Athletic women are generally looked upon as being fit and healthy. But a look from the inside out shows a different story.

Approximately 20 percent of active women are unhealthy to the point they stop having regular menstrual periods. This condition is called amenorrhea and is characterized by estrogen deficiency, similar to menopause. Although some women deem amenorrhea as a desirable side effect of exercise, because they no longer have to deal with the hassles and possible discomfort of monthly periods, others recognize absence of periods is linked with health problems:

* loss of calcium from the bones

* almost a three times higher incidence of stress fractures (24 percent of athletes with no or irregular periods experience stress fractures as compared to only 9 percent of regularly-menstruating athletes)

* long-term problems with osteoporosis starting at an early age.

Amenorrhea can also interfere with the ability to conceive easily should the athlete want to start a family. > Causes of Amenorrhea

Many women believe they have stopped menstruating because they are exercising too much. Not the case. Many very-thin athletes who exercise heavily have regular menses. Other women believe they have stopped menstruating because they are too thin. Also false. Studies suggest no differences in body fatness between athletic women who do and do not menstruate regularly.

The question remains unanswered: Why, given a group of women who have similar exercise programs and a low percent body fat, do some experience menstrual problems and others don't? The answer commonly relates to nutrition. The woman with amenorrhea may be striving to maintain a weight lower than what is appropriate for her genetics. When the cost of achieving this desired leanness is inadequate nutrition, she'll stop menstruating. > Prevalence of Amenorrhea

Although amenorrhea is not sport-specific, sports with the highest prevalence include ballet (19 to 44 percent of dancers) and competitive running (24 to 26 percent of runners). An estimated 3 to 5 percent of sedentary women also experience menstrual irregularity; this is strikingly less than what prevails among athletic women.

You are more likely to become amenorrheic if you have lost weight quickly, have a low body weight, low percent body fat, exercise very hard, had irregular menstrual periods even before you started to train hard, are feeling emotionally stressed, and have a restrictive diet. > Amenorrhea and Anorexia

Although amenorrhea exists among women with no eating disorders, loss of menses is certainly a red flag symptomatic of restrictive, anorectic-type eating behaviors. The American Psychiatric Association's definition of anorexia lists "absence of at least three consecutive menstrual cycles" among the criteria. Other criteria include: weight loss 15-percent below that expected, intense fear of gaining weight or becoming fat, and distorted body image (i.e., claiming to feel fat even when emaciated), all of which are concerns common to female athletes.

Resolving the Problem

If you feel as though you are struggling to balance food and exercise, you might want to seek counseling from a trusted physician, dietitian and/or counselor. To find a local sports nutritionist, call 800-366-1655 or visit www.eatright.org and use the American Dietetic Association's referral network.

The possible changes required to resume menses include exercising 5 to 15 percent less and eating a little more. If you totally stop training, such as may happen if you are injured, you may get your period within a few months. Some amenorrheic athletes have resumed menses by simply exercising less and gaining no weight or less than five pounds. This small amount of weight gain is enough to achieve better health yet does not result in your "getting fat."

The following tips may also help you resume menses or at least rule out nutrition-related factors.

1. Throw away the bathroom scale. Rather than striving to achieve a certain number on the scale, let your body weigh what it weighs.

2. If you have weight to lose, don't crash diet but rather moderately cut back on calories by about 20 percent. You are eating too few calories if you are always hungry and obsessing about food.

3. Eat adequate protein. Amenorrheic athletes tend to eat less protein than their regularly-menstruating counterparts. Even if you are a vegetarian, remember that you still need adequate protein. Eat additional yogurt, fish, beans, tofu and nuts.

4. Eat at least 20 percent of your calories from fat. If you believe you will get fat if you eat fat, think again. Although excess calories from fat are easily fattening,, some fat (20- to 30-percent of total calories; 40 to 60-plus grams of fat per day) is an appropriate part of a healthy sports diet. Nuts, peanut butter, salmon, olive oil are healthful choices.

5. Include small portions of red meat 2 to 3 times per week. Surveys suggest runners with amenorrhea tend to eat less red meat and are more likely to follow a vegetarian diet than their regularly-menstruating counterparts. Even in the general population, vegetarian women are five times more likely to have menstrual problems than meat eaters. It's unclear why meat seems to have a protective effect upon menses.

6. Maintain a calcium-rich diet to help maintain bone density. A safe target is the equivalent of 3 to 4 servings per day of low-fat milk, yogurt and other calcium-rich foods. Being athletic, your bones benefit from the protective effect of exercise, but this does not compensate for lack of calcium nor lack of estrogen. > Is There Long-Term Damage?

Amenorrheic women who resume menses can restore some but not all of the bone density lost during their months of amenorrhea. Hence, their goal should be to minimize the damages of amenorrhea by eating appropriately and taking the proper steps to resolve the problem. Every woman always wins with good nutrition!

Nancy Clark, MS, RD counsels people individually at Sports Medicine Associates in the Boston area. Her popular Sports Nutrition Guidebook offers additional information. It is available via www.nancyclarkrd.com or by sending $23 to Sports Nutrition Services, 830 Boylston St. #205, Brookline MA 02467.