Let's talk about what milk fats are, how they work, and what they can do for your baby. Milk fats are mostly insoluble (i.e. they don't dissolve in water), so milk has a great system to get fats from mothers to babies – the milk fat globule. Milk fat contains a very wide range of fatty acids from a mother’s diet. The mammary cell assembles these water-insoluble fats into particles and coats them with a layer to make them stable in water. But then mothers do something that is unique in biology: on the way out of the mammary cell, the fat globules are wrapped in an entire additional layer (a complex membrane) from the mother’s own mammary cell. Basically, mothers send a little bit of themselves to their babies like a gift wrapping on the globule. This coating consists of phospholipids, cholesterol, glycolipids, proteins, enzymes, complex carbohydrates, and a mucin coating. It's a major investment of the mammary cell, losing its own membrane as it secretes fat globules. Milk contains many many globules, in sizes that range from so large that they rise to the top of milk if stored for an hour to super small particles that can be stable for days. Scientific research is still making discoveries about the benefits to babies of this unique fat transfer system, but what we already know is amazing! First, fat globules contain the essential fatty acids and fat soluble vitamins that babies need to survive. These fatty acids provide essential substrates for the neurological tissues, brain, eyes, and peripheral nervous system that infants are developing very rapidly in the first months of life. Fat is a dense and effective fuel supply providing calories to every tissue including (and most importantly) the developing brain that uses fats early in life for fuel. The fat globules provide the entire repertoire of complex lipid components from the mother’s mammary cells to provide these as a base for infant cell to grow from, and infants are building all of their epithelial surfaces (i.e. thin lining tissue) rapidly as protection from the real world that they have just entered. Second, fat globules help a ton in the transportation and absorption of milks components. Once eaten, fat globules come apart in the gut of the baby and then spontaneously come back together into ordered structures that make absorption of all of the components of milk, both water soluble and insoluble, easier. Third, fat globules give baby the complex lipids that are used as packaging material that make up lipoproteins for moving lipids from their intestine to all of the tissues around their body. Milk also provides the complex lipids for babies to assemble high density lipoproteins that shuttle insoluble molecules around babies and protect those babies from infections and inflammation. Lastly, fat globules pass along the infant intestine looking from the outside as if they were cells. So, microorganisms bind to them along the gut, which helps to develop a stable and protective microbial community—especially in the lower gut, also known as the microbiome. So, it is increasingly clear from growing scientific evidence that the fat components of milk are the most expensive for mothers to make and among the most valuable for infants as they grow.