THEY are the people who don’t join their colleagues for after-work dinner parties and aren’t seen on the restaurant circuit over weekends.

People like office worker Qiao Qiao, 34. After dinner on Wednesday, she doesn’t eat anything until 3pm on Thursday.

She’s part of the new health trend of fasting, where people give up food for a prescribed stretch of time to lose weight and “detox” the body. There are now even weekend “detox camps” for those who need some professional help with fasting.

Is fasting healthy? Medical experts seem to agree that it’s a matter of how it’s done. They warn that those who fast need to know what they are doing and understand all the physical implications.

The idea of fasting for health and longevity isn’t particularly new. It has been around for centuries, often associated with religious beliefs. The concept is that we often don’t need to eat nearly as much as we do, and in many cases, we are choosing the wrong food to eat.

Qiao started the weekly fasting regime this year after frustrated attempts at bi gu (辟谷), a Taoist health philosophy which literally means “shunning food” to achieve longevity.

“For two years I fasted a few times a year to lose weight,” Qiao said. “The fasts lasted from a week to over 30 days, but my health deteriorated.”

Now she is an aficionado of the “fast diet” — an intermittent fasting regime popularized by a namesake book by British authors Michael Mosley and Mimi Spencer, published in 2013. Qiao, who said she prefers vegetarian food, said the new system seems beneficial.

“The only problem is I keep putting on weight,” she said. “I guess milder fasting schemes don’t work well for me.”

Reena, 40, a professional in the finance industry, fasts for 36 hours every two weeks, during which she drinks only water.

“I think I’ve found the right way to detox my body,” she said. “I feel energetic, both physically and mentally. But I find it hard to do a 36-hour fasting regularly.”

Reena said she was inspired by a health and diet guru named Qing’an, a mathematics professor at a college in Shandong Province, who recently began hosting free weekend fasting camps in a private villa on the outskirts of the Shandong capital Jinan.

Her camps have attracted more than 1,000 people from all over the country since they began in 2013.

Participants arrive for the fasting weekends on Friday evening. They have to get up before 5am on Saturday, when the day starts with some yoga exercises. After drinking water, they are taught how to make a breakfast recommended for weekdays back home. They spend the rest of the day practicing yoga and mediation, and they listen to lectures on healthy eating. No regular meal is served at the camp until Sunday noon.

Qing’an advises her devotees to maintain a largely vegetarian diet during the week and resume fasting and yoga on weekends. “The most obvious health benefit to my customers is the improvement of their digestion because many digestive problems are caused by wanton eating habits,” Qing’an said. “Basically you are what you eat.”

Last year, she said, she lived on water and fruit for 13 days without any desire to eat anything else.

“I often tell people that it’s okay for them to have some congee if they feel too weak to carry on with fasting,” she said.

The best way to fast, however, is to eat as little as possible every evening, according to the guru.

Medical experts say there’s no problem with skipping formal meals one or two days a week. It gives the digestive system a chance to rest. But water, they add, is indispensable during a fasting period as are vitamins and trace elements.

“Fasting should always be done warily because any diet of unbalanced nutrition that deprives the body of protein and trace elements will only result in illness,” said Xiao Ying, a member of the Shanghai Popular Science Committee of the Chinese Medical Association and a former pediatric doctor.

“A balanced diet, balanced health and a balanced mind are the best approach to detoxing your body, and regular physical exercise and bowel movements should serve that purpose,” Xiao said.

While some people who do the “fast diet” report a significant reduction in grocery bills, others are willing to pay up to 1,000 yuan (US$157) for a few bottles of special juices to aid them in fasting.

For about two years, Anne Pan, 28, an office worker, has been fasting one or two weeks every month, using three-day juice packages she orders on the Internet.

Pan, who drinks four bottles of the juice a day while fasting, said it relieves her hunger.

“I have a stressful job and that somehow tends to make one eat more,” she said. “But I feel more physically and mentally alert by eating less rather than desperately filling up my stomach like many of my colleagues do all the time.”

But Pan admitted that juice might not be a perfect fasting solution nutrition-wise, and a lack of exercise and irregular sleep patterns still take a toll on her health.

Hao Yan, founder of DASH, a subscription based healthy gourmet food company originated in New York, said so-called juice fasting is more of a marketing gimmick than a health science.

“Of course, people will feel full or even lose weight after drinking juice for a few days,” he said.

“But juices have sugar and with so much sugar intake, your body may tell you that you are full, but it doesn’t mean that your body is getting the proper nutrition. Furthermore, any weight loss from a three-day fast will be only temporary.”

He added, “Nothing beats the benefits of a healthy, balanced diet.”

His recommendation: Eat a bit less than you would like to — be 70 percent full.

Qing’an’s recommended diet for weekdays preceding a weekend fast:

Breakfast: salted congee with ginger powder and tofu skin, lettuce with jam made from peanuts, sesame or soybean, whole wheat steamed bun.

Lunch: balsam pear soup with black pepper, corn milk, lettuce with jam, stir-fried vegetables with bean products, steamed bun or brown rice.

Dinner: millet congee with wolfberries, pickled vegetables with seaweed, lettuce, sweet potato.

Hao Yan’s recommended breakfast drink, to be made in a standard home juicer:

1 green apple (red apple or pear to make it sweeter)

1 cucumber

200 grams of spinach or green leafy vegetable

1 piece of ginger

1 whole yellow lemon