The unfortunate truth is that protecting ourselves from colds isn’t quite so straightforward. Photo / 123rf
The most effective ways to stay healthy, according to an immunologist.
When it comes to our health, everyone loves a quick fix. Perhaps a vitamin C tablet or glass of orange juice to “boost” your immune system. Or maybe a spoonful of Manuka honey is your preferred salve for a sore throat. However, the unfortunate truth is that protecting ourselves from bugs isn’t quite so straightforward.
Professor Daniel Davis knows from experience how complex the immune system is. As the head of life sciences and a professor of immunology at Imperial College London, he has studied it for the past 25 years. In his laboratory, he uses high-resolution microscopes to examine what happens to immune cells when they interact with other stressors, with the aim of developing medicines. Here he explains the immune boosting “fads” to steer clear of this winter and what really works…
Davis explains that vitamin C makes no difference to whether you get a cold or not. Interestingly, vitamin C’s popularity as an immunity booster traces back to one man, he explains. Linus Pauling was a renowned Nobel Prize-winning scientist who enhanced our understanding of chemical bonds and – later in life – turned his attention to campaigning against nuclear war.
He was widely regarded and trusted by the public and when he released a book in 1970 titled Vitamin C and the Common Cold it was an instant hit. In fact, new factories were built to keep up with skyrocketing demand for vitamin C which he claimed could prevent colds and even cure cancer in high doses.
The reality was that he’d cherry-picked data and relied on anecdotal evidence to promote these claims. However, he was a massively respected and trusted public figure and – even 50 years later – the idea of vitamin C preventing a cold remains ingrained in our culture. Fortunately, his claims around cancer were rightly discredited.
“The data actually shows that people who regularly supplement themselves with a high dose of vitamin C will get over a cold just 8 per cent quicker,” Davis says. “So if you experience symptoms for a few days, you might get better a few hours earlier on average.” That’s an incredibly small gain – plus, the correlation is weak as most participants are likely to follow other health-boosting habits in their lives which are hard to account for.
What actually works: Vitamin D and A
However, there are two other vitamins with good evidence for supporting the immune system – vitamins D and A. A 2020 review in Nutrients concluded there is an “indisputable relation” between vitamin D and the immune system, with a deficiency possibly leading to infection or an autoimmune condition. Davis therefore recommends sticking to NHS guidance and taking a vitamin D tablet each day during the winter months.
Vitamin D exists in two main forms – D2 which comes from plants and fortified foods like plant-based cereals, and D3 which is produced in human skin and found in animal-based foods like fish and egg yolks. Evidence suggests that vitamin D3 is generally more effective than D2 at raising and maintaining vitamin levels in the blood.
What’s more, there is a wealth of evidence which suggests that vitamin A helps the immune system to work properly. Davis says deficiency is rare in the UK (but a significant issue in some parts of the world), as most people here consume enough vitamin A in their diets with cheese, eggs, oily fish, milk and liver products all being good sources.
Men need 700mcg of vitamin A and women 600mcg a day, according to the NHS. (One large hard-boiled egg contains around 75mcg, 100g of cooked salmon contains roughly 69mcg, while a tablespoon of cod liver oil contains a substantial 4080mcg of vitamin A.)
Yoghurts with health benefits are all the rage nowadays, but while they seem convenient, Davis urges caution against probiotic yoghurts that neatly package gut health and immunity-boosting benefits into an easily consumable bottle.
“These brands may create the impression that the bacteria in your yoghurt drink will boost your immune health but, quite simply, this isn’t yet clearly proven,” he says. “Whilst the experiments are clear in that the bacteria in your microbiome certainly do influence your immune system, the gap in our knowledge is exactly how this relationship works.”
In his most recent book – Self Defence: A Myth-busting Guide to Immune Health (Vintage, 2025) – Davis references a study where volunteers were fed a particular strain of live bacteria called Bifidobacterium infantis 35624 for eight weeks. In that time, the levels of T cells (white blood cells which help your immune system fight germs) in their blood increased.
This demonstrates that probiotics in the form of live bacteria could feasibly support immunity, however it stops short of proving a medical benefit because we don’t know if an increased number of T cells improved those participants’ protection from infections and viruses, Davis says.
When we’re scanning the supermarket shelves, he urges us to “beware of the hype” and take a step back to think about who is promoting a product with miraculous health benefits, if they have an agenda, and what the evidence really shows.
What actually works: Managing long-term stress
Rather than paying for products to “boost” your immune system, most scientists agree that one of the best things you can do for it is managing long-term stress.
“We have an almost molecular-level understanding of exactly why and how long-term stress can make you more susceptible to infection,” Davis says.
