Milk and milk products play an important role in the human diet. However the nutritional composition of cow’s milk can vary considerable depending on factors such breed, stage of lactation and diet of the animal. In order to increase the milk yields it has become the common practice to feed concentrates, based on grain products, and to some extent these have replaced the traditional grass products of hay and silage. However recent research indicates that this change in diet may have a detrimental impact on the content of some of the key nutrients for humans.
Milk fat is a valuable source the Essential Fatty Acids (EFAs) (1), Conjugated Linolenic Acid (CLA) (2) and vitamins (3).
There are 2 families of EFAs, namely the omega-6s and the omega-3s. Ideally the ratio of omega-6:omega-3 should be close to 1 and no higher than 4. Unfortunately in the typical Western diet this value is usually in the range 15 to 30, which means that there is an excessive amount of omega-6s and an inadequate supply of omega-3s. A relatively high intake of CLAs is associated with various health benefits including a reduced incidence of cancers and heart disease.
The fatty acid composition in beef produced with diets with varying amounts of grass and concentrates was studied in an investigation conducted in Ireland (4). The results (Table 1) show that the “Grass only” diet (22kg grass/day) had much higher contents of CLAs and omega-3 EFAs that the diet with “Concentrate only” (8kg concentrate plus 1kg hay/day ie mainly grain). It was suggested that the higher content of omega-3s in the meat from grass-fed animals was because the grass contained about 30 times more omega-3s than the concentrate. The results for CLA support the hypothesis that the relatively high levels of sugars and soluble fibre in the grass facilitate the development of the bacteria in the rumen which produce the CLA and/or discourage the development of those which utilise the CLA.
Table 1. Essential Fatty Acids in Beef from Animals Fed either Concentrate or Grain
|Concentrate only||Grass only|
These findings have been confirmed by Daley et al in a review published in 2010 (5). Grass- finished beef consistently contains a higher content of omega-3 fatty acids while the level of omega-6s remains virtually constant. This means that the ratio of omega-6:omega-3 is reduced and is therefore preferable from the perspective of human nutrition. The same review also reported that grass-fed ruminants produce 2 to 3 times more CLA than ruminants on a diet mainly consisting of grain. It was suggested that this is due to the higher rumen pH associated with grass consumption which favours the micro-organisms that produce the CLA.
This review also found that:
- The beef from animals fed on grass contains about 7 times more ?-carotene (Vitamin A precursor) than from those fed on grain
- There are much higher levels of Vitamin E and its precursors in the beef from animals fed on grass
- Grass-fed beef contains high levels of the antioxidants glutathione, superoxide dismutase and catalase which are particularly effective at mopping up free radicals.
Essentially similar results have been obtained for milk. The effect of varying the grass content in the diet of dairy cows on the composition of the fatty acids in milk was studied in an investigation conducted in France (6). Four groups of 2 cows each were fed 4 different diets in which corn silage was replaced by 30, 60 and 100% grass for a period of 2 weeks. The first week was for adjustment and the measurements were done during the second week. Table 2 confirms that the composition of these important fatty acids in milk replicate those which were found in the Irish study on beef with progressive increases in the contents of both omega-3s and CLAs with the proportions of grass in the diets.
Table 2. Changes in the Essential Fatty Acids of Milk with Variation in the Grass Content of the Diet
|% Grass in the diet|
Although there is little detailed information on sheep it is highly likely that the meat is a good source of omega-3s and CLAs since these animals have a diet which is predominantly based on grass.
Vitamin K2 (VK2)
In previous blogs (3, 7) I have described how VK2 has emerged as an important nutrient. For example epidemiological studies have shown that low intakes of this vitamin are associated with an increased incidence of chronic diseases including heart disease. One of the key functions of VK2 is to facilitate the utilisation of Vitamins A and D as well as calcium. This means it is particularly crucial for anyone who is taking these nutrients as supplements to ensure that there is an adequate intake of VK2. A person who is taking calcium supplements but lacks sufficient VK2 runs the risk of damaging the arteries by calcification.
Unfortunately as yet there is no reliable information generally available on the amounts of VK2 in different foods. However it has been established that the produce of animals which have been fed a diet that is predominantly grass-based are good sources of VK2. So this is another compelling reason why it is eminently sensible on nutritional grounds to choose milk and meat as well as the respective products from animals that have been fed a diet mainly of grass. Finally it must be emphasised that all the nutrients are present in the fat and therefore choosing low fat versions is not an option!
- P French et al (2000) Journal of Animal Science 78 (11) pp 2849-2855
- C A Daley et al (2010) Nutrition Journal 9 10
- S Couveur et al (2006) Journal of Dairy Science 89 pp 1956-1969