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By Juliette Frank

Although the threat of COVID-19 is slowly receding in many countries due to the help of mass vaccinations, many doctors and public health officials are still trying to make sense of the highest risk factors in order to reduce the burden of the pandemic as the world is still struggling to combat it.

It has become clear early on that individuals with underlying health conditions and chronic diseases (so called “comorbid conditions”) such as diabetes, heart disease, and hypertension were at an increased risk of death from COVID-19. Although there is extensive data suggesting unhealthy lifestyle behaviors are one of the leading causes of metabolic disorders and our chronic disease epidemic, there is little data around the association with diet quality and risk and severity of COVID-19. A preprint of a study conducted by Jordi Merino, Ph. D. from the Harvard Medical School and colleagues from the Department of Twin Research at King’s College London was just recently posted in June looking at the association between diet quality and risk and severity of COVID-19.

The study used data from more than half a million participants from the United Kingdom (UK) and United States (US) from the smartphone-based COVID Symptom Study from March 24, 2020 to December 2, 2020 to look at the association between diet quality and risk and severity of COVID-19 and its intersection with socioeconomic deprivation, i.e. the relative disadvantage an individual or community experiences in terms of access and control over economic, material or social resources and opportunities (Lamnisos et al., 2019). Diet quality was assessed using a healthful plant-based diet index (hPDI) which focuses on intake of nutritious foods such as fruits, vegetables, and whole grains which supports a healthy gut microbiome and has been associated with lower risk of metabolic diseases. Individuals with a high diet score reported eating a healthier, plant-based diet while individuals on the lower end of the diet score reported less healthy eating behaviors.

Participants in the highest 25% of the diet score, e.g. those eating the healthiest diet, tended to be older, female, healthcare workers, less obese (lower BMI), more likely to engage in regular physical activity, and have a higher socioeconomic status than participants in the lowest 25%. The study looked at over 3,886,274 participants within the ~9 months of the study of follow-up with 31,815 COVID-19 cases documented. Raw COVID-19 rates per 10,000 participants within the ~9 months of the study were 72.0 for participants in the highest quartile of the diet score and 104.1 for those in the lowest 25%. These results suggest a nutritious diet consisting largely of plant foods, was associated with lower risk and severity of COVID-19 and was particularly evident amongst individuals with a higher socioeconomic status.

In secondary analyses for COVID-19 risk based on a positive test, the study showed that raw COVID-19 incidence rates per 10,000 participants within the ~9 months of the study were 12.9 for individuals with high diet quality and 16.4 for individuals with low diet quality. Similarly, risk of severe COVID-19 was lower amongst individuals reporting high quality diets (1.6) compared to low quality diets (2.1). In stratified analyses, or a statistical approach that allows researchers to test the association between two variables (COVID-19 risk and diet quality) by adjusting for a third (deprivation), the inverse association between diet quality and COVID-19 risk was more evident in participants living in areas of high socioeconomic deprivation and those reporting low levels of physical activity. When looking at all three risk factors (low socioeconomic status, low physical activity, low diet quality), the proportion of contribution to excess COVID-19 risk was estimated to be 31.9% to diet quality, and 38.4% to socioeconomic deprivation and 29.7% to their interaction. The absolute excess rate of COVID-19 per 10,000 participants within the ~9 months of the study for lowest vs. highest quartile of the diet score was 22.5 among individuals living in areas with low socioeconomic deprivation and 40.8 for those with high deprivation making the additivity of diet and socioeconomic status the most significant risk determinant.

The findings of this study align with prior science-backed evidence that a healthy largely plant-based diet is associated with lower risk of metabolic diseases and even “reduces the burden of certain infectious diseases” due to increased gut health and immune resilience (reviewed in detail in The Gut Immune Connection). Not only does this study suggest that diet is associated with COVID-19, it emphasizes the additivity of poor diet and high socioeconomic deprivation which is a powerful contributor to severe COVID-19 risk. Just as these results prove, the burden of the COVID-19 pandemic has not been evenly felt and has disproportionately impacted low-income communities with less access to resources. Their models estimate that nearly a third of COVID-19 cases would have been prevented if one of two exposures (diet and socioeconomic deprivation) were not present. This is consistent with data from ecological studies that show higher rates of COVID-19 incidence and deaths in regions experiencing greater social inequalities, which further emphasizes how increased accessibility to nutrition, specifically in disinvested communities, could help decrease the burden of infectious diseases.

The findings of this study also align with a comparative risk assessment study suggesting that a 10% reduction in the prevalence of diet-related conditions such as obesity, type 2 diabetes and hypertension (often referred to as the metabolic syndrome) would have prevented ~11% of the COVID-19 hospitalizations that have occurred among US adults since November 2020. It is clear that individuals with an inadequate diet are more likely to live in areas of higher socioeconomic deprivation, which is why it is important to consider that the potential for residual confounding in this study still exists. Individuals who eat a healthy diet are much more likely to have other behavioral traits and lifestyles associated with a lower risk of infection such as more spacious and hygienic household and work conditions, time for regular exercise and access to adequate health care.

Due to the self-reported nature of this study, there are several limitations such as inaccuracies in the reflection of diversity to the general population and some measurement error and bias. In order to confirm direct causal association between diet and COVID-19 risk, further research needs to be done with random sampling and more accurate measurements of dietary intake. Even with the various limitations, the data provided offers consistent evidence that a healthier, largely plant-based diet is not only associated with better metabolic health, but also with a lower risk and severity of COVID-19. The findings suggest that when considering ways to reduce the risk for and burden of infectious diseases and to address the disparities the pandemic has exposed, there should be special attention to the accessibility of healthy foods as a social determinant of health.

Juliette Frank is a UCLA student majoring in Public Affairs with a minor in Food Studies. Her interests include the interrelation between food systems, digestive health and the environmental impacts of food production.