Cows fed rapeseed oil as part of their daily diet produce milk with significantly less saturated fat. Milk, butter and meats, known to contain high levels of saturated fats, have long been implicated in contributing to a higher risk of cardiovascular disease and high cholesterol in humans.
"This kind of tailored milk production could in future be applied to make any dairy product healthier, from cheese to ice cream," said Anna Fearon, one of the authors of the study.
Cows commonly eat a variety of plant-based feeds, which contain varying amounts of vegetable oils. The vegetable oils are naturally high in unsaturated fats but these fats are transformed into saturated fats during the digestive process by microorganisms that live in the cow's rumen, part of the animal's stomach.
The scientists report in the recent issue of the Journal of the Science of Food and Agriculture (Volume 84) that increasing the amount of rapeseed oil in the cows' diet had an effect on the kind of fats their milk contained - unsaturated fatty acids (the 'good' fats) went up while saturated fatty acids (the 'bad' fats) went down.
Butter made from the milk is also easier to spread at fridge temperatures because it is lower in saturated fat than ordinary butter, added the scientists.
Cows eating 600g of oil a day produced milk with 35 per cent more oleic acid, the unsaturated fat that is also present in olive oil, than cows on an ordinary diet, and 26 per cent less palmitic acid, the saturated fat which has been linked to heart disease and obesity.
The study is one of several published recently that look at ways of changing the composition of dairy products based on the herd's diet. Some believe that intervening at an earlier stage of food development may provide higher chances of success than adding ingredients to milk and dairy foods.
Fearon and her colleagues tested the effect of four different amounts of rapeseed oil - the control group received no rapeseed oil, while other groups of the herd were given 200g, 400g or 600g a day. The cows' milk got 'healthier' with each increase of oil in their diet, and the cows themselves also stayed healthy, they found.
"The milk fat changes showed no signs of plateauing-out as the amount of dietary rapeseed increased," said the authors.
"It seems likely that by giving dairy cows even more oil supplement in their diet, we can achieve an even more beneficial balance of fatty acids," they conclude.
Their research builds on earlier similar findings published by researchers in November last year. Scientists at the University of California developed a new cattle-feed supplement to boost the content of unsaturated fatty acids over saturated fats in cows' milk. At the time, the researchers said that the new supplement, based on naturally occurring proteins, could significantly improve the health value of milk.
It also provides dairy processors with the ability to modify various food qualities, such as the spreadability of butter and the flavour of cheese, they reported.
The supplement relies on proteins that occur naturally in milk and other foods, without using any synthetic chemicals. "By taking advantage of the unique properties of proteins we were able to form complexes between those proteins and lipids, which are high in unsaturated fatty acids. These complexes protect the lipids against modification by the microorgnisms that live in the rumen," said lead researcher Ed DePeters at the time.
During feeding trials, the researchers mixed the supplement with the cows' normal feed. The study involved more than 750 cow-days, with more than 1500 milk samples analysed. Within less than three days, they recorded as much as an 800 per cent increase in the proportion of specific unsaturated fatty acids, such as linolenic acid, in the cows' milk.
Although the supplement's effects have only been studied on the fatty-acid composition of milk, the researchers indicate that it might have similar benefits in reducing the level of saturated fats in meat.