During your 20s, it seems that you can eat what you please and be as sedentary as Mount Rushmore with nary an ill effect. Once you hit your 30s, though, the use-it-or-lose-it rule becomes urgent. If you continue your 20-something merrymaking, you turn into a blob. Keep at it until 40 or 50, and you'll be a grumpy blob at high risk for a smorgasbord of nasty diseases.

Luckily, there's a way to stay in shape after 30: walking. "It's practical, and you can fit it into your everyday routine," says Mark A. Pereira, epidemiology researcher at the Harvard School of Public Health.

Although walking is beneficial at any age, it's particularly sensible for women in their 30s, 40s and 50s. One reason: it's easy to start -- and stick to -- a walking program. Moreover, it's easy on the joints. "The impact is reduced by at least a third compared to running," says David K. Brennan, assistant clinical professor of physical medicine and rehabilitation at Baylor College of Medicine. Walking also protects women from nearly all the chronic diseases that can come along during those years -- and beyond.

Here's how walking can keep you fit as decades go by.

the 30s

Many women begin their 30s in complete control of their lives. Then come the babies, the responsibilities, the extra pounds. "Generally, women become a lot less active during their 30s," says Wayne C. Miller, program coordinator for the Women's Exercise Research Center at George Washington University Medical Center. "That's when they tend to gain weight." Thus, 30 is an excellent time to start walking, if you haven't already begun.

When you're sedentary, your metabolism slows and you burn fewer calories. Walking briskly not only burns calories while you're hitting the pavement but also after you finish. "By building lean muscle mass, regular exercise will help you maintain a high metabolism, particularly if you are trying to lose weight by cutting your caloric intake," Miller says. His prescription: walk up to one hour a day, three or four times a week, and strength train twice a week for 30 to 40 minutes.

Another benefit from walking is all-day energy. Physically fit people improve their body's aerobic capacity (its ability to use oxygen) and that translates into more energy -- which comes in handy when juggling a career, family and home. "If you're fit, not only are you more efficient at consuming oxygen while you're exercising, but you feel more energetic throughout the day," says Robert J. Moffatt, chair of the department of nutrition, food and exercise sciences at Florida State University.

the 40s

During their 40s, most women face constant stress as they care for their family and make important decisions about their own lives. To top it off, this is the time for women to think seriously about preventing disease and loss of muscle mass. Walking helps with all three:

Stress Relief: When you are stressed, your blood pressure rises, your heart rate increases, and a host of stress hormones are dumped into your blood, says Susan Johnson, director of continuing education at the Cooper Institute for Aerobics Research in Dallas. Chronic stress can weaken your immune system, making you more susceptible to anything from the common cold to cancer. Walking causes your brain to produce mood-elevating beta-endorphins, and may help prevent you from becoming depressed or anxious.

Disease Prevention: "Exercise reduces the risk of dying prematurely from all causes," Johnson says. Statistics back this up. A Cooper Institute study shows that women who do moderate exercise such as walking are less likely to die from cancer than sedentary women. According to Pereira, the physically active are 50-percent less likely to develop heart disease. A 1998 Harvard School of Public Health study found that moderate exercise can cut stroke risk in half. "We're not talking marathon runners," Johnson says. "We're talking moderate exercisers, like people who walk regularly. It's powerful."

Muscle Loss: "We lose muscle mass as we age," says Moffatt of Florida State. "Exercise can help diminish the effect of aging on muscle loss." Add strength training a few times a week, and not only can you manage muscle loss, but you can build it as well.

the 50s

Around age 50, women face menopause -- and an entirely new spectrum of health issues, since their bodies begin producing less estrogen. The aging process begins to accelerate at age 50, but women who walk tend to be less affected by it than are their sedentary sisters. If you're looking for an anti-aging elixir, exercise is it.

Menopause. Walking helps the menopausal woman in countless ways. Here are a few:

A loss of heart-protecting estrogen causes cardiovascular risk to rise, but walking -- which raises HDL (good cholesterol) and lowers LDL (bad cholesterol) as well as blood pressure -- offsets some of that risk.

The beta-endorphins that our brain releases during exercise can take some of the sting out of menopause's mood swings.

Estrogen loss can bring on insomnia, but exercise can improve both quality and amount of sleep.
Because exercise can offset so many of the problems of menopause, it makes good sense to start walking with or without hormone replacement therapy.

Osteoarthritis. An estimated 20 million Americans suffer from osteoarthritis, a painful disease in which joint cartilage degenerates. Common among the 50-and-over set, it tends to affect the hands, feet, knees, and hips. For a long time, doctors advised osteoarthritis sufferers to stay off their feet. But a 1997 study funded by the National Institute on Aging suggests that people with moderately severe osteoarthritis of the knee who exercise in moderation have less pain than sedentary sufferers. (To be safe, check with your doctor before beginning any exercise program.)

Osteoporosis. Some 28 million Americans -- 80 percent of them women -- have osteoporosis, a loss of bone that can lead to fractures. Our bones are protected before menopause by estrogen (the glue that keeps calcium and other minerals in the bones), but after menopause those minerals leak out, leaving bones porous and brittle. Studies show that weight-bearing exercise such as walking can increase bone density.

Beyond all of these health concerns, walking can add quality to your life. Any walker will reel off the benefits -- self-esteem, confidence, and a positive outlook. The Cooper Institute's Johnson sums it up: "There's a psychological and physical hardness that comes about with a regular exerciser that you simply don't see with a couch potato."

Source: Kelly, Alice Lesch. Well-Being magazine. Walking's Annual Diet (1999), Fitness Guide (Spring 1999).

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