You're dieting. You've eaten about 1,200 calories today, you think, as you sit in front of the computer, ready to enter every morsel you've swallowed. But when the computer tallies up the results, it tells a different story. You overshot your day's goals by 500 calories, the screen reads, and 40 percent of your intake was in fat.

This is the principle behind the Original Boston Computer Diet, a computer program that aims to help you lose weight and develop better eating habits.

OBCD is one of a growing number of computer programs designed to improve your health and fitness. While sophisticated software for health and sports professionals has been available for more than a decade, hundreds of health-oriented programs for consumers have reached the market in the past couple of years. Last year, sales of this software mounted to $70 million, according to Hilda Uribe, senior analyst at Future Computing, a Dallas market research firm specializing in the computer industry. Annual sales are projected to reach $300 million next year. Fledgling software firms, well-established companies and even organizations like the American Heart Association are introducting new programs.

Still in its infancy, health and fitness software accounts for a small slice of the software market, typically lumped in the category of self-help or life-enrichment software. Health-oriented computer programs can help you plan nutritious meals, improve your cardiovascular fitness or even figure out how long you can expect to live.

Many of the software programs on the market are targeted to the approximately 80 million Americans who are between 10 and 40 pounds overweight and to the country's 30 million or so runners. Others focus on general well-being, such as health-risk appraisals, which analyze a personal health profile to let you know which areas you can improve upon to maximize your chances of good health.

Health software places the burdens of record keeping, calculation and analysis on the computer, freeing the health-conscious user to concentrate on a diet or exercise regimen itself. With a diet program, for instance, you dispense with the usual log books and other tedious methods of counting calories. You still have to keep track of and enter what you eat into the computer every day. The program calculates the daily totals, stores the data, and often tells you how you're doing at the end of each week.

More sophisticated programs present graphs or charts. Programs for runners, such as "Be Your Own Coach," provide graphs that show you weekly summaries of your mileage, pace, weight, heart rate, workout time and more. Many programs offer personalized advice and feedback. "Nutri-Byte," a weight-loss program, takes the information you type in and offers feedback like "You ate more than half your snacks while watching TV" or "Last week you consumed especially large meals when you ate with Irene," then suggests ways to break those fattening habits. Nutrition and Diet Programs

Many of today's consumer-oriented packages provide basic nutritional information as well as a breakdown of the components and nutrients in the foods you eat. While some simply add up calories, fats, protein, carbohydrates and a variety of nutrients, others point out deficiencies and excesses in your diet and sometimes plan meals. A growing number of these programs include an exercise module in an effort to contribute to overall well-being.

At the heart of all these programs are food and nutrient data bases that catalogue 700 or more foods as well as the recommended dietary levels of nutrients. Most programs construct the food data bases from the U.S. Department of Agriculture's data on calories, carbohydrates, fats and proteins. Nutrient data bases most often rely on the Recommended Dietary Allowances established by the Food and Nutrition Board of the National Academy of Sciences. Using the information in these data bases, the programs calculate the nutritive value of the foods you eat.

One of the earliest and best-known nutrition-analysis programs is "Nutritionist I," which contains a data base of 730 foods and tracks as many as 19 nutrients. Packaged with the USDA Nutritive Value of Foods booklet, it computes calorie and other nutrient levels, compares your intake with the RDA, and lets you create low-sodium, low-calorie, and other special diets. To get an analysis of each meal you've eaten during the day, you enter a code number for each food and the serving size. You then see a table listing the amount of calories, protein, fat, iron and other components of the meals. You can also call up graphs displaying the percentage of RDA your meals supply, based on your age, sex, weight and height. If a graph shows an excess of fat, you can display a breakdown of the fat content of all foods you've eaten, sorted in order from highest to lowest fat content.

"Nutri-Byte," a program complemented by four-color graphs, records weight-loss goals, tracks and analyzes daily diets and suggests calorie levels and specific menus. This program also asks for information on your eating habits -- what mood you were in when you ate, how hungry you were, how much exercise you got, where and with whom you ate, and what else you did while eating. The program then correlates the calories you consume with all these factors to help you modify your eating habits.

The "Original Boston Computer Diet," one of the newest health programs, uses behavior modification to help those who are less than 40 pounds overweight. Developed by a group of medical and health professionals, this package features "counselors" who guide you through the program and mete out feedback and advice. The chief appeal of this program lies in its ease of use. Reporting the daily intake of food, often a tedious process, takes less than 15 minutes. While the meals the program plans won't win prizes for imagination -- bran cereal, skim milk and unsweetened orange juice make a well-balanced but boring 210-calorie breakfast -- they're certainly sound, based on guidelines from the American College of Sports Medicine.

