The study, published in the Journal of the American Dietetic Association, found that family members of obese patients in a Cognitive Behavioural Therapy programme also showed changes for the better.

CBT focuses on changing people's thoughts and attitudes to eating and other lifestyle habits, and giving them practical ways to make improvements.

"CBT for the loss of weight influences the lifestyle habits of family members of participants, reducing energy intake and promoting a more favourable attitude towards physical activity," wrote senior researcher Giulio Marchesini.

Marchesini and his colleagues surveyed the family members of 149 obese patients going through the CBT programme at the University of Bologna, which consisted of 12 to 15 weekly group meetings.

Six months after their relatives started the programme, family members - mainly spouses and adult children - were on a healthier diet.