Phospholipids and glycosphingolipids

Phospholipids and glycosphingolipids accounts to about 1% of total milk lipids [8]. These lipids contain relatively larger quantities of polyunsaturated fatty acids than the triacylglycerols. They have functional roles in a number of reactions, such as binding cations, help to stabilize emulsions, affect enzymes on the globule surface, cell-cell interactions, differentiation, proliferation, immune recognition, transmembrane signalling and as receptors for certain hormones and growth factors [6]. Gangliosides are one of these components found in milk. Gangliosides (with more than one sialic acid moiety) are mainly found in nerve tissues, and they have been demonstrated to play important roles in neonatal brain development, receptor functions, allergies, for bacterial toxins etc [94].

Protein

Bovine milk contains about 32 g protein/l [9] (Table 1). The milk protein has a high biological value, and milk is therefore a good source for essential amino acids. In addition, milk contains a wide array of proteins with biological activities ranging from antimicrobial ones to those facilitating absorption of nutrients, as well as acting as growth factors, hormones, enzymes, antibodies and immune stimulants [95,96]. The nitrogen in milk is distributed among caseins, whey proteins and non-protein nitrogen. The casein content of milk represents about 80% of milk proteins. Caseins biological function is to carry calcium and phosphate and to form a clot in the stomach for efficient digestion. The milk whey proteins are globular proteins that are more water soluble than caseins, and the principle fractions are beta-lactoglobin, alpha-lactalbumin, bovine serum albumin and immunoglobulins. Whey is the liquid remaining after milk has been curdled to produce cheese, and it is used in many products for human consumption, such as ricotta and brown cheese, and concentrated whey is an additive to several products e.g. bread, crackers, pastry and animal feed. The rate at which the amino acids are released during digestion and absorbed into the circulation may differ among the milk proteins, and whey proteins are considered as rapid digested protein that gives high concentrations of amino acids in postprandial plasma [97]. The benefit of drinking whey has been known for centuries, and two ancient proverbs from the Italian city of Florence say, "If you want to live a healthy and active life, drink whey" and, "If everyone was raised on whey, doctors would be bankrupt" [98].

Some of the milk proteins (e.g. secretory immunoglobulin A, lactoferrin, 1-antitrypsin, β-casein and lactalbumin) may be relatively resistant to digestive enzymes, and the whole protein or peptides derived from it, may exert their function in the small intestine before being fully digested [99].

As several bioactive proteins and peptides derived from milk proteins are potential modulators of various regulatory processes in the body, some of these are produced on an industrial scale, and are considered for application as ingredients in both 'functional foods' and pharmaceutical preparations. Although the physiological significance of several of these substances is not yet fully understood, both the mineral binding and cytomodulatory peptides derived from bovine milk proteins are now claimed to be health enhancing components that can be used to reduce the risk of disease or to enhance a certain physiological function [100]. Milk protein composition may differ among breeds [101]. For example the concentration of beta-casein A1 is low in milk from cows in Iceland and in New Zealand. It has been speculated that this proteins may have a role in the development of diabetes and cardiac disease [102]. However, later it was concluded in a review article that there is no convincing evidence that the A1 beta-casein of cow milk has any adverse effect in humans [103].

Milk peptides and blood pressure

Several studies has suggested that there is an association between milk consumption and blood pressure; as hypertension is inversely related to milk consumption in some epidemiological- and intervention studies [104]. It has been suggested that some milk peptides have antihypertensive effects, both by inhibiting angiotensin-converting enzyme, having opoid-like activities, antithrombotic properties and by binding minerals [104].