As long as people conscientiously report what they've eaten, all these programs will maintain accurate records, often pointing out excesses and deficiences previously unknown or ignored.

"The key thing is that most people don't know what they've eaten during the day," says Dr. Michael Davidson, an assistant professor specializing in the use of computers and health at the Department of Health Professions at Montclair State College in New Jersey. While a breakdown of a single day's diet reveals nothing about a person's general level of health, he notes, such a breakdown over time can show dietary trends.

The complex nutritional information some programs provide is difficult to interpret, says Robert Gold of the U.S. Public Health Service's Office of Disease Prevention and Health Promotion. He cautions against making long-term decisions or massive changes in diet based solely on these programs. Although he warns against the "potential for massive misinterpretation and health quackery," he acknowledges that many programs offer enormous benefits as well, when used with reasonable care.

Most consumers needn't track every nutrient in their diet, says Holly Irving, a nutrition research assist at the the U.S. Department of Agriculture's Food and Nutrition Information Center, which maintains a library of health programs. Most people, Irving says, can concentrate on a handful of dietary components such as calories, fat, carbohydrates, protein, fiber, calcium, sodium, iron and zinc. Health-Risk Appraisal Programs

Based on actuarial tables used by insurance companies, health-risk appraisal programs identify factors that reduce longevity and sometimes predict life span. For instance, "Health Risk Appraisal," a program developed in conjunction with the Unviersity of Minnesota, cites the 10 major causes of death and mortality rates for people of your age, race and sex. After quizzing you on your medical history and current behaviors ("How much of the time do you use a seatbelt?"), it adjust the average mortality figures according to your responses and computes the number of years you can expect to live. Finally, it recalculates your life span, based on the assumption that you correct risky behaviors.

While these predictions may reveal nothing new, seeing the facts on the screen of an impartial machine can spur some people into taking appropriate preventive action.

Still, as Gold points out, these programs rely on group statistics and probabilities, which limits the accurancy with which they can assess an individual's risk. Some people "tend to fly into panic because they treat computer printouts with reverence," Gold says, emphasizing that people should keep the limitations of this software in mind. Fitness Programs

While a few companies offer software for those devoted to aerobic exercises, most fitness software addresses the needs of runners. These programs maintain records of how far and how fast you run, usually provide weekly and monthly performance analysis and often produce customized training schedules.

Emphasizing long-term health rather than quick gains in speed and distance, "Running Your Best Race" offers advice from runner Joe Henderson. The program accompanies the college textbook of the same name and provides quick analysis of your training, testing and racing results, along with a computerized log. While simple enough for beginning runners to use, it's aimed chiefly at serious runners who want to improve performance.

"The James F. Fixx Runner Program," developed by the late runner and a group of medical specialists, is designed for all levels of runners -- from beginners to marathoners. The program helps you evaluate your current fitness level and assists you in setting goals, planning schedules and monitoring progress. The program comes with educational information about exercise, cardiovascular fitness, life style and nutrition.

Aided by the program's simple menus, windows and help messages, runners enter the distance covered, training time, weather and road conditions and calories consumed per day. The program spits out the number of calories burned, total monthly mileage and dietary breakdown. In all, it provides a level of guidance that most novice or intermediate runners will welcome. How to Choose

Because health and fitness programs differ widely in capabilities and ease of use, it can be difficult to select the appropriate package. Some guidelines:

*Pin down precisely what you want to do. Make sure the program will help you meet your goal, whether that goal is running a marathon or planning nutritious family meals.

*Consider its difficulty. If other family members, particularly children, plan to use the program, ease of use is essential. Many programs require little experience with personal computers, yet they can still intimidate those who consider themselves computer illiterate.

*Read the program manual carefully or call the company to check the source for the information used in the data base and the credentials of the program's developers. While a well-written, comprehensive manual is no guarantee of a program's accuracy, it often reflects the care with which a program is developed.

*Decide on a price you're willing to pay, but don't let price be your sole guide. Some inexpensive packages lack jazzy features but frequently are as useful as more expensive programs.

*Make sure the program will run on the computer you own or use. Most programs run on the IBM PC and compatibles and Apple II series, and many run on the Commodore 64, TRS-80 Models III/4, Atari, and other computers commonly sold for home use. Programs that display graphs may require a monochrome or color graphics adapter.