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Calories from fat may be one of the most misunderstood label in the entire dietary equation. While most of us know that calories are a measure of energy, most do not know what calories from fat are, how they are calculated, or what they really mean.
Calories from fat are, in some ways, no different than calories from any other source. They take no longer to burn than "regular" calories and their caloric value when compared the calories in the same food, is no different. However, where they differ is in how easily the body is able to store fat calories.
Calories from carbohydrates and proteins are harder to convert to fat for storage. Calories from fat are already in the form of fat and therefore are easier to store. Thus, fat calories tend to get stored quicker and with less energy expended by the body. This is often why athletes prefer to take in calories via carbs and proteins. They do not store as easily and are more apt to get expended. Additionally, even the process of converting protein and carb calories to fat requires burning some of those calories.
The general rule of thumb when calculating calories from fat is to look at the total grams of fat on a food, per serving, and multiply by 9. That will give the approximate calories from fat content. Of course, most packaging with nutritional information already includes a category marked calories from fat but some do not.
Why is it that calories from fat often seem to represent the majority of calories in a food? This is because carbs and proteins only pack 4 calories per gram, less than half the value of fat calories. This represents a big difference, especially if the food is high in fat.
This is probably why most dietitians encourage those concerned about calories to eat foods high in carbohydrates and proteins and low in fat. Not only does fat, per gram, double the amount of calories when compared to protein and carbohydrate grams, it is also easier to store. In some ways, calories from fat could be considered twice as bad as other calorie sources.
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