Skim milk is whole milk from dairy cows that has most or all of it’s fat removed.

Traditionally, this was done by letting milk settle, and then “skimming” the fat off the top of the milk. What is left is the protein-rich, low-fat liquid below the layer of fat. In modern milk processing, the de-fatting process is done with centrifuges (basically the milk is spun around inside a big stainless steel tank and the fat is separated and drained off.)

Skim milk (also labeled as “fat-free milk” or “non-fat” milk) generally has less than 0.5 percent milk fat. Low-fat, semi-skimmed milk or “1% milk” has between 1 and 2 percent fat. For comparisons sake, whole cows milk has around 3.5 percent fat, or 7.9 grams of fat (4.6 grams of which are the “bad” saturated type of fat) in a 1 cup (16 oz) serving. In terms of calories, whole milk weighs in at 147 calories, in comparison to the 91 calories in skim milk.

Clearly choosing skim milk over whole or even 2% milk makes the most sense from a fat and calorie perspective.