27-05-2025
Why vitamin C DOESN'T cure colds - but here are two others that WILL boost your immunity
No doubt your mother used to tell you that vitamin C-packed orange juice would help ward off colds.
You might pop probiotic-packed yoghurts into your shopping trolley to 'boost' your natural defences against illness. Or perhaps you've gone for a run to try to 'sweat out' a fever.
There's so much of this kind of received wisdom about how to support our immune systems. But do any of these tactics actually work?
I've been immersed in questions about immunity for 25 years. I have studied up close how certain immune cells latch on to cancers and then send toxic molecules to kill them, as well as watching others engulf dangerous bacteria to destroy them.
I'm also regularly asked about the best way to improve our natural immunity. Even as an expert I admit it's highly confusing, and there's also a lot of hype around improving immunity.
It's all become big business – one UK High Street chain has 276 products listed under 'immune support supplements' alone. Yet not a single one of these products is required to have scientific proof for their claims.
And, most importantly, none of us is the same as the next person: the fact is, your immune system is probably the single most unique thing about you. It's not a simple part of our anatomy, it's a multi-layered, dynamic lattice of interlocking genes, proteins and cells.
It means we all vary in how susceptible we are to illnesses – and in the symptoms we experience with infections.
Which means what might work for one person might not work for another. But from what we do know, some simple truths emerge about how we can really help our natural immunity – and what are simply myths.
VITAMIN C WON'T STOP YOU CATCHING A COLD
Vitamin C is important to health. A strong antioxidant, neutralising potentially dangerous substances known as 'free radicals' in our body, it also helps us absorb iron, and is involved in things such as collagen production, essential for tissue healing and more.
But there's no solid proof vitamin C can cure colds.
For more than 60 years, this idea has had a stranglehold on our imaginations – which is largely down to Linus Pauling, a double Nobel Prize-winning scientist who was based in the US.
He first championed this theory in the 1960s, after he and his wife took a megadose of vitamin C – some 3,000mg, though our current advised daily dose is just 40mg. They reported that it gave them extra energy and prevented further colds.
Pauling then scoured scientific journals, cherry-picking results to fit his idea. In 1970, his best-selling book Vitamin C And The Common Cold was published, claiming everyone's health could be improved by higher levels of the vitamin. And a huge myth was born.
However, a 2013 systematic review – which collects data from previous studies – concluded that, for the general population, vitamin C does not reduce the chance of catching a cold.
Yet for unknown reasons, people taking regular vitamin C supplements did experience cold symptoms for slightly less time.
To be precise, daily vitamin C reduced the duration of a cold by 8 per cent in adults and 14 per cent in children. In other words, for a cold lasting a few days, someone taking regular vitamin C supplements might feel better a few hours sooner.
. . . BUT HERE ARE TWO VITAMINS THAT DO HELP
Happily, there are two vitamins which can improve immune health: vitamins A and D.
We generate vitamin D in our skin with exposure to sunshine, and some from eating foods such as eggs and oily fish.
Once in the body, it's processed by immune cells. One of its roles is to dampen the activity of specific immune cells.
This might sound like something you don't want to happen, but it's extremely important for an immune response to quieten down when a threat has been removed.
If the immune system stayed in a heightened state after a threat was neutralised, there's more chance of collateral damage to the body, potentially increasing the risk of developing autoimmune diseases (such as type 1 diabetes).
Low levels of vitamin D are also linked with an increased susceptibility to infections.
In 2021, an analysis of 43 trials involving nearly 50,000 people found vitamin D supplements helped protect against respiratory infection – most likely relating to vitamin D being important in the body's response to germs, including the process by which immune cells destroy bacteria.
It's probably sensible to take it in the wintertime, as per government advice.
As for vitamin A – found in carrots, meat and eggs – it's vital for the development and functioning of many immune cells, and is crucial to our defences, playing a role in the body's production of mucus, which obviously helps us eject germs from our body in respiratory infections, for example in a runny nose.
As it happens, most people get the vitamin A they need from their normal diet.
TRUTH ABOUT KEFIR AND YOGHURT DRINKS
I've been asked many times to promote everything from health stores to a dating agency (as my first book included a section on whether the immune system affects who we find attractive – the jury's still out).
Most recently, I was asked to consider promoting yoghurts containing live bacteria.
I've never said yes to any such offers because I've spent nearly three decades studying immunology and still don't know the half of its complexities, so it seems unlikely I could ever get behind a simple advertising slogan.
