Food and physical activity

It’s that time of year again

You’ve just indulged in holiday cheer and food; friends and food; family and food; food and more food! And now the holiday fun is over and you’ve gained more weight than expected.

This month, I’ll explore how calories are measured and discuss how you can best balance calorie intake with calorie expenditure.

Your body extracts the energy trapped within carbohydrate, fat (correct name: lipid), and protein nutrients you consume via processes that power all biological work in your system. This includes muscle action, nerve transmission, and digestive action, as well as lung, heart, and kidney activity, plus all cell function throughout the body.

Since all bodily functions require and produce energy, both food and physical activity can be evaluated by one common denominator: energy.

What is a calorie?

A calorie represents a unit of heat transfer expressed in energy units. Scientists have determined that a calorie, abbreviated kcal for kilogram-calorie, represents the amount of heat required to increase the temperature of one kilogram of water (slightly more than one quart) by 1 degree Celsius.

For example, one McDonald’s Big Mac (7.6 oz. = 550 kcal) and medium fries (4.1 oz. = 380 kcal) contain about 930 kcal. Technically, this number of calories refers to the energy required to raise the temperature of about 930 quarts or 232.5 gallons of water 1 degree Celsius.

For convenience, we just say, “these foods contain 930 total calories.”

This 930 kcal represents the heat energy stored within the molecular bonds of the carbohydrate, lipid, and protein molecules that form these foods. When we speak of food calories or kcals, we are indirectly acknowledging that 930 kcals also equals the food’s total heat energy content.

How calories are measured

A heat-measuring instrument called a bomb calorimeter directly determines the amount of heat released when a food literally burns in the device shown here.

Graphic of a calorimeter.

To “burn” the food, an electric current ignites a fuse inside the chamber filled with oxygen. This causes the food to release all of its energy. A container of water that surrounds the calorimeter absorbs the heat released from the “exploded” food. A sensitive thermometer records the increase in water temperature to provide a precise measure of the amount of heat liberated. For example, when a four-inch sector of apple pie weighing 4.7 ounces (135 grams) completely burns in the calorimeter, it releases 350 kcal of heat energy. When we think of the calorie value of the pie, we say it contains 350 kcal.

Calories in foods

Foods generally consist of a mixture of three major macronutrients: carbohydrates, lipids, and proteins. The burning of one gram of pure carbohydrate in the bomb calorimeter produces, on average, 4.2 kcal, one gram of pure protein releases, on average, 5.65 kcal, and one gram of pure lipid yields, on average, 9.5 kcal.

Because the foods we consume contain different proportions of these three nutrients, the caloric value for a given food, such as apple pie and vanilla ice cream, is determined by the amount of carbohydrate, fat, and protein in an average serving. Thus, a food containing a large amount of lipid has a greater total energy (contains more calories) than a relatively fat-free food of the same weight. For example, one cup of whole milk contains 160 kcal, whereas one cup of skimmed, non-fat milk contains 44 percent fewer fat calories for a total of only 90 kcals (with all the other ingredients being the same).

When one gram of carbohydrate or fat “burns” or metabolizes in the body’s trillions of cells, it yields a slightly lower kcal value because of the body’s inefficiency in digesting and converting all of the stored potential energy in the nutrient to useful energy for the cell.

In the body, the average kcal yield for burning carbohydrate and protein is 4 kcal, while it is 9 kcal for lipid.

1 gram carbs = 4 calories

1 gram protein = 4 calories

1 gram lipid = 9 calories

Of note: foods with identical kcal values can vary greatly in quantity and/or weight. For example, if you consume one tablespoon of mayonnaise (equal to about 100 kcals), you would have to consume the equivalent of five carrots, 20 stalks of celery, seven green peppers, or one large grapefruit to consume 100 kcals of these foods. The mayonnaise has a high concentration of lipid, which explains why only a small amount yields such a large energy content, and why you have to consume much more of the other foods to consume 100 kcal (because they are essentially free of lipid).

Calories produced by the body

A whole-body, direct human calorimeter, similar in principle to the bomb calorimeter, measures the amount of energy expended by humans throughout the day. This includes a well-insulated, airtight chamber with an oxygen supply and sophisticated electronics so a person can live and work for extended time periods. Below is an illustration of a direct human calorimeter.

Graphic of a person burning calories.

A stream of cold water flowing at a constant rate through tubes coiled at the ceiling of the chamber removes the heat produced and radiated by the person inside the calorimeter. A temperature difference in the water entering and leaving the chamber reflects the person’s heat production (i.e., caloric production).

All energy-producing chemical reactions in the body depend on a continual use of oxygen; thus oxygen uptake by all of the body’s cells reflects energy expenditure.

For each one liter of oxygen consumed in the burning of a mixture of carbohydrate, lipid, and protein molecules during human energy metabolism, five kcals of heat energy is liberated. By measuring oxygen uptake in the laboratory during rest and different modes of physical activity (from archery to yoga), scientists accurately estimate the body’s total energy production or calorie expenditure.

How to use this information

It is possible to establish a caloric balance between energy input (food consumption) and energy expenditure (physical activity). This forms the first step of a successful weight control program.

The first step involves keeping track of all caloric intake and caloric expenditure. There are many excellent web pages (and books) that provide tables for both food and physical activity calories. Simply search for activity calorie counter and food calorie counter.

By keeping track of this data, you can take small but valuable steps to successfully manipulate your caloric intake and output.


  1. W. James Foster

    Excellent article, Professor Katch. Most articles today are written for commercial reasons to sell books or convince you to participate in some nutritionally unsound diet rather than to provide accurate and useful information regarding nutrition and metabolism. As a physician as well as someone actively engaged in the battle of the bulge, I have both a scientific curiosity and a practical need to better understand nutrition. Do you know of a well written book on this subject that would appeal to someone who understands the basic concepts but just hasn’t read about them for 43 years?


  2. Arthur Floor - classs of 44 grad 48

    It seems that the standard method of calculating calories does not take into account the variability of the digestive process. By strict following of bomb calorimeter results one could conclude that coal is a great source of food energy. Is there work in progress to address this?


  3. Denise Baron Parr - 1974

    Reading your article took me back a few years to your undergraduate exercise physiology class. I’m happy to say I have shared your wisdom with thousands of students, and have hopefully made an impact in reducing childhood obesity. I am considering retirement to spend more time with my grandchildren, but still am conflicted for I love being a teacher. Do you remember me?


  4. Victor Katch

    Dr. Foster, thanks for your question and kind comments. I’m glad you like my Michigan Today articles. Regarding a book for you, without trying to be self-serving, I might recommend one of our textbooks: Undergrad Introductory Text: “Essentials of Exercise Physiology.” Fourth Edition, 2011. Authors: V. Katch, W.D. McArdle, F.I. Katch. (Lippincott Williams & Wilkins. ISBN: 978-1-60831-267-2); Upper Level Undergrad/Graduate Level: (this book was awarded first prize in the Physiology/Medicine Textbook competition by the prestigious British Medical Association, 2002): “Exercise Physiology: Nutrition, Energy, and Human Performance.” Seventh Edition, 2010. Authors: W.D. McArdle, F.I. Katch, V. Katch. (Lippincott Williams & Wilkins. ISBN: 978-0-7817-9781-8); Undergraduate Text: “Sports Nutrition.” Fourth Edition, 2013. Authors: W.D. McArdle, F.I. Katch, V. Katch. (Lippincott Williams & Wilkins. ISBN: 978-1-4511-1806-3).


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