IT IS that time of year again. As the cold and flu season hits the northern hemisphere on top of the ongoing ravages of covid-19, many of us reach for something to give our body鈥檚 defences a helping hand. There is no shortage of foods and drinks that claim to be able to help. Supermarket shelves are heaving with 鈥渋mmunity support鈥� juices, yogurts and spice-infused teas. Comforting, yes, but can these foods really help stave off contagion?
The impact of diet on our natural defences is a topic rife with claims that don鈥檛 stand up to scrutiny, but we are finally developing a clearer picture of the powerful influence that food can have on our immunity. The surprising truth is that focusing on particular products or ingredients could be leading you down the wrong path. In fact, an immune 鈥渂oost鈥� might be the last thing you need. Here is a guide to the foods you really should be eating to keep your immune system on top form.
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The idea that certain foods can help us when we are ill goes back a long way. Think chicken soup or a drink of hot honey and lemon. Many different plants and plant products, such as spinach, blueberries, garlic, broccoli, cabbage and turmeric, have immune-boosting reputations. In recent years, new concoctions to supposedly help our immune health have been developed at a furious pace. In 2020, nearly 2 per cent of new foods and drinks sold globally , from 鈥渂reakfast on the go鈥� smoothies and probiotic yogurts to fermented drinks, such as kefir and kombucha, and so-called functional shots: juices made from ingredients like lime, beetroot and ginger that are designed to impart health benefits.
Numerous claims have also been made on , especially in response to the covid-19 pandemic. This has prompted the World 午夜福利1000集合 Organization to issue that explain how, for instance, hot peppers in food can鈥檛 prevent or cure covid-19.
In reality, teasing apart the impact of any food on your immunity is extremely challenging. Many substances that show promise in tests on cells or in lab animals don鈥檛 measure up in large-scale, real-world trials in people. One reason for this is the complexities of the immune system itself 鈥� an intricate network of cells, physical barriers and chemical signals.
Our innate immune system is the first to respond to an invader. It includes general-purpose cells such as macrophages and neutrophils that patrol the body, detecting and killing bacteria, viruses, fungi and parasites.
The next line of protection is the adaptive immune system, with a more specialised set of defences, such as white blood cells called B-cells and T-cells that learn through exposure to recognise and kill specific invaders. Hordes of immune cells rush to the site of an injury or infection, releasing chemicals called cytokines that recruit even more immune cells. A combination of heat, pain, redness and swelling, known as inflammation, builds at the infected site to aid the healing process.
It is tempting to think that supercharging this system might allow someone with, say, a cold to spring from their sickbed. But many symptoms of infections are actually signs that your immune system is actively working to fight them off. Inflammation can make your throat sore, for instance, and cytokines might trigger a fever to increase the rate of your immune response.
The key to a healthy immune system is to strike a balance between reactivity and restraint. This ensures your body is well equipped to both tackle an infection and avoid too strong a response. 鈥淚nflammation is a double-edged sword,鈥� says at the University of Minnesota. To limit unnecessary harm, the immune system must put checks in place. Some cytokines inhibit inflammation, and regulatory immune cells keep reactive ones under control.
If these checks become disrupted, the immune system can attack the body鈥檚 own cells and proteins, causing autoimmune diseases like rheumatoid arthritis, type 1 diabetes or multiple sclerosis, or can mistakenly react to harmless foreign substances, such as pollen or peanuts, resulting in allergies.
Inflamed response
But as well as these immune overreactions, there is the increasingly common problem of , which is linked to accelerated ageing and ailments such as stroke and heart disease. So while it is important to maintain a strong immune system to stave off infection, an ability to dampen it is equally important.
A crucial component of the immune system is the digestive tract. In fact, the in the body 鈥� and it needs to be. Eating is a perilous process that introduces around 100 grams of foreign proteins from plants and animals per day, as well as a myriad of microbes, all of which must be assessed by the immune system to sift the helpful from the harmful.
