Unsaturated fatty acids

Oleic acid (18:1c9) is the single unsaturated fatty acid with the highest concentration in milk accounting to about 8 g/litre whole milk [9] (Table 1). Accordingly milk and milk products contribute substantially to the dietary intake of oleic acid in many countries. In Norway about a quarter of the average intake of oleic acid comes from milk and milk products [3]. Oleic acid is considered to be favourable for health, as diets with high amounts of monounsaturated fatty acid will lower both plasma cholesterol, LDL-cholesterol and triacylglycerol concentrations [46], and replacement of saturated fatty acids with cis-unsaturated fatty acids reduces risk for coronary artery disease [13]. Several studies also indicate a cancer protective effect of oleic acid, but the data are not fully convincing [47].

Fatty acids are the main building material of cell membranes. The unsaturated fatty acids are reactive as they may give oxidative stress with free radicals and secondary peroxidation products (different aldehydes such as malonedialdehyde and 4-hydroxynonenale) that may be harmful to proteins and DNA in the cells [48,49]. This may contribute to cancer [49] and to mitochondrial aging processes caused by mutations in mitochondrial DNA [50]. The enzyme lechitin/cholesterol acyl transferace (LCAT), having an important role in reverse cholesterol transport, is sensitive to oxidative stress [51] and it is also inhibited by minimally oxidized LDL [51]. Oleic acid is more stable to oxidation than the omega-3 and omega-6 fatty acids, and it can partly replace these fatty acids in both triacylglycerols and membranelipids. A high ratio between oleic acid and polyunsaturated fatty acids will protect lipids in i.e. LDL towards attack from oxidative stressors such as cigarette smoke, ozone and other oxidants. Studies have shown that a diet rich in monounsaturated/polyunsaturated fatty acids give better protection against atheromatosis and CVD than a diet rich in polyunsaturated fatty acids [52,53].

Milk fat is rich in oleic acid (about 25 % oleic acid) and it has a very high ratio oleic acid/polyunsaturated fatty acids. A diet rich in milk fat therefore may help to increase this ratio in the total dietary fatty acids. A high intake of meat from i.e. sheep may be expected to have similar effect. This might partly explain why mortality by cardiac disease has been lower in Iceland compared to the other Scandinavian countries [54], and the average age of living has been higher [55] despite of higher intake of saturated fat (coming from both mutton and milk).

The concentration of PUFA in milk is about 2 g/l [9], and the main PUFA in milk are linoleic- (18:2 omega-6) and alpha-linolenic (18:3 omega-3) acid (Table 1). These fatty acids may be converted to fatty acids with 20 carbon atoms, i.e. arachidonic acid (20:4 omega-6) and eicosapentaenoic acid, (EPA) (20:5 omega-3), and further converted to eicosanoids; metabolically very active compounds with local functions. Eicosanoids derived from linoleic acid, via arachidonic acid, may enhance blood platelet aggregation and thereby increase the coronary risk, in contrary to eicosanoids produced form the long omega-3 fatty acids [56]. EPA has the ability to partially block the conversion of the omega-6 fatty acids to harmful eicosanoids, thereby reducing the cardiovascular risk and inhibiting tumour genesis. PUFA may also affect signal transduction and gene expression [57,58]. It is conceivable therefore that the type of fatty acid in the membrane governs several metabolic functions.

It has been argued that the Mesolithic man had a ratio of 1–4:1 between the omega-6 and omega-3 fatty acids, against now in most European diets 10–14:1 [59]. Eskimos, and some populations in Japan, having a high intake of omega-3 fatty acids, also have a low rate of coronary heart diseases, and of some cancers [59]. Conceivably, protection against cardiovascular diseases and cancer would be related to the ratio of EPA plus oleic acid to omega-6 fatty acids in the diet, and hence in the body.

In milk the ratio between omega-6 and omega-3- fatty acids is low and favourable compared to most other non-marine products (Table 1). This ratio is greatly influenced by the feeding regime, and may with favourable feeding be as low as 2:1 (see later). Comparing the omega-6 to omega-3 ratio in milk in the Nordic countries, Thorsdottir et al. [60] has reported the lowest ratio in Iceland: 2.1:1, compared to 4.7:1 in milk from the other Nordic countries. It has been suggested that the higher supply of omega-3 fatty acids from milk in Iceland might explain the lower prevalence of type-2 diabetes and CHD mortality in Iceland compared to the other Nordic countries [60]. A Norwegian study showed reduced risk of premenopausal breast cancer with milk intake [61]. With proper feeding regime, milk and meat from ruminants can in fact be the main source of omega-3 fatty acids in the human diet, as is the case in France [62].

According to the above considerations a favourable meal should be rich in oleic acid, and have a low ratio between omega-6 fatty acids and omega-3 fatty acids, perhaps near 1–2:1. Indeed, milk fat fits into this description probably better that any other food item.