Hear that crash? It's the health boom colliding with the personal-com-puter explosion, producing the latest permutation in pep-tech: fitness software.

Given America's delirious faith in a microchip apocalypse and the creeping math- atrophy of a populace barely able to change a quarter without groping for the calculator, the advent of computerized diet/exercise programs was inevitable.

In the past year, over 20 have hit the market--from the cringing modesty of the "Personal Weight Control Program" ($29.95) for your Timex Sinclair to a titanic $4,800 chow-checker called "Dietary Analysis and Assessment," which tells you everything but which fork to use. In between are a dozen screen gyms suitable for the average jocko prepared to lose some weight around the wallet.

Most of them are of two kinds. Nutritional analyzers break down your daily nosh into total calories; distribution of fat, protein and carbohydrates; percentage of Recommended Daily Allowances for major vitamins, salubrious goos such as calcium, sodium and iron--even exotic glops like Histadine and Tryptophan (amino acids, not Third World countries). Nothing you couldn't do by hand with a reference chart, a bucket of aspirin and a free weekend. But programs do it instantly. Among the best is "The Nutritionist" ($145 for 19 nutrients, $295 for 27 nutrients plus nine aminos; for Apple, IBM-PC or CP/M systems from N-Squared Computing of Silverton, Ore.). The second type adds a more or less complex method for recording daily exercise and linking it to food intake. Typical of these is "InShape" for the IBM-PC ($95 from DEG Software of Houston).

"The Nutritionist," despite a maundering, inept manual, is engagingly simple to operate. You enter each food and portion size by flipping through the Department of Agriculture's 730-item booklet of nutritive values, included with the program. (Everybody uses the USDA figures. "There's no use even talking about whether they're good or bad," says Dr. Aaron Altschul, director of Georgetown University's diet management/eating disorders program. "It's all there is.")

It took our 120-pound female tester 10 minutes of thumb-numbing aggravation to log a recent day's diet of tea with sugar, english muffins with cream cheese and tuna salad, a peach, four crackers with cheese, a hamburger, a Pepsi Free and popcorn at the movies.

The software digested that, burped once (understandably) and seconds later disgorged a graph of results: 1,325 calories; 19 percent protein, 45 carbohydrate, 37 fat; surpluses of protein, phosphorus and niacin; conspicuous deficits of calcium, iron, thiamin and vitamin C. It ranked each item by nutrient content; and when queried for substitute foods to rectify the menu, it became a veritable cybernetic mom, prescribing broccoli, spinach, turnip greens and kale--all high in calcium and vitamin C, but jeez!--and even offered sample diet plans.

Such analyses are vastly superior to simple calorie- counting. Consider our 185- pound male tester, whose day's worth of speed-grub and sundry fryables totaled only 1,870 calories and 119 grams of protein--ostensibly good news. But the program showed that he didn't beat the greasies: 41 percent of his diet was fat, with a whopping cholesterol count and low thiamin.

"The Nutritionist" has a few deficiencies. In calculating food needs, it lumps ages 23 to 50 in a single category, irrespective of body size or activity. Worse yet, it bytes off more than you can chew: using a single daily baseline of 2,000 calories for women, 2,700 for men. "That's high," says Altschul. "The national average intake is 1,600 for women in that age group, 2,100 for men." And it disregards significant variations in resting metabolism, which "contributes over half to the total output of calories per day."

"InShape," cleanly designed and lucidly documented, also tallies fat/carbo/protein ratios and total calories for nearly 1,000 items. But no table-thumbing here: The data are on the disk (including mass-market entities like the Big Mac, tipping the nutritional Toledos at 563 calories and 26 grams of protein). The program accepts daily entries of diet, exercise and weight; keeps a year's worth of records; and generates quarterly or yearly graphs comparing any two variables--exercise vs. calories, protein vs. weight, carbohydrates vs. food group, etc. It calculates exercise through a complex "point" system: 50 minutes of squash gets you 7.5; walking a mile in 10 minutes rates 4; nine holes of golf or an hour of tennis doubles are 1.5. But it ignores hugely differing levels of player intensity, and provides no way to compare workout with diet to determine whether you're eating too much or too little of what.

"Nutri-Calc" (unreviewed; $350 from PCD Systems in Penn Yan, N.Y.) attempts the latter by setting five levels of user bustle: sitting, standing, sleeping, walking and "vigorous work" (probably unusable in Congress, where these categories are often indistinguishable).

An ideal program would allow data entry by age, sex, body configuration, exercise intensity and basal metabolism. It would relate sports stress levels to diet and suggest changes in either to balance the biological books. "NutriQuest" (unreviewed; $350 for Apple II, TRS-80 and IBM-PC from Capital Systems Group in Rockville) purports to come close. It combines a 900-food data base with a Mayo Clinic formula for computing basal calories, then adds intake requirements based on six levels of activity. And doubtless even more sophisticated stat-grinders will arrive eventually.

But before you capitulate entirely to a manifest silicon destiny, remember that no amount of chip-chomped data can replace professional advice. Nor should a bunch of numbers--especially the trivial obsession with mere weight--ever distract you from the prime method of gathering information: Listening to your own bodys the most miraculous machine of them all.