November 9, 2017

expert reaction to study looking at eating antioxidant-rich foods and risk of type 2 diabetes

A new study published in Diabetologia reports that the consumption of antioxidant-rich foods is associated with a lower risk of type 2 diabetes.

Prof. Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“In statistical terms, this is a decent study based on data from large numbers of women.  But its findings can’t go further than suggesting a role for dietary antioxidants, as the study report clearly says.  An important reason is that the study was observational, that is, the researchers asked the women about their diets and then followed them up to see if they developed diabetes.  So one possibility is that eating more antioxidants reduces diabetes risk, but there are other possibilities.  Maybe there’s some other difference between those who ate high amounts of antioxidants and the others, and it is this difference that reduces diabetes risk, and not the antioxidants at all.  The researchers did allow statistically for various possible sources of difference (so-called confounders) like this, such as smoking and body mass index, but these corrections cannot deal with all possible confounders.  For instance, maybe eating more antioxidants happens to be associated with eating less of something else, and it is the consumption of the something else that affects one’s chances of diabetes.  The researchers did take account of some other aspects of diet, but it remains possible that they missed a crucial aspect.  Unfortunately, with observational studies, you can never be sure.

“There are some other points, made clear in the study report, that we should bear in mind.  First, how big is the reduction in diabetes risk for women who consumed the highest level of antioxidants?  The media release reports, accurately, that the risk was 27% less than in women who consumed the lowest amount of antioxidants – that is, roughly, the risk in the high consumers of antioxidants was about three quarters of the risk in the group who consumed the least antioxidants.  But the study doesn’t say directly what the actual risk of diabetes is in these groups.  In any case, the women who were studied were not typical of the French population – they were less likely to be socially deprived and more likely to be health-conscious.  We can’t be sure the results would look the same in a more typical population in France, and things may be different again in the UK, and perhaps different again in men.  Type 2 diabetes is a common disease, so if the position in the UK population is similar to that in this study (and that’s a very big if), eating more antioxidants could be important in relation to diabetes, but we can’t be sure without more research.

“The data on diet came only from a questionnaire that the women completed when they entered the study, so the researchers couldn’t take account of changes in the women’s diets after that (and they followed the women up for many years so there would be time for change in diet).  Also, crucially, the researchers point out that, if antioxidants are really reducing diabetes risk, they don’t yet know how this works biologically.”

Dr Ian Johnson, Nutrition researcher and Emeritus Fellow, Quadram Institute Bioscience, said:

“This study adds to the evidence that, as is already well known, diet influences the risk of type two diabetes.  Observational studies of this type are good at finding associations but poor at establishing causes.  The authors have shown an association between risk and dietary antioxidant levels, but foods are very complex, and diets defined in this way will have many other characteristics which may also be modifying risk.”

Dr Charlotte Mills, Postdoctoral Research Associate in the Division of Diabetes and Nutrition, King’s College London, said:

“Caution should be taken when interpreting such data as even though foods behave as antioxidants in the lab this does not necessary translate to the way they are used in the body after consumption.  Although polyphenols (such as flavonoids) may contribute to reducing the risk of type 2 diabetes, they are unlikely to act via antioxidant mechanisms due to extensive metabolism before being absorbed.”

* ‘Dietary antioxidant capacity and risk of type 2 diabetes in the large prospective E3N-EPIC cohort’ by Francesca Romana Mancini et al. published in Diabetologia on Thursday 9 November 2017.

Declared interests

Prof. Kevin McConway: “I am a member of the Science Media Centre Advisory Committee. (Nothing else relevant.)”

Dr Ian Johnson: “No interests to declare.”

None received.