Background: Weight loss in the obese improves risk factors for cardiovascular diseases and diabetes. However, several studies have shown inconsistent long-term effects of weight loss on mortality. We investigated the influence on mortality of intention to lose weight and subsequent weight changes among overweight individuals without known co-morbidities.

Methods and findings: In 1975, a cohort of individuals reported height, weight, and current attempts (defined as "intention") to lose weight, and in 1981, they reported current weight. Mortality of the 2,957 participants with body mass index > or = 25 kg/m2 in 1975 and without pre-existing or current diseases was followed from 1982 through 1999, and 268 participants died. The association of intention to lose weight in 1975 and actual weight change until 1981 with mortality was analysed while controlling for behavioural and psychosocial risk factors and hypertension as possible confounders. Compared with the group not intending to lose and able to maintain stable weight, the hazard ratios (with 95% confidence intervals) in the group intending to lose weight were 0.84 (0.49-1.48) for those with stable weight, 1.86 (1.22-2.87) for those losing weight, and 0.93 (0.55-1.56) for those gaining weight. In the group not intending to lose weight, hazard ratios were 1.17 (0.82-1.66) for those who did lose weight, and 1.57 (1.08-2.30) for those gaining weight.

Conclusion: Deliberate weight loss in overweight individuals without known co-morbidities may be hazardous in the long term. The health effects of weight loss are complex, possibly composed of oppositely acting processes, and need more research.