We spend a lot of energy on calories — counting them, avoiding them, feeling guilty about them. But what are calories, anyway? Well, they ARE energy. Specifically, calories are the energy we get from the food we consume. And that makes them a good and essential thing. Calories wouldn’t be a problem if we consumed them and then used them through physical activity. But when we don’t use as many calories as we consume, they accumulate and we gain weight.
When we count calories, we’re really counting how much energy we get from our food. One Twinkie has 150 calories. That’s the supply side. On the demand side, calories measure how much energy it takes to do things. If you raise a small apple one meter (39 inches), you’ll burn 2.4 calories. You’d have to raise an apple over 62 times to work off one Twinkie (and they usually come in packages of two).
It’s actually a bit more complicated than that because we burn calories just by being alive. It takes energy — calories — to breathe, circulate the blood, and heat or cool the body to 98.6° F. This is called the basal metabolic rate.
Something for nothing
Even if you do nothing more than lie on the couch, you’re using energy, burning calories. And your muscles are using more calories than your fat. Muscles are almost always slightly contracted, and that requires energy. It’s called muscle tone or residual muscle tension. Since men’s bodies tend to have a higher ratio of muscle to fat than women’s bodies, men generally have the advantage of a higher basal metabolic rate (BMR.) Pound for pound, men burn more calories just by being alive.
Even if two people have the same ratio of muscle to fat, they could have different BMRs. Some people simply have faster or slower basic metabolisms. If you go on a low calorie diet that doesn’t include enough essential nutrients, your metabolic rate will drop. This conserves energy in times of famine, but makes it harder to lose weight if you’re dieting voluntarily.
Your metabolic rate is directly related to how much you weigh. That makes sense, since a larger body has to circulate blood through more cubic inches and has to heat or cool a larger mass. BMR also depends on your age – metabolic rate decreases with age. As a rough estimate, the BMR for a 155-pound man between the ages of 30 and 60 would be 1691 calories. For a 132 pound woman it would be 1351 calories. (Details below.)
Calories in, calories out
We also burn calories by digesting the food we eat. Food needs to be broken down before it can be used, and that takes energy. This is called the thermic effect of food. As a general estimate, the thermic effect uses about 10% of the calories we take in just to put those calories to work in the first place.
So, if our 155-pound man ate 3000 calories in a day, the combination of BMR and food digestion would use 1991 calories. In order not to gain weight, he would need to burn off the difference (1009 calories) through activity. For example, he could burn 1000 calories in 2 hours of jogging, 3.6 hours of golf, 4 hours walking the dog, 4.7 hours bowling, 5.6 hours of slow ballroom dancing, or 6.8 hours of light household cleaning. Who has time for that? To avoid gaining weight, he would need to eat fewer calories in excess of the requirements of his metabolic rate and digestion.
Over the last few decades, Americans have been eating more not fewer calories, and now we find ourselves in an “obesity epidemic.” It doesn’t take much to gain weight. In 1995, women reported eating 143 more calories per day than they did in 1977. It takes an extra 3500 calories to add a pound. So if activity level remained the same, those additional 143 daily calories would add 24 pounds in a year. But if you add a daily activity that uses 143 calories, you can maintain the same weight. A 132-pound woman can burn 143 calories in 36 minutes of brisk walking, 29 minutes of low-impact aerobics, 18 minutes of swimming, or 15 minutes on a ski machine.
To avoid gaining weight, it’s important to make calories count and not just count calories. Empty calorie foods, like french fries, candy bars, and alcohol, have plenty of calories, but they’re “empty” of nutrients, such as vitamins, minerals, antioxidants, and amino acids. The food industry likes to call these foods energy dense. It puts a nice spin on a Butterfinger. Soft drinks — colored water flavored with high fructose corn syrup – are energy dense, but the calories are empty.
The technical stuff
What I’ve been calling a calorie is technically a Calorie, with a capital C, or a Kilo-calorie (1000 calories). Here in the U.S. we don’t bother with the distinction. We use calorie to mean K-cal or Calorie when referring to food energy. The term calorie was originally used in physics and chemistry to measure energy, but the preferred term in science today is joule. A calorie (small c) or 4.1858 joules is the amount of energy required to raise a gram of water (at 15° C) one degree Centigrade. If you buy food in a country that uses the highly sensible metric system, the label will indicate Kcal. It might also list kilo-joules, as does this label from Southeast Asia.
Precise methods to determine basal metabolic rate will vary, but here are the numbers I used in my calculation.
For a 155-pound (70 kilogram) man, BMR = 879 + 11.6 * 70 = 1691 calories.
For a 132-pound (60 kilogram) woman, BMR = 829 + 8.7 * 60 = 1351 calories.
Here’s how I got 1991 calories when I added the thermal effect:
BMR calories + 10% of 3000 food calories = Total calories
1691 + 300 = 1991
For calories burned during exercise, I used a table at NutriStrategy. They offer software that will make these calculations for you.
The coefficients I used for calculating BMR are from Schofield, Schofield and James, 1985, “Basal Metabolic Rate – Review and Prediction together with Annotated Bibliography of Source Material,” Human Nutrition: Clinical Nutrition, Supplement 1, as quoted in David M. Cutler, Edward L. Glaeser and Jesse M Shapiro, Why Have Americans Become More Obese?, Journal of Economic Perspectives, Vo. 17, No. 3, Summer 2004, pages 93-118. (PDF)
The statistic of 143 more calories consumed by women in 1995 than in 1977 is from the USDA’s Continuing Survey of Food Intake 1977-1978 and 1994-1996, also taken from Why Have Americans Become More Obese? (PDF)