Counting Calories

by Paul Doherty

From Exploring Food Magazine Vol. 14 #4 1990

In my undergraduate biophysics physics course at MIT, Professor George Benedek burned a peanut. That may not sound impressive, but it was. Professor Benedek stood in the front of a small 50 seat lecture hall. He was a middle age man who had the build of a swimmer under a tweed suit, and he always wore white socks. He held the peanut in a loop of wire made from a bent paper clip and held the bent paper clip in a pair of pliers. He positioned the peanut under a test tube which contained ten grams of water.

Beneath the peanut was a large pan filled with water. A very large fire extinguisher stood on the floor nearby. I thought the fire extinguisher was excessive for a single peanut. For that matter, so was the pan of water.

Then professor Benedek set the peanut on fire. The peanut burned, and burned, and burned, and then burned some more. Drops of flaming oil oozed from the nut and dripped into the pan of water. The water in the test tube started to boil. When the peanut finally burned out, there were only eight grams of water left. Not only had the peanut heated the water from room temperature to 100 degrees Celsius, it had also boiled away two grams of water.

Heat flowed from that burning peanut as combustion converted the hidden chemical energy stored in the nut into the easily measured energy of heat flow. When you eat a peanut, your body does the same sort of thing: it converts the energy stored in the peanut into the energy it needs to keep running. As professor Benedek's demonstration showed, a little bit of food stores a great deal of energy in its chemical bonds.


More energy than a stick of Dynamite

Physicists and dieters are both concerned with the energy contained in food. And they both count calories.

Physicists measure the energy content of food by burning the food. To a physicist, a calorie is the heat flow needed to raise the temperature of one gram of water by one degree Celsius. After burning that peanut, professor Benedek turned to the blackboard and calculated the calories that the peanut had produced. The burning peanut warmed ten grams of water from tap water temperature, 20 degrees Celsius, to boiling, 100 degrees Celsius&emdash; a temperature increase of 80 degrees Celsius. This temperature increase required 800 calories of heat flow. The heat flowing from the peanut then boiled away two grams of water, which took 1080 calories more, since 540 calories are needed to boil a gram. All in all, one burning peanut delivered 1880 calories to the test tube of water.

A single peanut contains 1880 calories? Those of you who know about food calories may be shocked by this figure. After all, an entire lunch doesn't contain 1800 food Calories. The explanation lies in the capital C. One food Calorie, spelled with a capital C, is 1000 times larger than one physicist's calorie, spelled with a small c. A peanut actually contains 1.8 food Calories.

Having the same name for two different units that differ in size by a thousand is one of the less logical parts of the language of science. From now on, when I talk about Calories, or calories, watch for that capital C.

To measure the Calorie content of food accurately, scientists use a bomb. A bomb calorimeter to be precise. The food sample to be measured is dried and ground into a powder. Then it is placed into the bomb calorimeter, a strong metal container surrounded by a water bath. The metal container is pumped full of pure oxygen at 30 atmospheres pressure and the food is ignited. The resulting energy release is fast and violent &emdash; just like a bomb. The steel container holds in the explosion. In air, not all of the peanut burns, but this isn't a problem in the bomb. Pure oxygen promotes combustion, and high-pressure oxygen greatly enhances combustion. All of the burnable parts of a dried and powdered peanut will burn in a calorimeter, leaving just a touch of ash. The calorimeter turns the energy stored in the peanut into heat flow. The temperature increase of the water and metal of the calorimeter reveals how many calories the food contained.

Since food energy and heat flow are both forms of energy, they can be measured in the same unit: the calorie. In the metric system, however, the unit of energy is neither the calorie nor the Calorie; it is the joule. For a physicist, the benefit of converting calories to joules is that work and all other forms of energy are also measured in joules. When food energy is measured in joules, we can estimate immediately how much work can be extracted from the food. Or conversely, we can estimate how much work it takes to burn off the food energy.

