Toronto, May 9 (IANS): Women with obesity and overweight, particularly women with high waist circumference, are more susceptible to fractures than those with normal weight, finds a study.

In men, however, underweight, not overweight, is associated with a greater risk of broken bones, according to the research presented at the European Congress on Obesity (ECO) in the Netherlands.

Obesity has long been thought to help protect against fractures. This is because mechanical loading on bones, which increases with body weight, helps increase bone mineral density, an important determinant of bone strength.

However, recent studies have suggested that the relationship between obesity and fracture risk varies depending on sex, the skeletal site studied and definition of obesity used (body mass index BMI vs. waist circumference).

To find out more, a team from the CHU de Quebec Research Center in Canada analysed data from almost 20,000 individuals aged 40-70 years from Quebec, Canada. The participant's BMI and waist circumference (a measure of abdominal obesity) were measured.

During a median follow-up of 5.8 years, 497 women and 323 men sustained a fracture.

In women, greater waist circumference was linearly associated with an increased risk of fracture. For each 5cm (two inch) increase in waist circumference, the risk of fracture at any site was 3 per cent higher and the risk of a distal lower limb fracture (the part of the leg below the knee) was 7 per cent higher.

The association between waist circumference and ankle fractures was particularly strong.

"Waist circumference was more strongly associated with fractures in women than BMI. This may be due to visceral fat, fat that is very metabolically active and stored deep within the abdomen, wrapped around the organs, secreting compounds that adversely affect bone strength," said Dr. Anne-Frederique Turcotte, Endocrinology and Nephrology Unit at the Centre.

Further, women with greater BMI were associated with a high risk of distal lower limb fractures. Compared with women with a BMI of 25 kg/m2,A the increase in risk rose linearly from 5 per cent in those with a BMI of 27.5 kg/mA, to 40 per cent in those with a BMI of 40 kg/mA.

While the risk of obesity's link with a higher risk of fractures in women remains unknown, the researchers said most fractures are a result of a fall and falls are more common in people with obesity. The ankle, unlike the hip and thigh bone, is not protected by soft tissue, which could make it more prone to breaking during a fall.

"We also know that people with obesity take longer to stabilise their body, when they trip, for example. This is particularly pronounced when weight is concentrated at the front of the body, suggesting that individuals with distribution of body fat in the abdominal area may be at higher risk of falling," Turcotte said.

The study also found that men with a BMI below 17.5 kg/m2 were twice as likely to have distal upper limb fracture as men with a BMI of 25 kg/m2.

The researchers noted a larger number of fractures in men is needed to determine whether this is a true result or whether the pattern for men follows that for women.