Fats are vital for normal growth and development, and are the main energy source of the newborn infant. In addition to providing 40%–50% of the total calories in human milk or formula, fats are an integral part of all cell membranes, provide fatty acids necessary for brain development, and are the sole vehicle for fat-soluble vitamins and hormones in milk. Furthermore, these energy-rich lipids can be stored in the body in nearly unlimited amounts, in contrast to the limited storage capacity for carbohydrates and proteins. Before birth, glucose is the major energy source for the fetus, with the fetal requirement for fatty acids supplied mainly as free fatty acids from the maternal circulation. After birth, fat is supplied chiefly in the form of milk or formula triglycerides.

Lipids are nonpolar or amphipathic substances that are insoluble in aqueous media (Figure 23.1). Absorption of fat permits the efficient assimilation of a great number of hydrophobic (fat-soluble) chemicals, some beneficial (such as the fat-soluble vitamins) and some detrimental (such as hydrophobic xenobiotics, drugs, and food additives).

Major lipids in infant nutrition

The major lipid classes are glycerides, phospholipids, sterols (cholesterol), and free fatty acids (Figure 23.1).

Glycerides are nonphosphorus-containing lipids that result from the esterification of glycerol and fatty acids (Figure 23.1). Triglycerides (neutral fat) are the most abundant lipids in animal tissue and serve as an important energy source. In triglycerides all three of the carbon molecules of glycerol are esterified with fatty acids.