Happy New Year! Healthy New Year! These are the things we want most from life: happiness and health. And the second of these, a reasonable degree of good health, can help us achieve the first, a reasonable degree of contentment and adjustment in this uncertain world.

Even if we must endure some illnesses -- and sooner or later, we must -- we can do a great deal to keep as fit as possible. And this will help us endure whatever afflictions, physical or emotional, we must face.

Sound grim? Not at all. People who look after their health without becoming utter fanatics are generally people who face all of life with courage and zest. This I say from a lifetime of observation -- and an eye on my 91-year-old mother. She has whirled through the decades eating sensibly (she was one of the first to worry about "cholesterol"; she started skimming the fat off her chicken soup); keeping on the move (until recent years she seldom sat still; constant activity was her "jogging") and mingling with friends, old and new (she rarely dwells on herself, instead combats troubles, losses and loneliness with laughter and people).

Fortunately, medical science can now tell us more than ever about staying zestfully alive -- to reach a goal set by anthropologist Ashley Montagu: "To die young at as old an age as possible."

We have only one life. We have only one body. Keeping it well is largely a matter of:

*What we put into it: what we eat.

*What we don't put into it: what we avoid.

*How we exercise it, as sport or part of our daily lives and work.

*How we rest and relax it. Life's not all jogging.

*How we manage -- yes, we can manage -- our mind and emotions. Our emotions trigger complex chemicals that affect every organ.

*How we use modern medicine to help us.

I have been mingling with doctors and biological scientists for nearly 40 years. Their knowledge about all these things is far more developed and sophisticated than it was even a decade ago.

But all of science is a continuing story. Most of the science of staying alive is either continuing discussion or outright controversy, with different people holding different views, since they must argue on the basis of still incomplete evidence.

This means almost all the advice they can give us is provisional. It all can change. Different authorities will almost always be saying different things on different days.

Given this incomplete nature of medical knowledge, let me try, then, to sum up the sensible consensus on the best ways of life to enjoy healthful life. Here is what the best and best-informed people in each area now think and try to do.

But remember, nobody's perfect. We all, including nutritionists and other experts, let down sometimes. It's only human to pig out or dodge exercising once in a while.

So we can do that and be comfortable, too, if we pay attention to most of the rules most of the time. One smoker I know quit three years ago, but still will bum a couple of cigarettes when she goes to a party. Many ex-smokers can't handle that, but she apparently can. Smoking two cigarettes once or twice a month isn't likely to kill her, and her lungs will continue to get progressively healthier despite a smoking history of 20 years.

Even an elderly smoker will benefit from quitting within a mere matter of weeks, if only because the lungs will soon be in healthier shape to resist any infection or other challenge.

In short, the more healthy things we do, the better, even if we don't do them all the time or haven't done them all our lives. We all have our little weaknesses. But we can't afford the really big ones. What to Avoid

This is the one area of virtually universal agreement among scientists, and by now we're all experts on this subject, whether or not we take heed of it.

*Tobacco. Tobacco is Public Health Enemy No. 1. Shun smoking and tobacco in all forms. Smoking provokes even more heart attacks than it does lung cancers, and it contributes to many other diseases.

*Alcohol. There is some evidence that very moderate daily drinking provides some protection against heart disease. But there is also evidence that even moderate drinking adds a bit to the risk of cancer and high blood pressure. Certainly, shun alcohol in excess. How much is excess? Anything more than a drink or at most two a day (with a "drink" defined as 1.5 ounces of hard liquor or five ounces of wine or a 12-ounce can of beer). .

And beware! Some people cannot imbibe even this much alcohol without the danger of falling into alcoholism, and it is not necessary to drink a little to maintain a healthy heart. Our health and longevity depend mainly on two influences: heredity, which we can't influence, and our habits, which we can affect. Our health habits consist of a balance of many pluses (such as wise eating and healthy exercise) and many minuses (such as excess drinking, unwise eating or lack of exercise). Keep the balance on the plus side.

*Drugs. Shun harmful drugs. Jogging and avoiding cholesterol may give you a trimmer body and stronger heart, but won't counteract the dangers of cocaine to your mind. What to Eat and How Much

First of all, not too much. There is some debate about whether in later life -- after the mid-forties -- it might not be best to be 10 or 20 pounds "overweight," by average weight tables. Dr. Reuben Andres of the National Institutes of Health is the main proponent of this point of view.

