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Soy protein, not isoflavones, lower blood lipids - study

By Stephen Daniells , 02-Mar-2006

Soy protein isolates (SPI), and not soy isoflavones, are associated with lowering blood fat levels, and reduce the risk of cardiovascular disease (CVD), claims a new study.

A recent scientific statement by the American Heart Association in the journal Circulation concluded that soy had little effect on cholesterol levels, and raised doubts about health claims associated with soy.

"[The new] results support the ability of soy protein to modulate the ratios of serum lipids in healthy young men in a beneficial direction for CVD risk, irrespective of the isoflavone content of the soy protein," wrote lead author Brianne McVeigh from the University of Guelph, Canada.

The new study, published in The American Journal of Clinical Nutrition (Vol. 83, pp. 244-251), looked at the effects of three different types of soy extracts on the blood lipid levels of 35 young, healthy men - a study population that is different than most reports which looked at people with diagnosed hypercholesterolemia (high cholesterol levels).

The three soy extracts used were a milk protein isolate (MPI) with about 32 grams of protein and no isoflavones, a low-isoflavone soy protein isolate (low-iso SPI) with about 32 grams protein and 1.6 milligrams of isoflavones, and a high-isoflavone soy protein isolate (high-iso SPI) with about 32 grams protein and 62 milligrams of isoflavones.

The blinded study involved three 57-day supplementation periods separated by 28-day washouts. Blood lipid levels were found to be similar at day one for each of the study groups - evidence that the washout periods were sufficient.

Blood samples were taken to measure serum levels of high-density lipoprotein (HDL) cholesterol, of low-density lipoprotein (LDL) cholesterol, total cholesterol and different apolipoproteins (apo A-I and apo B).

"When ratios of serum lipids were evaluated, results showed that total:HDL, LDL:HDL, and apo B:apo A-I were significantly lower with consumption of the low-iso SPI and the high-iso SPI than with that of the MPI," reported McVeigh.

Decreases of similar magnitude were seen for all three ratios for both the low-iso SPI and the high-iso SPI, while the protein from 'normal' milk raised the ratios, particularly the ratio of LDL:HDL.

"Elevated total:HDL was shown to predict cardiac events and HDL:LDL, and apo A-I:apo B are considered to be more accurate than are individual lipids for predicting coronary heart disease," wrote McVeigh.

The MPI also raised individual levels of triacylglycerides by about five per cent.

Since no difference was observed between the low- and high-iso SPI supplements, the researchers concluded that the effects of the soy was due to the presence of the soy protein and not the isoflavones.

"Further research is needed to evaluate the effects of processing techniques on the composition of soy protein and its influence on lipid lowering," concluded McVeigh.

Although this is further support for the soy industry, it does not fully contradict the claims by the American Heart Association (AHA).

Dr Frank Sacks, a member of the AHA panel, said in January: "It's really clear that isoflavones don't contribute anything to cardiovascular benefits."

CVD causes almost 50 per cent of deaths in Europe, and is reported to cost the EU economy an estimated €169 billion ($202 billion) per year. According to the American Heart Association, 34.2 percent of Americans (70.1 million people) suffered from some form of cardiovascular disease (CVD) in 2002.

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