The Imprecise Nature of Calories

Calories in < calories out = weight loss. This ubiquitous diet formula has been around for years, and is still propagated by reliable organizations today. According to the CDC, “To lose weight, you must use up more calories than you take in.” Is it really that simple? Do our bodies operate like machines that perform precise mathematical calculations every time we eat?

Calorie counting and dieting are rampant in our society. According to the Boston Medical Center, 45 million Americans go on a diet every year. Many of these people count calories because they think that consuming fewer calories will result in weight loss. However, recent research indicates that the process of caloric absorption is much more complex than we think. It turns out that all calories are not created equal, and that nutrition labels are often inaccurate measurements of the amount of energy actually extracted from food.

Chemically speaking, a calorie is the amount of energy required to raise the temperature of one gram of water by one degree Celsius. The word “calorie” is actually derived from the Latin word calor, which means “heat.” Calorie content can be calculated a number of different ways, and the FDA currently permits food manufacturers to use any of five different methods. Only some of these methods take into account the body’s inability to digest certain dietary fibers, which means that many nutrition labels are inaccurate estimates of caloric content. This is why different brands of cashews, for example, can have different calorie contents - even though they are the exact same food, the calories were calculated using different methods.

In addition, there is a significant difference between the amount of calories listed on a label and the amount of calories actually absorbed. Research conducted last year at the Beltsville Human Nutrition Research Center in Maryland found that people absorb about 30% fewer calories than what is listed on the nutrition facts for almonds, and about 20% fewer calories than the number listed for walnuts. Dr. David Baer, a physiologist at the research center, says this is because of the structure of nuts: “All the nutrients – the fat and the protein and things like that – they’re inside this plant cell wall.” The cell walls must be broken down through chewing and digestion for the calories to be absorbed. And because the cell walls aren’t completely broken down, some of the calories are not absorbed and are eliminated.

Another factor to consider is that not all calories are created equal. After studying wild chimps in Africa in the 1970’s, Dr. Richard Wrangham, an anthropologist at Harvard University, discovered that the chimps ate a completely raw diet of fruits, vegetables, seeds, and insects. Wrangham attempted to adopt this diet himself, but it left him ravenous. In his novel Catching Fire: How Cooking Made Us Human (2009), he argues that cooking breaks down certain molecular structures in food, making it easier for people to digest. In one of his experiments, he fed raw peanuts to one group of rats and roasted peanut butter to a second group of rats. After a number of weeks, the rats fed the raw peanuts weighed less than the rats fed the peanut butter.

Wrangham’s research demonstrates that cooking methods can significantly alter the amount of calories we extract from our food, yet nutrition labels do not take this into account. Based on his research, Wrangham proposed that the more processed a food is, the easier it is for our bodies to extract and absorb all of the calories, since processing involves exposing foods to extremely high temperatures and pressures. In other words, a 100-calorie bag of potato chips is not metabolized in the same way as a 100-calorie apple, which is why looking at calories alone can be misleading.

Another problem with counting calories is individual variation. Every person has a unique genetic makeup that determines his or her caloric needs and expenditure. Gender, height, weight, liver size, and stress level are only a few of the many factors that affect how many calories an individual burns. For example, the caloric requirements for two people of the same gender, age, and weight can vary by more than 600 calories!

One reason for this is that we have different gut microbes. Dr. Peter Turnbaugh of the University of California transplanted gut microbes from a pair of twins into two groups of mice. One of the twins was obese and the other was average weight. All of the mice were fed the same diet, yet the mice who received the microbes from the obese twin gained considerably more weight. Turnbaugh concluded that “microbes might actually be contributing to the energy that we gain from our diet.”

There is also considerable variability in individual glycemic response, as demonstrated by a recent study conducted in Israel. Over the course of a week, the researchers measured the blood sugar levels of 800 participants. They found that even when the participants consumed the exact same foods, their glycemic responses varied greatly. This provides evidence that the glycemic index (GI), used to determine how specific foods will affect blood sugar, cannot be applied universally since glycemic response is such an individual phenomenon.

The presentation of these research findings is not intended to imply that calories are completely worthless, rather that calorie counts are merely estimates and should be regarded as such. They are far from being a precise measurement of energy absorption. Dr. Adam Drewnowski, an epidemiologist at the University of Washington, proposed that nutrition labels include a nutrient density score that considers the amount of nutrition per calorie, as opposed to only the number of calories.

It is important to remember that the human body is a complex organism that utilizes food in a variety of ways, only some of which are understood by scientists today. By turning eating into a numbers game, we fail to consider the nuances of nutrition and risk missing out on important vitamins and minerals that are present in many calorically dense foods, like nuts and seeds. These nutritious foods are metabolized differently by the body than their processed counterparts, and should certainly be included as part of a healthy diet.