Good news and bad news about that old rocking chair:

There is some evidence that rocking helps lower blood pressure, ease anxiety, promote digestion.

But letting that old rocking chair get you can promote clogged arteries, arthritis, osteoporosis, constipation and a host of other ills doctors used to think were no more than the wages of age. Now they are considered more the wages of inactivity and poor nutrition--no matter the age -- especially if you keep eating as you did when you were 20.

At a conference here on Physicial Fitness and Aging, sponsored by the President's Council on Physical Fitness and Sports (and underwritten by General Foods) a group of experts spent two days exhorting America's mellowing population to straighten up (and run and dance and swim and jump) and eat right.

Arthritis specialists, for example, have mostly switched from advising patients to take to their beds to telling them to take to the parcours, the track, the swimming pool, the tennis court or the gym.

For women, aging carries the additional problem of osteoporosis, the loss of calcium from the bones that takes place after menopause or hysterectomy. This thinning of bone density makes fractures virtually inevitable in some women.

Barbara Drinkwater, research physiologist at the University of California's Institute of Environmental Stress, told the conference that some 200,000 women break their hips every year and 15 to 30 percent of these may have complications ending in death.

Many older women traditionally have been given estrogen therapy which appears to retard bone loss -- but will not restore bone already lost--so long as the therapy is continued. But with new evidence tending to link the use of estrogen to cancers, many women may not consider the trade-off a good one.

Exercise (combined with supplements of calcium and vitamin D) is a reasonable alternative, noted Drinkwater: Not only does it increase resistance to cardiovascular disease in both men and women, it increases bone density.

Exercise is not the whole story, however, for fit aging.

Consider nutrition, a field only recently accepted by the medical establishment as a proper medical discipline. There are still plenty of medical schools where no courses on nutrition exist.

Dr. Myron Winick, a pediatrician and director of the Institute of Human Nutrition at Columbia University Medical School in New York, is concerned with eating habits from cradle to, well, beyond maturity. His observations on nutritional good sense are being syndicated to nearly 150 radio stations around the country. (In the Washington area the Columbia Nutrition Bulletin will be broadcast on WASH-FM at 11 a.m. Mondays, Wednesdays and Fridays.)

The last major breakthrough in nutrition was probably the actual discovery of vitamins and their relation to such diseases as scurvy, pellagra, beriberi and rickets, diseases now so rare in most parts of the world many doctors have never seen them.

But work at Winick's institute and at other research centers has produced a wealth of knowledge about more subtle and incredibly complex nutritional processes. However, as one authority on aging has said, "what we still don't know about nutrition is encyclopedic."

Winick warned at the conference that many Americans eat too much of the wrong things and not enough of the right ones. Moreover, new scientific concepts on the relation of nutrition and aging suggest a link between lifelong eating habits and quality of elderly life.

"We always made the assumption," says Winick, "that the nutritional requirements for an elderly person were the same as the nutritional requirements for the young adult. Most of the measurements for nutritional requirements are made on healthy college students because that's where we get our volunteers . . .

"It's becoming more and more obvious that this is not true." Although scientists cannot say with certainty that underfeeding young humans will increase longevity -- as with young rats--"the lessons we may learn from this in terms of humans," says Winick, "indicate at least that even very early in life there are nutritional considerations."

There is plenty of evidence, he says, that too much salt, too high a consumption of saturated fats and too many calories can contribute to the development of heart disease, hypertension and diabetes.

At the same time, a reduced calorie diet, especially in diet-conscious women, can produce a long-term deficiency of certain nutrients (like calcium and iron) and many so-called micro-nutrients. There may be few dramatic symptoms until the damage is done. For example, you may not notice that your bone calcium reservoir is being used up until your hip breaks.

Complicating the calcium situation was the discovery that the amount of calcium in the body depends on how much is absorbed through the gastrointestinal system. This process requires a form of Vitamin D which older people are often less well able to produce. Moreover, if too much phosphorus is present in the body, calcium absorption is restricted, suggesting another change in diet for an older population. (Phosphorus is found in high protein foods, especially meats, and in some soft drinks.)

In general, an older individual requires fewer calories, says Winick, for two reasons.

l) Older people have a greater percentage of fat tissue in their bodies, even if they weigh less than they did when they were younger.

2) Their metabolism will convert more calories to fat, especially if they are sedentary.

Which brings us back to exercise.

Says Winick, "Here is where exercise becomes extremely important because the more they exercise, the greater number of calories they can consume (without being stored as fat) which gives them a greater source for all the nutrients they may need."

Winick knows the final word is far from being spoken on nutrition for the elderly. After all, he notes with some horror, it was only last year that the National Research Council Food and Nutrition Board saw fit to publish dietary allowances for older people, "and even here," he says, "they are estimates, at best."

But now that doctors see nutrition as a subject of proper study at last, and exercise as a necessity -- instead of a rather odd option -- the older we get, the better we should know how to stay that way, longer.