Nutritional and Health Aspects of Milk Fat
So far we have seen that the characteristic milk fatty acid profile can be modified by manipulating the cow's diet. Different fatty acid profiles in turn influence the physical properties of the fat which has implications for the quality of manufactured products. We will leave that subject to others and turn to the question of the nutritional value of milk fat and how that may be changed by feeding the cow.
Milk fat has long been under attack by the medical community and others as being undesirably high in cholesterol and saturated fatty acids, both believed to contribute to atherosclerosis and cardiovascular disease (Havel, 1997). The status of milk fat as an agent in cardiovascular diseases and cancer was challenged objectively and the role of milk fat in balanced diets was demonstrated clearly in an indepth review by Berner (1993). Clearly, cholesterol in milk fat is not a major factor influencing its total dietary intake; with exception of full fat ice cream, a serving size of any dairy food provides 2 to 11% of the recommended (300 mg/day) intake of cholesterol (recall that dietary cholesterol is provided only by animal products). Further, a serving of these foods, including butter, provides less than 15% of the recommended daily fat intake (Berner, 1993). Among saturated fats, only lauric (12:0), myristic (14:0) and palmitic acids (16:0) are considered to be hypercholesteremic; these constitute about 40% of milk fat. Berner is critical of studies which used single dietary fat sources to compare effects of fats on blood lipids; obviously, effects of any single fat source will be diluted when in a mixed diet. Quoting Ramsay et al. (1991), many experts have unrealistic expectations from diet interventions because of "over reliance on short-term experiments, controlled studies of rigorous diets in captive populations, and uncontrolled observations". Berner (1993) further concluded that the current consensus recommendations for 10% of diet energy each from saturated, monounsaturated and polyunsaturated fatty acids, with total fat intake not to exceed 30% of diet energy, though perhaps acceptable for effects on serum total and low density lipoprotein (LDL) cholesterol levels, have several weaknesses: 1) these recommendations do not consider effects on high density lipoproteins (HDL) cholesterol and apoprotein levels; 2) recommendations do not consider effects of changes in fatty acid intake on parameters other than serum lipids and lipoproteins, for example, thrombotic tendencies and LDL oxidation; and 3) categorizing fatty acids as saturated, mono-unsaturated, or polyunsaturated ignores the fact that not all fatty acids within a group have similar effects. Also, these recommendations ignore possible anticarcinogenic effects of fat components, such as CLA, sphingomeyelin, butyric acid, and glycerol ethers for which milk fat is a key dietary source (Parodi, 1997).
Additional perspective on the role of public policy in recommending dietary intakes is given by Scott (1997) who discusses the impact of individual responses to dietary intervention, and by Harper (1992) who vigorously attacks the political correctness effects on policy making and effectively illustrates that the supposed epidemics of cardiovascular disease and cancer in the 20th century are not diet-related, but effects of improved health and well-being so that life expectancy is increased. These are diseases of aging, not of poor diet.