The fundamental difference between skim milk and raw milk is that raw milk comes directly from a cow, while skim milk is the end product of putting raw milk through various processes. Although advocates of each claim their milk is healthier and the other is dangerous, the U.S. Food and Drug Administration maintains that raw milk is the more dangerous of the two.

Pasteurization and homogenization make up the primary differences between raw milk and skim milk. Raw milk is not pasteurized or homogenized, but skim milk is. Pasteurization and homogenization are processes used by milk manufacturers to keep milk from spoiling. Pasteurization involves heating raw milk to a high enough temperature to kill off many of the molecules and bacteria that can cause it to spoil more quickly. But in killing off the bad molecules, pasteurization also kills the good probiotic bacteria and molecules. These molecules can help digestion and aid the immune system.

Homogenization is similar to pasteurization in that it involves altering raw milk to make it more appealing to the consumer market. In homogenization, raw milk is forced through a filter with tiny openings at high pressure. The filter breaks up larger fat molecules in the milk, which makes the milk’s taste slightly blander but also more consistent. Homogenization also stops cream from forming.

The FDA released a position paper warning consumers to be wary of raw milk and products made from it. The FDA stated that, because raw milk does not undergo the process of pasteurization, it can contain seriously harmful to potentially life-threatening bacteria, including Salmonella, E. coli and Listeria. The FDA paper included a section refuting the idea that pasteurization can be harmful, focusing on the argument that pasteurization does not reduce milk’s nutritional benefits.

Raw milk advocates hold that pasteurization does indeed reduce the nutritional value of milk. First of all, they maintain that pasteurization kills of probiotic bacteria the body requires to maintain a healthy digestive system. Second, they say that skimming milk removes vitamins and fats critical for a healthy body.

Dr. Ganmaa Davaasambuu, a researcher at Harvard University, has linked the hormones in milk to cancers that rely on such hormones to thrive. She found that hormone-dependent cancer is more prevalent in countries where more milk is consumed. However, skim milk contains fewer of these hormones, because the hormones occupy fat molecules in milk, and skim milk has little to no fat molecules.