Background: The strong association between socio-economic disadvantage and poor health in northern Europe and North America causes great public health concern but as the mechanism is not understood, government actions have been completely unsuccessful. The social gap continues to widen.
Aim: I propose that an important mechanism involved is relative deficiency of vitamin D in the socio-economically disadvantaged people.
Design: The evidence is presented by explaining apparent paradoxes, together with the analogies of geography, season and ethnicity. Vitamin D studies are also demonstrated.
Method: The picture is put together according to Hill's criteria of causation: strength of association, consistency of association, temporality, biological gradient, plausibility, coherence, experimentation and analogy.
Results: Hill's criteria of causation are met, acknowledging that they are pragmatic and not absolute. The associations between vitamin D deficiency and socio-economic deprivation are supported by analogies of geography, seasonality and ethnicity. There is also biological plausibility. The paradoxes of Albania and India show that health disadvantage and socio-economic disadvantage are not necessarily connected, if relative deficiency of vitamin D is the mediating biological factor.
Conclusion: The poor health and average early death of the socio-economically disadvantaged in the UK, together with the widening social gap of health, can be explained by relative deficiency of vitamin D. At present there is no explanation and attempts by government to close the social gap have failed. The understanding presented gives an important opportunity to improve the health disadvantages of the socio-economically disadvantaged.