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Alex Hutchinson draws on the latest research to answer your fitness and workout questions in this biweekly column on the science of sport.

THE QUESTION

Will taking a few weeks off from exercise hurt me?

THE ANSWER

That depends on what you mean by "a few" and "off." Taking an occasional break from your usual routine can be an important refresher, both mentally and physically. Spending the month of December on the couch, on the other hand, could be a problem - especially if your usual routine is not that rigorous.

Earlier this year, Paul Williams of the Lawrence Berkeley National Laboratory in California published some surprising results about a phenomenon he called "asymmetric weight gain and loss," based on a group of 55,000 runners he has been following since 1991. Put simply, he found that you gain more weight when you stop exercising than you lose when you subsequently resume the identical exercise program.

"In other words," Dr. Williams says, "if you stop exercising you don't get to resume where you left off."

The difference may be just a pound or two if bad weather and busy holidays force you to temporarily abandon the gym. But if you extrapolate that over a decade, it begins to add up. Fortunately, the effect wasn't seen at higher levels of exercise: Men running more than 32 kilometres a week and women running more than 16 kilometres a week were able to reverse their weight gains.

Given that we all occasionally face unavoidable time crunches, the question is how long we can afford to slack off. The literature on "detraining," or loss of fitness, is surprisingly complex, because different adaptations to your muscles, heart and metabolism fade away at different rates.

One pattern that emerges is that people who have taken up exercise relatively recently lose their fitness quite quickly; by many measures, they're most of the way back to their sedentary selves within a few weeks. Those who have been exercising for a long time, on the other hand, have structural adaptations like a larger heart and more capillaries to take oxygen to their muscles, which will endure for several months.

A series of classic studies in the 1980s explored the best way to maintain fitness during a break in your routine. You can get away with working out fewer times a week, and with doing shorter workouts, the researchers found - but if you reduce the intensity, your fitness quickly evaporates. In fact, subjects who were used to training six times a week were able to maintain key fitness indicators such as heart size and oxygen uptake by exercising just twice a week, provided that the intensity was high enough.

That's consistent with more recent research by exercise physiologist Martin Gibala and his colleagues at McMaster University in Hamilton - and it matches what elite athletes and their coaches have been doing. "I got an e-mail from a British track-and-field coach," Dr. Gibala recalls. "He said, 'You know, I never bought an excuse from my athletes that they couldn't maintain fitness during busy exam times.' "

The British coach advocated a short but hard workout, perhaps five repetitions of a one-minute sprint. The details, though, aren't as important as the principle: You can always find a bit of time, and you can pack a lot of effort into a short time.

Alex Hutchinson is a former member of Canada's long-distance running team, and has a PhD in physics.

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Doing the minimum to stay in shape

Paul Williams's findings about "asymmetric weight gain and loss" suggest that light and heavy exercisers need different strategies to cope with breaks in their exercise routines

Light exercisers

If you exercise for less than two to three hours a week, stopping completely for a few weeks could add pounds that you won't later be able to shed. This means it's crucial to stay active by any means necessary - taking stairs instead of elevators, getting off the subway a few stops early and walking, or simply making a trip to the gym a higher priority.

Heavy exercisers

If you exercise more than three hours a week, and particularly if you exercise at a high intensity, taking an occasional break from your routine isn't a bad idea. But don't stop completely: Try alternative activities (for example, biking instead of running) to give your body a break.

High performance

If you need to stay in shape for a game or race, but are tight for time, try "high-intensity interval training." A simple cardio workout of 10 times one minute hard, with one to two minutes of recovery, done two to three times a week, will keep you in shape. But remember: It has to be hard.

Alex Hutchinson