When you’re stressed and your body senses a threat, it prepares itself for action with the famous fight or flight response. Your adrenal gland – which sits on top of your kidneys – secretes cortisol and adrenaline and these all-important hormones prepare you for action. As part of this preparation, your body quietens your immune system – the thinking being that if you’re running away from a lion or fighting for your life, your immune system doesn’t need to fight off an infection in that particular moment.
That’s all fine and makes perfect sense if your stress response is over and done with after an hour or two. However, if you’re stressed and your cortisol levels are heightened for an extended period of time, then this dampens your immune system and could make you more susceptible to infection, as demonstrated in Davis’ research below with immune cells and cancer cells.
In a 2024 review in the Journal of Clinical Medicine, chronic stress was found to significantly disrupt immune function which heightens our susceptibility to infections and can exacerbate autoimmune conditions, while also influencing the progression of cardiovascular diseases and other health outcomes. Therefore, managing long-term stress is incredibly important for maintaining a strong immune system – particularly during the winter months – whether you prefer a walk in the park, spending time with friends and family, or reading a good book.
You may hear anecdotally that a mug of tea can soothe a sore throat or perhaps you swear by a spoonful of honey to support your immune health. While Davis clarifies that he would never tell anyone not to eat a food that makes them feel good, he warns that there’s no scientific consensus on these remedies.
It’s almost impossible to test the vitality of so-called “immune boosting” remedies, he argues, without actually injecting people with an infection (obviously not at all ethically appropriate, Davis notes) and then blindly handing out beneficial honeys alongside placebos to examine the effects.
“Even then, for something like this to be widely accepted, we’d need to be able to track a detailed molecular pathway to demonstrate how that particular product might be beneficial for the immune system,” he explains.
“That’s not to say you shouldn’t use them if they help pep up your energy in the winter months, or if you find them soothing. A number of studies suggest that honey may help soothe a cough, while this review in Herbal Medicine: Biomolecular and Clinical Aspects links herbal teas to supporting immune function as they contain anti-inflammatory properties.
What actually works: Uninterrupted sleep and moderate exercise
Once again, lifestyle interventions have a role to play and sleep is key. Davis cites a study that compared participants who slept normally through the night, with those who had disrupted or shortened sleep. They were all given a vaccine, and then the amount of antibodies they produced in response was used as a measure of how well their immune system was operating.
The researchers found that those who slept through the night produced more antibodies while those with disrupted sleep produced a lower amount. In other words, uninterrupted sleep is important for our immunity.
Coupled with this, is the importance of regular exercise, Davis believes – although not too much of it – and not at too intense a pace. When it comes to colds and flu, we know that exercise can’t physically stop germs and infections from entering the body, so the question is whether it can help us to fight and lessen the severity of them.
Numerous studies suggest that the answer is yes – either to the extent that the germs don’t develop into a fully-fledged infection or severe symptoms are less likely. One large study in China, for instance, found that exercising moderately for three or more days per week was associated with 26% less risk of having at least one cold a year. This is likely because exercise enhances blood flow which improves the circulation of immune cells and can also reduce inflammation.
“However, there’s a threshold to this,” Davis notes, “because people that do very strenuous exercise like running marathons, completing an Iron Man, or very competitive sports, may find that stress comes into play which we know can negatively impact the immune system.”
Rather than strenuous exercise, moderate exercise is key: brisk walking, cycling or tennis may give you better protection from the winter nasties. Once again, Davis says, a balance is key. There is some evidence that extreme levels of exercise may weaken immune health in the short term but can be beneficial later in life.
Plus, we’re all individuals. As Davis says: “Your immune system is fundamentally different from mine. Everything is personal and there’s no ‘one-size-fits-all’ path to immune health. I’m on a mission to guide people through the science, hopefully empowering people to make more informed choices about their immune health.”
“If I took cells out of your blood right now and mixed them up with certain types of cancer cells which are easily detected by immune cells, we could look through a microscope and see how many cancer cells are being killed,” Davis says.
“We’d see that your immune cells are doing a fairly good job. However, if I added cortisol (a stress hormone) into the mix, then your immune cells would find it a lot harder to kill the cancer cells. It’s one reason why managing long-term stress is so important for our immune health.”
Scientists have reached a consensus on the impact that stress, sleep, ageing and body weight have on the immune system, however the same is not true when it comes to supplements and other supposedly “immune boosting” products, Davis explains. In fact, he thinks the idea of “boosting” the immune system is problematic in itself.
While part of its job is defending the body from infections and viruses, the immune system also needs to regulate this response against perceived “invaders” to ensure it doesn’t unwittingly attack any normal, healthy cells. If your immune system reacts to something which isn’t actually harmful (like healthy cells, foods or pollen) then you could develop an autoimmune disease or allergy. Therefore, rather than “boosting” the immune system, maintaining its delicate balance is far more crucial.