Breast milk contains ingredients that formula will never duplicate. Unlike formula, breast milk composition can change over time and adapts to your baby's needs. Cow's milk formula manufacturers have gotten better at matching their formulas to breast milk, but fundamental differences between the two remain, both in the macronutrients such as protein, fats and carbohydrates and micronutrients such as vitamins and minerals.
If you remember your nursery rhymes, you might remember that protein consists of curds -- also known as casein -- and whey. Casein is the tougher curd, while whey is the watery portion. Breast milk contains more whey than formula, which makes it more easily digestible. Human milk also contains slightly less protein than formula -- 1.1 grams per deciliter compared to 1.5 in formula, according to the University of Hawaii John A. Burns School of Medicine. One type of protein found in breast milk but not formula, lactoferrin, has antibacterial and immunological properties. Another enzyme found only in breast milk, lysozyme, also acts as an antimicrobial against E. coli and salmonella. The protein content in breast milk adjusts to your baby's needs.
Both cow's milk and breast milk contain the milk sugar lactose, but breast milk supplies more lactose than formula; around 37 percent of breast milk consists of lactose compared to less than 10 percent of formula, according to the North Clinic of Minneapolis. Lactose, a disaccharide, breaks down into two simple sugars, galactose and glucose, which your baby absorbs easily. Breast milk contains more oligosaccharides, which are carbohydrates containing three to 10 simple sugars. Oligosaccharides have anti-infective properties that act as prebiotics and help keep the intestinal flora in balance.
The fats in breast milk are its most important ingredients, according to pediatrician and author Dr. William Sears. Fat makes up 50 percent of the calories in breast milk, the University of Hawaii John A. Burns School of Medicine reports. Breast milk contains omega-3 fatty acids DHA and AA, as well as cholesterol, not found in most formulas. Both help your baby's brain growth and development. Breast milk also contains lipase, which helps break down fats. The stools of bottle-fed babies often have an unpleasant odor because they don't contain lipase, so the stools contain more undigested fat.
Formula manufacturers add iron to their product, so that formula contains more iron than breast milk. However, the iron that formula contains is less easily absorbed; your baby absorbs 50 to 75 percent of iron from breast milk, but just 5 to 10 percent from formula, Dr. Sears explains. Your doctor might suggest starting your baby on iron supplements at age 4 to 6 months, if you breastfeed. Your baby also absorbs other vitamins and minerals more efficiently from breast milk than from formula; manufacturers add larger quantities of vitamins and minerals to compensate. The larger amounts of vitamins and minerals make formula more difficult to digest.