There is a weird trend among women in China: posting photographs showing off their thin waists, thinner than the width of an A4 size paper sheet. People, especially the young, could go on extreme diet influenced by such bizarre trends.

When people - particularly adolescents who want to be excessively slim - follow an unsupervised diet, they become vulnerable to health problems. Although teenagers may have many reasons for dieting, dissatisfaction with their bodies and the desire to be thinner are the usual motivating factors. Adolescents, especially girls, are the target of advertisements showing unrealistically thin women, which they wrongly equate with beauty, health and personal success.

Since society places a high value on youth and physical beauty, adolescents try to imitate the images they see in the media. As a result, they often engage in unhealthy, unnecessary and unsupervised attempts to lose weight.

To lose weight, many teenagers adopt a series of behavioral changes and alter their eating habits. They resort to fad dieting, fasting, skipping meals, using laxatives, practicing self-provoked vomiting and using dangerous supplements or drugs to reduce weight. Studies show such behaviors are more prevalent among girls. But there is no socio-economic or ethnic group immune to this trend.

Teenagers not consuming enough calories could become weak, tired and moody. Also, since insufficient calories can affect the functioning of their brains, they may not be able to make the right food decisions, which in turn could even affect their learning process. Severe lack of food could even affect their hearts, bones and other organs, and in extreme cases they could even die because of malnourishment.

Some teens may use diuretics (water pills), diet pills, tobacco or other drugs to control their weight. But these substances can cause serious damage to their organs and, the nicotine in cigarettes or amphetamines in certain diet pills could turn them into addicts and cause permanent damage to their health.

Research shows teenagers suffering from chronic illnesses such as diabetes, asthma, attention deficit disorder or epilepsy are more prone to be dissatisfied with their bodies and practice unhealthy weight loss habits. Teenagers with significant psychiatric symptoms such as depression and anxiety, too, are likely to engage in unhealthy dieting practices. And adolescents who use "quick" weight-loss strategies, particularly when they are unsupervised, are also more likely to indulge in other risky behaviors such as substance abuse, unprotected sex and suicide attempts. Besides, girls overly concerned about their weight are more likely to start smoking.

To prevent youngsters from following dangerous dieting practices, parents and teachers should teach them the difference between "healthy weight" and "cosmetically desirable weight", and encourage them to accept a realistic weight for themselves.

The author is an international public health consultant.