Nobody has ever seen a calorie. But calories are agonizingly visible when they settle down on you in the wrong places. It may seem strange that a thing which has no physical existence can make you fat. A square inch has no flavor and neither has a calorie. You can't touch a calorie any more than you can put your finger on a heat wave. It is as invisible as a fever. A calorie, in short, is merely a measuring unit of energy in terms of heat. It is a thermal yardstick applied to the exchanges of energy in your body and to the foods that provide such energy.

In a furnace, coal gives off heat which could be measured in calories but is usually calculated according to a different yardstick called British Thermal Units. In you, food gives off heat measured in calories, one calorie being the amount of heat required to raise the temperature of a pint of water 4 degrees Fahrenheit.Now you may promptly forget that definition. Weight control arithmetic is so simple that it is understood as easily as Mother Goose.

Coal, if mixed with slate, does not give off very much heat. Some foods contain much water, inert fiber, and other unburnable elements so that they do not yield much heat either. You cannot judge, from the mere physical bulk of a meal, how much energy value it contains.

It is possible to eat small-size meals and grow in-gloriously fat on them. It is also possible to eat meals of huge bulk and grow thin as a rail. The differences are matters of calories.

All calories are alike, but there are three different kinds. That sounds like a paradox. Let's be scientific, then, and explain that although any calorie represents the same amount of heat, it can be furnished by three very different types of food elements: carbohydrates, proteins, and fats. And oh, the difference to you! The modern way to weight control lays great stress on these differences. Mere calorie-counting is no longer enough. One kind of calorie is far and away more efficient as a weight-reducer than the other two kinds.

Carbohydrates is a resounding fifty-cent word that sounds a lot less formidable when you think of these food elements as being sugar and starch, which they are. Fruits, vegetables, cereals and sweets contain them in relative abundance.

They are the quickest food-fuels for muscular energy. When carbohydrate is digested, a simple form of sugar is poured into the blood. If you think that sugar in the blood (or even a little bit in the urine) is bad business and means you are coming down with diabetes, be advised that you can't get along without it. It is indispensable for muscular exertion, as marathon athletes who munch chocolate bars for quick energy have long since learned.

But your blood can hold only a limited amount of sugar. A considerable proportion of the surplus is tucked away, mostly in the liver but also in the muscles, in the form of glycogen a starch-like substance that can be drawn upon quickly for energy. When the liver storehouse is fully loaded, surplus carbohydrate is simply laid down the truth may as well be known as fat.

Most of us hold carbohydrates in high gustatorial esteem. A "sweet tooth" is common among the over weight. Alcoholics who are on the wagon exhibit a pronounced craving for candy as a substitute for the cup that cheers. There is also such a thing as a starch craving, often shown in the form of an exaggerated consumption of breads, pastries, and potatoes. Dr. Hugh Rony has reported the case of an obese laundress who had no liking whatever for sugars but who did manage to consume one pound of laundry starch a day!