Department of Food Science and Human Nutrition
Extension
Colorado State University
Fort Collins, CO 80523-1571
Did You Know?....
January/February 2005
Vol 26 No. 1
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The format for this section is altered this month from the usual Q/A format to an interview Q/A format with Dr. Kenneth Allen who is featured in our Spotlight this month. Dr. Allen comments on a recently published study [Journal of Nutrition, 2004; 134(11):2991-2997 (November)] in which researchers reported that a diet rich in alpha-linolenic acid from walnuts, walnut oil and flaxseed oil not only lowered bad cholesterol but also decreased markers for blood vessel inflammation in men and women at risk of heart disease.
Q: According to a recently published study on walnuts, alpha-linolenic acid from walnuts, walnut oil, and flaxseed oil not only lowers bad cholesterol, but also decreases cardiovascular risk by decreasing C-reactive protein. Can you clarify the differences between linoleic acid (LA) and linolenic acid (LnA) for the purpose of understanding the benefits of including walnuts in the diet?
A: Both linoleic (LA) acid and linolenic (LnA) acid are essential polyunsaturated fatty acids. The body cannot make them; they must be obtained from food. LA acid has 18 carbons with 2 double bonds at positions 9 and 12. LnA acid also has 18 carbons with 3 double bonds at positions 9, 12 and 15. LA is an omega-6 fatty acid while LnA is an omega-3 fatty acid. Both LA and LnA are essential for cell membrane structure and function, but for this role they are both elongated (additional carbons added) and new double bonds (desaturation) are also inserted. LA is also used for the production of important hormones such as prostaglandins, but once again it is the elongated and desaturated product of LA (called arachidonic acid) that is used to produce these hormones.
Since both LA and LnA are polyunsaturated fatty acids the finding that they both decreased serum total and LDL cholesterol and serum triglycerides is not surprising as this is a well established response in humans.
The serum level of C-reactive protein (CRP) is a marker of inflammation and cardiovascular heart disease is an inflammatory disease. People with increased serum CRP have increased risk of cardiovascular disease, and this increased risk appears to be independent of their serum cholesterol and LDL cholesterol levels. The study shows that both the LA and the LnA diets lowered serum CRP - in both cases this was shown by correlation analysis. However, while the LnA diet appeared to exert a greater decrease, this was not significantly different from the LA diet.
I'd like to make one final point about this article and the source of LnA. Both experimental diets contained the same amounts of walnuts and walnut oil, i.e., approximately half of the fat in both diets was provided by walnuts and walnut oil. The high LnA diet was formulated by adding flaxseed oil which has a very high LnA content of approximately 50%.
Q: There is often confusion as regards omega-3 fatty acids in walnuts, e.g. as compared to the omega-3 fatty acids from marine sources. Are they equally beneficial for improving cardiovascular risk?
A: The most potent omega-3 fatty acids in terms of human health are the long chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Both EPA and DHA are found in high quantities in certain fish (sardines, salmon) and fish oils. While humans can make DHA from the LnA in their diet, the pathway is not very efficient. Apparently, the high LA content of typical diets in the U.S. (averaging 7% of calories) inhibits the conversion of LnA to DHA. Numerous studies in humans have shown that even high quantities of LnA are not converted to DHA in appreciable amounts. In the case of cardiovascular disease risk EPA and DHA are very effective in reducing serum triglycerides and in reducing some of the prostaglandins that cause arterial clotting.
Q: Please define EPA and DHA as they relate to research results on walnuts.
A: Regarding walnuts, or any vegetable oils or nut oils for that matter, there is no EPA and DHA in these oils. Walnuts do contain LnA but the amount of LnA in walnut oil is quite similar to that of soybean oil.
Q: What quantity of walnuts would one have to eat to achieve this cardiovascular health benefit?
A: Based on the results of the study one can do a simple calculation to determine the quantity of walnuts or walnut oil that would need to be consumed to achieve the LnA content of the high LnA diet. Remember, both experimental diets contained the same amounts of walnuts and walnut oil, but the high LnA diet was formulated by the addition of flaxseed oil. The high LnA diet provided 6.5 % of calories as LnA. Assuming a 2000 calorie diet, 6.5% is 130 calories from LnA. There are 9 calories per gram of fat so this represents 14.44 g per day of LnA. Walnuts contain 9 g LnA per 100g serving (USDA Nutrient Data Base). Thus one would need to consume 160.44 g of walnuts per day - about 6 ounces. However, this amount of walnuts would provide 1,050 calories per day - about half of the daily caloric intake!
How about walnut oil? Walnut oil contains 10 g LnA per 100 g walnut oil (USDA Nutrient Data Base). One would have to consume 144.4 g of walnut oil to achieve the desired LnA intake. Once again this comes at a considerable cost of about 1,300 calories per day!
Q: As of April, 2004 walnuts received approval for a 'qualified health claim' from the FDA. Sellers of whole and chopped walnuts can claim "supportive but not conclusive research shows that eating 1.5 oz. (a little more than a handful) of walnuts per day as part of a low saturated fat and low cholesterol diet, and not resulting in increased caloric intake, may reduce the risk of coronary heart disease." Does current research conflict or support the approval of this qualified health claim?
A: The qualified health claim that sellers of whole and chopped walnuts may now assert - "supportive but not conclusive research shows that eating 1.5 ounces (a little more than a handful) of walnuts per day as part of a low saturated fat and low cholesterol diet, and not resulting in increased caloric intake, may reduce the risk of coronary heart disease"- was approved in April of 2004. To me it seems to be a reach. After all, the LnA content of walnut oil is very similar to soybean oil.
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