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Counting calories in food...
To determine the amount of calories contained in any food, we have to know it's composition. Specifically, we have to know how many grams of carbohydrates, fats, and proteins are contained within the food. We only need to concern ourselves with these three macronutrients because as we've already noted, of the six basic nutrients all foods are made from-- water, vitamins, and minerals contain zero calories. The other thing we need to know to determine the calories in food, is how many calories are contained in each gram of a carbohydrate, a fat, and a protein. If we know that, and we know the food's composition, it becomes a simple matter to calculate the total food calories.
--But is a gram of food burned in a bomb calorimeter worth the same amount of calories as a gram of food burned by the body? --Furthermore, do all the different kinds of carbohydrates contain the same amount of food calories? For instance, will a gram of carbohydrate in legumes contain the same amount of calories as a gram of carbohydrate in fruits? --Do all fats contain an equal amount of calories? Will a gram of fat found in dairy products contain the same amount of food calories as a gram of fat found in meat? --Likewise, do all proteins? Is a gram of protein contained in vegetables worth the same amount of calories as a gram of protein in eggs? The answer is, Yes. Well, no actually. But for all intents and purposes, we say, Yes. Those interested in counting calories in food will be relieved to know that in fact, we use 'average' values. It's not an exact science but it makes the task of counting calories a really simple one. Here's the explanation...
However, depending on the arrangement of the atoms in the molecules of any food, the heat of combustion will differ slightly. This means that not all carbohydrates contain the same amount of calories. Neither do all fats. And neither do all proteins. For instance, a gram of carbohydrate from vegetables can have a heat of combustion of 4.2 calories, while sugars can have a heat of combustion as low as 3.95 calories. Similarly, some proteins in vegetables have a heat of combustion of 5 calories per gram whereas protein from eggs may have a heat of combustion of 5.75 calories per gram. Likewise for fats. But on average, by placing a gram of a macronutrient inside a bomb calorimeter and burning it, it's been determined that the average calories in food macronutrients are:
The heat of combustion of the macronutrients can be considered the 'gross' energy value. Of course, this is because in the human body, digestion impacts the ultimate energy value of these foods. Indeed, not all of the food we eat is digested and absorbed to serve the body's biological needs. So there is a difference, albeit a small one, between a gram of food 'burned' in a bomb calorimeter and a gram of the same food 'burned' in the body. The amount of food that is actually digested and absorbed by the body is numerically defined as the coefficient of digestibility. This number is expressed in relative percentages. In general, approximately 97% of carbohydrates, 95% of fats, and 92% of proteins are 'usable' for the body's biological needs.
They are called the Atwater General Factors. These are the average values that many of us are familiar with since they are widely known today. The Atwater General Factors simply state that:
Thus, this is why we say there are 4 calories per gram of carbohydrates and proteins, and 9 calories per gram of fat. And incidentally, 1 gram of alcohol contains 7 calories of energy.
If you want to count the calories in food, this is really easy. Obviously, how much food you eat as part of your meal is a big factor, but as we stated earlier, you also need to know the composition of foods, i.e., how many grams of carbohydrates, fats, and proteins are contained in each of your foods.
It turns out an average apple contains about 20 grams of carbohydrates, 0 grams of fats, and 0 grams of protein, therefore, this apple contains about 80 calories, i.e., 20 g x 4 kcal/g = 80 kcal If you ate 2 apples as your meal, the total energy value would therefore be 160 calories. Of course, not all apples are the same size. A medium-sized apple, 3 inches in diameter, weighing approximately 182 grams, contains about 95 calories. You should also know that whatever sized apple it is, there is always a tiny bit of fat and protein in it-- perhaps 0.3 g and 0.4 g, respectively. This amount is so small, most sources will simply state it as 0 grams from fats and proteins. And it's not just the calories in food that matter to our health either. It might excite you to know that an apple also contains various amounts of vitamin A, vitamin C, vitamin E, vitamin K, vitamins B1, B2, B3, B6, and folate-- as well as a few minerals such as calcium, iron, magnesium, phosphorus, potassium, copper, and manganese! So, how do you find out about the calories in food? Where can you get all of this food nutrition information? Luckily for us, the exact composition of thousands of foods have been analyzed, evaluated, and compiled by various governmental agencies. So determining the calories in food we eat and all of its food nutrition information is a fairly simple task. Finding out the nutrition data of specific foods is the subject of our next topic...
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