Illini DairyNet Papers
TAKE HOME MESSAGE
- Milk fat has a negative image with many consumers
- Conjugated linoleic acid (CLA), a component of milk fat, has potential health benefits.
- The content of CLA in milk fat has been shown to change under certain dietary conditions.
- Our research will investigate if milk CLA content can be increased by feeding soybean oil to cows.
Whole milk consumption in the US has declined dramatically in the past 20 years. This has been attributed to health concerns about the content and composition of milk fat. The public has been exposed to negative messages about milk fat, specifically regarding the low ratio of poly-unsaturated to saturated fatty acids. However, this is an oversimplification. Hydrogenated vegetable oil, which has a favorable polyunsaturated: saturated ratio, may contain 50 percent trans fatty acids. Linoleic acid, a major polyunsaturated fatty acid of vegetable oils, has been linked to increased risk for some cancers. Moreover, some minor components of milk fat, notably CLA, sphingomyelin, butyric acid, and ether lipids have beneficial effects on health, especially as anticarcinogens. Research at the University of Illinois is presently focusing on one of these compounds, CLA. This report briefly describes some previous research investigating health benefits of CLA (references available upon request ) and the strategies of our research program.
What is CLA and where does it come from?
The acronym CLA stands for conjugated linoleic acid. It is found in milk and meat products of ruminant origin because it is synthesized in the rumen during the biohydrogenation of linoleic acid. It is not an end-product of this reaction and is quickly metabolized to other compounds. However, research has shown that it is possible to influence the extent of ruminal biohydrogenation and the concentration of CLA absorbed and incorporated into milk fat.
What are the health benefits of CLA?
The anticarcinogenic activity of CLA was discovered when researchers who were investigating tumor-causing compounds in cooked meat noticed that one of the components exhibited anticarcinogenic activity. This compound was later identified as CLA. Since that research was published the health benefits of CLA have garnered a great deal of attention. The research summarized in Table 1 is only a representative sample. Although it has been necessary to conduct this research using animal and cell culture models, the mechanisms of action are applicable to humans. CLA has been identified in human plasma and breast milk in concentrations that are influenced by diet.
Table 1 illustrates two important points. First, the potential health benefits are diverse and many have only recently been investigated. Second, CLA is effective at very low concentrations in the diet. It has been estimated that a human consuming about 3.5 grams CLA/day is ingesting an amount comparable to a rat consuming 0.1% CLA in the diet. The CLA content of milk fat was 18 mg CLA/g lipid when cows consumed a diet containing sunflower oil (high in linoleic acid).
Table 1. Health benefits of CLA
|Chemically induced mammary tumorigenesis in rats||CLA, 0.05% to 0.5% of the diet||dose dependent9in mammary tumor yield|
|Chemically induced stomach tumors in mice||CLA, 0.8 ml in olive oil, administered by gavage||CLA-treated mice developed only 50% as many tumors|
|Chemically induced skin tumors in mice||CLA up to 1.5% of the diet||9 in tumors with 1% dietary CLA|
|Chemically induced colon cancer in rats||CLA, (0.5% of the diet) by gavage||CLA inhibited the formation of aberrant crypts in the colon|
|Rabbits||CLA, 0.5 g/rabbit/day for 12 weeks||9 in total and LDL cholesterol, 9 in atherosclerotic plaques in the aorta|
|Pregnant and lactating rats||CLA, 0.25% or 0.5% of the diet||CLA improved postnatal growth and feed efficiency of pups born to CLA-supplemented dams|
|Chicks, rats, and mice||CLA, 0.5% of the diet||9 in body fat and 8 in lean body mass and carcass water|
|Chicks and rats injected with endotoxin||CLA, 0.5% of the diet||9 in post-injection weight loss|
A 12-ounce glass of this milk, if it contained 3.5% milk fat, would contain about 225 mg of CLA. It is evident that if the CLA content of ruminant products is increased a consumer may select a diet containing a beneficial amount of CLA.
Can the CLA content of dairy products be increased?
There is limited work examining factors which specifically affect the concentration of CLA synthesized in the rumen, and/or transferred to milk fat. It was shown more than 30 years ago that the conjugated diene (probably CLA) concentration of Canadian milk fat exhibited seasonal variation, most likely due to dietary changes. When cows were fed sunflower oil (5.3% diet DM), which is high in linoleic acid, the CLA content of the milk fat was 18.1 mg/g, almost double the 9.9 mg/g lipid produced by cows consuming peanut oil (low in linoleic acid). A change in the forage: concentrate ratio from 50:50 to 35:65 resulted in CLA content of the milk fat increasing from 6.6 to 11.3 mg/g lipid.
In research at the University of Illinois, we will be take advantage of factors described above by dietary supplementation with oils high in linoleic acid (soybean oil) and a high-grain ration that depresses rumen pH. We will monitor the synthesis of CLA in the rumen and its incorporation into milkfat. The overall objective of this work is to utilize the rumen microbes to synthesize a beneficial compound, CLA, from linoleic acid that is found in soybean oil. This work has the potential to benefit both the consumer and the dairy industry. Watch for results to be presented in a future edition of the Illinois Dairy Report!