Technically, yes, a calorie is a calorie is a calorie, but when one examines the macronutrient composition of food—the carbohydrates, protein and fats contained in foods—more in depth, all calories may not, in fact, be exactly equal.
A calorie is a unit of energy. One calorie is the equivalent of 4.19 joules, or the energy needed to raise the temperature of one gram of water by one degree Celsius. This scientific jargon doesn't mean much to many, but the important aspect to extract is that calories provide you with energy to breathe, walk, run, think and even sleep!
The quantity of calories your body needs to sustain its bodily functions is your resting metabolic rate. This rate is determined based on your age, sex, muscle mass and activity level. But this doesn't tell the entire picture, as various health conditions, medications and information also influence your caloric needs.
How Does All of This Affect My Weight Loss?So if you're looking to lose weight, does it matter if you meet your daily caloric needs from vegetables or a candy bar?
Physiologically speaking, it does.
When your stomach is empty, it releases the hormone ghrelin, which signals to your brain that it is time to eat. Consuming 100 calories of vegetables occupies a greater volume in your stomach than if you were to eat 100 calories of potato chips. The stretch of your stomach when it's full, signals to your brain to turn off
The way in which the three macronutrients—carbohydrates, proteins and fats—are broken down and utilized in the human body varies. This is called the thermic effect of food, in which various amounts of heat are released by the body when digesting and metabolizing each
For example, protein and carbohydrates both contain four calories per one gram of the nutrient, but more heat is lost in the metabolism of protein than in the metabolism of carbohydrates. In other words, carbohydrates are easier for the body to digest. It takes more energy to digest and metabolize protein than it does carbohydrates, which means
Even further, the way in which various types of carbohydrates, proteins and fats are metabolized differs. One of the most studied examples is the difference in the body's metabolism of glucose and fructose, two types of simple carbohydrates. On a biochemical level, the difference between glucose and fructose appears minimal—they both provide four calories per one gram consumed. However, the ways in which these two nutrients are metabolized, taken up by cells in the body to be used, and the tissues in which they are needed most, differ drastically.
Glucose is much more widely used in cells throughout the body, and it is the brain's preferred source of energy. Fructose, on the other hand, is only used by certain tissues within the body. Studies repeatedly show high fructose consumption (versus glucose intake) increases insulin resistance and increases fat stores in the body, making the intake of these very similar calories not equal at all. Since fructose is almost never eaten by itself and is usually in approximately equal amounts with glucose, the current recommendation is to make an effort to cut back on all added sugar.
So even though you may be able to attain weight loss at a sub-caloric 1,200 calorie diet composed of fast food, it definitely will not lead to optimal energy levels, reduce your risk of chronic diseases, contribute to restorative sleep or aid optimal hormone concentrations in the body. Instead of purely focusing on calories consumed, focus on the quality of foods you chose to eat. From a dietitian's perspective, everyone can benefit from choosing foods that are closest to how they're found in nature—more plants, less processed.