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Two weeks ago, Jockology examined the evidence behind claims that supplements can enhance athletic performance. This is the second part of Alex Hutchinson's analysis.

Any substance that corrects a deficiency has a good chance of enhancing your performance. Even a glass of water can work wonders if you happen to be dehydrated.

The problem is that scientists aren't quite sure whether exercise heightens your need for certain micronutrients - and if so, whether those additional needs will be taken care of by simply eating more calories over all.

Reviewing a long list of conflicting studies in 2007, University of Pennsylvania nutrition professor Stella Volpe concluded that, "in most cases, if energy intakes are sufficient, the micronutrient requirements of athletes are similar to healthy, fairly active individuals." But for many of the supplements targeted at athletes, the question is far from settled.

Here are three examples in which some people believe athletes need a little extra.

SUPPLEMENT: PROBIOTICS

Used for: Recovery

The claim: Probiotics are live bacteria found in certain foods such as specially labelled yogurt and milk, and in capsule form. They are thought to enhance digestion and immune function, protecting exhausted athletes from getting sick.

The evidence: A 2007 study of 141 Finnish runners, half of whom received probiotics, found no reduction in the number of respiratory infections or gastrointestinal problems in the two weeks after a marathon. However, the length of the average GI episode was shortened from 2.3 to 1.0 days. Even better, a British Journal of Sports Medicine study last year reported that a group of highly trained Australian distance runners taking a common probiotic supplement cut their number of sick days from respiratory infections in half.

The verdict: TRUE

The research into probiotics and exercise is still in its infancy, but the signs are encouraging. Given the relative lack of side effects, they're worth a try if you're putting your body through a period of extreme physical stress. Get them, ideally, from whole foods rather than pills, which lose their potency over the course of their shelf life.

SUPPLEMENT: VITAMIN D

Used for: strength

The claim: No supplement has had more positive buzz of late than vitamin D. Cancer, diabetes, heart disease, schizophrenia - you name it, epidemiologists think vitamin D helps protect against it. Some advocates are now touting it as a way to enhance athletic markers such as speed, strength, balance and reaction time.

The evidence: Scientists from the University of Manchester in Britain tackled this question in the latest issue of the Journal of Clinical Endocrinology and Metabolism. "Our study found that vitamin D is positively related to muscle power, force, velocity and jump height in adolescent girls," lead author Kate Ward reported. There's an important caveat, though: When tested, 75 per cent of the 99 subjects were found to be vitamin D deficient.

The verdict: FALSE.

It's clear that a vitamin D deficiency will hurt your performance, but there's no evidence that extra doses in a healthy person will enhance your performance. There are plenty of reasons for sun-starved Canadians to consider taking a supplement - but improving your slapshot isn't one of them.

SUPPLEMENT: ZMA (ZINC, MAGNESIUM, VITAMIN B6)

Used for: strength, recovery

The claim: ZMA helped Barry Bonds hit 73 home runs in a Major League Baseball season - or so the story goes. Developed by Victor Conte, the disgraced steroid mastermind, ZMA's initial athletic claims focused on raising testosterone levels and packing on muscle; current advertising is focused on "sleep enhancement" and better recovery, and it still has a strong following.

The evidence: The most commonly cited study, published in 2000, found that a small group of U.S. college football players using ZMA increased their testosterone by over 30 per cent and their strength by over 10 per cent. That study was co-authored by Mr. Conte, and its results have yet to be replicated. A 2004 study from Baylor University in Texas found no significant change in hormone levels, strength or anaerobic capacity. A new study from the German Research Centre of Elite Sport, to be published in the European Journal of Clinical Nutrition, found no effect on testosterone levels - though they did report that "urine flow was almost doubled after eight weeks of ZMA use," putting users at risk of dehydration.

The verdict: FALSE

This is a classic example of a substance with a strong effect if you have a deficiency, but little impact otherwise. There have been suggestions that some athletes may lose large amounts of zinc and magnesium through sweating. That's possible, but unsubstantiated. The claims about better sleep have not been demonstrated either. One thing is clear, though: A lack of evidence won't slow the sales of any substance promising better athletic performance.

jockology@globeandmail.com

Alex Hutchinson blogs about

research on exercise and athletic performance at sweatscience.com.