As the dietitian for MyFitnessPal, I’m often asked, “Is a calorie a calorie?” Well, according to the laws of thermodymanics, yes, all calories are created equal (at least on paper). But—and this is a big but—the way the body breaks down carbohydrates, protein and fat, the three main sources of calories in our diet (four if you count alcohol), and the effect they have on our bodies differ vastly. There are semester-long courses that explain just how our bodies break down, burn and store each of these four calorie sources differently (I know because I took one in graduate school), but since this is a 750-word blog post and not a 750-page textbook, I’ll try do my best to briefly explain why not all calories are created equal.
In addition to being a potent and flavorful source of energy, fats slow digestion, deliver important fat-soluble vitamins to the body, and provide important building blocks for every one of our cells.
All dietary fats provide about 9 calories per gram but, as you likely already know, some fats are better for our health than others. For example, polyunsaturated omega-3 fats, found in foods like wild salmon and flaxseed, have protective, anti-inflammatory properties, whereas artificial trans fats have been linked to increased inflammation and heart disease.
Protein also keeps us feeling fuller for longer by slowing digestion, but its primary role in the body is to maintain and build new cells. Protein needs are greatest during childhood, adolescence and pregnancy, when the body is growing and adding new tissues. But we now also know that protein is beneficial during weight loss, as it contributes to satiety and offsets the amount of lean muscle that is burned for energy, in addition to fat, during a calorie deficit.
All proteins provide about 4 calories per gram but there are higher quality proteins, which may reduce appetite and optimize muscle repair and recovery (think: fish or eggs), and lower quality proteins (think: hamburger meat) that are loaded with branched-chain amino acids, which have been linked to metabolic disease and insulin resistance. In this case, you get more nutritional bang for your buck if you consume 4 calories of high quality protein.
When it comes to differentiating calories, carbohydrates are by far the most complex (pardon the pun) mostly because our bodies use the different types of carbohydrates (such as fiber, starch and sugar) in very different ways.
Carbohydrates are used by the body as a quick source of energy, particularly for the brain, liver and muscles. All carbohydrates (with the exception of fiber, which our body can’t digest) provide 4 calories per gram. But just as there are healthier fats and higher-quality proteins, there are varying degrees of carbohydrate quality.
Though not a source of calories, fiber is considered a high-quality carbohydrate since it slows digestion (thus making you feel fuller, longer) and can moderate the absorption of other nutrients, like sugar. For this reason, high-quality carbohydrates typically contain fiber and are minimally processed. These include fruits, vegetables, whole grains and legumes. Lower-quality carbohydrates almost always lack fiber (with the exception of dairy which contains natural sugars packaged with protein) and add little more than “empty calories” to our diets.
By now it’s probably clear that a calorie from fat is not the same as a calorie from protein or carbohydrate. But let’s take it a step further and compare calories from two different types of sugar: glucose and fructose.
Starchy foods like rice, potatoes and pasta, are predominantly made up of glucose, a simple sugar that that can be burned for energy by every cell in our bodies. It’s stored in our liver and muscles for a quick source of energy during exercise or while we sleep. Unprocessed starchy foods, like brown rice, potatoes with the skin on and 100% whole-wheat pasta, contain the food’s natural fiber as well as some vitamins and minerals.
Unlike glucose, which can be burned for energy by all organs, fructose can really only be broken down in the liver. It’s also the sweetest tasting of the three simple sugars which makes it enjoyable on the tastebuds. In nature, fructose is found in fruits bound tightly to indigestible fiber that, as we already know, reduces and slows its absorption. Unfortunately, the majority of fructose in our diets isn’t from fruits–it’s from calorie-containing sweeteners added to sweetened beverages and the majority of processed foods—including these 10 foods that might surprise you.
Here’s the main difference between these two sugars: While too many calories from glucose can lead to weight gain and accumulation of the less harmful subcutaneous fat, too many calories from fructose (found in calorie-containing sweeteners like sugar, honey, high fructose corn syrup etc…) can overwhelm the liver, contributing to nonalcoholic fatty liver disease, insulin resistance and more.
As you can see, a calorie of carbohydrate is not the same as a calorie from fat or protein, nor are all carbohydrate calories created equal. As a general rule of thumb, I recommend consuming the majority of your calories from minimally or unprocessed whole foods since, ultimately, the quality of what we eat determines the quantity of calories we consume, which impacts not only our weight but also our overall health and well-being.