3.8: Caloric Value of Food

When it’s said that an apple has 80 calories, this isn’t exactly correct. It’s more accurate to say that an apple provides the body with about 80 calories. As measured in a bomb calorimeter, an apple has more calories than the 80 that the body can use. All of a food’s calories can’t be used if the body can’t digest it or break it down to carbon dioxide and water.

Figure 3.4: Bomb Calorimeter

Fiber in apples has calories as measured in a bomb calorimeter, but we don’t have the enzymes to digest it. Thus, fiber doesn’t provide the body with energy. Similarly, wood burns and gives off heat, but we don’t get any calories from eating it.

Another example is saccharin, the non-caloric sweetener. It has calories as measured in a bomb calorimeter and is absorbed by the body, but we don’t have the necessary enzymes to break it down (i.e., our body can’t release any of its energy). We don’t get any calories from it, and it’s excreted unchanged in the urine. When it’s said that fiber and saccharin don’t have calories, what’s meant is that we can’t use those calories. The caloric values we’re familiar with are actually physiological calories (calories that the body can use).

Carbohydrate, protein, and fat are called energy-providing nutrients because the body gets energy from them. Conversely, vitamins, minerals, and substances like saccharin don’t have calories because the body doesn’t get energy from them. The [physiological] caloric values of the energy-providing nutrients are:

Carbohydrate* = 4 calories per gram

Protein = 4 calories per gram

Fat = 9 calories per gram

Alcohol = 7 calories per gram

Calculating Food Calories

Knowing the amounts of carbohydrate, protein, fat, and alcohol in a food, we can calculate its calories. For example, a cup of whole milk that has 11 grams (g) carbohydrate, 8 g protein, and 8 g fat has 148 calories (usually rounded to 150):

It’s also useful to calculate how calories are distributed in a food, e.g., what percent of calories comes from fat. It’s easier to compare foods when fat content is given as a percent of total calories, rather than as a percent of total weight. When given as a percent of total weight, comparisons are distorted by the amount of water in the food.

For example, whole milk has, by weight, about 3.5% fat, lower-fat milk 1-2% fat, and fat-free milk almost none. It isn’t obvious that switching from whole milk to lower-fat milk would make much difference in % calories from fat. But milk is mostly water, so its fat content, by weight, is low and misleading.

Calculating % calories from fat, we see that about 50% of the calories comes from fat in whole milk, whereas it’s 35% for reduced-fat (2% fat) milk, 15% for low-fat (1% fat) milk, and 5% for fat-free milk. Looking at milk’s fat content this way, the advice to switch to a lower-fat milk is obviously a good way to lower fat intake.

For this calculation, you only need to know the grams of fat and the total calories in the food, numbers which are on food labels and in food composition tables:

1 cup whole milk (3.5% fat by weight): 8 g fat, 150 calories: 8 g fat X 9 cal/g X 100 = 48% of total calories from fat 150 total calories

1 cup reduced-fat milk (2% fat by weight): 5 g fat, 125 cal 5 g fat X 9 cal/g X 100 = 36% of total calories from fat 125 total calories

1 cup low-fat milk (1% fat by weight): 2 g fat, 120 calories: 2 g fat X 9 cal/g X 100 = 15% of total calories from fat 120 total calories

1 cup of fat-free milk (0% fat by weight): 0.5 g fat, 90 cal 0.5 g fat X 9 cal/g X 100 = 5% of total calories from fat 90 total calories

On a package of frankfurters, does 80% fat-free mean that 80% of the fat was taken out, making them low in fat? Looking more carefully at the label, we find that one of these frankfurter has 100 calories and 8 grams of fat. This means that 72% of the calories comes from fat!

8 g fat X 9 cal/g X 100 = 72% of total cal from fat 100 total calories

The label is misleading but technically correct because fat is given as a percent of total weight, and over half of the frankfurter is water. The label says that each frankfurter weighs 45 g and has 8 g of fat. This means that, by weight, the frankfurter is 18% fat [(8 g fat/45 g total weight) X 100 = 18% fat by weight]. The other 82% is fat-free! It would be more to the point to say, more than 70% of the calories come from fat. This, of course, would be bad for sales.

U.S. government standards allow a maximum of 30% fat (by weight) in frankfurters. If it’s less than 30%, we’d expect this to be emphasized on the label. A label stating 33% less fat means that the frankfurters are 20% fat by weight (80% fat-free!) instead of the usual 30% (70% fat-free!). Fortunately, food labels are required to give nutrition information, and claims are more tightly regulated. Unfortunately, the percent of calories from fat isn’t given on the label (it can be calculated). Guess which food companies object to listing fat this way!

^{*When talking about carbohydrate’s calories or the carbohydrate content of foods, only digestible carbohydrates are considered. Indigestible carbohydrates—dietary fiber—are simply called fiber.}