Analysing 22 clinical trials, researchers from the American Heart Association's nutrition committee found that large amounts of isolated soy protein in the diet only lowered LDL cholesterol by about 3 percent on average.
The scientific statement, published in the January 23 issue of the association's journal, Circulation, throws some doubt over the health claim approved by FDA in 1999 and now widely used by makers of soy products.
"The problem is that they [FDA] say soy protein is something special. But it isn't any better than other proteins, although soyfoods should still be recognized as good foods," said Dr Frank Sacks, a member of the panel.
He added that many of the studies tested doses of up to 50g a day. The FDA health claim suggests that consuming 25g soy protein daily can help reduce cholesterol levels.
An FDA spokesperson told us that the agency is evaluating the science. "FDA may determine that a re-evaluation of a health claim may be needed when new scientific evidence does not support the current claim," he said.
Dr Sacks, a professor of nutrition at the Harvard School of Public Health in Boston, said that several of the new studies reviewed were 'very well-controlled'.
"There were early studies back in the 80's that showed a much larger effect but these were not that well controlled," he told NutraIngredients-USA.com.
The review also found that soy protein had no effect on HDL cholesterol or blood pressure. And a further analysis of 19 studies showed that soy isoflavones, the bioactive molecules found in soy, had no effect on lowering LDL cholesterol either.
"Its really clear that isoflavones don't contribute anything to cardiovascular benefits," said Dr Sacks.
The AHA had previously recommended in a 2000 statement that soy protein should be added to a low-fat diet for good heart health. However it decided to review the evidence on soy following a number of new studies in the last few years.
The authors acknowledge however that soy protein products such as tofu, soy butter, soy nuts or some soy burgers should still be considered healthy foods as they are high in polyunsaturated fats, fiber, vitamins and minerals and could replace other high-fat proteins in the diet.
"Soy products may have benefits when replacing other foods such as hamburgers," Sacks said.
The committee also found that soy protein and isoflavones did not lessen symptoms such as hot flashes due to menopause and results were mixed with regard to soy's ability to slow postmenopausal bone loss.
Evidence of protection against cancer is also weak, the committee reported.