I know that a calorie is a unit of heat, but why does eating heat make me fat?

A calorie is a much broader (if you'll pardon the expression) concept than just heat. A calorie is an amount of any kind of energy. We could measure the energy of a speeding Mack truck in calories, if we wanted to.

Energy is whatever makes things happen; call it "oomph" if you wish. It comes in many forms: physical motion (think Mack truck), chemical energy (think dynamite), nuclear energy (think reactor), electrical energy (think battery), gravitational energy (think waterfall) and yes, the most common form of all, heat.

It's not heat that's your enemy; it's energy--the amount of energy-for- living that your body gets by metabolizing food. And if metabolizing that cheesecake produces more energy than you use up in walking to the TV couch, your body will store the excess as fat. Fat is a concentrated storehouse of energy, because it has the potential of giving off lots of heat when burned. But don't jump to any conclusions. When an advertisement promises to "burn off fat," it's only a metaphor; a blow torch is not a feasible weight-loss device.

A calorie is . . .

How much energy is a calorie, and why do different foods "contain" (that is, produce) various numbers of calories when metabolized?

Since heat is the most common and familiar form of energy, the calorie is defined in terms of heat--how much heat it takes to raise the temperature of water. Specifically, as the term is used by nutritionists (chemists use it differently), it's the amount of heat it takes to raise the temperature of a kilogram of water by 1 degree Celsius. For us Americans, who insist upon clinging to the ancient and clumsy English system of measurement (in the entire world, only the U.S. and three other great powers--Brunei, Burma and Yemen--continue to use the English system), a calorie is the amount of heat it takes to raise the temperature of about 2 1/2 cups of water by 1 degree Fahrenheit. For example, if you want to bring 2 1/2 cups of water from 70 degrees up to boiling temperature (212 degrees), you would need to add 212 less 70, or 142 calories of heat.

Different foods, as everyone knows, provide us with different amounts of food energy. The calorie contents of foods were originally found by actually burning them in an oxygen-filled container immersed in water and measuring by how many degrees the water was heated. (The apparatus is called a calorimeter.) You could do the same thing with a serving of apple pie to find out how many calories it releases.

But is the amount of energy released when pie is burned in a laboratory the same as the amount of energy released when it is metabolized in the body? Remarkably, it is, even though metabolism releases its energy much more slowly, and mercifully without flames (heartburn doesn't count). The overall chemical results are exactly the same: Food plus oxygen produces energy, and it's a principle of chemistry that if the initial and final substances are the same, then the energy change is the same, regardless of how the reaction took place. The only problem is that foods aren't digested and oxidized completely in the body, so we actually get out of them somewhat less than their total energy content.

On the average, we wind up getting 9 calories per gram out of fat and 4 calories per gram each from both proteins and carbohydrates. So instead of running into the lab and setting fire to every food in sight, nutritionists these days just add up the numbers of grams of fat, protein and carbohydrate in a serving and multiply by 9 or 4.

How many calories?

Your normal basal metabolism rate--the minimum amount of energy you use up just by breathing, circulating your blood, digesting your food, repairing your tissues, keeping your body temperature normal and keeping your liver and kidneys, etc., doing their jobs--is about 1 calorie per hour for every kilogram (2.2 pounds) that you weigh. That's about 1,600 calories per day for a 150-pound male. But that can vary quite a bit depending on sex (women about 10 percent less), age, health, body size and shape, and so on.

Your bottom line (again, please excuse the expression) is how your intake of food energy above and beyond your basal metabolism rate compares with your expenditure of energy by exercise, not counting fork-lifting. For an average healthy adult, the National Academy of Sciences recommends a daily intake of 2,700 calories for men and 2,000 for women--more for jocks and fewer for couch potatoes.

One fallacy that I've heard is that drinking ice water will help you lose weight because calories are expended in warming the water up to body temperature. Maybe true, but trivial. Warming 8 ounces of ice water up to body temperature uses only 8.75 calories. If dieting were that simple, "fat-farm" spas would have ice-water swimming pools. (Shivering also uses up energy.)

Finally, what about the often- heard conundrum, "I eat a pound of fudge and I swear I gain four pounds. Doesn't that violate some law of physics?"

Unfortunately, there is no Law of Conservation of Mass for fudge and fat. Yes, the moment after you consumed the fudge you weighed 1 pound more. But then the fudge was processed by your metabolism machine, and everybody's machine is different. Some are more efficient at metabolizing and/or storing fat than others. The real questions are, How many calories were in that box of fudge, and How much did they boost your caloric intake above your basal metabolism needs? And did you run a marathon that day, or just watch one on TV?

I've heard people say that when they eat a pound of candy it goes straight to their hips, and that they might as well apply it directly. Depending on their metabolism, they may be better off applying it directly and gaining one pound, rather than eating it and gaining four.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh. His latest book, "What Einstein Didn't Know--Scientific Answers to Everyday Questions," will be published this month in paperback by Dell. Send your food or cooking question to the Food Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or e-mail it to wolke@pop.pitt.edu. Individual responses cannot be made, but we will try to address your questions in this column.

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