During adolescence, Wong Hoi-yee, now 23, had a normal body mass index (BMI). But like many teenagers, she was conscious about her weight and keeping fashionably slim. When she turned 19, Wong (name changed for reasons of patient confidentiality) became increasingly unhappy with her weight and started dieting to lose unwanted kilograms. She would never sit down to a proper meal, and lunches and dinners consisted mainly of small bites. One morning, she slipped and fell on a freshly mopped floor. X-rays showed that she had fractured the thigh bone, or femur, in her left leg. Setting the bone required open surgery. The surgeons operating on Wong found that the bone had fractured in three places, and noted that it was abnormal to have a severe fracture at such a young age. Wong was diagnosed with osteoporosis and referred to Professor Annie Kung Wai-chee, a specialist in endocrinology and diabetes and former director of the Osteoporosis Centre at Queen Mary Hospital. On referral, Wong's weight was 36.3kg. She was seriously underweight for her height with a BMI of 14. Healthy adults should have a BMI of between 18.5 and 22.9. Osteoporosis is a disease in which the bones become brittle and fragile, leading to a higher risk of fractures. Bone is a living tissue. Throughout a person's life, old bone is continuously broken down and replaced by new bone in a process called remodelling. Osteoporosis mainly occurs when people get older because this remodelling slows down. When the rate of bone loss is faster than that of new bone formation, the bones become thinner and weaker. When the amount of bone (bone mass) has been reduced to a significant level, fracture or breaking of bones may occur in the absence of trauma or in response to trivial trauma. Calcium is essential for many bodily processes, including building strong bones and contributing to and maintaining optimal bone mass. Dietary calcium that's not required immediately is transported to the bones, where it's added to bone mass and stored there until it's needed by the other parts of the body. If there's insufficient calcium from the diet, as in Wong's case, any stored calcium will be removed to maintain other essential functions. This can result in the loss of bone strength and mass, and can ultimately lead to osteoporosis. Wong's bone mineral density was evaluated using dual energy X-ray absorptiometry (DEXA), a simple radiology scan that takes measurements at the spine, hip and wrist. The scan results are given as two different scores: T-score and Z-score. The Z-score compares the bone density with that of a person of the same age. The T-score compares the patient's bone density with that of a person who is 30 years old, which is the peak age of bone mass. A minus one deviation is roughly equal to a 10 to 15 per cent decrease in bone density. The World Health Organisation classifies T-score values above minus one as normal, between minus one and minus 2.5 as osteopenia, and any score below minus 2.5 as osteoporosis. Wong's T-score was minus 3. Says Kung: 'This very low score was unreasonable for someone of her age and suggested a very low bone density and a very high risk of further fracture.' Wong was referred to a dietitian, who advised an individualised regime based on balanced and regular meals, as well as calcium and vitamin D supplements. Vitamin D is needed for the absorption of calcium, and is mostly obtained via the action of sunlight on the skin. After a year, Wong's weight increased by only a few kilograms, and the DEXA scan showed that her bone mass density was still low. The treatment team decided to start her on bisphosphonates, a group of drugs used for bone building. Biphosphonates are traditionally prescribed to post-menopausal men and women to improve bone density. She now weighs 41kg - still underweight, but an improvement. Kung warns that weight-conscious teens and young women could be exposing themselves to an increased risk of osteoporosis. In a recent study, Kung found that almost 40 per cent of 20- to 29-year-old women in Hong Kong monitored from 2005 to 2010 were classified as having a low body weight. 'When a female is in puberty and she tries to control her body weight by eating less, she can't get enough calcium and vitamins to sustain bone growth,' she says. More than 90 per cent of a person's bone mass develops before the age of 20. Kung says that it's vital for women to have adequate nutrition through eating a balanced diet and doing regular weight-bearing exercises. She adds that half of the calories should come from carbohydrates. 'Asians have a higher risk of developing osteoporosis because we don't exercise regularly, and we tend to have a diet which is low in calcium,' Kung says. 'Also, the Asian craving for 'whiteness' has left 60 per cent of the Hong Kong population deficient in vitamin D due to low sunlight exposure.' Optimal amounts of calcium can be obtained by eating green leafy vegetables, nuts, soya beans and fish with bones. Many Asians, Kung says, can't tolerate milk, but digestibility can be improved by heating the milk up first or including dried milk powder.