For example, one food Calorie can be converted into enough work to lift an adult human two stories into the air. Professor Benedek would have calculated this as follows: one Calorie equals 1000 calories, which converts to 4200 joules. One joule of energy will lift a tenth of a kilogram, (a quarter pound, though my professors never used pounds) one meter in the air. So one Calorie or 4200 joules will lift a 70 kilogram (155 pound) person six meters into the air. To work off the 1.8 Calories from one peanut a dieter will have to climb to the top of a four-story high building.

Calorimetry reveals that a Milky Way® candy bar contains more energy than a stick of dynamite. The candy bar contains 200 food Calories. That's 200,000 physicist calories or about 840,000 joules! Nearly a megajoule! A megajoule of energy from a candybar can perform enough work to lift an average 70-kilogram human being 1200 meters in the air. That's higher than the cliff face of Yosemite's El Capitan. No stick of dynamite can do that! In fact,an ounce of dynamite produces only one-quarter as many calories when it explodes as an ounce of sugar does when it burns.

Of course, the body cannot convert all of the energy from one candybar into work. A lot of the energy goes into heat, and some of the work goes into digestion. Overall, less than 20 percent of the energy contained in food can be converted into work. (So don't try to climb El Capitan armed with only one candybar.)

Some countries that use the metric system have completely abandoned the Calorie as a measure of food energy. In Australia, for example, diet soda sports the label "Low joule Cola." In the United States advertisers can claim a can of diet soda has "less than one Calorie." In Australia the equivalent statement just doesn't sound as impressive: "less than 5000 joules." On the other hand, candy advertisements in Australia can say that you'll "get a megajoule jolt from our candybar!"


Burning peanuts in your body

When a peanut or a candybar burns, chemical bonds in their molecules break under the high temperatures of combustion, and then combine with oxygen from the air in a reaction called oxidation. In this reaction, fats and carbohydrates are converted to carbon dioxide( CO2) and water( H2O). The oxygen bonds in these compounds have lower energy than the original bonds holding together the fat and carbohydrate molecules. The difference in energy is released into the air as heat flow.

Your body also oxidizes the peanut to release energy and produce carbon dioxide and water. Your body does not burn the peanut&emdash; although, if the peanut was part of a hot and spicy kung pao dish you might feel like it was flaming away in your stomach. But the peanut releases the same amount of energy when it is oxidized in the body as when it is burned. Your body oxidizes the peanut with a low temperature process which is more complicated than the reactions that occur in a fire.

Consider how a muscle in your body gets the energy it needs to contract. Your body begins by breaking the peanut into chemical components during digestion. Next, these components are reassembled into other chemicals, such as glucose, a simple sugar. The circulatory system carries this glucose to the muscle. The lungs take in the oxygen, and hemoglobin transports it via the circulatory system to the muscle. Finally, the oxygen combines with the glucose, producing energy without a flame. The overall reaction is

glucose (C6H12O6) + 6O2 = 6CO2 + 6H2O + energy.

Part of this energy is released as heat which helps a warm blooded animal stay warm. Most of the rest is used to energize molecules of Adenosine triphosphate, or ATP. The energy released by oxidizing one glucose molecule energizes 36 molecules of ATP. The ATP molecules power your muscles. One ATP molecule is used each time a muscle fiber ratchets one step shorter.

Peanuts &emdash;as any dieter could tell you&emdash; are high in Calories. To figure out why some foods are so high in calories, you need to look at the chemicals from which they are made. A peanut is 48 percent fat, 26 percent protein, 16 percent digestible carbohydrates, 2 percent indigestible carbohydrates (like cellulose), 6 percent water, and 2 percent ash.

Calorimeter studies show that all dried carbohydrates and proteins produce close to the same amount of energy: about 100 Calories per ounce or 4.2 Calories per gram. So a 2-ounce carbohydrate-loaded candybar contains 200 Calories. Fats, on the other hand, contain a little more than double the number of calories per ounce as carbohydrates and proteins: 220 Calories per ounce or 9.3 Calories per gram. Peanuts are a high calorie food because they are nearly 50 percent fat. For comparison, celery is low in calories; it contains 94 percent water, no fat, 1 percent protein, 4 percent carbohydrate and 1 percent indigestible fiber.

