Soy protein and omega 3 fatty acids, which are found in abundance in fish, flax seed and some nuts, long have been touted for their health benefits. But researchers suggest they also produce physiological effects that benefit blood flow.
In the first study, scientists led by David Jenkins, a professor of medicine and nutritional science at the University of Toronto, looked at 23 men and 18 postmenopausal women all with elevated cholesterol levels. Study participants rotated through three one-month diets low in saturated fat.
In the first diet, the main sources of protein such as meats and fish were replaced with low-fat dairy products and an egg substitute. The other two diets consisted of low-fat soy milk and a variety of soy-base meat substitutes, such as tofu burgers and soy hot dogs -- with one diet containing high levels of isoflavones, a plant-based compound, and the other featuring low isoflavone levels. Body weight, blood lipids and blood pressure were measured before and after the diets.
Study results showed no significant differences in blood lipids and blood pressure between the high- and low-isoflavone soy protein diets. Low-density lipoprotein, or LDL, cholesterol -- the so-called "bad" cholesterol that clogs arteries -- was lower after the soy diets compared to the dairy-and-egg-substitute diet. One difference was blood concentrations of LDL were lower after the high-isoflavone diet.
The only significant difference between the sexes was greater blood pressure drops among the men after the high-soy isoflavone diet.
Jenkins said it might be the whole soy protein and not just the isoflavones that are having this cholesterol-fighting benefit.
"I'm not saying isoflavones may not have effects, but I think these effects are relatively mild," Jenkins told United Press International. "We believe the soy protein itself has major effects (and) it's the protein composition that may be very important or the nature of the amino acids."
Jenkins added, "We're using our soy experience to build it into diets which we believe will be able rival those starting doses of cholesterol-lowering drugs."
A second study, from Baker Medical Research Institute in Melbourne, Australia, compared the effects of two types of omega 3 fatty acids, one called eicosapentaenoic acid or EPA, and the other called docosahexaenoic acid or DHA. Thirty-eight middle-aged men and women with elevated cholesterol consumed either an EPA supplement, a DHA supplement or a placebo for seven weeks.
Researchers found arterial elasticity improved 36 percent in the EPA group and 27 percent in the DHA group, but there was no change in the placebo group. If an artery is too stiff, blood flow is unable to pass smoothly through it and cholesterol plaques can build up more easily. Triacylglycerol or TAG levels also were lower among the omega 3 supplement groups. Lower TAG means reduced risk for heart disease.
"The greater the elasticity, the less likelihood of hypertension," Keith Ayoob, a spokesman for the American Dietetic Association and a professor at Albert Einstein Medical College in New York, told UPI. "Any benefit would be speculative at this point, but we know omega 3 fatty acids from fish oil seem to have an effect of lubricating the arteries."
Ayoob cautioned, however, the Australian study was small and brief and therefore more research is needed. Symptoms can improve on the short-term, he said, and then rebound, so it is unclear how long omega 3s could have this effect.
Ruth Kava, director of nutrition at the American Council on Science and Health in New York, agreed. "But maybe if you'd eat these constituents for six months, you'd really pick up a difference."
The bottom line, Kava told UPI, is supplements -- not whole foods -- offer the benefits. This holds true for getting enough omega 3 fatty acids and isoflavones. She also said it might not be only the isoflavones in soy protein that are producing a cholesterol-lowering effect, but other compounds as well.
"It may not be the isoflavones," Kava said. "It may be something else that has to do with soy foods."
Both studies are published in the July 25 issue of American Journal of Clinical Nutrition.
(Reported by Katrina Woznicki, UPI Science News, in Washington)