How many calories are there are in six macadamia nuts? A table-spoon of peanut butter? An ounce of Cheddar cheese?Five cups of mushrooms?

Most people are surprised to learn that the answer for all the foods is the same: 100 calories.

It's even more unlikely that most people would know how long you have to walk or play tennis to work off those 100 calories.

That answer depends partly on whether you weigh 117 pounds or 196 pounds. At 117 pounds you could knock off 100 calories in about 20 minutes of a moderate tennis game, but at 196 pounds it would take only about half that time.

The answers to these questions are important to people who diet but they are hard to keep straight. It just seems easier to go an a fad diet and let someone else do the work for you. Fad diets also seem less boring. though not necessarily more successful.

All of this accounts, for the fact that dieters spend $10 billion a year trying to get slim. And women spend much more than men. There's good reason for that.

How easily and quickly you lose weight depends on your sex. According to Dr. Barbara Edelstein, author of "The Woman Doctor's Diet for Women", (Prentice-Hall, $8.95). "Men lose weight almost twice as fast as women do.

"They burn calories twice as fast for the same amount of exertion". Edelstein explains that "a woman's body is naturally composed of a higher proportion of fat to music tissue than a man's and muscle mass burns five more calories per pound to maintain itself than fat or connective tissue."

This is discouraging for women and may explain why more of them diet than men. In general women are ready targets for the thousands of gimmicks that are supposed to make losing weight easier and they know much more about losing (and gaining) weight than do men.

At the same time women appear to do a slightly better a job at keeping their weight down than men, even though it is more difficult.

According to the National Center for Health Statistics, the average man is now 20 to 30 pounds over his ideal weight, while the average women is 15 to 20 pounds heavier than she should be. Men are four pounds heavier today than they were 10 years ago.

Perennial dieters are always looking for the silver bullet. But there is none. Though various gimmicks may help certain people to stay on diets, the hard part really comes after the weight is lost. The recidivism rate for dieters is higher than it is for repeat offenders in the country's prisons.

In recent years there has been a growing realization that in order to keep off the weight once it is lost, new behaviour patterns are usually necessary. This has led to diet and behaviour modification groups. Even the old-time diet organizations, such as TOPS, (Take Off Pounds Sensibly) are into "behaviour mod". They now have a follow-up program called KOPS (Keep Off Pounds Sensibly).

But more people try crazy crash diets rather than the sensible ones like TOPS, hoping for an easy cure. Some of those diets have proved extremely dangerous.

The liquid protein diet comes to mind immediately, not only because of its recent popularity, but because a number of deaths have been attributed of it, are Part of the fasting method for losing weight.

Total fasting for extended periods requires a stay in the hospital and must never be undertaken without medical supervision.

Even modified fasting requires the constant monitoring of a qualified physician. The liquid protien diet is a modified fast which includes some protein, plus vitamin and mineral supplements. In addition to possible minor side effects, like nausea, bad breath and hair loss, the drastic changes in the body chemistry due to a long-term diet of only "liquid protein" apparently proved to be too great for about 50 people. Their deaths have been linked, though not conclusively, to liquid protein dieting.

While high fat diets, which have been around for about 100 years, are not considered as dangerous, physicians say they are not particularly safe either. One of the most recent versions was the Dr. Atkin's diet in which the dieter could eat all the fat desired, but no carboydrates. While there may be an initial weight loss, mostly due to loss of body water, the diet, experts explain, puts a great a burden on the kidneys.

The protein diets, like Dr. Stillman's also preclude carbohydrates-with the similar possible consequences. Low carboyhdrates diets of any kind are particularly dangerous for people with cholesterol or heart problems.

Then there are the diets which go by names such as Grapefruit or Banana.

Too often, they are just gimmicks for diet-bored people. The Grapefruit Diet, first unveiled by Helen Rubenstein in 1938, called for eating nothing but grapefruit and drinking only back coffee for one week. It is bound to work. What happens after, if you survive it, is another matter.

Perhaps the most absurd diet is the one that calls for injections from the hormones of a pregnant woman six times a week for a total of 40 injections. The HCG diet made quite a splash about two years ago. The injections are supposed to make "abnormal deposits of fat easier to get rid of. But, just to be on the safe side, the dieter eats only 500 calories a day. Anyone would lose weight eating so little. On a diet like that you'd use up all your calories with just 30 macadamia nuts.

It isn't difficult to eat 30 macadamia nuts once you start on them. And it's hard to believe they contain 500 calories. One of the greatest problems for dieters is to figure out is the calorie count of most foods.

Shirely Matson, a nutritionist at the Golden Door, a health spa in Escondido, Calif., has devised a graphic display for 100-calorid portions of more than 50 different foods. The display is set up each week for the guests and Matson says most people are shocked by how little it takes of some foods to add up to 100 calories. They are particularly chagrined to find that a single ounce of cheese, a dozen almonds and just 13/4 ounces of ground beef add up to 100 calories. They are usually delighted to discover that you can eat a whole cantaloupe or a pound of tomatoes (3 medium) and consume only 100 calories.

This kind of information is very useful when you are "psyced" to do the right thing: Eat a well-balanced diet, consume less food and exercise more. To quote from one of the foremost authorities on dieting, Tufts Universiting President Dr. Jean Mayer: When it comes to dieting, remember the tortoise and the hare: Slow and steady thins the waist.

Evidently even Mayer has trouble following his own advice. According to Family Health Magazine, for whom Mayer writes a monthly column, ". . . the results of Dr. Mayer's demanding schedule have started to show, leaving the famous Frenchman with an expanding waistline and a doubling chin. 'I just don' have time for exercise', he admits, 'and I have far too many official dinners.'"