But when it comes to health and body energy balance, not all calories are created equal.
For example, some studies have reported that diets that are high in protein, low in carbohydrates, or a combination of the two, lead to greater weight loss than diets that contain other levels of fat, protein, and carbohydrates.
If all the calories in food were the same, you wouldn’t expect to see differences in weight loss between people who eat the same number of calories distributed in different types of food.
Dietitians like me know that there are many factors that affect what calories mean to your body. Here’s what we understand about calories and nutrition so far.
Energy is already available to your body
In the late 1800s, chemist W.O. Atwater and his colleagues devised a system for figuring out how much energy—that is, how many calories—different foods contain. Basically, he burned food samples and recorded how much energy he was releasing in the form of heat.
However, not every bit of the energy in food that can be burned in a lab is actually available to your body. What scientists call metabolizable energy is the difference between the total energy of food consumed and the energy that leaves your body, undigested, in feces and urine. For each of the three macronutrients—proteins, carbohydrates, and fats—Atwater created a percentage of the calories they contain that can actually be metabolized. Per gram, protein has the lowest available energy at 92 percent, fat in the middle at 95 percent, and carbohydrates, which turn into blood sugar, at 97 percent.
According to the Atwater Diet, it is estimated that one gram of each large nutrient provides a certain number of calories. The USDA still uses these calculations today to come up with an official calorie figure for each food.
How much energy do you use
What you eat can affect what scientists call your body’s energy expenditure. This is how much energy you need to keep you alive — the energy you use for breathing, digestion, maintaining blood flow, etc. — along with what you do to move your body. You may have heard this referred to as metabolism, although metabolism is actually a somewhat broader concept.
The quality of the diet can alter the body’s energy expenditure, which is also called the thermic effect of food. For example, in one study, people who ate the same number of calories per day, but on a low-carb or low-fat diet, had differences in total energy expenditure of about 300 calories per day. Those on a low-carb diet consumed the most energy, while those on a low-fat diet used the least amount of energy.
In another study, high-fat diets resulted in lower total energy expenditure — the number of calories you burn — compared to high-carb diets. Other researchers reported that although replacing carbohydrates with fats did not alter energy expenditure, people who increased protein intake to 30 percent to 35 percent of their diet used more energy.
In general, diets high in carbohydrates, fats, or both result in a 4 to 8 percent increase in energy expenditure (calorie burning), while high-protein meals cause an 11 to 14 percent increase above resting metabolic rate. Protein has a higher thermogenic effect because it is more difficult for the body to break down. Although these differences are not huge, they can contribute to the obesity epidemic by delicately encouraging weight gain.
The quality of the calories you eat
Dietitians are concerned with a food’s glycemic index and glycemic load—that is, how quickly and by how much your blood glucose levels increase. High blood glucose triggers the release of insulin, which in turn affects energy metabolism and the storage of excess energy as fat.
Foods like cake, cookies, and potato chips all have a high glycemic index/load. Green vegetables, raw peppers, mushrooms, and legumes are all low on the glycemic index/load. There is some evidence to suggest that foods lower on the glycemic index/load may be better for keeping blood sugar levels in order – no matter how many calories they contain.
The brain’s reward centers light up when people eat high glycemic index/load foods, highlighting the pleasurable and addictive effect of foods like candy or white bread.
The fiber content in food is another thing to consider. Your body can’t digest fiber — which is found in plant foods like fruits, vegetables, whole grains, and beans — for energy. So foods high in fiber tend to have less metabolic energy and can help you feel full when eating fewer calories.
Empty calories – those found in foods with minimal or no nutritional value – are another factor to consider. Things like white sugar, soda, and many ultra-processed snacks don’t offer much, if any, benefits in the form of protein, vitamins or minerals despite their high calorie count. The opposite would be nutrient-dense foods that are high in nutrients or fiber, while still being relatively low in calories. Examples include spinach, apples, and beans.
And don’t think of empty calories as neutral. Nutritionists consider them harmful calories because they can have a negative effect on health. Potato chips and sugar-sweetened beverages are one of the biggest contributors to weight gain. On the other hand, the foods that are inversely related to weight gain are vegetables, whole grains, fruits, nuts, and yogurt.
More health than calories and weight
It is indisputable that for weight loss, the difference between the number of calories consumed and the number of calories expended through exercise is the most important factor. But don’t fool yourself. While weight plays a role in health and longevity, losing weight alone does not imply health.
Yes, some high-protein diets seem to promote weight loss at least in the short term. But epidemiologists do know that in the areas where people live the longest — nearly 100 years on average — they eat a primarily plant-based diet, with less animal protein and monounsaturated and polyunsaturated fats.
I often hear friends or clients say things like, “It’s the carbs that make me fat” or “I need to go on a low-carb diet.” But these complaints drive nutritionists like me, well, nuts. Carbohydrates include foods like Coca-Cola and candy cane, as well as apples and spinach. Reducing simple carbohydrates such as soft drinks, bakery products made with refined flour, pasta and sweets will certainly have a positive effect on health. But cutting out carbohydrates like vegetables and fruits will have the opposite effect.
A diet rich in plant proteins and carbohydrates mostly from vegetables, fruits, nuts, and legumes is the healthiest diet researchers know about longevity and prevention of chronic diseases such as heart disease, cancer, high blood pressure, and many other conditions.
The modern Western diet suffers from an increase in the amount of calories consumed with a simultaneous decrease in the quality of calories consumed. Researchers now know that calories from different foods have different effects on fullness, insulin response, the process of converting carbohydrates into body fat, and metabolic energy expenditure.
When it comes to your health, rely on the quality of the calories you consume more than the number of calories.
Assistant Professor of Nutrition at Mississippi State University. This article was first published in The Conversation.