Healing Foods

Natto: The Newest Soy

by Charles K. Rosenberg, CN

The incidence of breast, colon and prostate cancer is significantly higher in the United States than in Japan. In fact, breast cancer kills three times as many American women as Japanese women.¹ American women also have quite a different menopausal experience than women in Japan. Some 80 to 85 percent of U.S. women experience unpleasant hot flashes compared with less than 25 percent of their Japanese counterparts.² Scientists attribute some of the Japanese population's protection against breast, colon and prostate cancer, as well as the symptoms of menopause, to diets rich in soy phytoestrogens.

Many Americans are now incorporating more traditional Japanese soy foods, such as tofu and miso, into their diets. But these are not the only soy options. The third-most popular soy food in Japan is natto, a product practically unknown in the States. Natto is a fermented soy food produced by adding the bacteria Bacillus natto to lightly cooked soybeans. After fermenting for 24 hours, the soybeans develop a brown color, a sticky texture and a distinctive fermented odor and taste. Natto has been described as the Roquefort of soy foods. And just as Roquefort is not, to many Americans, the most palatable of cheeses, natto may never be as ubiquitous as tofu. But Americans—particularly women—would do well to acquire a taste for natto, since its potential health benefits are as extraordinary as its flavor.

Natto packs a punch of genistein, the isoflavone to which soy's anti-cancer benefits are largely attributed. It contains up to five times more genistein than either tofu or soy milk.³ In soybeans, genistein occurs in a glycosidic form, meaning it is bound to a sugar molecule. As such it is referred to as genistin. But genistin, unlike the unbound genistein, is not as bioavailable. Genistein is absorbed and used by the body at a significantly higher rate than genistin.⁴ However, there is little free genistein, the unbound form, in whole soybeans.³

Fermented soy foods, such as miso, tempeh and natto, have a decided advantage over unfermented ones such as tofu. During fermentation, the natural microbes that act as fermenting agents cleave genistin's sugar molecule, converting it to the better-absorbed genistein.³

Once genistein is absorbed by the body, it functions as a phytoestrogen. Phytoestrogens are plant compounds structurally similar to the body's natural estrogen. As such, they can occupy estrogen receptor sites on cells. But because phytoestrogens are weaker than natural estrogen, they only displace it and do not exert an estrogenic effect. By displacing natural estrogen, phytoestrogenic isoflavones may reduce the risk of breast cancer. In studies conducted in Asia, women eating the most soy food tend to have the least breast cancer risk.⁵ For example, premenopausal women who ate more than 55 g of soy products daily had 60 percent less chance of developing breast cancer compared with women who ate less than 20 g.⁶

The phytoestrogenic effect of genistein also helps reduce menopausal hot flashes, according to a 12-week, double-blind, placebo-controlled, randomized study of 104 Italian postmenopausal women. The women added to their daily diet 76 mg of soy isoflavones, which provided 40 mg of naturally occurring genistein, and experienced a 45 percent reduction of hot flashes.² The Italian researchers at the University of Ferrara used a powdered soy protein drink that the women were not too enthusiastic about. Perhaps if they had tried spaghetti and natto, a popular dish in Japan, the women would have enjoyed their isoflavones a bit more.

Japanese restaurants in the United States commonly serve natto in a nattomaki sushi roll or with rice, mustard and soy sauce. Natto is available in a wide range of varieties and flavors, including some manufactured domestically. It is packaged in individual servings, which typically contain 30 mg of isoflavones, half of which are genistein.⁷

In Japan, natto is associated with a wide range of health benefits, but its potential anti-cancer power alone should encourage Americans to give it a try. One caution: Anyone taking anticoagulant drugs such as warfarin should avoid natto because it has a high vitamin K content, which could interfere with these drugs' effectiveness.

Charles K. Rosenberg, M.S., C.N., is an adjunct faculty member in the nutrition department at Bastyr University in Kenmore, Wash., and a certified nutritionist practicing in Seattle.

References

1. American Cancer Society. Graphical data, cancer around the world, 1992-1995. In Cancer facts and figures; 1998. http://www.cancer.org/statistics/cff98/graphicaldata.html#catw.

2. Albertazzi P. The effect of dietary soy supplementation on hot flushes. Obstet & Gynecol 1998 Jan;91(1):6-11.

3. Fukutake M. Quantification of genistein and genistin in soybeans and soybean products. Food & Chem Toxicol 1996;34:457-61.

4. Hutchins AM. Urinary isoflavonoid phytoestrogen and lignan excretion after consumption of fermented and unfermented soy products. J Am Dietetic Assoc 1995;95:545-51.

5. Zheng W. Urinary excretion of isoflavonoids and the risk of breast cancer. Cancer Epidemiol Biomarkers Prev 1999;8:35-40.

6. Wu AH. Soy intake and risk of breast cancer in Asians and Asian Americans. Am J Clin Nutr 1998;68(Suppl):1437S-1443S.

7. USDA. Iowa State University database on the isoflavone content of foods; 1999. www.nalusda.gov/fnic/foodcomp/Data/isoflav/isfl_tbl.pdf.