typical gains of 1 to 3 per cent in races lasting longer than 12 minutes; the caveat is that beet juice triggers gastrointestinal symptoms in some people.
  • Baking soda: During all-out exercise lasting between about one and 10 minutes, your muscles and blood become increasingly acidic. Baking soda buffers (counteracts) this rising acidity, boosting race performance by about 2 per cent. As my McGill teammate discovered, it can also trigger gastrointestinal problems. There’s emerging evidence that an amino acid called beta-alanine, taken daily for several months, can play a similar role in counteracting rising acidity without the GI risks.

There are other supplements that may turn out to be useful, such as probiotics to enhance immune functions, though the current evidence remains patchy.

But the overall message from the consensus statement is one of caution. Even seemingly benign supplements such as vitamins C and E, the authors points out, seem to interfere with training-induced fitness gains. A worrying number of sports supplements turn out to contain unlabelled and sometimes dangerous ingredients, other studies have found.

In other words, as I discovered in that long-ago relay race, sometimes the most reliable performance-booster you can take is nothing at all.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.