Consuming a diet high in whole grains in addition to at least five portions of fruit and vegetables a day has long been promoted as beneficial for overall health. However, the specific effects of these dietary factors on the risk of type 2 diabetes mellitus (T2DM) have not been clear; two new papers published in the British Medical Journal set out to address these questions.To counteract the potential biases and errors of traditional nutrition studies, which rely on self-reported diet information, Nita Forouhi, Nick Wareham and colleagues used blood levels of vitamin C and six carotenoids (the pigments that give fruit and vegetables colour) as objective biomarkers of fruit and vegetable consumption. They worked within the framework of the EPIC study with collaborators in eight European countries to set up a case–cohort study with the >300,000 EPIC participants for whom baseline blood samples were available. Over 10 years, the team identified 9,754 cases of new-onset T2DM, and a reference set of 13,662 participants without T2DM.
Credit: SCIENCE PHOTO LIBRARY/Getty“We tested the association of the blood biomarkers individually and as the composite biomarker score with the risk of new-onset T2DM,” explains Forouhi. “We adjusted these analyses for a large number of other factors that could potentially be alternative explanations for any observed association.” The analysis showed that participants with the highest composite biomarker score had a 50% lower risk of developing T2DM than participants with the lowest composite biomarker score.The researchers had also collected self-reported data on fruit and vegetable consumption, which they then combined with information on the composite biomarker score and development of T2DM. “We found that habitually eating a diet including a higher intake of around 66 grams of fruit and/or vegetables per day (or around one portion per day) could potentially cut the relative risk of T2DM by a quarter over time,” explains Forouhi.The researchers emphasise that their research does not support the increased consumption of vitamin supplements. Rather, they suggest that even small increases in the consumption of fruit and vegetables can reduce the risk of T2DM. Forouhi and colleagues recognise the limits of their study, such as that it is observational and that the benefits of specific fruits and vegetables could not be identified. Further research is needed to establish cause and effect. “It would be important to replicate this type of research in other populations including people of different ethnic groups in different geographical regions,” adds Forouhi. Additional research could also help to identify more specific biomarkers. “This is some way off at present, but there is the possibility that with greater investment in research we might be able to test a set of biomarkers in urine, saliva or blood to judge the healthfulness or otherwise of diets we consume,” concludes Forouhi.Whole grain foods are heterogeneous in terms of their glycaemic index and their fibre, mineral, vitamin and phytochemical content. “Despite this heterogeneity that might potentially lead to differential health effects, no study has comprehensively examined individual whole grain foods in relation to disease outcomes,” says Qi Sun, author of the second paper.From a cohort of >200,000 participants, Sun and colleagues collected data on dietary intake, lifestyle and medical history (including self-reported diagnosis of T2DM) every 2–4 years for ~30 years. A supplementary questionnaire was sent to those who reported having T2DM to collect data on date of diagnosis, blood levels of glucose, complications of T2DM and medication use.“The most significant finding is that the nutritional and glycaemic differences among individual whole grain foods do not lead to differential associations with T2DM risk,” says Sun. Indeed, they found that most of the commonly consumed whole grain foods and ingredients (whole grain breakfast cereals, oatmeal, dark bread, brown rice, bran and wheat germ) were inversely associated with the risk of T2DM. “These data confirm the inverse association between total whole grain intake and T2DM risk and meanwhile go beyond prior studies by demonstrating associations for major individual whole grain foods,” explains Sun.“even small increases in the consumption of fruit and vegetables can reduce the risk of T2DM”
Sun and colleagues are now hoping to take this work forward by considering other cardiometabolic disease outcomes, such as obesity. “I feel it’s time to perhaps think of doing clinical trials to establish the causal relationship between whole grain intake and these cardiometabolic conditions,” says Sun. “Another implication is for public health nutrition researchers to further their research for promoting whole grain intake in general populations, whose whole grain intake is still less than optimal.”
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Claire Greenhill.About this articleCite this articleGreenhill, C. Dietary factors in the risk of T2DM.
Nat Rev Endocrinol (2020). https://doi.org/10.1038/s41574-020-0401-5Download citation