Data from the long-running Whitehall II study of 9,453 civil servants showed that, between 1985 and 2004, there was a 74 per cent reduction in the risk of a first MI.
Most of this reduction could be explained by reduced non-HDL cholesterol levels, increased HDL levels, lowered BP and reduced smoking.
But without rises in BMI, a further decrease of 11 per cent in risk of first MI would have occurred, researchers said. The rise in BMI would have triggered an increase in MI risk were it not for the risk reduction from other factors.
The researchers warned that the influence of BMI may soon counteract benefits achieved by reducing other risk factors.
Lead researcher Sarah Hardoon, of University College London Medical School, commented: 'Although these favourable trends seem to have outweighed the negative contribution of rising BMI over recent decades, continued increases in BMI may reduce further, and even reverse, the decline in the incidence of heart attacks in the future.'
She added: 'The rising BMI in the UK and in other countries needs urgent attention.'
The researchers said research was needed to determine factors behind the MI risk reduction not explained by changes in cholesterol, BP and smoking.