Is a Calorie a Calorie?

Is a Calorie a Calorie?

By: Jeremy Partl, RD and Gillian Ward

“A calorie is a calorie” is a common statement in the nutrition sphere. Some people say that food is simply a part of a thermodynamic equation, and regardless of where your calories come from, they will have the same effect on your weight. Others disagree. Just like with all nutrition information, we have to question the validity of the statement. Is a calorie just a calorie?

In this article, I try to provide you with an evidence-based answer, one that may help you determine whether it’s worth focusing on the sources of the energy you consume.

Calories 101

A food calorie is a unit of energy equal to the amount of energy required to increase the temperature of one kilogram of water by one degree Celsius. Fun fact: a food calorie—those listed in the nutrition facts on food labels—is actually a kilocalorie; one food calorie equals one-thousand calories. Confused yet? Let’s try to simplify. Calories are units to quantify the energy that is stored in the chemical bonds of the foods we eat. Our metabolism comprises the chemical reactions that break down these foods and release that energy, which gets recaptured as ATP or lost as heat.

From a thermodynamic viewpoint, a calorie is, of course, just a calorie. Our bodies can’t defy the first law of thermodynamics. We are constantly transforming energy into ATP and heat from the food we eat.

The nuance comes in when we dive into nutrition—because we don’t just get calories from food. That’s where the simplicity of “a calorie is a calorie” becomes inadequate for our purposes. Not all sources of calories are created equally.

Calories vs. Macronutrients

We don’t eat calories; we eat foods that are composed of macronutrients in varying amounts—the proteins, carbohydrates, and fats that we see on the nutrition label.

The different macronutrients follow different pathways in how they are broken down or stored. The sum of calories available in food depends on how much carbohydrates, protein, and fat it contains. We get the common values for these macronutrients, which you see on nutrition labels, from extensive studies on how many calories a macronutrient contains versus how much the body can extract:

• 1 gram of carbohydrate = 4 calories
• 1 gram of protein = 4 calories
• 1 gram of fat = 9 calories

From an energy standpoint, the macronutrients are pretty similar (fats being denser), but there are a variety of reasons we have to consider the differences between macronutrients for nutritional purposes.

Function

Each macronutrient has its own function. While the body prefers to use carbohydrates for energy, it can utilize any macronutrient, and each is necessary. For example, you won’t be able to build muscle without consuming protein, because the primary function of amino acids is to repair and build muscle tissue. Dietary fats are necessary for normal hormonal secretion and must be present to help absorb vital nutrients.

While there may be some overlap in functionality between macronutrients, without all three, our bodies would not survive.

Thermic Effect and Storage

The thermic effect of food (TEF) is the scientific term that describes the amount of energy required for the digestion, absorption, and metabolism of macronutrients; it takes energy to just break them down and create and store energy. The TEF for each macronutrient is 2-3% for lipids (fats), 6-8% for carbohydrates, and 25-30% for proteins.¹

In other words, it takes much more energy to digest, absorb, and metabolize protein than the dietary fats and carbohydrates you eat. All else equal, consuming 100 grams of protein would result in greater energy expenditure than consuming the same number of calories in carbohydrates or fats.

In addition, dietary proteins and carbohydrates are preferentially oxidized, and dietary fats are preferentially stored in adipose tissue. That is, fats are much more likely to be stored in adipose tissue versus the other macronutrients.

Amount of Fiber

Fiber is an indigestible carbohydrate that bucks the notion that every gram of carbohydrate contains four calories per gram. Because this component is not fully digested and absorbed, the caloric content of a complex carbohydrate source would be different from a simple carbohydrate source. According to the Food and Drug Administration, soluble fibers provide about two calories per gram and insoluble fibers do not contribute any calories.²

Other Components of Food

While vitamins and minerals are not broken down for energy, our bodies need them just as much as macronutrients. These micronutrients cover a slew of different utilities in the body.

For instance, the B vitamins primarily help the body convert food into energy (metabolism), but they also help create new blood cells and maintain healthy skin cells, brain cells, and other body tissues.³ Vitamin D, which is found in foods primarily of animal origin, doesn’t do these things for the body, aiding primarily in calcium absorption and bone mineralization, among other things.

Given some unique functions of micronutrients—a lack of overlap for some very necessary utilities—the potential for nutrient deficiencies is real. While these have declined over the years—long gone are the days when sailors drank grog to prevent scurvy—it’s important to consider micronutrient content, especially when going on restricted diets, like a vegan diet.

One of the differences between animal and plant foods is that animal foods do not contain phytochemicals (also known as phytonutrients). These unique compounds are part of plants’ defense systems that help them to resist fungi, bacteria, viral infections, insects, and other animals. When we eat plants, these compounds (like antioxidants) help prevent disease and promote health.

The difference should be clear: 100 calories of sugar is not equal to 100 calories from spinach. Their difference, in part, is because of everything outside of energy (phytochemicals, vitamins and minerals, fiber, water, etc.) that you get along with those calories.

Food Volume, Energy Density, and Satiety

The two primary factors that influence your hunger and satiety signals after meals are macronutrient content and volume. You could eat 500 calories of three different foods and find that you feel full much longer when you eat foods that have more volume and are higher in protein.

