Response to: The Microbiome: A Biological Mechanism Underpinning the Social Gradient of Musculoskeletal Conditions

We thank Dr Brennan-Olsen and her colleagues for their thought-provoking letter stimulated by our article on microbiome and musculoskeletal disorders of aging. They highlight the importance of the social gradient in health inequalities, which is observed in osteoporosis as well as many health issues of our time. Indeed, we would agree that social health inequality is “an elephant in the room,” the mechanisms of which deserve much greater scrutiny if we are to address it. As towhether the humanmicrobiomemay play a role in social inequalities of health, unfortunately there is sparse data to date and more research is clearly indicated. It is clear that some societal practices, which may differ by socioeconomic status, impact the human microbiota (recently reviewed in O’Doherty and colleagues), but whether the humanmicrobiome is on the causal pathway is not known. The social gradient of health inequality does appear to operate across the life course, which has led investigators such as Brennan-Olsen to investigate epigenetic mechanisms. Equally, there is growing evidence that the gut microbiome may be “set” from early in childhood, and this may be related to socioeconomic factors (expertly reviewed in Subramanian and colleagues). Therefore, we would agree that the human microbiome is a promising area of investigation into the mechanisms behind life social gradients in health outcomes, which may have impact across the life course.