Sales of soy rose by 17.9 percent between 2001 and 2002, says Mintel, but growth of 12.6 percent the following year showed that the slow-down had begun. The trend became even more pronounced between 2003 and 2004, when the market increased by just 6.1 percent.
Consumer research carried out last year also showed a decline in usage: 27 percent of adult respondents said they were likely to buy soy-based versions of popular food and drinks, compared with 30 percent in 2002.
Mintel suggests several causes from the drift away from soy.
For several years soy has enjoyed urban legend status as a natural estrogen replacement with health benefits for menopausal women. Between 2002 and 2003 soy products marketed as 'menopause friendly' began to appear.
Although the FDA cleared claims that a diet rich in soy protein may reduce the risk of coronary heart disease in 1999, the estrogen replacement claim has not been substantiated.
A study carried out in 2003 by the American Council on Science and Health indicated only moderate and inconsistent connections between soy isoflavones and relief from menopausal complaints - insufficient evidence to merit FDA approval.
As consumers become more aware of the administration's role in approving health claims made by certain foods, they are less likely to be convinced by myth and hearsay. Thus less menopausal women are buying soy products in the hope that it will alleviate their complaints.
Another reason cited by Mintel is that more organic meat and dairy products have begun to appear in mainstream outlets. Whereas consumers might previously have opted for soy alternatives due to publicity over disease or contamination, they are now turning back to traditional products, their fears allayed by organic assurances.
"The manufacturers and marketers behind these products are up against some significant competition from non-soy products, but have managed to expand the category year after year," said Erik Thoresen, senior research analyst at Mintel.
Mintel forecasts that the soy market will move into a more stable, mature phase as manufacturers diversify into new food segments. Over 435 new soy-based products introduced to the market in 2004, compared with 329 in 2003.
Energy bars and gels made up 41.3 percent of the market last year - more than dairy and meat alternatives combined, which accounted for 22.9 and 14.3 percent respectively.
Cold cereal made up 11.2 percent of the market, frozen entrees, pizza, and convenience foods 5.4 percent and condiments 4.9 percent.
A separate report from Business Communications Company in December suggested that soy-based foods were no longer closeted in specialist health food shops but were starting to appear on the shelves of mainstream outlets in increasing volumes.
"The ascendancy of functional snacks, especially low-carbohydrate, soy-containing ones, should set the pace for growth over the next five years," it said.
The FDA is currently reviewing a qualified health claim linking soy protein-containing foods with a reduced risk of certain types of cancers, submitted by Solae in April 2004.