The Harvard School of Public Health study suggested their weight gain may be due to marriage improving men's 'bachelor' eating habits.

But the marital demands on their time appear to prevent them making it to the gym to work off the calories.

The study is published in the Journal of Epidemiology and Community Health.

It followed over 38,000 men aged between 40 and 75 between 1986 and 1994.

All the men were healthcare professionals taking part in a long term study into chronic illness.

The men were asked about their marital status at least twice during this period.

They were also asked to complete detailed questionnaires about their dietary and exercise habits, and how much they smoke and drank.

The study did not look at how marital status affected women's health.

Unhealthy divorcees

It was found that remarriage did appear to improve men's diets.

They reduced their alcohol intake, ate more vegetables and lean poultry and had fewer sugary drinks than peers who were divorced or bereaved.

The dietary benefits were greater for younger men who remarried after the loss of a spouse.

But, in contrast to the positive change in eating habits, men who remarried did appear to exercise less, and to gain weight.

When the researchers studied the health of widowers they found that, compared to those who stayed married over a four-year period, they drank more and ate far fewer vegetables.

Divorced men lost weight, but also smoke and drank more than other groups.

Writing in the Journal of Epidemiology and Community Health, the research team led by Dr Patricia Mona Eng, said: "Remarriage was not linked to an increase in activity levels.

"Instead, formerly solitary men experienced relative decline along with weight gain upon remarrying."

They speculate: "Time demands of a new spousal role may preclude routine exercise.

"Married life may also bring regularity to meal patterns and increased food intake via social facilitation."

Professor Ben Fletcher, of the British Psychological Society, said: "The men who remarry are likely to become more sedentary, and there's an increase in social support - which can have a powerful effect on health and well-being."

Professor Fletcher, head of psychology at the University of Hertfordshire, added that there were other studies showing men coped less well with losing a partner.

"With men, unlike women, you do see an increase in mortality risk in the six months after a bereavement."