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On average, caffeine makes athletes faster – but for people with slower metabolisms, its effects may be muted.Getty Images/iStockphoto

In sports, as in life, caffeine is the pick-me-up of choice. When Spanish researchers analyzed more than 20,000 urine samples from national and international athletic competitions a few years ago, three-quarters of the samples contained caffeine.

But does a cup of coffee really make you stronger and faster? That may depend on your genes, according to surprising new data presented at a conference earlier this summer by researchers at the University of Toronto.

Some people metabolize caffeine quickly, while others metabolize it slowly – and there's growing evidence that the difference can affect not only athletic performance, but also your risk of health problems such as heart attacks.

The new study was led by Nanci Guest, a dietitian and doctoral candidate, working with her adviser Dr. Ahmed El-Sohemy, a professor in the University of Toronto's department of nutritional sciences who studies nutrigenomics, the study of how genes and nutrition interact.

Guest recruited 100 athletes from various sports, and asked them to complete a series of 10-kilometre cycling time trials after taking pills that contained either a placebo, a low caffeine dose (2 milligrams per kilogram of body weight) or a higher dose (4 mg/kg). Over all, the subjects cycled 3.5-per-cent faster with the full caffeine dose than with the placebo, a boost that is consistent with previous studies.

But the plot thickened when Guest tested her subjects for variants in a gene called CYP1A2 that helps determine how fast your body breaks down caffeine. Those with the "GG" variant of the gene, who make up about half the population, metabolize caffeine quickly. Those with the "GA" or "AA" variants metabolize it slowly.

The results, which haven't yet been peer reviewed but were presented at the American College of Sports Medicine's annual meeting in Boston in June, showed that the 49 fast metabolizers in the group sped up by an average of 1.2 minutes on caffeine. The 43 slow metabolizers with the GA genotype sped up by a statistically insignificant 0.5 minutes; and the eight slowest metabolizers with AA genotype actually slowed down by 2.5 minutes after taking caffeine.

Why the difference? Caffeine's stimulating power results in part from its ability to bind to adenosine receptors in the brain, which are related to fatigue. Everyone receives that benefit; but in slow metabolizers, the caffeine then stays in the body for longer, allowing negative side effects to accumulate. Increased heart rate, jitters and anxiety could all contribute to worsened athletic performance, Guest speculates, though the exact mechanisms haven't yet been studied.

The new study's large sample group bolsters earlier studies that hinted at the same conclusion. "Looking at all available data, I think that Nanci's findings and our findings point to a preferential effect" for fast metabolizers, says Dr. Christopher Womack, a researcher at James Madison University in Virginia who has published two previous studies on the topic.

The results also fit with a larger body of research on caffeine's health benefits and risks. In 2006, El-Sohemy and his colleagues published data showing that fast metabolizers had no increased risk of heart attack even if they drank four or more cups of coffee a day. In fact, drinking one to three cups a day lowered their risk, perhaps thanks to the health-promoting polyphenols and antioxidants in coffee.

Slow metabolizers, in contrast, had a 36-per-cent higher risk of heart attack if they drank two to three cups of coffee a day, and 64-per-cent higher for four or more cups a day. Since the caffeine lingered in their bodies, its negative effects outweighed any polyphenol and antioxidant benefits.

Interestingly, El-Sohemy pointed out at the conference, there is no difference in consumption levels between fast and slow metabolizers. How much people like coffee doesn't seem to be affected by how long it lingers in their bodies; instead, it's affected by other genes related to adenosine receptors.

To El-Sohemy, the findings are a clear indication that DNA-based dietary advice is ready for prime time: "If you'd asked me five or six years ago, I would have said the science was in its infancy," he said – but that's no longer the case.

Four years ago, he launched a company called Nutrigenomix that tests a battery of "modifier genes" that predict individual variations in response such as weight loss on a low-carb diet or blood pressure on a low-salt diet. Earlier this summer, the company added a sports-specific test; both tests are available only through health professionals.

Whether or not getting tested seems worthwhile to you, the research offers a powerful illustration of the difference between average and individual results. On average, caffeine makes athletes faster – but for half of us, its effects may be muted or perhaps even counterproductive.

So if your preworkout coffee doesn't give you the same jolt that your friends seem to get, trust your instincts.

Alex Hutchinson's latest book is Which Comes First, Cardio or Weights? Follow him on Twitter @sweatscience

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