By: Sarah-Jane Reilly, RD
The role of nutrition in relation to immunity remains a contentious subject. During the pandemic, specific dietary advice and nutrition myths relating to COVID-19 were circulated online.¹,² This prompted the World Health Organisation (WHO) to publish nutrition advice for adults, recommending a well-balanced diet; rich in fresh fruit and vegetables, legumes, nuts, and wholegrains and low consumption of processed foods containing high amounts of saturated fat, salt, and refined sugar.³
Good nutrition is vital for the optimum functioning of the immune system in fighting against infectious disease.⁴ According to the European Food Safety Authority, (EFSA), there is a clear cause-effect relationship between the daily intake of six vitamins (A, C, D, Folate, B6, B12), four key minerals (zinc, iron, copper, and selenium) and the normal functioning of the immune system. ⁵-⁷ Similarly, there is growing interest in the relationship between nutritional status and impaired immune function as a result of malnutrition. ⁸,⁹
The epidemiological factors underpinning susceptibility to COVID-19 are multi-faceted. Not all studies examining the relationship between nutrition and COVID-19 are well conducted. For example, a widely reported preprint paper¹⁰ examining patterns of COVID-19 mortality and vitamin D status, was highly criticised as the paper was not peer-reviewed and the qualifications of the authors were unknown.¹¹ The preprint had been used more than 100 000 times, downloaded more than 17 000 times and mentioned in social media over 8000 times, yet was considered to be fraudulent. ¹¹ This highlights how nutrition misinformation can be heightened during a pandemic.
Plant-based diets promote the consumption of foods primarily from plant sources such as fruit and vegetables, nuts and seeds, oils, wholegrains, and pulses.¹² These diets are not strictly defined as vegetarian or vegan as they offer dietary flexibility, with the consumption of meat or dairy being optional. Interest in plant-based eating has grown in recent years in an attempt to optimise human health, while also encouraging environmental sustainability. Most notably, the EAT-Lancet report has advocated for the global adoption of a plant-based diet to reduce environmental degradation and prevent premature deaths associated with chronic disease. ¹³
Plant-based diets have been associated with more pronounced metabolic improvements in weight status, glucose, plasma lipids and inflammatory markers when compared to other dietary patterns in the short-term (3-24 months). ¹⁴ Compared to omnivore diets, evidence from the Oxford-EPIC trial showed higher intakes of fibre, magnesium, iron, folic acid, vitamins B1, C and E and lower intakes of nutrients such as saturated fatty acids (SFA) among vegan dieters.¹⁵
Lower intakes of saturated fat may play an important role in metabolic control by reducing systemic hyperlipidemia and subsequent cardiovascular disease risk.¹⁶ Similarly, high fibre intake associated with plant-based diets could give rise to metabolic benefits such as upregulated carbohydrate fermentation and downregulated protein fermentation,¹⁷ which has recently been associated with increased microbiota diversity and a reduction in inflammatory markers.¹⁸
A recent study¹⁹ published in BMJ Nutrition, Prevention & Health aimed to investigate the association between dietary patterns and COVID-19 by asking healthcare professionals from six countries, with a high frequency of exposure to COVID-19 patients, to complete a web-based questionnaire. The questionnaire collected information on personal demographics, past medical history, medications, lifestyle information, COVID-19 symptoms and diet using a food frequency questionnaire (FFQ) adapted from a validated FFQ. Multivariable logistic regression models were used to adjust for potential confounding factors such as age, sex, ethnicity, smoking status, medical specialty and medical conditions associated with worse COVID-19 outcomes, such as obesity. A total of 2,884 healthcare workers were included in the study (568 COVID-19 cases and 2316 controls).
Overall, the study revealed that frontline workers who reported a ‘plant-based diet’ and a ‘plant-based or pescatarian diet’ for at least one year previously had 73% and 59% lower odds of developing moderate-severe COVID-19, when compared to controls. Interestingly, participants who followed a low-carb, high protein diet had nearly a four times greater risk of moderate-severe COVID-19 (OR 3.86).
While the results of this study are promising, it is important to be aware of the short-comings of the case-control study design. This was an observational study. Therefore, the findings do not establish causation. The self-reported questionnaire used in the study is likely to introduce a degree of recall bias into the study design. FFQs cannot provide absolute nutrient values and therefore, it is unclear from the findings what nutritional aspects of a plant-based diet confer the benefit reported.
Many of the study participants were male (70%) and physicians (95%) which limits the generalisability of the findings to the public. Most of the COVID-19 cases in the study were reported as very mild to mild in severity (430/568 cases) which does not strengthen the quality of the results obtained. Considering these findings, future studies should recruit a more representative group of participants with a higher number of moderate-severe COVID-19 cases to show concordance with the results obtained.
Nonetheless, these findings add to the growing evidence base in support of following a predominantly plant-based diet. Healthcare professionals can incorporate this research into practice by encouraging their patients to decrease their consumption of red and processed meat and increase their consumption of foods from plants.