Bryan Wang, 24-year-old financial analyst, likes sporty girls who are fit and trim.

"They look energetic and I like the way they walk," says Wang. "Skinny models are just beautiful on the runway. Few guys like that style in real life."

Extremely thin girls seem unhealthy, according to 27-year-old Jeff Jiang, a bank employee.

"A skinny girl colleague of mine has legs as thin as my arms," says Jiang. "I'm always afraid she'll get blown away in strong wind."

And in fact, the skinny girl often calls in sick, he says.

"It is well-known that overweight people are more vulnerable to the 'three highs' (high blood pressure, high cholesterol and high blood sugar) and other ailments, including diabetes," says Dr Yang Kefeng of the Nutrition Department of the Medical School of Jiao Tong University.

So taking off the extra weight is important for health.

BMI (body mass index) is an internationally accepted measurement for fitness,  it compares height and weight.

According to the World Health Organization, people with a BMI from 18.5 to 24.9 are fit, those with a BMI over 25 should lose weight; a BMI below 18.5 is too thin.

Dr Yang says he sees many young women with a good BMI who still want to lose weight. Their ideal is not within the BMI healthy range.

Considerable research shows that women become overweight and obese more easily than men. In China, however, that's not the case and the difference is only a few percentage points, says Yang.

The reason why so many more women try to lose weight is because they are highly self-critical and influenced by advertising with unrealistically thin models, say most nutritionists and women themselves.

Though the rate of cancer is lower among lean or thin older people than among the obese, too-thin teenagers and young women may have health problems, says Dr Yang.

"They tend to have lower immunity and the lack of nutrition can lead to physical problems and illness," he says.

The problem is not ordinary thin or slim,  it's ultra-thin.

Girls who are too thin may be low on estrogen, which has many consequences, including poor calcium absorption and brittle bones.

People, especially the elderly, who are very thin tend to have more memory problems than their heftier peers, as fat comprises 50 percent of the brain. Brain cells need fat for nutrition, metabolism and cognition.

Growing children and teenagers need a healthy diet for proper physical and mental development.