by Tamara Scully
Changing cow diets changes the amount of fatty acids in their milk. Fatty acids found in cow milk, which are thought to play an important role in human health, include linoleic and alpha-linoleic (ALA) – or omega 6 and omega 3 – as well as conjugated linoleic acid (CLA). All cow milk, no matter what the animals are fed, contains these and many other fatty acids. But the amounts and proportions of these fatty acids can differ based on cow diet.
“The milk fat profile can definitely be influenced by the composition of the diet,” said Elle Andreen, USDA-ARS support specialist who works in dairy grazing research. Andreen presented information on the “dairy cow side of fatty acids” during a recent webinar. “Scientists are starting to realize that milk, and milk fat, is healthier than previously thought. All milk, regardless of its production method, has healthy fats and other nutrients.”
What are fatty acids?
Fatty acids are “the building blocks of lipids,” of which triglycerides are the most common, Andreen explained. Triglycerides are composed of a glycerol backbone, with three fatty acids attached. The physical characteristics of the fatty acids impact human health.
Fatty acids are classified based on the length of the carbon chain, as well as their saturation. Fatty acids with 18 or more carbons in their chain are long chain fatty acids. Saturated fatty acids have no double bonds between the carbons; monounsaturated ones have one double bond. Those with more than one double bond are the polyunsaturated fats.
Trans and cis fatty acids are identified based on the position of hydrogen atoms around the double bonds in the carbon chains. While these can be manufactured in a lab, artificial versions are not the same as those that occur naturally. Artificial trans fatty acids, made by adding hydrogen to liquid vegetable oils, are detrimental to human health.
Cow milk contains more than 400 fatty acids, but only 10% or so are quantified for dairy research. These range in size from two to 22 carbons in their chains. About 70% of these are saturated fats. Gas chromatography is the traditional way of mapping fatty acids, although a newer technique of MIR spectroscopy (similar to technology used in forage analysis and milk component testing) can identify the percentage of unsaturated and saturated fats in a milk sample. It’s faster and less expensive than traditional fatty acid testing.
Fatty acids are ingested by the cow in the form of lipids, and can be stored in body fat. When cows are in a negative energy balance, they will utilize these body fat stores.
The majority of ingested lipids are directly metabolized in the rumen, where they are hydrolyzed. This process releases free fatty acids, which are no longer attached to any type of backbone, such as glycerol. Some of these enter unchanged into circulation. Most, however, are processed further and biohydrogenated into saturated fatty acids prior to entering into the cow’s circulatory system, where they are partitioned for the mammary gland.
Fatty acids with less than 16 carbons, known as de novo fatty acids, will be repackaged by the mammary gland into new fatty acids. The longer or pre-formed fatty acids will be incorporated, basically unchanged, into milk fat.
“That is why we have a lot of saturated fat in meat and dairy products,” Andreen said.
Cow dietary influences
Conventional feeds of corn, soybean and distillers’ grain have high omega 6 fatty acid content and very low omega 3 content. Pasture and hay are just the opposite. This is significant, as the ratio of omega 6 to omega 3 in the human diet is important. Most people consume too much omega 6, so increasing omega 3 in the diet can have a beneficial effect on health. ALA is also the precursor to EPA and DHA, two other important omega 3 fatty acids which humans cannot synthesize and must obtain from food sources.
Dairy and meat products are also important sources of CLA. CLA is stable throughout processing, so it is unaffected by pasteurization. Thought to be anti-carcinogenic, CLA is highly influenced by dairy cow diet.
“The pasture-based ingredients have a lot more alpha-linoleic acid,” Andreen said. “The concentration of CLA could be modified by management. The consumption of pasture can really modify CLA.”
A review of research data covering 260 conventional herds and 19 pasture-based herds showed that pastured herds have almost twice as much CLA content in their milk than those fed conventionally. Levels of CLA can be increased by switching herds to pasture-based management, and the increase can be equal to feeding various supplements in order to enhance CLA levels, she said.
Pastured cows don’t produce as much CLA on a gram per day basis, however. This discrepancy is likely due to less milk being produced per cow per day than in conventionally managed herds.
There is also a seasonality to milk fat production. CLA production on pasture increases gradually over a three-week period during spring, falling rapidly in autumn. Conventionally-fed herds also show seasonality to their CLA production, but the difference from peak to low levels are not as drastic as seen in grass-based herds.
The CLA levels in organic milk increase 55% in summer, while CLA levels in conventional herds only increase 12% during the same timeframe. Organic milk’s CLA levels are 42% higher in summer than CLA levels in milk from conventional herds. Organic and conventional herds are at similar CLA levels during the non-grazing season.
Supplements such as flaxseed and fish oil can enhance CLA, but content in these supplements can be variable and they can be expensive. There may also be cow palatability concerns and storage issues, and they can depress milk fat. Increasing pasture and forage intake will increase the content ALA and CLA.
Holsteins have shown slightly increased levels of CLA in their milk in both conventional and pasture-based herds when compared to other breeds under the same management. The heritability of CLA has been shown to be low to moderate, but it is also negatively correlated with milk fat production.
A survey of retail milk showed that ALA content in organic milk was 60% higher than in the conventional brands when averaged across 12 months. The EPA and CLA in the organic milk were both about 20% higher than that of conventional samples too.
A small increase in the percentage of CLA or ALA in milk fat would help people to meet the guidelines for omega 3 intake, which is 1.0 to 1.5 grams per day, as recommended by the National Institute of Health.
To determine the actual levels in each carton of milk, the industry would need to implement expensive testing to account for herd and seasonal changes seen. Potentially, elevated omega 3 levels could be connected to a value-added premium offered to producers, as consumers are willing to pay more for these attributes.
“The impact on human health depends on specific fatty acid content of milk and how much milk fat is consumed,” Andreen said. “That consumer awareness is important to us, because that demand could help drive how cooperatives value the fatty acids in pasture based milk.”