If you don't have time to exercise for a full hour, rev up the intensity of a shorter workout, experts advise.

Oct. 29 — What’s behind the recent doubling of the government’s exercise recommendation? Smart Fitness explains the new one-hour guideline and shares tips from the experts on how to fit fitness into hectic schedules.

Q: I heard something recently about the government raising exercise recommendations from 30 minutes a day to a full hour. Why? I have struggled for years to fit a half-hour of daily exercise into my extremely hectic schedule and now they have raised the bar. What am I supposed to do? I work up to 12 hours a day and simply don’t have time to exercise for an hour.

A: Last month, the government’s health advisers at the Institute of Medicine upped the exercise ante with a report saying that American adults and children should exercise for at least an hour a day to maintain a healthy weight and a healthy heart.

That’s twice the previous recommendation of 30 minutes a day, originally urged by the Surgeon General’s landmark report in 1996.

And while it may feel like the medical establishment has suddenly reversed itself and snatched back the carrot, that’s not exactly the case. The surgeon general’s 30-minute recommendation was actually a minimum requirement for gaining health benefits. The one-hour guideline, however, is put forth as an optimal goal. The Institute of Medicine’s 21 scientists and physicians concluded that the previously recommended half-hour was insufficient to maintain a healthy weight and reap maximum cardiovascular health benefits. People who want to stay healthy — both in terms of their weight and cardiovascular fitness — are urged to step it up to a total of 60 minutes of moderate exercise per day, such as swimming, brisk walking or jogging.

“The guidelines don’t really contradict each other as much as they complement each other,” says Cedric Bryant, chief exercise physiologist for the American Council on Exercise. “The Surgeon General report tells inactive people how to start on a course to get initial benefits and the Institute of Medicine is really focusing on telling people how to get optimal benefits.”

The combined message of both reports, according to Bryant: “More exercise is better than some, but some is better than none.”

Just because you don’t reach the 60-minute level doesn’t mean your efforts are wasted, he emphasizes.

The new guideline stems from studies of how much energy is expended on average each day by people who maintain a healthy weight. Research shows that those who are active at least one hour a day do best.

ADDING UP ACTIVITIES

It’s important to realize that daily energy expenditure is cumulative — it includes all movements, from low-intensity activities of daily life like walking up stairs to more intense, deliberate workouts, such as sweating through an aerobics class.

“Exercise is like loose change in your pocket — it all adds up,” Bryant says.

So don’t be intimidated by the idea of a 60-minute workout. Think of the guideline instead, in terms incorporating movement into your daily activities, such as walking across the hall rather than sending an interoffice e-mail, taking the stairs at the mall, parking your car at the far end of the lot, pacing while you’re on the phone or clenching your abs while you’re online. Why not flex those abs right now?

And for those people who, no matter how many mini-activities they try to cram in, just can’t accumulate a total of one hour, then rev it up, says Mirabai Holland, director of fitness at the 92nd Street Y in New York. Instead walking for one hour at moderate intensity, try jogging or running for 30 minutes. She also encourages interval training: Add in some hills and sprints in your 30-minute jog and you’ll be reaping even more benefits than someone who’s walking for an hour.

But how can you measure the intensity of your workout? Holland suggests using your heart rate as a guide. To find out your maximum heart rate, subtract your age from 220.

So if you were 50, your maximum heart rate would be 170 beats per minute. Your target heart rate zone — 55 percent to 85 percent of the maximum — is the range you should try to stay within. The middle of that zone is considered moderate intensity, the upper end is high intensity.