It's been called the disease of little old ladies. Actually, if it can be called a disease at all, it's the one that causes little old ladies.

Osteoporosis, the progressive degeneration of bone mass, affects about one out of every four (mostly white) women over the age of 60. More than 5 million American women have it now and it is believed to be a major factor in virtually all of the 200,000 broken hips American women suffer every year.

Because complications will cause the deaths of nearly 40,000 of those suffering hip fractures, osteoporosis is the 12th leading cause of death in the United States. It is one of the so-called "big three" reasons for entering a nursing home, along with senile dementia and urinary incontinence.

Yet, says Dr. Morris Notelovitz, it is truly the "silent epidemic," because "people do not have symptoms until such time as they actually have a fracture. Then it is often too late to do anything constructive about it."

Even more common than broken hips, Notelovitz believes, are spontaneous fractures of the vertebrae of the spine, which cause a loss of height in older women as the spine quite literally collapses on itself. Eventually it results in the so-called "dowager's hump."

"It is very difficult to define these in terms of numbers," says Notelovitz, "because unfortunately we have been led to believe or to accept the concept of the little-old-lady syndrome--and that the bent-over lady with the hump is just a part of normal aging. Indeed, it is not."

Notelovitz is a gynecologist who is trying to make the study of middle years in both men and women as much a speciality as pediatrics or geriatrics.

He has just written a book on osteoporosis--Stand Tall! The Informed Woman's Guide to Preventing Osteoporosis (Triad, $12.95, hardcover; $6.95, paper), co-authored by Marsha Ware. Most of the royalties will be donated, the book states, to the research and scholarship fund of the Center for Climacteric Studies at the University of Florida. Notelovitz is director of the center, which specializes in the study, treatment and prevention--especially prevention--of disorders that occur or begin in the middle years of life.

Prevention of osteoporosis should begin in young womanhood, Notelovitz and most other specialists in the field are beginning to believe. Too little calcium intake throughout life appears to be one predictor of degenerative bone disease later on. After menopause, a lack of estrogen is a major factor. Through a complicated set of biochemical relationships, estrogen prevents other hormones from adversely affecting bone density.

"It is useful to start early in osteoporosis prevention ," says Dr. G. Donald Whedon, former director of what is now the National Institute of Arthritis, Diabetes, Digestive and Kidney Diseases. He is now associated with the Kroc (scientific and educational) Foundation in California.

"Both men and women should pack in the milk in their growing stages and should pay attention to getting adequate calcium right through their thirties and forties."

Virtually all researchers agree that the currently recommended daily allowance of 800 mg. of calcium is too low. But precisely how much higher it should be for anyone at any given age has not been thoroughly established, although most agree it should be between 1,200 and 1,500 mg. (About 1 1/2 quarts of milk or three cans of sardines.)

Those, usually older people, identified as being unable to activate Vitamin D, which enhances the absorption of calcium, may need even more calcium supplementation.

Dr. A. Michael Parfitt, of the Henry Ford Hospital in Detroit, believes that the "single most important causative agent in osteoporosis is estrogen deficiency." However, he agrees with many colleagues that it is too soon to "make the recommendation that all women should take estrogen replacement therapy ." Rather, researchers should be trying to identify those women at greatest risk:

* Women with a positive family history of osteoporotic fractures.

* Women who have had a wrist fracture--a common early manifestation of osteoporosis.

* Women who have a low measurement of bone mass, shown by whatever test available. (Such as an X-ray taken for something else, chest, for example.)

Because estrogen-replacement therapy has been linked to certain cancers, of the endometrium and possibly of the breast, the individual decision must be weighed carefully. New studies suggest that cyclical estrogen replacement, along with some form of progesterone (or the synthetic progestogen) supplement, may eliminate the cancer risk almost entirely.

Notelovitz feels strongly that younger women who have had hysterectomies in which their ovaries have been removed should receive estrogen therapy. One concensus conference at the National Institutes of Health has already approved estrogen-replacement therapy as a preventive measure in high-risk women. Another conference is scheduled for next year to examine the latest findings on the subject.

Other more easily correctible factors that appear to contribute to the syndrome are lack of exercise, smoking, diets containing too much protein, especially from red meats--although there are some conflicting studies on the last.

Notelovitz adds that black women are less susceptible (but not totally immune) to osteoporosis than Caucausians or Orientals for reasons that have not been established. Also, women who have been moderately overweight during most of their lives have usually built up denser bones from carrying around the extra poundage and are therefore somewhat protected against osteoporosis, although they may be more susceptible to problems with weight-bearing joints, as, for example, osteo-arthritis. He also notes studies suggesting that vegetarians lose bone mass at a slower rate than meat-eaters.

Says Dr. Robert P. Heaney of Creighton University in Omaha, who recently headed a review of nutritional factors--especially calcium--related to osteoporosis: "Most people nowadays believe that increasing calcium intake by one means or another is certainly safe and probably effective . . . it certainly can't hurt."

Heaney and his team write in the report, published in the American Journal of Clinical Nutrition:

"The available evidence, taken together, does not indicate that raising the calcium intake will abolish the problem of osteoporosis. It does indicate, however, that calcium nutrition is considerably more important in the genesis of osteoporosis than has been commonly thought for the past 35 years."

His own personal bias, says Heaney, is against pills and he urges increasing the amount of calcium in the diet. Dairy products are, of course, full of calcium, but it also is found abundantly in fish like sardines where the bones are soft enough to eat, and in green vegetables like broccoli. On the other hand, Notelovitz believes a calcium supplement is necessary, certainly when dairy products cannot be consumed in quantities large enough to provide the needed calcium.

Authorities tend to differ on the usefulness of exercise. Notelovitz has received a grant from the manufacturers of Nautilus exercise equipment for an eight-year study of the effects of that exercise program on bone density. Heaney notes that studies indicate "even mild exercise programs for little old ladies tended to increase bone mass."

However, "what we don't know in the long run is how much or what kind of exercise . . . stevedores, of course, who carry 100-pound bags out of the holds of ships have heavy bone structure, but you can't ask little old ladies to carry 100-pound bags."

Other NIH-financed studies--one at the Henry Ford Hospital and another at the Mayo Clinic--are examining the potential of sodium fluoride in building bone mass in women already diagnosed as osteoporotic. But results from these studies are several years away.