An often-overlooked reward of cold-weather activities is that you can eat large quantities of food and not gain weight. It may be the only time in our diet-conscious society that you can, without excessive feelings of guilt, pig out.

The reason is fairly obvious. In cold weather your body uses calories not only to stay warm but also to make up for the calories burned in exercising. Skiing, for example, is one of the most calorie-demanding sports there is.

Also, for cold-weather activities -- skiing, hiking, skating or just plain sightseeing -- you need to drink large quantities of liquids. Your body loses fluids in a variety of ways:

Burning calories to stay warm requires water.

Dry air pulls moisture out of the body.

Deep breathing results in the loss of much water vapor through your lungs.

Modern winter clothing is so effective in keeping you warm, that you sweat more.

Therefore, to stay warm and healthy in cold weather you need three hefty meals a day, high-calorie snacks in between and several quarts of water. (Be sure to drink whether you are thirsty or not. Thirst is a poor and delayed indicator of your state of hydration, and you simply cannot drink too much. Electrolyte solutions such as GatorAde are fine but not necessary.)

Lest you think that skipping meals and snacks while skiing may be an easy way to shed a few pounds or a clever way to cram in an extra run or two -- lift fees being what they are -- it is not. Insufficient calories increase your susceptibility to hypothermia and, possibly, to frostbite, and cause fatigue. And fatigue results in poor judgment and accidents.

An insufficient fluid intake causes the same problems.

The number of calories you need depends on a host of factors: your weight and size, the outdoor temperature, your degree of exertion, even the altitude at which you are.

But unless you are climbing Mount Everest or training for the Olympic cross-country ski team, there is no need to calculate caloric intake. Under average conditions, for six hours of outdoor activities, for example, your caloric requirements are probably about twice what you need for a routine day of office work or, very roughly, 3,000 to 5,000 calories per day.

While hot meals give many people an important psychological lift, they are, in fact, only marginally better for the body than cold food. And hot beverages give the same psychological lifts but do not make you warmer. So don't spend time and energy (calories) to heat fluids if only cold ones are readily available.

Most experts recommend that for days spent outdoors you maintain a diet consisting of carbohydrates, fats and proteins in about a 3-1-1 ratio. At mealtime, bread, pasta and cereal are good sources of carbohydrates. A milk product -- milk, yogurt, cottage cheese, ice cream -- supplies the fat. Meat, eggs or beans supply protein.

Snacks too should supply ready energy and therefore should be high in complex carbohydrates. Some old-timers recommend constant nibbling -- sucking on hard candies, for example. But hard candies are simple carbohydrates, sugars, and are apparently not as efficient. And there is no evidence that constant eating is any better than occasional, every hour or two, snacking.

Many people prefer snacks of what hikers call gorp -- the various mixtures of raisins, dried fruits, chocolate and nuts that are readily available at supermarkets. (Or you can, of course, make your own mixture.) These mixtures supply the carbohydrates (raisins, fruits), proteins (nuts) and fats (chocolate) you need. Such mixtures and candy bars yield about 100 calories per ounce.

Many health food stores and some outdoor magazines recommend vitamin supplements for winter activities to help keep you warm and healthy. The consensus of nutritionists is that vitamins outside your food are not necessary and that the so-called mega-vitamin formulations for winter activities -- or for any other activities -- may be dangerous.

Stay away from alcohol and drugs. These seem to predispose one to hypothermia.

Also stay away from snow and ice. Even when handy, snow and ice are inefficient ways to get water. A mouthful of snow yields only a few drops of water while using many calories to melt the snow. Ice is not much better. And snow and ice, just like any natural water, may be contaminated. Dr. Karl Neumann is a pediatrician in Forest Hills, N.Y., who writes about travel and health.