We should shift some of our attention from total fat to the composition of fatty acids in milk.
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By Jim Linn
Total milk fat has economic importance and is a monitor for cow health and good nutrition. But we should shift some of our attention from total fat to the composition of fatty acids in milk.
Why? Consumers are becoming increasingly aware of diet and health. Studies are finding that fat intake alone is less linked to heart disease and other problems than are certain fatty acids such as trans fat.
Today, producers are paid on pounds of fat produced and, therefore, it makes good economic sense to maximize fat production. However, someday and in some markets, it may be the composition of fatty acids in milk that has the value and not just total fat.
Milk fat originates from two sources: the de novo, or synthesis, of fatty acid in the mammary gland; and the direct uptake of fatty acid circulating in blood by the mammary gland.
Milk fatty acids with a carbon chain of 14 or less (short-chain fatty acids) are almost exclusively synthesized in the mammary gland. Fatty acids with 18 or more carbons (long-chain fatty acids) originate from the diet or mobilization of body fat.
The 16-carbon fatty acids (principally palmitic) can either be synthesized in the mammary gland or taken up from the blood. Either way, the fatty acids found in milk are influenced by the diet being fed.
The short-chain fatty acids synthesized in the mammary gland originate from the substrates acetate and beta-hydroxybutyrate. These milk fat precursors are produced during the fermentation of fiber in the rumen. Feeding diets containing high-quality forages, adequate amounts of both total fiber and physical fiber, highly digestible fiber sources both forage and nonforage, and limited starchy grains is the best way to maximize short-chain fatty acid production and total milk fat. The short-chain fatty acids--lauric, myristic and palmitic--are saturated fatty acids which are often accused of increasing cholesterol. However, recent studies have shown these fatty acids increase the "good" cholesterol to help reduce the risk of heart disease and not the "bad" cholesterol.
Long-chain unsaturated fats in milk are primarily of diet origin, but up to 20% can come from body fat during times of negative energy balance. Those recognized for human health benefits are oleic acid and the omega fatty acids (linolenic and linoleic) leading to conjugated linoleic acid (CLA).
Approximately 25% of the fatty acids in milk are oleic and only 1% to 2% are linoleic or linolenic. The challenge in increasing these acids in milk is that rumen bacteria convert unsaturated fatty acids into saturated fatty acids unless they are rumen-protected. Thus, only about a third of the fatty acids in milk are unsaturated.
We can change the fatty acid composition of milk fat, as well as maintain or increase total milk fat produced, through the type and composition of rumen-protected or inert fat that is fed. Here are some things to consider about protected fat sources if you want to change the fatty acid composition of milk.
Feeding palmitic acid directly appears to be a good way of increasing it in milk. The palmitic acid in diets, once absorbed into the body, appears to go almost exclusively into milk fat. The mammary gland decreases its synthesis of palmitic acid when dietary sources increase, but total milk fat often increases with palmitic acid feeding.
To increase oleic acid and CLA, feeding 18-carbon saturated stearic acid is recommended. The mammary gland converts stearic acid to oleic and CLA through the enzyme delta 9 desaturase. Research from the United Kingdom has shown this enzyme is variable among cows and is heritable. Increasing this enzyme in cows allows for greater conversion of stearic acid into CLA and other healthy fatty acids.
A small amount of polyunsaturated fatty acids (vegetable oils) or unprotected fats such as tallow can be tolerated in diets and may slightly increase the healthy fats in milk. However, unless unsaturated fatty acids are rumen-protected, feeding more than 1 lb. in total of unsaturated fats from all feeds in the diet often leads to milk fat depression and possibly decreased feed intake.
Rumen-protected or inert fat sources that are digestible and contain healthy fatty acids or precursors for synthesis of healthy fats by the mammary gland are the best feeding alternative today for achieving both total fats and healthy fats in milk.
Cost is important in selecting fat sources, but the total economic value in terms of milk production, fat yield, adding body condition and reproduction benefits needs to be considered in the selection of a fat supplement for feeding.
Someday, milk price may include the composition of fat and not just total fat. Until then, consumers will get an added health benefit by consuming dairy products with increased levels of healthy fats.