People sunbathe their backs at a public square in Huaibei city, Anhui province, on July 29. WAN SHANCHAO/FOR CHINA DAILY

While westerners normally sunbathe to get tanned, TCM specialists have long considered sun exposure on the back — a body part containing a rich network of acupuncture points and channels where vital energy and blood flow — as an effective means to bolster immunity.

The best time to sunbathe the back is during the sanfu days — the hottest periods of summer based on the Chinese lunar calendar that runs from July 15 to Aug 14 this year.

On the micro-messaging platform Sina Weibo, two topics related to young adults relying on sunbathing their backs to shed weight surged to become one of the most-searched keywords recently.

On Xiaohongshu, an online lifestyle promotion platform, netizens have also shared pictures of them practicing the TCM therapy by lying on their stomach on a yoga mat or simply a piece of clothing. They often don a hat or place an umbrella on the ground to shade their heads and expose the rest of their bodies directly under intense sunlight.

Zhang Jin, head of the department of preventive treatment of disease at Xiyuan Hospital of the Chinese Academy of Chinese Medicine Sciences in Beijing, said that sunbathing during sweltering summer days can increase energy and boost immunity.

"But two kinds of people — those who are allergic to ultraviolet light and those who tend to have excess internal heat, suffer swollen throat or constipation frequently — are advised against conducting the therapy," she said in an article released on the official website of the National Health Commission.

"There is also no need for people with preexisting illnesses, especially high blood pressure and coronary heart disease, as well as pregnant women and toddlers, to sunbathe intentionally. It is suggested that they get sunlight casually during the daytime," she added.

Zhang suggested people who stay indoors in air-conditioned rooms for long periods, as well as seniors with chronic lung diseases or decreased bone strength, should tan their backs for 15 to 20 minutes at one go.

"Sunbathing sessions should take place before 9 am or after 3 pm, and each should last no longer than 30 minutes," she said, adding that a slight sweat would suffice and sweating heavily should be avoided.

Zhou Tianmei, a TCM specialist at Tongde Hospital of Zhejiang province in Hangzhou, Zhejiang province, said during an interview with People's Daily that weight loss from sunbathing mostly comes from the utilization of carbohydrates stored in the body, and lost weight will be quickly regained following water intake.

"Sunbathing the back is not aimed at losing weight, but dispelling chill and internal dampness in the body," she said.