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"Previous reports suggest that poor nutrition is a common feature among groups disproportionately affected by the pandemic, but data on the association between diet and COVID-19 risk and severity are lacking," said lead author Jordi Merino, a research associate at the Diabetes Unit and Center for Genomic Medicine at MGH and an instructor in medicine at Harvard Medical School.

For the study, Merino and his colleagues examined data on 592,571 participants of the smartphone-based Covid-19 Symptom Study. Participants lived in the UK and the US, and they were recruited from 24 March 2020, and followed until 2 December 2020.

At the start of the study, participants completed a questionnaire that asked about their dietary habits before the pandemic. Diet quality was assessed using a healthful Plant-BasedDiet Score that emphasises healthy plant foods such as fruits and vegetables.

During follow-up, 31,831 participants developed Covid-19. Compared with individuals in the lowest quartile of the diet score, those in the highest quartile had a 9 per cent lower risk of developing Covid-19 and a 41 per cent lower risk of developing severe Covid-19.

"These findings were consistent across a range of sensitivity analysis accounting for other healthy behaviors, social determinants of health and community virus transmission rates," said Merino.

"Although we cannot emphasize enough the importance of getting vaccinated and wearing a mask in crowded indoor settings, our study suggests that individuals can also potentially reduce their risk of getting Covid-19 or having poor outcomes by paying attention to their diet," said co-senior author Andrew Chan, a gastroenterologist and chief of the Clinical and Translational Epidemiology Unit at MGH.

The researchers also found a synergistic relationship between poor diet and increased socioeconomic deprivation with Covid-19 risk that was higher than the sum of the risk associated with each factor alone.

"Our models estimate that nearly a third of Covid-19 cases would have been prevented if one of two exposures--diet or deprivation--were not present," said Merino.

The results also suggest that public health strategies that improve access to healthy foods and address social determinants of health may help to reduce the burden of the Covid-19 pandemic.

"Our findings are a call to governments and stakeholders to prioritize healthy diets and wellbeing with impactful policies, otherwise we risk losing decades of economic progress and a substantial increase in health disparities," said Merino.

The study was co-led by investigators at Kings College London. Co-authors include Amit D Joshi, Long H Nguyen, Emily R Leeming , Mohsen Mazidi, David A Drew, Rachel Gibson, Mark S Graham, Chun-Han Lo, Joan Capdevila, Benjamin Murray, Christina Hu, Somesh Selvachandran, Alexander Hammers, Shilpa N Bhupathiraju, Shreela V Sharma, Carole Sudre, Christina M Astley, Jorge E Chavarro, Sohee Kwon, Wenjie Ma, Cristina Menni, Walter C Willett, Sebastien Ourselin, Claire J Steves, Jonathan Wolf, Paul W Franks, Timothy D Spector, Sarah Berry, and Andrew T Chan.

Funding for the study was provided by the National Institutes of Health, the National Institute for Health Research, the UK Medical Research Council/Engineering and Physical Sciences Research Council, the Wellcome Trust, the Massachusetts Consortium on Pathogen Readiness, the American Gastroenterological Association, the American Diabetes Association, the Alzheimer's Society and Zoe Ltd.

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