A calorie is a calorie, at least from a thermodynamic point of view. It is defined as the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius (2.2 pounds by 1.8 degrees F).
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 contained that could actually be metabolized.
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 the amount of energy needed to keep you alive – the energy you use for breathing, digestion, maintaining blood flow and so on – along with what you do to move your body. You may have heard this referred to as metabolism.
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 compared to high-carbohydrate diets. Other researchers reported that although replacing carbohydrates with fats did not alter energy expenditure, people who increased their protein intake to 30%-35% of their diet used more energy.
Overall, diets high in carbohydrates, fats, or both result in a 4%-8% increase in energy intake, while high-protein meals cause an 11%-14% increase over 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 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 white rice, cakes, crackers, and chips, all have a high glycemic index/load. 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 provide much, if any, benefits in the form of protein, vitamins or minerals besides their calories. 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. The foods that are the biggest contributor to weight gain are potato chips, chips, sugar-sweetened beverages, and meat, both processed and unprocessed. 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 vegetarian diet, with very low or no animal protein and low or moderate fat in the form of monounsaturated fat. .
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 vegan diet rich in plant proteins and carbohydrates mostly from vegetables, fruits, nuts and legumes is the healthiest diet researchers know about for 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.
Written by Teresi Tolar Peterson, Associate Professor of Food Science, Nutrition and Health Promotion, Mississippi State University.
This article was first published in The Conversation.