Most informed scientists believe otherwise, and a panel at a recent NIH consensus conference said "even five pounds" overweight can be a cause for concern.

At the same time, however, there is an underweight issue. Our current worship of slimness may be causing worse problems: under-nutrition, anorexia and bulemia. I attended a 13-year-old girl's bat mitzvah celebration recently. When the food was set out, the boys rushed to fill up. The girls stayed away from the table and ate almost nothing. In years of attending bar and bat mitzvahs, I had never seen anything like it. Better some pudginess than nutritional disorders.

Okay, what to eat?

The great cholesterol controversy remains one of medical science's unfinished stories. But the strong concensus is that a diet heavy in saturated fats (mainly meat and dairy fats) and dietary cholesterol (prominent in egg yolks) can cause a cholesterol buildup in the bloodstream, which eventually can clog arteries and lead to heart attacks.

Some consumption of unsaturated fats (such as cottonseed and corn oil) can even lower body cholesterol levels. But we are also told by students of cancer and diet that over-consumption of all fats and oils seems to predispose you to some kinds of cancer. So these anti-cancer scientists propose a slightly different and in some ways conflicting diet list than the anti-heart disease scientists.

And other experts tell us: avoid salt, avoid too much sugar, avoid chemical additives.

With the help of some dietary guidelines from the American Health Foundation, a group that looks at all diseases and their causes, I arrived at my own "12 Rules for a Healthier Body," printed above.

In summary, these rules say: use some of the healthier unsaturated oils but go light on all fats and oils; eat healthy, energy-rich starches like potatoes and pasta but don't swamp them in fats; go light on eggs; increase intake of fibers (a good bet, but not a sure one, since knowledge here is less than complete; but it almost certainly can't hurt); eat a lot of green and yellow fruits and vegetables; limit salt, sugar and preserved or cured meats; keep any alcohol use moderate; keep all food intake moderate.

What's left to eat? A lot that tastes good. Also, I repeat, it won't kill us to eat most "bad" things once in a while. I caught one of the healthiest-looking, trimmest, exercise-and-diet conscious newswomen I know gobbling some junk food the other day. "Deadlines!" she explained.

A nice thing: if we follow most of these "12 Rules," we almost automatically follow the others. If we cut down on pastries to avoid sugar, we also eat fewer fats -- pastries are loaded with fats. If we eat the good starches -- scientifically, complex carbohydrates -- we don't have room for too much else. If we eat fresh fruits and vegeatables, we consume fewer fats and chemicals. In most of these ways, if we're sensible and eat slowly and take care to savor and appreciate rather than gulping what we do eat, we also consume fewer calories.

There is also recent evidence that most of us should add calcium to our diet -- women to avoid osteoporosis or bone degeneration in later years and everyone (if a new study is correct) to help prevent both colon cancer and hypertension (high blood pressure). Dairy foods are the most obvious way to add calcium. Most cheeses (except cottage cheese) have a lot of fat (and almost all are very salty). But most people can drink fat-free or low-fat milk, and most of the many who can't digest lactose (milk sugar) can drink milk spiked with lactase (the enzyme that digests lactose).

What about coffee? Or strong tea, which has almost as much caffeine? A Johns Hopkins study of more than 1,100 doctors recently found that those who drank five or more cups of coffee daily had a 2.8 times greater risk of heart disease than non-coffee users. Some other studies have found no such result.

One of the Johns Hopkins authors said he has cut coffee consumption to two cups a day as part of a "prudent life style." Dr. William Kannell, former director of the famed Framingham, Mass., heart study, said, "There's not enough consistent evidence to make that kind of recommendation," and he favors Mark Twain's advice for "moderation in all things, including moderation."

I'm now trying to switch from five or six to no more than three or sometimes four mugs of my usual black-as-sin tea daily.

What about vitamins and minerals? The right levels for health are a current topic of fierce debate at the National Academy of Sciences. Most of the scientists working in this field say: You get enough if you eat an adequate and varied daily diet with enough vegetables and fruits; if you don't eat like that, take a daily multi-vitamin supplement.

Most advise against megadoses of vitamins without good reason. Quite a few, while telling you that the evidence that vitamin C can help prevent colds is at best thin, quietly take some vitamin C. Exercise

Exercise is a current craze. But the movement still has not gone far enough. Average American weights are up in recent years and remain up.

This is partly because of improved nutrition and increased height. Also, however, we are far more sedentary than our grandparents. We drive instead of walking. We use machines for housework. More of us work sitting down. Less drudgery but more flab.

