Skim milk and light soy milk have a lot in common: both have a similar number of calories; both are low in fat; and both have the potential to be good sources of a range of vitamins and minerals. One characteristic they do not share relates to shelf life. The shelf life of soy milk is longer than skim milk, which generally lasts a little over one week. Soy milk benefits from processing that can keep it fresh in your refrigerator for at least two weeks.
Calories
Skim milk and light soy milk are both smart choices if you're on a diet. One 8-oz. serving of skim milk has 90 calories, while one 8 oz. serving of light soy milk has around 70 calories. Both products are sold sweetened or flavored, which has the potential to add more calories. Check the nutrition label to verify whether the milk has added sugars.
Fat
Despite its name, skim milk is not devoid of fat. The U.S. Department of Agriculture allows milk containing up to 0.5g fat per 8-oz. serving to be labeled as fat-free or skim. Light soy milk, on the other hand, has about 1.5g fat per cup or three times the fat content of skim milk, but still a small amount of fat.
Protein
Skim milk has more protein than light soy milk, with 8.75g protein per 8-oz. serving. Light soy milk has approximately 6g protein per serving. Milk protein is not a balanced protein in that it is missing at least one of the 16 essential amino acids your body needs for health. Soy milk protein is a complete protein, containing trace levels of all amino acids.
Calcium and Minerals
Light soy milk must be enriched to be a good source of calcium. Naturally, an 8-oz. serving of light soy milk contains about 60mg calcium. Manufacturers use powdered calcium to bring this amount of calcium up to 300mg in 8 oz. of enriched light soy milk. This is closer to the amount of calcium in one serving of skim milk. Skim milk must be fortified with vitamins A and D because otherwise it does not have much of these nutrients. Soy milk has double the folate of skim milk, with 24mg per 8-oz. serving. Both skim milk and light soy milk have about 85mg sodium per cup.
Shelf-Life
Perhaps one of the least appreciated differences between the two products is shelf life. Cow's milk has a short lifespan of a little over one week. Pasteurization, which is responsible for killing off initially harmful bacteria and pathogens, is also responsible for milk taking on an odor and going sour after one week. Soy milk, on the other hand, is often sold in aluminum-lined containers that can sit on your shelf for months. Once opened, soy milk is best consumed within 14 days.
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