You have seen the advertisements: Eat anything you want and as much as you want! Revolutionary new diet plan just burns away the fat! No exercise required! One pill a day is all you need! Lose 30 pounds in 45 days!

It amazes me, but does not upset me, that this sort of offer does not seem to break state and interstate laws. What does upset me is the way that these offers violate the laws of physics and chemistry.

Human beings, as animals, are subject to the laws of biology. This does not, however, exempt us from other natural laws. For instance, you and your automobile have much in common. In order to function, you both need fuel. The fuel that you use happens to be called food, but it serves many of the same purposes as the gasoline that you feed your car. First, it provides the energy you or your car need to move around. In addition, it provides the energy to perform other subsidiary functions, which I will term "overhead." In the case of your car, overhead includes the energy needed to operate everything from the clock to the lights to the turn signals.

In the case of your body, the overhead functions are more diverse. First, your food/fuel must provide the heat to maintain a constant body temperature. Think of this as equivalent to your car's heating and air-conditioning. Second, your food must provide the energy needed to digest other food. Think of this as similar to the electrical energy needed to operate the car's spark plugs (unless you happen to drive a diesel).

Third, and something different in kind from the fuel that you feed your car, the food that you eat has to provide the materials with which your body repairs and maintains itself. It would be nice if your car could do this. You would put an additive in with the gasoline, and that additive would take care of the wear and tear on brakes and windshield wipers and tires, so you would never have to worry about replacing them or performing oil changes. In our food, energy comes mainly from fats and carbohydrates. Proteins provide both energy and the raw materials for body maintenance and growth.

Your body does one other thing with its fuel, and this one is an astonishing achievement. You combine your food with oxygen, which is to say you burn it. You are a specialized form of furnace. However, through the use of proteins called enzymes, your furnace operates at much lower temperatures than the one in your basement. This is just as well. Without the enzymes, your body temperature would be seven or eight hundred degrees, rather than a cool 98.6.

Comparing foods with other forms of fuel can be confusing, because one word is used with two different meanings. When we speak of a calorie in physics we are referring to a precisely defined unit of heat. It is, in fact, the amount of heat needed to raise one gram of water by one degree Celsius. However, this is a very small amount, so in biology and in nutrition the unit employed is a thousand times as big. Unfortunately, it is still called a calorie. In what follows we use the word to mean the big unit, the one used in nutrition.

It may seem like an odd question, but we can ask how much fuel of the conventional kind it would take to provide our bodies with their daily energy needs. The answer is, surprisingly little. Were we able to digest petroleum products, a pint of heating oil would provide about 4,400 calories. In solid fuels, a pound of anthracite coal gives 3,100 calories, a pound of oven coke about 2,700. A pound of dry oak wood (digestible by termites, and not by us) produces about 1,400 calories, a pint of 100 percent proof liquor (digestible by humans, and not by termites) about the same. A person sitting around and doing absolutely nothing needs maybe 1,500 calories a day just to survive. A person working in an underwater environment may consume 6,000 calories a day and still lose weight. A person who runs marathons will often eat 5,000 calories a day and remain as thin as a rail.

This brings us to the heart of the diet claims. Having taken in food with a given calorific value, how might we dispose of the energy that it would normally provide?

There are five possible ways. First, food can pass through you partly undigested, with its energy content not utilized. This is medically regarded as a bad thing, and unfortunates suffering ailments such as Crohn's disease or a spastic colon often have this problem. They eat lots of food that goes straight through them, and they remain malnourished.

Second, you can exercise hard, the equivalent of driving your car several hundred miles a day. This will allow your car to burn more gasoline, or you to burn more calories. This is the one your doctor will probably recommend.

Third, you can live in a cold climate, so that your body burns more energy just to keep you warm. I have not seen any diet plan that included a move to Antarctica, though that would seem an ethically acceptable answer.

Fourth, you could artificially raise your body temperature. This again is not recommended medically. It would be likely to reduce your life span more noticeably than your weight.

Fifth, and perhaps the most common though least desired solution: you can store any unused energy in the form of fat.

Unless the diet advertisements mentioned in the first paragraph recommend one of the first four methods (they are unlikely ever to mention the fifth), they cannot succeed. You are taking in fuel with a certain calorific content. Those calories have to go somewhere. Energy can be converted to heat, or to movement, or food/fuel can be rejected without its energy content ever being realized. Physics does not admit other options, and like it or not, we are all subject to the laws of physics.

Actually I can think of another diet plan, one almost guaranteed to let you lose weight. I hope that it has never been tried, although with today's dieting mania, who knows? Let us call it the "bionic diet." It falls into the "one pill a day is all you need" class.

The pill contains live tapeworm eggs.

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