"The only safe and sensible way to lose weight involves eating a wide variety of low-calorie foods and planning for a slow, steady weight loss of about two pounds per week," concludes "Popular Diets: How They Rate," by the Los Angeles District of the California Dietetic Association, a division of the American Dietetic Association.

Two behaviors are also tied to successful weight loss, dietitians note: Breaking the habits which lead to excess weight, such as indulgence in fattening foods and eating too much, and following a safe, effective exercise program.

Diets should offer at least 1,200 calories for women and 1,600 calories for men, dietitians say. Food plans with fewer calories are almost always lacking in important nutrients needed to maintain health and should only be followed under close medical supervision. Usually, such plans require a multivitamin and mineral supplement, dietitians say.

Good advice about nutrition is often available from hospital outpatient clinics, local or state health departments and extension services, local dietetic associations, medical associations and nutrition consultants.

But beware, anyone can use the title "nutritionist." Only a registered dietitian (RD), however, is required to meet strict educational requirements, including a dietary internship, and pass an exam. Other recognized credentials include membership in the American Society for Clinical Nutrition or certification by the American Board of Nutrition.

It's always smart to check with your doctor before starting a weight-loss program.

But, unfortunately, you can't always count on your physician for expertise in weight loss. Although some doctors have nutrition training, most receive little if any, says Dr. Myron Winick, director of the Institute of Human Nutrition at Columbia University and author of "Your Personalized Health Profile: Choosing the Diet That's Right for You" (William Morrow and Co., $16.95). Winick recently chaired a Food and Nutrition Board panel that recommended improved nutrition education in medical schools.

Other sources for weight-loss help:

"The LEARN Program for Weight Control -- Lifestyle, Exercise, Attitudes, Relationships, Nutrition." This book is a clinically tested guide to weight loss, developed by Kelly D. Brownell, PhD, of the University of Pennsylvania's obesity research group. Send $15 to Dr. Kelly D. Brownell, Department of Psychiatry, University of Pennsylvania, 133 S. 36th St., Philadelphia, Pa. 19104.

"A Good Nutrition Reading List" is available free from the American Dietetic Association. Send a self-addressed, stamped envelope to ADA, 430 N. Michigan Ave., Chicago, Ill. 60611.