Health Aspects of New Developments in Milk Fat Research
Parodi (1997) succinctly summarized the contribution of cow's milk fat components as potential anticarcinogenic agents. The best-characterized anticarcinogenic agent presently is conjugated linoleic acid - a collective term for a group of conjugated dienoic isomers with both positional and cis/trans variants. The best-characterized of these, as well as the most abundant isomer in cow's milk fat, is cis-9, trans-11 octadecadienoic acid. This has been given the trivial name rumenic acid because it is found most abundantly in ruminant fats and is known to be an intermediate in the ruminal biohydrogenation of linoleic acid. In mouse models, dietary CLA has been shown to protect against the induction and proliferation of chemically-induced skin, forestomach, colon, prostate and mammary tumors. Recently, mice fed butter during the time of pubescent mammary development were shown to have higher CLA in tissues, less mammary epithelial cell mass, and chemically-induced mammary tumor yield was decreased by 53% (Ip et al., 1999). It was shown that CLA inhibited proliferation of terminal end bud cells, which are the target sites of mammary carcinogenesis. High CLA butter was more effective than feeding an equal amount of synthetic CLA. It was proposed that the higher tissue concentration of CLA observed when feeding high CLA butter was caused by endogenous synthesis of CLA, occurring by desaturation of vaccenic acid (trans-11 18:1) which was 5-fold higher than CLA in the milk fat. Their study also conclusively showed that the cis-9, trans-11 isomer was biologically active as an anti-cancer agent. Effectiveness of other isomers remains to be determined.
Vaccenic acid (this name also derives from its predominant occurrence in ruminant fats - vaca, cow in Spanish) occurs in milk fat commonly at about 2% of the total fatty acids (4 to 5-fold that of CLA in milk fat) and can be increased to 5 to 10% by selective feeding. It is potentially a very important contributor to CLA available to the body. It is desaturated by )-9 desaturase, and rumenic acid is the product. We have found that 50% of vaccenic acid stored in tissues is desaturated in mice (Santora et al., 2000). Vaccenic acid is desaturated by bovine mammary gland, contributing as much as 2/3 of milk CLA (Griinari et al., 2000). Desaturation may be lower in adipose tissue (Palmquist et al., 2000). Intake of CLA in typical US diets is estimated to be 100 to 200 mg/day (McGuire et al., 1999); vaccenic acid in dairy and other ruminant products potentially could at least double the amount of CLA available for human tissues. We are currently collaborating in a study in Helsinki to estimate desaturation of vaccenic acid in humans.
Though CLA has drawn a great deal of attention among dairy, medical and nutrition scientists, other components of milk fat also have biological effects with anti-cancer properties. Among these, sphingomeyelin, a component of the milk fat globule membrane, and thus particularly rich in buttermilk, contains the biologically-active components ceramide and sphingosine. These are active in three major antiproliferative pathways B inhibition of cell growth and cell differentiation, and promotion of apoptosis (programmed cell death).
Butyric acid, present in milk at 3 to 5% by weight and about 10% of the fatty acid molecules potently inhibits growth and differentiation, and promotes apoptosis in several cancer cell lines. Greatest interest has been given to its role in colon cancer; effectiveness of butyrate may be greatest when provided with fat-soluble vitamins, also abundant in fortified milk fat (Parodi, 1997). Conclusive evidence for butyrate as an anti-cancer agent remains to be found.
Ether lipids, found in milk fat in rather trace amounts are readily absorbed and have anti-cancer properties against cultured cancer cells. No role for ether lipids in cancer prevention has been reported (Parodi, 1997).
Parodi (1997) cited further evidence, though limited, that fewer tumors were induced in experimental animals fed milk fat than those fed other fats. Of particular interest is the evidence that CLA fed before puberty may impart long-lasting protection against induction and proliferation of tumors. Perhaps this is a result of the inhibition of terminal end bud development by CLA (Ip et al., 1999).