Soy Protein Did Not Reduce LDL Cholesterol in Study of Adults With Mildly High Cholesterol
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A new study shows that eating soy protein didn't cut LDL cholesterol levels in adults with mildly high cholesterol levels. The findings appear in the August edition of the American Journal of Clinical Nutrition.
The FDA allows foods containing at least 6.25 grams of soy protein to carry a health claim stating that consuming 25 grams of soy protein per day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.
The new study, conducted in Australia, tested the impact on cholesterol from eating that much soy protein in 90 adults with borderline high cholesterol levels who weren't soy eaters.
How high was their cholesterol? On average, their total cholesterol was 217 and their LDL cholesterol was 139 when the study began. Ideally, total cholesterol should be under 200 and LDL under 130.
For six weeks, participants ate 24 grams of soy protein per day, split into three daily servings. They got their soy protein in drinks, custard, cookies, snack bars, and pasta supplied by the researchers.
For comparison, participants ate half as much soy protein for another six weeks, and then no soy protein for a final six-week period. They took blood tests before and after each six-week diet.
While on the higher-dose soy protein diet, participants had a 3% drop in their total cholesterol and a slight drop in their LDL cholesterol. And although their LDL cholesterol dipped slightly, that change wasn't statistically significant, meaning that it might have been due to chance.
The 3% drop in total cholesterol may have been because they were eating less saturated fat, note the researchers, who included graduate student Alicia Thorp of the University of South Australia.
Thorp's team also went one step further and checked whether people's ability to digest certain soy nutrients into a compound called equol made any difference. It didn't.
WebMD asked Alice Lichtenstein, DSc, who is the Stanley N. Gershoff Professor of Nutrition Science and Policy at Tufts University in Boston, and Nancy Chapman, RD, MPH, executive director of the Soyfoods Association of North America, to review the study.
The study is "excellent" and in line with findings from her own lab and recent work by other researchers, Lichtenstein tells WebMD.
In a statement emailed to WebMD, Chapman says the study "confirms that soy foods lower total cholesterol even though the drop in LDL cholesterol wasn't significant. She points out that the participants' LDL levels were "nearly" normal when the study started, and that the study might not have picked up on small changes in LDL cholesterol.
Lichtenstein counters, "I don't think you'd find people to study that had extremely high cholesterol levels that were going to be treated just with soy, because it's not consistent with current guidelines," which involve cholesterol-lowering drugs, as well as dietary changes. Lichtenstein also says the 3% drop in total cholesterol wasn't enough to bring participants' total cholesterol down to the normal range.
Chapman says other research has linked soy consumption to cholesterol improvements.
Lichtenstein sees some benefit in soy foods. "If you use a soy burger or a soy frankfurter to displace a hamburger or frankfurter, you would likely see a drop in LDL cholesterol because you're decreasing saturated fat," says Lichtenstein. "From that perspective, using soy-containing foods to displace foods with animal fat is probably advantageous."
But Lichtenstein says no recent data in people with mildly high cholesterol levels show that soy protein or other soy nutrients called isoflavones go above and beyond that to independently reduce heart disease risk.
"I think whenever there is a claim that a food is going to do something positive like lower cholesterol, we all become very enthusiastic and it usually gets embraced without too much hesitation," Lichtenstein says. "However, when data starts emerging that perhaps the original assessment was not accurate, the bar is much higher as far as what's necessary to establish lack of effect as opposed to an effect."
SOURCES: Thorp, A. American Journal of Clinical Nutrition, August 2008; vol 88: pp 298-304. FDA: "A Food Labeling Guide: Appendix C: Health Claims." WebMD Medical Reference from Healthwise: "High Cholesterol: Exams and Tests." Alice Lichtenstein, DSc, Stanley N. Gershoff Professor of Nutrition Science and Policy, Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy, Tufts University; senior scientist and director, Cardiovascual Nutrition Laboratory, Jean Mayer USDA Human Nutrition Research Center on Aging, Tufts University. Statement from Nancy Chapman, RD, MPH, executive director, Soyfoods Association of North America.
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