Overweight and obesity are independent risk factors for the development of disease and death in the general population. However, in people with various conditions (old age, wasting diseases, heart diseases or renal dialysis) overweight and obesity are associated with a higher survival rate. The terms "reverse epidemiology" or "obesity paradox" have been suggested to describe this finding. However, it still remains uncertain, whether this phenomenon is attributable to a real protective effect of high body fat mass. Methodological problems in studies suggesting an obesity paradox such as survivor bias, selection bias, lead time bias or, in meta analyses, publication bias and confounders have been discussed. These cannot, however, entirely explain the observed phenomenon. Biological models, examining possible explanations for the protective effect of high body mass, for instance, in wasting diseases and elderly patients, have also been produced. In particular high inflammation markers combined with malnutrition predict a high mortality rate among patients with various medical conditions: overweight and obesity could counter these effects. Possible implications for clinical and public health recommendations regarding weight management and nutrition are issues for future research. In elderly subjects and patients with a poor prognosis the impact of weight management on quality of life should also be taken into account.