Interestingly, many past studies comparing brief HIIT routines and longer, moderate workouts did not hew to formal exercise recommendations, because scientists wished to match the workouts’ frequency or other measures. So, volunteers in these studies typically worked out three times a week, whether completing a few minutes of HIIT or half an hour of brisk walking.

But the exercise guidelines for each type of activity differ. Medical and sports groups suggest we interval train no more than three times a week, to avoid overstraining muscles and cardiovascular systems, meaning if we exercise only with HIIT, we are inactive four days a week or so. Comparable guidelines for moderate exercise suggest getting out and moving at least five times a week and for at least 30 minutes each time.

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So, the Guelph scientists thought, what happens if people HIIT three days a week and do not otherwise exercise on the other four, or train moderately five times a week?

To find out, they first recruited 23 sedentary, overweight adult men. (They did not include women, because of concerns about menstrual cycles affecting metabolic results, but hope to involve women in any larger, future experiments.) They invited these men to the lab, measured their fitness, body compositions and blood pressures, and asked them to down shakes laden with big glops of fat to see how their metabolisms responded to the nutrient. They also fitted them with blood-sugar monitors to wear at home for a week to gauge their everyday blood-sugar control, a measure of metabolic health.

Then they asked half of the men to start interval training three times a week on stationary bicycles at the lab, riding as hard as possible for 30 seconds, resting for two minutes, and repeating that sequence four to six times.

The other men began a typical moderate-exercise program, riding bikes at the lab five times a week at a pace they could comfortably sustain for 30 to 40 minutes.

Over the course of the next six weeks, the HIIT group pedalled intensely for a grand total of less than an hour, while the moderate-intensity group worked out for at least 2.5 hours each week for the same period.

At the end of the six weeks, both groups returned to the lab for retesting, after which the scientists combed through their results for disparities. They found plenty.

The men almost all were fitter, and to about the same extent, however they had exercised. But only those in the moderate-exercise group had shed much body fat, improved their blood pressures or become better able to metabolise the extra fat from the unctuous shake.

Perhaps most interesting, everyone’s blood-sugar control at home was best only on the days when they exercised, meaning three times a week for the HIIT riders and five for the moderate group. On the remaining days, blood sugar levels tended to rise.

Taken as a whole, the results indicate that intervals and traditional exercise alter our bodies in divergent ways, and we may want to consider what we hope to achieve with exercise when choosing how best to exercise, says Jamie Burr, a professor at the University of Guelph, who conducted the new study with his graduate student Heather Petrick and other colleagues.

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“All exercise is good,” Burr said. But “there are nuances”. Frequent, almost-daily moderate exercise may be preferable for improving blood pressure and ongoing blood-sugar control, compared with infrequent intervals, he says, while a little HIIT is likely to get you in shape as effectively as hours and hours of easier cycling or similar exertion.

Of course, this study was small-scale and short-term and involved only overweight, out-of-shape men, so we cannot be sure the findings apply to the rest of us. But the primary lesson seems widely applicable. “Move often,” Burr said, meaning if you HIIT today, walk tomorrow, and repeat.

The New York Times