There are a number of ways that particular foods might be able to support the immune system. The first is via the vitamins and minerals needed for it to function: vitamins A, C, D and E, as well as some of the B vitamins, plus zinc, copper, selenium, magnesium and iron. A deficiency in any of these micronutrients can undermine immune function. And if you read the labels on almost all 鈥渋mmunity鈥� food products, you will see that this claim is due to the nutrients within them that 鈥� a factor that has been , such as sugary breakfast cereals.
Cynical marketing aside, the question is, if we are consuming the recommended amounts of these nutrients and don鈥檛 have deficiencies, will extra doses give additional benefits?
Many of us reach for vitamin C 鈥� either in pill form or from a glass of orange juice -when we feel a cold coming on, fuelled by an understanding that it helps our body fight the viruses responsible. Yet many studies show that an for someone already eating sufficient quantities, and it is the same story for many of the other nutrients typically added to food products to bolster their 鈥渋mmunity鈥� status.
More promising is vitamin D, which has recently attracted much attention. This vitamin, found in foods such as egg yolks, liver, red meat and salmon, can 鈥渢une the functions of many immune cells鈥�, says at the University of Ni拧 in Serbia. Around a billion people worldwide are deficient, including . Recent studies show how vitamin D reduces the risk of developing an autoimmune disease and can , although on mortality or requirement for intensive care due to covid-19.
Antioxidants are another factor commonly linked with immunity. These encompass a wide range of compounds, including vitamins C and E, plus plant-based polyphenols such as flavonoids, that can neutralise highly reactive molecules called free radicals. These metabolic products can wreak havoc on our tissues by causing oxidative stress, contributing to disease and ageing.
Although this neutralising power has been demonstrated in numerous lab tests, the health impacts of dietary antioxidants in real-world studies . One explanation is that each dietary antioxidant molecule can only neutralise one free radical, whereas the body produces antioxidant enzymes and each of these can neutralise thousands of them. 鈥淭he effect of dietary antioxidants inside the cell is trivial compared with antioxidant enzymes,鈥� says at University of California, Merced. This could be a reason why the spice turmeric 鈥� one of the world鈥檚 most revered immunity foods 鈥� and its active ingredient curcumin, an antioxidant, .
These findings pose a puzzle: while it appears that individual products containing high doses of vitamins and antioxidants are unlikely to deliver a health boost, diets rich in these nutrients are linked to healthier immune function. The Mediterranean diet, for instance 鈥� replete with antioxidant and vitamin-rich cereals, fruits, legumes, nuts, olive oil and vegetables 鈥� dampens down harmful inflammation, whereas the so-called Western diet 鈥� rich in sugars and saturated fats, and low in complex carbohydrates, fibre and vegetables 鈥� seems to be recognised by the immune system as a threat and is linked with inflammation.
For example, a 2016 trial of 25 adults (pasta, salad, nuts and olive oil vs a burger and fries). The fast food meal led to elevated inflammation and oxidative stress in participants, whereas the Mediterranean one dampened these responses.
A number of factors could explain these effects. One is that the Mediterranean diet is , a type of healthy fat found in dairy, fish, nuts, plant oils, seafood and seeds, and contains less omega-6 than a typical Western diet. 鈥淭his ratio of omega-3 and 6 is associated with lower inflammation, but in Western diets, the ratio is usually flipped,鈥� says at the University of Barcelona in Spain. Another factor is the way that many Western-type meals are prepared. Broiling, frying or roasting processed meat causes sugars to react with proteins, .
At the heart of these immune effects is our microbiome, the vast bacterial community that resides in our gut. 鈥淭hese communities of bacteria form a delicate ecosystem,鈥� says at the University of Oxford.
Microbiome response
The intricate details of the interaction between the microbiome and immunity were revealed in a 2020 study of more than 2000 people by a team of researchers led by at the Memorial Sloan Kettering Cancer Center in New York. This showed how the in the blood changed in relation to the presence of different bacterial strains in the gut. For example, a relatively high abundance of the bacterium Ruminococcus 2 was associated with increased neutrophil levels.
So, by virtue of the close interactions between gut immune cells and the trillions of microbes living among them, foods that affect these bacteria may have knock-on effects on our defences. 鈥淭he foods we choose to eat impact our gut ecology,鈥� says Powrie.
