The risk of dying from heart disease because of excess weight may be higher than thought.

A study published in Wednesday's issue of the British Medical Journal measured body mass index (BMI) and mortality among more than one million pairs of Swedish sons and their parents over 50 years.

The team from Bristol University and the Karolinska Institute in Sweden focused on offspring because the BMI of parents could be linked to illnesses they suffer rather than the extra weight itself.

The researchers found strong links between offspring BMI and parental mortality. For example, the risk of coronary heart disease for mothers increased by 1.15 times and 1.10 times for fathers depending on offspring BMI.

The link to heart-related deaths was much stronger than suggested by previous studies.

Using the father's BMI measurements alone, the risk of heart deaths increased by 45 per cent for every three-point increase in BMI level. But using a measurement based on the child's BMI, the risk increased by 82 per cent.

Unlike other studies of BMI in adults, the British and Swedish researchers found no link between a son's low BMI and an increased risk of respiratory disease and lung disease for fathers.

There was a positive link between a mother's risk of respiratory disease mortality and her son's BMI, which the researchers said presumably reflects maternal smoking.

"These data suggest that the adverse influence of higher BMI and obesity in a population is of greater magnitude than previously thought," George Davey Smith, a professor of clinical epidemiology at the University of Bristol and his colleagues concluded.

The findings also imply that programs to tackle obesity could offer major health benefits, they added.

Researchers have said that waist-to-hip ratio measurements are a better way of predicting obesity-related deaths than the standard BMI, since the distribution of fat also affects the risk of premature death.