DEAR DOCTOR K:

I just turned 40, and I have finally accepted that I need to make regular exercise part of my life. I'm in pretty good physical shape, if slightly overweight. How much exercise do I need to stay healthy and maybe drop a few pounds?

DEAR READER:

It's easier than you might think. It also depends on your goals.

You've probably heard that regular exercise protects you against many of the major diseases. I'm talking about heart disease, diabetes, osteoporosis, depression and even some cancers. When I tell my patients this, they often get a certain expression on their face that says, "Not this exercise-is-good-for-you stuff again." In other words, I'm boring them.

So the next thing I do is ask them this question: "Do you know just how much protection you get from regular exercise?" If they answer at all, they usually guess that exercise reduces their risk by about 10 percent.

In fact, regular exercise can reduce your risk of developing the most common kind of diabetes by 70 percent, for example. There is no medicine yet invented that can give you such protection. In other words, you can do more to protect yourself from diabetes than your doctor can do for you.

So the first important thing about regular exercise is how much good it can do for you. The second important thing is how little exercise you need to get powerful health benefits.

How much do you need to achieve your goals -- "to stay healthy and maybe drop a few pounds"? I usually "prescribe" at least 30 minutes of moderate exercise, or 15 minutes of intense exercise, at least five times a week. If you wanted to run a marathon, my advice would be different.

What qualifies as "moderate exercise"? Your heart rate should be at 50 percent to 75 percent of your maximum heart rate the whole time you are exercising.

What's your max? The standard formula is 220 beats per minute minus your age in years. So, a healthy 40-year-old like you would use 180 beats per minute as an initial heart-rate maximum. This formula doesn't work if you're on a medicine that slows your heart rate, like a beta blocker.

Another way of defining "moderate exercise" is walking briskly, at 3 to 4 miles per hour. That's like walking 1.5 to 2 miles in 30 minutes.

Furthermore, you don't need to get into sweat clothes, and you don't need to do all 30 minutes of exercise at one time. For example, suppose it takes you 15 minutes of brisk walking to get from where you park your car to your workplace. You've done your daily exercise just by walking to and from work.

If you want to lose more than a few pounds, exercise longer than 30 minutes a day -- or exercise more intensively, getting your heart rate above 75 percent of your max (for this you will need to be in sweats). And cut down on the calories.

After talking with your doctor, start slowly and build up gradually. Above all, stick with it!

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

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