While yuzu seed oil is processed, Mochifumi Totani, who heads the Umaji agricultural cooperative, talks about the research results and products in Umaji, Kochi Prefecture. Shunji Mizobuchi, a professor at Kochi Medical School, is also interviewed in Nankoku in the prefecture in December. (Mizuho Morioka)

A dozen or so yuzu floating in the bathtub while taking a soak is a time-honored Japanese remedy to combat flaky skin in winter.

Tiny shavings of peel from the small ball-shaped citrus fruit rich in vitamin C and antioxidants are also widely used to give pep to "nabe" hot pot dishes as well as steaming bowls of "soba" noodles.

For most people, that's as far as it goes with yuzu, which has a unique taste. However, research shows that the pips buried in the flesh of the fruit can be used to make oil extract to treat atopic dermatitis, or itchy skin, and perhaps even metabolic syndrome.

Kochi Prefecture, the nation's top producer of yuzu, also boasts the medical school of Kochi University, which teamed up with the agricultural cooperative of Umaji, a village in the prefecture, in 2009 to research the health benefits of yuzu.

In 2016, Kochi University obtained a patent on a drug to treat allergic dermatitis externally with an essential oil derived from yuzu seed extract.

The researchers are hoping to prove its efficacy against metabolic syndrome, which manifests itself in excess body fat around the waist and is a trigger for a range of serious disorders such as stroke and diabetes.

Mochifumi Totani, who heads the Umaji agricultural cooperative, is delighted that the research team found a use for yuzu seeds.

"Previously, yuzu pips were the only part of the fruit that was of no use," Totani noted.

The goal of the researchers is to be granted authorization to label their products as Foods with Function Claims, a certification issued by the governmental Consumer Affairs Agency.

The certification pins the responsibility on food business operators for the safety and functionality of labeled products based on scientific grounds.

Most yuzu weigh about 100 grams. The seeds, normally deemed to have no use, account for 10 percent of the bulk.

The Umaji agricultural cooperative started to produce yuzu seed oil extract in 2006 for use as a skin lotion.

In a good harvest, when the annual yuzu yield is around 1,000 tons, the cooperative can produce between 1.6 to 2 tons of oil extract.

Yuzu was listed in a volume of Chinese herbology known as "Bencao Gangmu" during the Ming Dynasty (1368-1644) as an effective medicine for hangovers and gastric pain, and used in ancient Japan to treat various ailments.

During trials to test its efficacy against metabolic syndrome, researchers fed mice with yuzu oil extract and found that the concentration of Adiponectin, a beneficial hormone in the blood, increased by 75 percent. Adiponectin burns fat in blood and helps insulin function to decompose sugar.

The finding suggested that yuzu oil extract is effective against metabolic syndrome. Researchers plan to conduct human clinical trials shortly.

They also found that yuzu extract taken in capsule form works to reduce the blood concentration of lipid peroxide, which contributes to the aging process.

The university obtained a patent for the antioxidant effect of yuzu seed oil extract in March 2017.

“We discovered through our research that (yuzu seeds) work against a wide variety of ailments. We were stunned,” said Shunji Mizobuchi, a professor of digestive surgery and clinical nutrition at Kochi Medical School. “We intend to do more research to grasp the power of yuzu.”

“In the past, there was not a lot of scientific evidence available to prove the health efficacy (of yuzu seeds),” said Kimito Asano, a graduate student at Kochi University and member of the Umaji agricultural cooperative who was involved in the development of yuzu seed oil lotion.

"We can promote yuzu focusing not only on its taste (but also on health benefits)," Asano said.

In Totani's view, more research will be good for yuzu farmers as well as society as a whole as "we now know that the pips are good for health."

Totani was certified as a sightseeing contributor with rare personal magnetism by the Japan Tourism Agency in recognition of his efforts to rejuvenate depopulated Umaji through the marketing of processed yuzu goods, which resulted in sales exceeding 2.9 billion yen ($27 million) in 2003.

For information about products made from yuzu (only in Japanese) check the website of the Umaji agricultural cooperative (https://www.yuzu.or.jp/).