Dr Shireen Kassam is a Consultant Haematologist and Honorary Senior Lecturer at King’s College Hospital, London with a specialist interest in the treatment of lymphoma.
Six years ago, for personal reasons, Dr Kassam started to delve into the data on diet and health. She was truly shocked to learn for the first time about the power of nutrition.
We are facing an unprecedented health crisis with COVID-19 -classified as a pandemic. I am unable to offer any guarantees and don’t mean to appear trite in writing this article. My aim is to offer some common sense, evidence-based advice on maintaining a healthy immune system.
The reason why older adults and those with chronic illness are at higher risk from respiratory viruses is because of the effects of chronic inflammation and the consequent reduced functioning of the immune system. We already know that diet and lifestyle choices can have an anti-inflammatory effect and boost immunity, thus promoting healthy aging and lowering the risk of chronic disease. The data on the effect of diet and lifestyle on preventing acute infections is less robust, but several lines of evidence suggest there may be a benefit.
Here is a list of my top diet and lifestyle tips based on the scientific evidence.
EAT THE RAINBOW: Fruits and vegetables are packed full of anti-oxidants, anti-inflammatory compounds and an array of phytonutrients, which all act to boost the function our immune system. The best evidence is for the beneficial role of flavonoids, a large group of polyphenol compounds found in an array of plant foods. Flavonoids have been found to significantly reduce the risk of upper respiratory tract infections. The dose required may be higher than that obtained in a typical diet, but diet can be used to boost levels. High doses of flavonoids are found in green tea, berries and dark chocolate!
One promising flavonoid that has been used at high doses in supplemental form is quercetin. It has shown activity against Zika and Ebola and SARS viruses by preventing entry of the virus into the cells and boosting the immune response to the infection. There appears to be no toxicity in studies performed to date and the U.S.Food and Drug Administration has already approved quercetin as safe for human consumption. Studies in humans suggest supplementation may reduce the risk of upper respiratory tract infections and human studies against COVID-19 are to be commenced in China. There are not enough data to support supplementation for everyone at the moment. However, foods high in quercetin include, tea, onions, apples, peppers, blueberries and dark green vegetables.
Vitamin C is an essential component of the diet and only found in fruits and vegetables. An adequate intake is required for the optimal functioning of the immune system and we should be aiming for 100–200mg a day from the diet. Top food sources of vitamin C include red and green peppers, oranges, kiwi fruit and broccoli.
Other specific fruits and vegetables have been shown to boost the immune system although direct anti-viral activity has not been conclusively proven in human studies. These include cruciferous vegetables, for which there is some supportive data for prevention of influenza, mushrooms, garlic and onions.
Nitrate-rich vegetables have a number of benefits for health. They release nitric oxide, which is important for the health of blood vessels. Several studies have suggested that nitric oxide may inhibit the replication cycle of viruses such as SARS, another coronavirus. There are no convincing human data to support the role of nitric oxide as an anti-viral agent, nonetheless, eating your green leafy vegetables has no downsides!
Foods high in nitrates. Source ‘How not to die’ by Michael Greger.
Make sure your diet is optimal in micronutrients such as selenium and zinc. Both are important in maintaining a health immune system. One brazil nut a day will take care of your selenium needs. Foods rich in zinc include, whole grains, beans and nuts. Soaking grains and beans before cooking can increase the bioavailibity of zinc and other nutrients in the food.
Herbs and spices in various forms have some of the highest concentrations of anti-oxidant compounds. Many show anti-infective properties, at least in the laboratory. So use a variety of herbs and spices liberally in cooking every day.
TURMERIC and its active component, Curcumin, has an array of health promoting effects. This includes anti-inflammatory, anti-oxidant and anti-infective properties. A number of studies have shown that that curcumin, at least in the laboratory, is active against a number of different viruses. The dose used in studies varies. In general, a dose of 1000mg of Curcumin combined with black pepper (piperine) to increase bioavailability is a safe option. The concern about supplements however is that there is very little regulation and you made not be getting what is advertised on the label.
