The existence of a social gradient in health, i.e. a graduated relationship between socioeconomic position (SEP)) and health, is well documented for chronic diseases. A major difficulty is to explicit how social determinants including physical, chemical or psycho-behavioural exposures are interlinked over time from childhood to influence subsequent health. Highlighting these causal chains helps to explain how the social environment becomes biological and how it can alter biological functioning to promote the development of diseases in the long term. Analysing the role of social determinants of health through their potential biological effects, including the growing field of epigenetics effects, is an innovative approach in the understanding of social gradient in health. The presentation will expose this new approach of embodiment of social environment and how the study of the social to biological transition rises important issues in terms of social justice and health policies.