For good effects on the heart and lungs, the Public Health Service recommends exercise at least three times a week that boosts heart and lung performance to 60 percent or more of capacity for at least 20 minutes (some authorities say 30 minutes). By running or swimming or hard walking, for examples.

The PHS urged five years ago that 60 percent of Americans 18 to 64 try to meet that "aerobic exercise" minimum by 1990. Data thus far indicate that 80 to 90 percent of us still do not meet it.

Is hard aerobic exercise, the kind that makes you puff after a while, really necessary? Yes, if at all possible. A Veterans Administration scientist conducted what he called the first controlled study to compare aerobic with less strenuous exercise. Ten men with mild high blood pressure rode stationary bicycles, walked on treadmills, jogged slowly and walked briskly. Nine others did simple toe-touches, arm circling, stretching and the like.

Only the harder exercisers saw their blood pressure fall to normal within two months. The same happened to the lesser exercisers when they did the more strenuous stuff.

At the same time, doctors agree, all exercise is beneficial, even slow walking, even calisthenics from a chair for the chair-bound. Simple range-of-motion exercises at least keep muscles and tendons supple. In addition to building up the cardiovascular system with aerobic exercise, it's a good idea to do some stretches for flexibility and calisthenics to build strength.

Caring for the body also means not injuring it. Our indispensable motor vehicles both carry and imperil us. The most important preventives: wear seat and shoulder belts, helmets while motorcycling and bicycling, and drive at moderate speeds.

When seat belts first came out, I was sloppy about using one. Then I watched a plastic surgeon do his third or fourth of what would be 10 or 12 operations to try to repair the mutilated face of a girl whose head had gone through a windshield in an automobile collision. I started fastening my belts. Managing Stress and Relaxing

Accidents, work, marriage, divorce, adolescence, raising children loneliness, youth, aging. Every part of life has its stresses, and sometimes they seem to outweigh the rewards.

How to handle them? We've all heard the recipes: find work or other activities we enjoy; make time for them; especially make time for them at the end of the day to ensure adequate relaxation and rest.

Persons who have trouble relaxing can now learn behavior modification, meditation or relaxation techniques that have powerful effects in easing the mind and preparing mind and body for new assaults. A book I recommend, if you're in need, is "The Relaxation Response" by Harvard's Dr. Herbert Benson (Morrow, 1976), which teaches simple relaxation techniques. If you think you need to read more, he's also written "The Mind/Body Effect" (Simon & Schuster, 1979) and "Beyond the Relaxation Response" (Times Books, 1984).

Like exercise, relaxation also helps lower blood pressure. It also seems to lessen the need for the kind of compulsive, stressful eating that increases weight and burdens the body and heart.

Interestingly, all the things I've been talking about work together. One helps the other. Using Doctors to Help

We can't always be our body's only keeper. We need a sensible amount of medical care, and some checkups.

Doctors once almost universally recommended a "routine annual physical" for everyone as the cornerstone of "the best" medical care. No longer.

The annual physical for all, it has become clear, is too expensive and too time-consuming for the small number of real disorders it turns up -- especially problems or potential problems you can do something about. It doesn't help to know you have some possible "pre-condition" if there's nothing you can or need to do about it at that point.

But there are a few conditions that, caught early or in an incipient stage, may be arrested or controlled. One is cancer. Another is high blood pressure, a dangerously silent disease that often leads to strokes or heart disease.

Most doctors now recommend some tests yearly, some not as often and a complete physical every so often, depending on age, sex and other factors.

For example, Group Health Association, the Washington area prepayment medical plan, urges its 145,000 members to follow a set of "Health Assessment Guidelines" reprinted here. A "Health Assessment" is GHA's term for a complete physical, with lesser tests recommended at more frequent intervals. Other authorities' similar guidelines may differ slightly.

It should be added that some doctors are wary of even this much testing in healthy people, since many medical tests sometimes produce "false positive" results, which may then lead to possibly harmful surgery or other treatments.

I suggest: At least get periodic cancer and blood pressure tests. Then, if anyone wants to cut into or otherwise invade your body merely as the result of some "positive" test, be careful. Get a second opinion before any elective surgery. And ask for a reasoned discussion of the pros and cons and possible risks of any recommended invasive procedure.

To sum up, as valuable as doctors can be, don't live your life in a doctor's office. There are too many other ways to be healthy in 1986.