HIGH FIBRE FOODS FOR A HEALTHY GUT: The health of our gut, in particular the microbes that live within it, is crucial for maintaining a healthy immune system. A healthy diet, full of fibre-rich foods is the best way to promote the growth of healthy gut bacteria and increase bacterial diversity. A plant-based diet consisting of fruits, vegetables, whole grains and beans is the best way to maintain a healthy gut and data are emerging to support the role of the gut microbiome in our defence against viral infections. There is not enough data yet to recommend probiotics for prevention of infection, but we do know that changing your diet can have beneficial effects on the gut microbiome in a matter of days to weeks.
NUTRITIONAL YEAST is a species of yeast known as Saccharomyces cerevisiae, which is grown specifically to be used as a food product. The yeast cells are killed during the manufacturing process so this is not a live product. It can be used in all sorts of dishes to add flavour and is also a great source of nutrients. Beta-glucan, one of the polysaccharide carbohydrates within the yeast, has been associated with a number of immune boosting effects. Of note, it is the beta-glucan in mushrooms that is thought to be one of the main reasons for the beneficial effects on the immune system. So add nutritional yeast to your soups and sauces daily.
GREEN TEA is high in polyphenolic catechins, which have a number of potential health benefits. Studies have also demonstrated anti-viral properties, including efficacy against influenza. In general 2–5 cups a day of green tea is needed. There are no data that green tea can protect against coronavirus infection, but no harm in preventing flu in the meantime.
VITAMIN D supplementation has been shown to reduce the risk of acute respiratory tract infection. In a meta-analysis of 10 933 participants in 25 randomised controlled trials, vitamin D supplementation showed an overall protective effect against acute respiratory tract infection. To prevent 1 episode of infection, 33 people would have to receive vitamin D supplementation. However, if you have severe vitamin D deficiency, there is a greater chance of benefit from supplementation with only 4 people needing to use supplements to prevent 1 infection.
The advice for UK citizens is to supplement with vitamin D in the winter months (October to March). Public Health England recommends a dose of 10mcg/400IU but it may be advisable to take higher doses, up to 25mcg/1000IU. However, take care not to overdo it, as more if not necessarily better and very high doses can have an adverse effect on health. For now, I would recommend supplementing longer than March whilst the pandemic continues given that most of us will require isolation at some point.
Vitamin D comes in two forms; vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is more effective at raising blood levels. If you are vegan, be sure to choose a plant-based source of vitamin D3, made from lichen.
ADEQUATE AND GOOD QUALITY SLEEP is essential for a healthy immune system. We should all be aiming to get between 7–9 hours sleep per night. To help ensure good quality sleep, the following can help:- maintain a regular sleep schedule; avoid daytime naps and if you do nap keep this to less than 30 minutes; make sure your bedroom is at a cool temperature, around 15–19 degrees Celsius; ensure a dark room without noise; avoid bright lights in the evening and expose yourself to sunlight in the morning; avoid heavy, carbohydrate-loaded meals 2–3 hours before bedtime; minimise screen-time/use of electronic devices in the evenings and avoid after 9pm; avoid caffeinated drinks and alcohol after 3pm.
REGULAR PHYSICAL ACTIVITY has so many benefits, including maintaining a healthy immune system and helping achieve good quality sleep. The recommended amount for adults is 150 minutes of moderate-intensity physical activity a week and strength building exercises twice per week. Even if you find yourselves having to stay indoors, everyday household activities can be of benefit to health in general.
MANAGING STRESS is really important for maintaining your immune system. Chronic psychological stress promotes inflammation and reduces your defence against infection. Useful approaches for managing stress include mindfulness and meditation, which may have a role in reducing the risk of infection. Don’t underestimate the effect of loving relationships either as studies have shown them to have a beneficial effect on the immune system. Remember to be kind, look after your neighbours and those who are vunerable in your communities.
PUTTING THIS ALL INTO PRACTICE. There are a number of useful apps for putting all this information into practice. One of the most useful is Dr. Greger’s Daily Dozen to ensure you are getting all your nutrients and more. Try headspace or calm for some guided meditation.
These recommendation do not guarantee freedom from infection or COVID-19, but they are side-effect free suggestions, which are also in line with general recommendations for healthy living.
Dr Shireen Kassam founded Plant-Based Health Professionals UK in 2017. This is a non-profit, membership organisation whose mission is to provide evidence-based education on whole food plant-based nutrition for the prevention and treatment of chronic disease.
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