That feeling you have after you scarf down a large salad, a ton of raw vegetables, or even something like a bowl of oatmeal? It’s the distension of your stomach and the subsequent fullness you feel, primarily impacted by the stretching of the stomach (but also fiber and water). Compare that to how you feel when you eat something that’s very calorie-dense and low in volume, like dried fruit, pizza, or even cooked vegetables. You don’t get nearly as much of the impact on making you feel full and satiated for hours.

The scientific term for these differences is energy density, or the amount of energy in a particular weight of food (typically the number of calories per gram). Foods with a higher energy density provide more calories per gram than foods with a lower energy density.⁴ It’s possible to feel just as full with fewer calories if you choose the right foods.

Certain macronutrients can have an impact on hunger hormones, influencing the subsequent feelings you have. I will let the words of a paper on the topic reinforce my point:

“Foods, and more specifically macronutrients, with the same caloric content exert different effects on satiation and satiety independent of their caloric value. In other words, not all calories are treated equally by the body.” ⁵

Numerous lines of research have suggested that the order of satiety from specific macronutrients goes protein > carbohydrate > fat. In other words, higher-protein diets tend to be more satiating than isocaloric diets that have a greater percentage of calories coming from carbohydrates and fats.⁶

These effects are primarily regulated by the positive impacts of protein on gastrointestinal hormones such as cholecystokinin (CCK) and glucagon-like peptide-1 (GLP-1) and ghrelin.⁷

There are certain exceptions to this that have become evident as different dietary patterns have evolved. For example, while weight loss tends to result in increases in hunger hormone levels, a ketogenic diet has been shown to consistently reduce appetite, even though fat has been found to be one of the least satiating macronutrients. While not fully conclusive, researchers theorize that getting into a state of ketosis and shifting fuel substrates can create an environment that bucks the notion that high-fat diets will not satiate you. Interestingly, it seems that once the body gets out of that state of ketosis, the typical understanding of the impact of macronutrients on appetite regulation returns to normal.⁸

A Calorie Does Not Equal a Calorie

We do get calories out of the foods we eat. But the foods we eat also have properties that affect our health and eating behaviors. It’s never one thing. We have to zoom out from just focusing on calories and see the whole picture when thinking about a well-rounded nutrition approach.

Gillian Ward Weighs In

While calories are an important consideration, the quality of your weight loss/gain will suffer if you ignore the macronutrients and the composition of what is on your plate. This is true whether we like it or not.

You may know that I am a former professional bodybuilder. There is a reason why bodybuilders and other athletes that seek to manipulate body composition to the highest degree eat specific whole foods and do not take an “if it fits your macros” or “just count calories” approach. If I could have achieved contest condition (lean enough to be competitive on stage) eating 1,800 calories per day worth of jelly donuts and pizza, don’t you think I would have?

The result of my sustained 1,800 calories per day diet of quality, whole foods in balanced ratios was very different looking and feeling than simply consuming any 1,800 calories. To optimize health, performance, and aesthetics goals the what is just as important as the how much.

We’ve known this for almost 200 years! Have you ever heard the phrase, “you are what you eat?” It is believed that Anthelme Brillat-Savarin, a French lawyer and politician, first used this phrase in his book “Physiologie du Gout, ou Meditations de Gastronomie Transcendante,” which was published in 1826. It translates in English as, “Tell me what you eat, and I will tell you what you are.”

Simply said, the food we eat has a bearing on our mental and physical conditions. Going deeper, the metaphor implies that what people eat could eventually affect their lives. If they eat healthy food, they will feel healthy—and if they eat highly processed junk food, they will not feel healthy. Types of foods affect body composition and consequently affect overall health and appearance, including hair condition, organs, teeth, skin, and brain cells.

So, ask yourself: Do you want to be a jelly donut or a finely marbled slab of beef? Creative liberties aside, a calorie is not just a calorie!

References

¹ Jéquier, E. (2002). Pathways to obesity. International Journal of Obesity, 26(2), S12-S17.

² https://www.fiberfacts.org/fibers-count-calories-carbohydrates

³https://www.medicalnewstoday.com/articles/325292#:~:text=B%20vitamins%20are%20important%20for,cells%2C%20and%20other%20body%20tissues.

⁴ https://www.cdc.gov/nccdphp/dnpa/nutrition/pdf/r2p_energy_density.pdf

⁵ Gerstein, D. E., Woodward-Lopez, G., Evans, A. E., Kelsey, K., & Drewnowski, A. (2004). Clarifying concepts about macronutrients’ effects on satiation and satiety. Journal of the American Dietetic Association, 104(7), 1151-1153.

⁶ Tremblay, A., & Bellisle, F. (2015). Nutrients, satiety, and control of energy intake. Applied Physiology, Nutrition, and Metabolism, 40(10), 971-979.

⁷ Paddon-Jones, D., Westman, E., Mattes, R. D., Wolfe, R. R., Astrup, A., & Westerterp-Plantenga, M. (2008). Protein, weight management, and satiety. The American journal of clinical nutrition, 87(5), 1558S-1561S.

⁸ Roekenes, J., & Martins, C. (2021). Ketogenic diets and appetite regulation. Current Opinion in Clinical Nutrition & Metabolic Care, 24(4), 359-363.