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Runners on Toronto’s Lakeshore Boulevard during the Scotiabank Waterfront Marathon.

Michelle Siu/The Globe and Mail

We all know that life is unfair. Put a dozen people through identical exercise programs and some will thrive, most will make modest improvements and a few – perhaps two or three – won't see any gains at all. That's just the way it goes.

Now we can quantify that unfairness, thanks to a new genetic test offered by a British company called XRGenomics.

The test measures your genetic responsiveness (or lack thereof) to aerobic training, based on an analysis of 27 distinct DNA markers. It's backed by robust scienti-fic research and has real predictive power, marking a significant step forward compared to earlier fitness-oriented genetic tests aimed at consumers. The question is: Will people use this information to tailor their workouts to their DNA? Or will they let the tests become self-fulfilling prophecies?

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Most people are understandably skeptical of tests that promise instant insight into your destiny. But the genetic component of exercise response has been firmly established by several decades of research. DNA determines about 60 per cent of the variation in how people respond to aerobic exercise, says Jamie Timmons, a professor of systems biology at Loughborough University and the director of XRGenomics. Another 20 per cent of the variation can be explained by factors like age, gender and baseline fitness (which contributes a surprisingly paltry 4 per cent); the final 20 per cent remains a mystery.

The XRGenomics test involves a simple cheek swab that you mail back to the company, and costs £199 (about $315). Based on an analysis of 27 different "single nucleotide polymorphisms" – changes of a single letter in the genetic code written in your DNA, also known as SNPs – you're classified into one of four categories of exercise response.

"Only those that have a score in the lowest 18 per cent of the distribution found in the population are labelled as 'low responders,' " Timmons says. "Our advice then focuses on what these people could and should make as a health target."

XRGenomics isn't the first to wade into fitness-oriented genetic testing. For example, several companies offer testing for a single SNP that has been shown to influence whether you're best suited to speed and power sports or endurance sports.

But as Timothy Caulfield, a professor of health law and science policy at the University of Alberta, points out in his 2012 book The Cure for Everything!, such rudimentary tests have only weak predictive power. Caulfield himself was a successful sprinter at the university level and beyond, but the genetic testing he undertook while writing the book labelled him an "unlikely sprinter" and suggested he pursue endurance sports.

The XRGenomics test, with its 27 SNPs, has much better predictive power than the test Caulfield took. But the factor that remains hotly debated – not just for sports, but for health-related genetic testing in general – is how people will make use of information about their personal risk factors and responses.

The obvious audience for the new test is people who are frustrated with their workout results – a group that sometimes appears to include just about everyone. For frustrated exercisers classified as "good responders," the company suggests, the results should serve as a signal that their workout routine isn't adequate and needs adjusting.

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On the other hand, it's easy to imagine someone who receives a diagnosis of "low responder" using the test as justification to stop exercising entirely. That would be a mistake, since the test only looks at aerobic fitness (as measured by VO2max, which describes how much oxygen your heart and lungs can deliver to your muscles while you exercise), and ignores positive changes in other parameters like blood pressure and blood-sugar regulation.

"A number of clients have said, 'That explains a lot,' in terms of their personal experience," Timmons says. "Nobody expects to be discouraged – just refocused on achievable goals."

Those refocused goals could involve regularly keeping track of alternate outcomes like blood pressure, in order to see tangible progress. Or it could involve a shift to resistance exercise in order to see gains in strength and muscle size.

Of course, it's also possible to be a non-responder to strength training. An international collaboration including Timmons and researchers at McMaster University has shown that a relatively small number of genetic markers can identify the people whose muscles grow big very quickly when they lift weights. The company hopes to develop this research into another consumer test.

"The hard part for some to swallow is that of course not everybody has the genetic potential to have a high peak VO2 or a big muscular response," says Stuart Phillips, an exercise metabolism researcher at McMaster and part of the research team.

We all know this already in abstract and general terms – the test just makes that knowledge concrete and personal.

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For some, that might be a valuable spur to work harder or differently; for others, it might be demoralizing. The key is to figure out which it is for you before you order.

Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com. His latest book is Which Comes First, Cardio or Weights?

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