What is the Role of Calorie Intake in Weight Management?

The relationship between calorie intake and weight management has always been a controversial one. Within the general U.S. public, the number of people who have tried calorie counting as a means of weight loss—and failed—is surely a very high number. In some ways it is similar to trying to decrease your cholesterol levels— eating less cholesterol-containing foods does not guarantee your blood cholesterol levels will decrease.

It should not be surprising that calorie counting has always been a topic of controversy in the field of nutrition. The concept of a “calorie” is a one that is difficult to understand. When organizations like the American Heart Association, American Diabetes Association, and American Dietetic Association all endorse approaches to weight loss that focus, in part, on calorie counting and calorie intake, it is not surprising that these public health recommendations are frequently misunderstood or misinterpreted.

What Is a Calorie?

In simplest terms, a calorie isn’t any kind of “thing” whatsoever. Calories are not like proteins, or carbohydrates, or vitamins, or any kind of nutrient. You can find protein in food. You can find vitamins in food. Yet, you cannot find a calorie in any food at all. Calories do not exist in that way.

Calories are units of measurement. They are like inches, miles, ounces, degrees of temperature, pounds, tons, gallons, and acres. They are just a way of understanding how much of something is present. In the case of calories, this something is energy. The amount of energy associated with any set of events can be measured in terms of calories. Calories don’t have to involve food. For example, there are a specific number of calories that any electrical wire can carry without catching fire. There are a specific number of calories that strike the earth each day in the form of sunlight. Calories are not found in food. They are only related to food insofar as food has the potential to be measured as a form of energy.

Can Food Calories Be Accurately Measured?

Hundreds of Internet website post lists of foods and calories. The U.S. Department of Agriculture publishes a searchable online database (http://www.nal.usda.gov/fnic/foodcomp/search/) with calorie information on thousands of food.

Is the information provided by the USDA and other websites accurate? Unfortunately, the answer is both yes and no. Yes, there are solid scientific studies using real foods and real laboratory conditions to support the specific calorie numbers that appear in the USDA database and in other published lists of food and calories. This research can be very high quality, sophisticated, and scientifically sound. But it is research based on laboratory analysis—not research based on the passage of real food through a person’s digestive tract. Unless food gets digested, it cannot provide us with any calories (energy).

When food calories are measured in a lab, a device called a bomb calorimeter is used. This device measures energy in the form of heat. Within this device, a highly oxygenated, sealed chamber containing a food sample is floated in water. An electrical current is used to ignite the food-oxygen mixture, and as it burns, the water surrounding the floating chamber heats up. The number of calories in the food is determined by the change in water temperature. A highcalorie food gets the water hotter by releasing more heat energy than a low-calorie one.

The human body, of course, is not nearly as simple as a lab device. We don’t digest food by setting it on fire. We digest chemically, and our biochemistry is highly individual—in fact, unique. The calories of energy we obtain (or don’t obtain) from food can vary significantly, and some individuals are better matched to one kind of food versus another. Even though calories can be measured accurately in a lab where they appear to be a fixed attribute of food, once we get inside a living person, and a uniquely biochemical digestive tract, all bets are off when it comes to a rigid set of calorie predictions.

How Is Weight Related to Energy?

Our body weight consists of three main components: water, muscle mass, and fat mass. With respect to water weight, we’re usually within the vicinity of 60% total weight. A person weighing 150 pounds would be expected to contain about 60% of those pounds, or 90 pounds, in the form of water.

Living, moving, staying warm, and all other body functions require energy. At least some of this energy must come to us daily in the form of food. Other portions of this energy can come from combustion of fat in our fat cells. If our bodies need more energy than we provide ourselves through food, our bodies obtain this needed energy from stored fat. (In certain situations, including emergency situations, our bodies also use starches stored in our muscles and liver and proteins found in the muscles themselves.)

When any component of our total body weight goes down (water, muscle, or fat) while the other components remain steady, we lose weight. When one component goes up to the same extent that another goes down, we remain the same weight. Over time, if our bodies require more energy than we provide ourselves through food, we always lose weight. This “always” cannot be measured in terms of hours, or even a few days. But over the course of time, it is not possible for us to remain the same weight if we maintain the same percent water weight and expend more energy than our digested food provides.

Proteins, Carbohydrates, Fats, and Calories

The laboratory-based rules in nutrition have always been simple: proteins and carbohydrates have traditionally been said to contain 4 calories per gram. Fats have been said to contain 9 calories per gram This calorie-based description of the three primary macronutrients has been used as the basis for dozens of weight loss programs, especially programs that advocate low-fat, reduced-calorie intake. These programs are based on sound science, but once again, the science is laboratory science, not human digestive tract science.

The reasoning behind these low-fat, calorie-based approaches to weight loss has been simple. Why risk consumption of one macronutrient type (fat) when that nutrient type contains more than twice as many calories (9 per gram) as the other two basic types (protein and carbohydrate at 4 per gram)? While this reasoning seems sound in terms of the mathematics, the successful weight loss experience of many individuals on high-fat, low-carbohydrate diets has seemed to contradict it. But there is not really a contradiction here at all. Individuals are not identical in their digestion. They are differently matched to different foods. Some individuals clearly do better on higher fat, lower carbohydrate diets—even if those diets contain the exact same number of calories as higher carbohydrate, lower fat diets! Figuring out the best dietary balance for your weight management—especially the best balance of proteins, carbohydrates, and fats—is important. It’s also a task that is separate from the task of counting calories.

Do Calories Matter?

If human digestion of food is so individualized and different from the laboratory analyses, do the lab analyses of food calories really matter? Yes, they do! No matter how well matched you are to your weight management meal plan, you simply cannot lose weight if you do not pay any attention whatsoever to calorie intake. You cannot lose weight if your body digests food and releases the exact same amount of energy from the food needed to maintain your muscle mass, fat mass, and water weight. In this sense, calories definitely matter. Paying attention to calories is worthwhile. But counting calories isn’t the whole story, and it doesn’t take the place of health-promoting nutrients that you need to burn fat. The key is that you need more nutrients and fewer calories.