According to the American College of Sports Medicine, you don't have to engage in vigorous exercise for sustained periods to gain substantial health benefits.
Scientists measure fitness with a test of heart strength called "VO2 max" -- the maximum amount of oxygen your body can take in and use. Research shows that to increase VO2 max, you have to exercise vigorously enough to raise your pulse at least 20 beats above its resting rate, and you must keep it there for several minutes three times a week. Since almost 80 percent of American adults can't exercise vigorously or long enough to increase VO2 max significantly, they may feel that it's a waste of time to exercise at all.
The new recommendation is to use cross-training and intervals. Use several different sports or activities and stop exercising when you feel the least bit of discomfort. Resume exercise in another activity when you have recovered.
For example, go to an aerobic dance class and stop when you feel the least bit tired, even if you have to quit after two minutes. Then ride a stationary bike until your legs start to feel heavy, perhaps for three minutes. Then walk on the track until you feel tired.
Try for a combined total time of 30 minutes of exercise per day, five days a week. You can count as exercise any physical activity that keeps you moving constantly, such as walking, climbing stairs or gardening.
Q: Should my teen-agers drink the new soft drinks that contain calcium?
A: A recent study in the journal of the American Medical Association shows that teen-age girls who take in more than the recommended dietary allowance for calcium have stronger and larger bones, which can help to protect them from developing osteoporosis later in life.
If they live long enough, all men and women will develop osteoporosis, a condition that weakens bones so that they are liable to break with the slightest impact.
A man's bones are strongest when he is about 30, and a woman's bones are strongest when she is about 20. Every year after that, their bones lose calcium and become weaker. Those who have the thinnest, weakest bones in their teens are those who are most likely to develop osteoporosis when they grow older. Lack of calcium during the teen years increases a person's chances of developing osteoporosis later on.
A very high protein diet with lots of hamburgers and lunch meats can weaken bones by leaching out calcium.
The most healthful way for children and teens to meet their needs for calcium is to eat a varied diet that is loaded with vegetables, beans, nuts and dairy products. They can also strengthen their bones by lifting weights.
Calcium-fortified soft drinks will not compensate for a poor diet and lack of exercise.
Q: Do the products that claim to get rid of fat while you sleep work?
A: I hope you don't believe those ads. They claim to help you burn fat instead of sugar while you sleep. Even if the product worked, you burn so few calories during sleep that it is irrelevant whether you are burning primarily sugar or fat.
Your body burns primarily fat and sugar for energy. The harder you exercise, the more sugar your muscles use. It uses the lowest percentage of fat when you exercise and the highest percentage when you sleep. The crucial factor is how many calories you burn, and not whether you burn more fat or sugar.
For example, the average 150-pound person burns about 60 calories an hour during sleep, compared to more than 600 calories per hour while jogging. If you burn 80 percent fat during sleep, you will burn 48 calories of fat or one fifth of an ounce. If you burn 50 percent fat when you exercise, you will burn 1.3 ounces of fat per hour or more than six times as much.
There are many other products on the market today that promise to help you lose weight. None will help you unless they also get you to exercise more and consume fewer calories. You must learn to reduce your intake of fat severely and eat lots of fruits, vegetables, whole grains and beans. Start an exercise program that alternates upper and lower body sports.
Dr. Mirkin is a practicing physician in Silver Spring specializing in sports medicine and nutrition.