Prevention is an important strategy for conquering cancer. Milk fat contains a number of components, such as conjugated linoleic acid, sphingomyelin, butyric acid, ether lipids, beta-carotene, and vitamins A and D that have anticancer potential. Conjugated linoleic acid inhibits the growth of a number of human cancer cell lines and suppresses chemically-induced tumor development at a number of sites in animal models. As little as 0.1% of dietary conjugated linoleic acid inhibits the development of rat mammary tumors, independent of the amount and type of fat in the diet. Sphingomyelin, through its metabolites ceramide and sphingosine, participates in multiple antiproliferative pathways associated with suppression of carcinogenesis. Dietary sphingomyelin inhibits murine colon tumor development. Butyric acid, uniquely present in ruminant milk, is a potent antineoplastic agent and may ameliorate its potency through synergy with other milk fat components. Dietary butyric acid inhibits mammary carcinoma development in rats. In humans, ether lipids, beta-carotene, and vitamins A and D are associated with anticancer effects. Cows have the ability to extract anticarcinogenic components from pasture and feed and transfer them to milk. Use of genetic engineering and other techniques to increase the range and level of anticarcinogens in pasture and supplements may increase the anticancer potential of milk.

The optimum approach to conquering cancer is prevention. Although the human diet contains components which promote cancer, it also contains components with the potential to prevent it. Recent research shows that milk fat contains a number of potential anticarcinogenic components including conjugated linoleic acid, sphingomyelin, butyric acid and ether lipids. Conjugated linoleic acid inhibited proliferation of human malignant melanoma, colorectal, breast and lung cancer cell lines. In animals, it reduced the incidence of chemically induced mouse epidermal tumors, mouse forestomach neoplasia and aberrant crypt foci in the rat colon. In a number of studies, conjugated linoleic acid, at near-physiological concentrations, inhibited mammary tumorigenesis independently of the amount and type of fat in the diet. In vitro studies showed that the milk phospholipid, sphingomyelin, through its biologically active metabolites ceramide and sphingosine, participates in three major antiproliferative pathways influencing oncogenesis, namely, inhibition of cell growth, and induction of differentiation and apoptosis. Mice fed sphingomyelin had fewer colon tumors and aberrant crypt foci than control animals. About one third of all milk triacylglycerols contain one molecule of butyric acid, a potent inhibitor of proliferation and inducer of differentiation and apoptosis in a wide range of neoplastic cell lines. Although butyrate produced by colonic fermentation is considered important for colon cancer protection, an animal study suggests dietary butyrate may inhibit mammary tumorigenesis. The dairy cow also has the ability to extract other potential anticarcinogenic agents such as beta-carotene, beta-ionone and gossypol from its feed and transfer them to milk. Animal studies comparing the tumorigenic potential of milk fat or butter with linoleic acid-rich vegetable oils or margarines are reviewed. They clearly show less tumor development with dairy products.

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A role for the amount and type of dietary protein in the etiology of cancer has not been studied extensively. Nevertheless, there is no compelling evidence from epidemiological studies to indicate that protein, at levels usually consumed, is a risk factor for cancer. On the other hand, animal studies suggest that certain peptides and amino acids derived from dietary proteins may influence carcinogenesis. The predominant protein in milk, casein, its peptides, but not liberated amino acids, have antimutagenic properties. Animal models, usually for colon and mammary tumorigenesis, nearly always show that whey protein is superior to other dietary proteins for suppression of tumour development. This benefit is attributed to its high content of cystine/cysteine and gamma-glutamylcyst(e)ine dipeptides, which are efficient substrates for the synthesis of glutathione. Glutathione is an ubiquitous cellular antioxidant that directly or through its associated enzymes destroys reactive oxygen species, detoxifies carcinogens, maintains proteins in a reduced state and ensures a competent immune system. Various experiments showed that tumour prevention by dietary whey protein was accompanied by increased glutathione levels in serum and tissues as well as enhanced splenic lymphocyte proliferation, phagocytosis and natural killer, T helper and cytotoxic T cell activity. Whey protein components, beta-lactoglobulin, alpha-lactalbumin and serum albumin were studied infrequently, but results suggest they have anticancer potential. The minor component lactoferrin has received the most attention; it inhibits intestinal tumours and perhaps tumours at other sites. Lactoferrin acts by induction of apoptosis, inhibition of angiogenesis, modulation of carcinogen metabolising enzymes and perhaps acting as an iron scavenger. Supplementing cows with selenium increases the content of selenoproteins in milk, which on isolation inhibited colon tumorigenesis in rats.

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