So how does the dairy industry get away with claiming that the reduced fat milk has only 2% milk fat? It turns out that they are not exactly lying. The dairy industry reports the percent fat in reduced fat milk in terms of weight rather than calories because 2% fat sounds much better than 35% fat! Going back to the label, the weight of a serving size of one cup is 240 grams (g), assuming that 1 mL of milk weighs 1 g (this is a good assumption), of which 5 g comes from fat. So the percentage of weight that comes from fat = (5 g/240 g)*100 = 2%.
The problem with using percent of total weight instead of percent of total calories is that the weight of the foods that we eat is meaningless in terms of our diets, which are based on calories. Additionally, the percentage of the total weight of any nutrient in a food, including fat, can be manipulated by adding water to the food. Adding water increases the total weight of the food without changing the actual weight of the other nutrients. So the percentage by weight of the other nutrients, including fat, goes down. This is an industry trick sometimes used for reporting reduced-fat foods. On the other hand, water has no calories, so adding water does not increase the total calories, so the percentage by calories of the other nutrients does not change.
If you’re trying to lose weight by eating low-fat foods and are having difficulties, you could be consuming a lot more fat than you think. So if you really want to know how much fat is in the foods you are eating in order to make better choices, calculate the percentage of calories that come from fat using the equation above to learn the truth.