Abstract: COVID-19 Original research Jordi Merino ‍ ‍,1,2,3 Amit D Joshi ‍ ‍,4,5 Long H Nguyen ‍ ‍,4,5,6 Emily R Leeming ‍ ‍,7 Mohsen Mazidi,7 David A Drew ‍ ‍,4,5 Rachel Gibson,8 Mark S Graham,9 Chun-­Han Lo ‍ ‍,4,5 Joan Capdevila,10 Benjamin Murray,9 Christina Hu,10 Somesh Selvachandran,10 Alexander Hammers,9,11 Shilpa N Bhupathiraju,3,12 Shreela V Sharma,13 Carole Sudre,9 Christina M Astley,2,14 Jorge E Chavarro,12,15,16 Sohee Kwon,4,5 Wenjie Ma,4,5 Cristina Menni ‍ ‍,7 Walter C Willett,12,15,16 Sebastien Ourselin,9 Claire J Steves,7 Jonathan Wolf,10 Paul W Franks,12,17 Timothy D Spector,8 Sarah Berry,8 Andrew T Chan4,5,18 ►► Additional supplemental material is published online only. To view, please visit the journal online (http://d​ x.​doi.​org/​ 10.1​ 136/​gutjnl-2​ 021-​325353). For numbered affiliations see end of article. Correspondence to Dr Andrew T Chan, Harvard Medical School, Boston, USA; a​ chan@​mgh.h​ arvard.​edu JM, ADJ and LHN are joint first authors. TDS, SB and ATC are joint senior authors. Received 7 June 2021 Accepted 19 August 2021 Published Online First 6 September 2021 © Author(s) (or their employer(s)) 2021. No commercial re-­use. See rights and permissions. Published by BMJ. To cite: Merino J, Joshi AD, Nguyen LH, et al. Gut 2021;70:2096–2104. 2096   ABSTRACT Objective Poor metabolic health and unhealthy lifestyle factors have been associated with risk and severity of COVID-19, but data for diet are lacking. We aimed to investigate the association of diet quality with risk and severity of COVID-19 and its interaction with socioeconomic deprivation. Design We used data from 592 571 participants of the smartphone-­based COVID-19 Symptom Study. Diet information was collected for the prepandemic period using a short food frequency questionnaire, and diet quality was assessed using a healthful Plant-­Based Diet Score, which emphasises healthy plant foods such as fruits or vegetables. Multivariable Cox models were fitted to calculate HRs and 95% CIs for COVID-19 risk and severity defined using a validated symptom-­based algorithm or hospitalisation with oxygen support, respectively. Results Over 3 886 274 person-­months of follow-­up, 31 815 COVID-19 cases were documented. Compared with individuals in the lowest quartile of the diet score, high diet quality was associated with lower risk of COVID-19 (HR 0.91; 95% CI 0.88 to 0.94) and severe COVID-19 (HR 0.59; 95% CI 0.47 to 0.74). The joint association of low diet quality and increased deprivation on COVID-19 risk was higher than the sum of the risk associated with each factor alone (Pinteraction=0.005). The corresponding absolute excess rate per 10 000 person/months for lowest vs highest quartile of diet score was 22.5 (95% CI 18.8 to 26.3) among persons living in areas with low deprivation and 40.8 (95% CI 31.7 to 49.8) among persons living in areas with high deprivation. Conclusions A diet characterised by healthy plant-­ based foods was associated with lower risk and severity of COVID-19. This association may be particularly evident among individuals living in areas with higher socioeconomic deprivation.