Many modern diets are totally different from the ones humans evolved with over thousands of years. 鈥淥n evolutionary timescales, we have changed our microbiome in the blink of an eye,鈥� says at Stanford University in California. He believes the increasingly common inflammatory diseases of the modern world, including diabetes, atherosclerosis and inflammatory bowel disease, may be partly caused by damage to our gut microbiome from consuming a high-fat, low-fibre diet.
One 2015 study, for instance, when the typical high-fibre, low-fat diet of 20 rural South African people and low-fibre, high-fat meals of 20 African American people were switched. Within two weeks, a profound change in the participants鈥� microbiomes was observed, with the Western-style food associated with inflammation of the gut鈥檚 mucous lining 鈥� a risk factor for the development of colon cancer.
This is one of a number of studies showing how high-fibre foods, such as vegetables, wholegrains and legumes, are key for an immune-healthy microbiome. For instance, the products of fibre fermentation by gut bacteria 鈥� short-chain fatty acids (SCFAs), such as butyrate and acetate 鈥� are anti-inflammatory. 鈥淲e can鈥檛 digest fibre into these compounds ourselves and eating meals rich in fibre helps to support bacterial populations that can,鈥� says P茅rez-Cano.
This takes us back to the question of whether dietary interventions could help tip our immune system in a healthier direction.
To find out, Sonnenburg and his team set up a clinical trial to investigate whether . For this, 39 participants were randomly assigned to a high-fibre diet (adding at least 20 grams of fibre per day to their normal meals) or a diet high in fermented foods (with six servings per day of kimchi, kombucha, yogurt or fermented dairy products). Immune markers such as cytokine levels were monitored, along with the diversity and functioning of gut bacteria.
This study, published last year, found that those consuming fermented foods developed more diverse microbiomes and decreased markers of inflammation. A different pattern was seen in the high-fibre cohort. Although the diversity of their gut microbiome didn鈥檛 increase, it was better able to produce SCFAs. Interestingly, the immune response of the participants on a high-fibre diet varied according to the initial diversity of their gut microbes, highlighting the need for a personalised approach for any microbiome interventions.
Another 2021 study, this time of nearly 1000 participants in the UK-based PREDICT-1 nutritional research programme, found that lower levels of markers of chronic inflammation were , but not fruit.
The exact mechanisms of how different species of gut bacteria affect immunity are being elucidated too. Beneficial microbes, such as Bifidobacterium, for example, influence immune cells via signals like the SCFAs that they produce. These compounds bind to receptors on immune cells and prompt them to produce cytokines that favour immune restraint over reactivity.
We are now realising that the beneficial effects of antioxidants also seem to be due to the microbiome. Plant-based foods are rich in polyphenols that were initially thought to dampen inflammation through antioxidant activity. New research, however, suggests that these compounds influence our immune system by favouring the growth of some bacteria over others. For instance, trials in humans showed that consumption of and , both rich in polyphenols, resulted in an increase in Bifidobacterium.
What about short-cutting this process and ingesting the right kind of bacteria for healthy gut function? This idea has fuelled a boom in probiotic supplements 鈥� drinks containing live bacteria to fortify our gut microbiome. But just as with vitamin supplements, the efficacy of these products is under question. 鈥淭he adult gut microbiome is resilient to changes and often prevents probiotic strains from becoming resident in the gut,鈥� says Powrie.
What does all this mean for when you are in the supermarket faced with an array of supposedly immune-boosting products? To keep your natural defences in good shape, what really seems to matter is a healthy gut microbiome. For this, the most important factor is the nature of your overall diet.
鈥淲e can try to improve our diets by increasing our intake of fibre and fermented foods to support our microbial populations,鈥� says Sonnenburg. A 2018 study from the American Gut citizen science research programme found that for maximum gut microbiome diversity, .
Although a turmeric and beetroot 鈥渨ellness鈥� shot or a kombucha fermented tea drink contains the kind of ingredients linked to a healthier microbiome, this isn鈥檛 going to move the dial if you live mostly on burgers and fries. And watch out for products boasting their immune-support potential when they are really just an unhealthy snack with a few added vitamins. But most of all, remember to nurture the cultures within your gut 鈥� they may be key to keeping your immune system in perfect balance.
