Thigh circumference is an indicator of physical activity levels. (Ron Heflin/Associated Press)

The thinner your thighs, the greater your risk of heart disease, a new study suggests.

The research, published in Friday's edition of the British Medical Journal, reveals that thigh circumference is linked to the risk of heart disease and premature death.

The study by Berit Heitmann, a professor of nutritional epidemiology at Copenhagen University Hospital, and her colleague looked at more than 2,800 men and women with an average age of around 50 who were followed for more than 12 years.

They found that the risk of heart disease more than doubled for both men and women who had a thigh circumference of less than 55 centimetres.

Those participants with thighs between 55 and 60 centimetres received a protective effect against heart disease, the study reports.

But that protective effect reduced for people with thighs above 60 centimetres in circumference.

The researchers took abdominal and general obesity, lifestyle factors such as smoking, and cardiovascular risk factors such as blood pressure into account.

Thigh size reflects activity

Prof. David Cameron-Smith, of the school of exercise and nutrition sciences at Deakin University in Melbourne, says this is very powerful research. He says a growing body of research is showing the increased risk of heart disease associated with living a sedentary lifestyle.

According to Cameron-Smith, thigh circumference is a broad indicator of physical activity and muscle mass is related to how much exercise you do.

"[If] you don't use it, you lose it," he says.

Cameron-Smith says using muscles has a very strong protective effect against heart disease and diabetes.

"It's been known for a long time that muscle mass and strength are important determinants of longevity and health," he says. "Even moving from no activity to some activity has a dramatic effect."

Cameron-Smith says muscle mass also protects people, particularly older people, from falls.

Support for weight training

He says a surprising finding in the research is that the risk associated with thigh circumference and heart disease is almost the same for men and women.

Cameron-Smith says the most likely reason is that most women in the study would have been on the cusp of menopause or post-menopause, which would explain the similar results between the sexes.

After menopause men and women start to display similar body morphologies.

Cameron-Smith says regardless of age, thin, weak thighs are a risk factor for heart disease.

"If you've got thin, frail thighs as a 30-year-old it's a sign that you've got to keep working at it."

Professor Tim Olds of the University of South Australia's School of Health Sciences, says the study is really interesting and solid.

He says previous research he has conducted found about 75 per cent of men and 80 per cent of women aged 18 and over have thigh girths below the "cut-off" suggested in the paper.

Clinical value doubted

Olds says the research suggests that "interventions which protect or increase muscle mass, such as weight training, may be effective in reducing cardiovascular disease even if no loss of body fat occurs."

The study's authors suggest that thigh circumference could be used in combination with other measurements, such as body mass index (BMI) and waist-to-hip ratio, to assess an individual's risk of heart disease.

But Dr. Ian Scott, director of internal medicine and clinical epidemiology at the Princess Alexandra Hospital in Brisbane, disagrees, saying "it seems unlikely that thigh circumference will be clinically useful."

Scott wrote a journal editorial accompanying the study that asked if the association is plausible. The answer was maybe, given that the link was no longer statistically significant as more variables were added to the models, but that may not have happened if the sample had been larger, he said.

"More research is needed to see whether measuring thigh circumference with a tape measure adds anything more to our clinical management than eliciting risk factors from the history, examining the cardiovascular system and measuring serum lipids," like cholesterol.

The researchers also called for more studies to check if changing thigh size improves health.