Wednesday May 28 2008
Nutritional content of milk was influenced by many factors
“Organic milk is much richer in nutrients linked with reducing the risk of cancer”, says the Daily Express today. Many newspapers report on research that has found that organic milk contains higher levels of vitamins, antioxidants and “healthy” fats than those found in ordinary milk. The level of “linoleic acid – which is thought to ward off cancer – was 60 per cent higher in organic milk during the summer”, the Express adds. The nutritional benefits of the milk are reportedly because the cows graze on fresh grass and clover.
The newspaper reports are based on research that analysed milk from 25 farms and showed that, although there are differences in the nutritional content of different milks, these are due to various factors, not only whether the farm is organic. The study did not look at the effect of the various types of milk on human health. At present, any benefit to health from the balance of nutrients found in milk from either organic or non-organic sources is only theoretical
Where did the story come from?
Gillian Butler and colleagues of School of Agriculture and Institute for Research on Environment and Sustainability, Newcastle University and the Danish Institute for Agricultural Science, carried out this research. The study was funded by the European Community and the UK Red Meat Industry Forum. It was published in the peer-reviewed: Journal of the Science of Food and Agriculture.
What kind of scientific study was this?
This was a cross sectional study in which the authors aimed to look at the differences in the fatty acid and antioxidant content of organic and conventional milk, and how these are affected by seasonal differences in farming (e.g. predominantly outdoor grazing or indoor forage feeding) and whether the farm uses low- or high-input methods.
The researchers collected 109 milk samples from 25 UK farms of three production types: low-input organic, low-input non-organic and high-input non-organic. A standard questionnaire was used to collect management and production information from the farms. The questionnaire covered dairy herd size, average cow weight and breeding details, total dry food intake per day and composition of diet during the outdoor period and indoor periods, i.e. the proportion of fresh grass or conserved forage such as grass silage or hay, cereals, by-products and vitamin and mineral supplements.
Ten of the farms were high-input and these used mainly ryegrass during the outdoor grazing period and grass silage during the indoor winter period, with a higher amount of concentrate (such as cereals) during both indoor and outdoor periods compared with the low-input farms. Ten of the farms were low-input organic, five of which calved all year round and so fed the cows both outdoor fresh diets and indoor forage diets during the winter, and five of which calved only during the spring so that cows were only producing milk in the outdoor period when they were able to eat fresh grass. The outdoor diet in both types of organic farms was predominantly grass-clover (with no nitrates or phosphate fertilisers added to the ground); the indoor diet was mostly conserved grass silage, but a quarter of the diet was still fresh grass. The five non-organic low-input farms also used only spring calving so, again, milk production only occurred while the cow was outside eating fresh grass. As with the organic farms, this was predominantly grass-clover, however nitrate and phosphate could be added to the ground.
Samples of milk were extracted from the bulk tank in March, May, August and October in all farms, and also in January from the high-input and low-input organic farms that used year-round calving. The researchers extracted fat from the milk samples and, using chemical methods, analysed the fatty acid chains that were present to determine the concentrations that were saturated, monounsaturated and polyunsaturated. They also analysed the ratio of omega-3 to omega-6 fatty acids (particular molecular forms of linoleic acid) and the concentration of certain antioxidants.
What were the results of the study?
Milk from low-input farms had a higher total fat content than the high-input farms with a significant difference between the non-organic low-input farms (highest fat) and the high-input farms. Looking at the composition of the fat, the high-input farms had significantly higher saturated fatty acid (“bad” fat) content than either of the low-input farms, while monounsaturated and polyunsaturated fatty acid content was higher in both types of low-input farm compared with high-input farms, but this was only significant in the non-organic low-input farms.
The ratio of omega-3 to omega-6 fatty acids was significantly higher in both types of low-input farm compared with the high-input farms. Overall, the percentage of a specific form of conjugated linoleic acid that is thought to have possible anticancer properties (CLA9) was higher for low-input than for high-input farms, however, this was only significant for non-organic low-input farms. The concentrations of most antioxidants were highest in the milk from the non-organic low-input farms, intermediate for the organic low-input farms and lowest for the high-input farms.
Comparing the farms that used year-round calving (organic low-input and high-input), there were few differences seen in the composition of milk produced during the outdoor and indoor periods, with saturated fatty acid (“bad” fat) content significantly higher in the organic low-input farms and omega-6 fatty acids and monounsaturated fatty acids significantly lower.
When the researchers compared the farms that used a spring calving system (non-organic low-input and organic low-input), they found conjugated linoleic acid to be at a significantly higher level in the non-organic low-input milk, along with monounsaturated and polyunsaturated fatty acid content. Saturated fatty acids and omega-3 fatty acid were higher in the organic low-input farms. However, the content differed depending on which month the milk sample was taken.
What interpretations did the researchers draw from these results?
The researchers conclude that milk composition is affected by the farm production systems, which include the length of the grazing period and composition of the diet. These factors will affect potential nutritional qualities of the milk.
What does the NHS Knowledge Service make of this study?
The research has analysed milk from 25 farms and demonstrates that, although there are clear differences in the nutritional content of different milks, this seems to be affected by a number of factors. These include whether the cows produced milk all year round or in the spring only, and whether the farm used a high- or low-input system, not only whether the farm was organic or not. It does not seem surprising that the quality of food that cows themselves eat will have some effect of the quality of milk that they produce.
- The results do not demonstrate conclusively that organic milk contains the best nutritional properties. In fact, the most favourable nutritional properties seemed to be found in milk from the non-organic low-input farms that used a spring calving system.
- Only a small number of farms of each type have been examined and milk samples were taken on one occasion in each specified month. As the researchers say, milk content in the same farm differed depending on the month that it was taken. It cannot be assumed that these results reflect the exact composition of the milk at other times in that farm, or from different farms. Further analysis of milk samples from other farms will be needed to try and more clearly establish the differences in nutritional content of different types of milk.
- It is unclear whether nutritional content was measured before or after pasteurisation. It is possible that pasteurisation will affect nutritional content, as would differences in how the milk was stored and processed (e.g., whether it was skimmed or converted to 2% fat milk).
- The benefits to health from the balance of nutrients found in milk from either organic or non-organic low-input farms compared with high-input farms are, at the moment, theoretical only. It has not been demonstrated that any type of milk protects against cancer or heart disease. It would only be with carefully conducted trials, where people were randomised to drink different types of milk and then followed over time, that it would be possible to gain any possible insight into their relative benefits.
For now, the decision to drink organic or non-organic milk should continue to be a personal lifestyle choice only.
Sir Muir Gray adds...
The key question is really: whole, semi skimmed or skimmed; for me it is the semi.