Japanese health enthusiasts are pursuing another lead in their quest for healthi er living. Following the green-tea boom, they are now drinking a much darker “tea,” prepared not from tea leaves but from black soybeans.

The infusion has a savory aroma of roasted beans and a slightly sweet taste. The left-over beans are soft enough to eat as a snack, thereby making them “a great way to consume all the nutrients in soybeans,” as a flier advertising black-soybean tea points out.

The black soybean, a rare legume native to China, has long been used in Chinese medicine to clear toxins from the body and promote urination. According to Japanese folklore, the bean infusion is also a cure for sore throat.

Its consumption as a food has been limited, though. In contrast to the yellow soybean, a variety the Japanese have utilized for everything from soy sauce to natto, the black soybean is consumed by many Japanese only once a year in a traditional New Year’s dish.

Now, however, several companies have developed a method for roasting the legume, making it easy to make black soybean tea and eat the beans as a snack.

The simplicity of preparation is appealing to consumers: The beans become tender after soaking in a cup of boiling water for just a few minutes.

At least part of the enthusiasm for black-bean tea seems to come from the fact that it is prepared in a similar way to many traditional medicines, where active ingredients are extracted by pouring boiling water over roasted or dehydrated materials. Bean-tea fans say it is better to drink the infusion as well as eat the softened beans since large amounts of soluble ingredients go into the water, which is often discarded after the cooking process.

Preliminary studies suggest the black soybean may have more health benefits than the conventional soybean.

One animal study conducted by researchers at Shizuoka Prefectural University and the food-processing company Fujicco Co., showed that the black soybean is more effective than the yellow soybean in preventing menopausal symptoms. The study found that menopausal rats, whose ovaries had been removed, showed a significantly higher decline in their blood-cholesterol level when given feed containing black soybeans, compared to those fed yellow soybeans. After four weeks, the blood cholesterol level of the black soybean group was up to 31 percent lower than a third group fed without soybeans, while the yellow soybean group was only a maximum of 16 percent lower.

“There is little nutritional difference between black and yellow soybeans except in the pigment of the hull,” says Takenori Okuhira of Fujicco’s research and development section. “So we are now checking what substances the hull contains.”

Studies have found that black soybeans contain anthocyanin, a type of polyphenol found in high concentrations in blueberries and raspberries. The substance is known to work in the body as an antioxidant, which helps neutralize unstable oxygen molecules that can damage cells. Okuhira says anthocyanin may be what his team is looking for, but they are also exploring other ingredients.

Some anecdotal evidence also indicates the health benefits in the seed coat. Physician Yutaka Nozaki of Hyogo Prefecture, a place famous for producing Japan’s finest black soybean crops, reported at a prefecture-sponsored conference that black soybean stock (prepared by boiling 50 grams of beans in 2 1/2 cups of water) was beneficial for decreasing blood pressure among his hypertension patients, lowering the blood-sugar level among his diabetic patients and even reversing graying hair. The effects were also confirmed using bean hulls only, but not with soybean flour made of hulled beans, he noted.

Such unrestrained praise for the bean extract is tantalizing. But experts warn that consumers should not see the beverage as cure-all. When it comes to the brewed tea in particular, there has been no laboratory or clinical research about the health benefits of the infusion published in peer-reviewed journals.

Overall, health claims made by retailers that black-soybean tea products are beneficial for such chronic diseases as hypertension, diabetes and osteoporosis are mostly based either on folklore, anecdotes or the findings of research into the yellow soybean. Retailers themselves admit it is not known for sure exactly how much, if any, nutritional value is contained in the beans, recommending consumers eat the cooked beans and drink the tea as part of a balanced, calorie-controlled diet.

It is true that studies have shown wild plants such as black beans and black rice have more health benefits than lighter-colored crops, says Toshihiko Osawa, professor at Nagoya University and a leading food expert.

“But the darkish color of the tea doesn’t necessarily mean it contains a high enough level of substances such as anthocyanin to be beneficial to the body,” he says. “Drinking black soybean tea is good for people’s health if it provides people with more opportunities to consume soybeans.”

Indeed, all varieties of soybean are good sources of protein and are now attracting worldwide attention for their potential to prevent various diseases, including heart disease and cancer, and the symptoms associated with menopause.

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