A peanut has a mass of about one gram. Using the above figures we can calculate that a peanut should produce about 6.3 Calories when it is burned. Yet Professor Benedek measured only 1.8 Calories. In the classroom demonstration, a great deal of energy from the burning peanut was lost. This energy would not be lost in a calorimeter.

Of course, there is more to food than just its Calorie content. The body requires a balanced diet. For example, over month long intervals, the body needs a full range of amino acids to build new proteins so the body craves variety in its foods. I found this out on a winter backpacking trip. I wanted to reduce my weight to the minimum, so I decided to carry almost pure fat as food. After all, each ounce of fat contained twice the calories as an ounce of carbohydrates. So I combined butter with a little powdered sugar until it tasted like frosting. Needless to say, after three days of eating buttery frosting on the trail I couldn't face a birthday cake for years.

Dieters often forget that the alcohol in drinks or ethanol (C2H5OH) is a source of food energy. If you burn pure alcohol, it produces about 150 Calories per ounce or 7 calories per gram &emdash; about half way between the Calorie content of carbohydrates and fats. A one-ounce shot of 100-proof alcohol is half water, yet it still contains 75 calories, almost as much energy as an ounce of sugar.


The Dieters Law

All of this brings us to one of the most important laws of nature&emdash;the law of conservation of energy, also known as the first law of thermodynamics. This is a law that anyone on a diet should know. It says that energy cannot be created or destroyed, only converted from one form to another. When you eat kung pao peanuts for lunch, the energy of those peanuts will be converted into another form. You can use the food energy to do work, or it can flow out of you as heat. If neither of these things happen, the energy will be added to your body as protein, carbohydrate, or body fat.

Your body stores the fat as an emergency food supply to carry it through times of famine. Once you've stored the energy it's difficult to get rid of the fat. Your body uses its fat reserves only when it is forced to do so&emdash; only after your body has used up its other food stores.

Using up fat reserves is also difficult because fat stores so much energy: 220 Calories per ounce. One pound of body fat will provide a person with 3500 Calories ( nearly 15 megajoules). If you didn't eat anything at all for nearly two days of normal activity, your body could power itself by using up just one pound of stored fat. Losing one pound through exercise requires working hard&emdash;playing soccer for 8 hours would burn up only one pound of fat. This is why rapid weight loss, more than 1/2 pound a day, is almost always the result of changes in the water content of the body. That weight can be added back just as fast as you lost it.

The law of conservation of energy raises a question: where did the energy in the peanut come from in the first place? When I was in grade school in the early 1950's, my teacher told me that all of our energy comes from the sun. She was right, mostly. Most of our energy, particularly our food energy, still comes from the sun. Food energy begins its trip up the food chain when light falls on a green plant. The plant stores the energy of sunlight in high-energy chemical bonds. Today, however, some plants are grown under artificial lighting, which is powered in part by nuclear power plants and by geothermal power. Some of the energy stored in the food we eat comes from the nuclear energy stored by exploding supernovas billions of years ago.

Nowadays, I'm a professor myself. If you visit my class when I'm teaching about food energy, you'll see a middle-aged professor with the build of a mountain climber, dressed in comfortable cotton shirt and pants. In my hand there will be a pair of pliers holding a burning peanut.

Going Further

Learn how to do the burning peanut experiment safely on my Burning Peanut web page.


Handbook of the Nutritional Content of Food. Bernice K. Watt, Dover Books, New York, 1975

This classic text breaks down foods into their fat, protein, and carbohydrate constituents and lists the calorie content per 100 grams and per pound,It also lists vitamins and minerals contained in foods and summarizes food analysis techniques.

Scientific Explorations with Paul Doherty

© 2000

19 October 2000