Low-carb proponents claim that eating a low-carb diet enhances weight loss irrespective of caloric content. Low-fat proponents often make the same claim. Many other advocates of special diets make similar claims: It’s not calories, it’s something else causing weight loss.
In support of their diet’s efficacy, proponents often cite their own successes or the success of other followers. However, they often fail to acknowledge that many other people lose weight following radically different weight-loss plans. And never mind the scientific research, as it provides evidence that all successful weight loss programs share a common characteristic: create a calorie deficit on a consistent basis and weight loss follows.
A calorie is a unit of energy. It is the amount of energy, or heat, that it takes to raise the temperature of 1 gram of water 1 degree Celsius (1.8 degrees Fahrenheit). The energy derived from foods when they are burned in the body is measured in kilocalories (thousands of calories). A kilocalorie is the amount of energy required to raise 1,000 grams of water 1 degree Celsius. Kilocalorie is written as “Calorie” (with a capital C) or it may be abbreviated to “Kcalorie” or “Kcal.” Therefore, whenever the word calorie is used in connection with food or nutrition, the meaning is always “kilocalorie” or “calorie.”
In terms of fat loss, a low-calorie diet consisting of Twinkies will result in weight loss, just like a low-calorie diet consisting of fruits and vegetables will. When considering which is more nutritious, fruits and vegetables win, hands down. Which is more filling? Fruits and vegetables win again. When considering issues other than energy content, it’s not just about calories.
Calories come from four sources: Protein (4 Kcal per gram), fat (9 Kcal per gram), carbohydrate (4 Kcal per gram), and alcohol (7 Kcal per gram).
To reiterate, technically, a calorie is a calorie. The definition of a calorie does not change, but different calorie sources have different properties and they affect the body in various ways. So when considering nutrient value, effects on appetite, intolerances, and allergies, there are differences in calorie sources.
Some low-carbohydrate enthusiasts claim that calories from fat are different from calories from carbohydrates, and this is evident due to greater weight loss that often accompanies low-carb diets in short-term trials. It is true that short-term weight loss is usually greater with a low-carbohydrate diet than a higher-carbohydrate diet, even when the calories are equal. This, however, is largely due to water loss.
Freedman et al. (2001). Popular Diets: A Scientific Review. Obesity Research Vol.9, Suppl.1 March.