Still simple slogans are rife – and probiotic yoghurts or kefir commonly feature scientific-sounding phrases such as 'supports immunity', 'protects against colds' and so on. One reason for this being they contain 'good bacteria' that are apparently beneficial for our gut health.
But do probiotic yoghurts really turbocharge immunity?
There are two ways live bacteria in probiotics could feasibly support immune health: by acting directly on our immune system, or by affecting the gut microbiome, the community of microbes in the gut that play a role in immunity.
There are signs both can happen. In a 2012 study published in the journal Gut, when volunteers were fed a particular strain of bacteria called Bifidobacterium infantis 35624 (a billion of them once per day for eight weeks), it increased levels of 'Tregs' in their blood.
Tregs are regulatory T cells: immune cells that specialise in turning off other immune cells.
They ensure an immune response doesn't happen against something which doesn't warrant it or that an ongoing immune reaction doesn't run out of control. In other words, they help keep our immune system balanced. It's also clear that ingesting bacteria affects the gut microbiome.
In one study, probiotic yoghurt consumed by patients who had inflammatory bowel disease changed the make-up of their microbiome.
In another study, the negative effect of antibiotics on the diversity of a person's gut microbiome could be avoided if they took probiotics at the same time.
However, it's notable that your environment – where you live – may be more important than your diet in terms of gut health.
Interestingly, when a hospital in Calgary, Canada, used a probiotic for infants, bacteria from it were often detected in the faeces of nearby babies who had not yet been given it themselves. In other words, cross-contamination can happen in a shared environment.
Indeed, some people living in the same place share features of their microbiome.
BEING OVERWEIGHT AND THE RISK OF INFECTION
Carrying a few extra pounds can affect the immune system because fat cells do much more than just store fat.
Perhaps surprisingly, there is a vast array of immune cells that live within body fat. If we have an excess of fat, the immune cells residing within it can become more active. These immune cells can then trigger unwanted inflammation, which in turn increases our risk of disease such as cardiovascular problems or rheumatoid arthritis.
As well as this, fat cells produce various molecules, including those called cytokines, which also trigger a background inflammation in the body, which in turn reduces your immunity's sensitivity to a real threat such as an actual viral infection.
To make matters worse, people living with obesity produce other kinds of cytokines in an attempt to counteract this unwanted background inflammation caused by excess fat, by dialling down the immune system.
Yet the very presence of these other cytokines then works against us if an immune response is needed to fight an actual infection.
However, being underweight can be just as perilous for our immune systems for a very simple reason: not getting enough food makes us deficient in all sorts of important nutrients, such as minerals, vitamins, amino acids, cholesterol and fatty acids.
HOW FASTING CAN IMPACT IMMUNE CELLS
Restricting food entirely for a period to lose weight is hugely popular – but can impact your immune cells.
During a period of fasting, we know the human body limits its energy use, which has knock-on consequences for immune health.
Indeed, fasting for just one day reduces the numbers of a type of immune cell called a monocyte in the blood. Experiments on mice show these cells moving into bone marrow, where they effectively hibernate to conserve energy. Eating again sees these immune cells immediately mobilise back into the blood.
We don't yet know what effect this has on immune health or the symptoms of immune-related diseases, but it's clear the immune system is very much affected by fasting.
ALWAYS AIM TO GET A GOOD NIGHT'S SLEEP
Sleep affects the symptoms of illnesses, and most often this comes down to fluctuating levels of hormones or cytokines.
This may explain why problems from asthma, for example, are more common at night, and deaths due to asthma are more likely around 4am.
Indeed, unwanted immune responses tend generally to worsen at night, at least in part because cortisol – the stress hormone – is kept low while we sleep, which means immune responses are not suppressed by it.
This is important for rheumatoid arthritis, where the immune system attacks the tissue lining of joints, causing unwanted inflammation and pain.
Again, because cortisol is kept low, inflammation can build up while we sleep, leading to stiff joints in the morning.
There's a paradox here, though. If sleep makes symptoms from unwanted immune responses worse, having less sleep should help. But it doesn't. Disrupted sleep is especially problematic and can lead to worse symptoms from autoimmune diseases, including rheumatoid arthritis and psoriasis.
AVOID HEAVY EXERCISE WHEN YOU'RE FEELING UNWELL
The evidence that hitting the gym helps the immune system is strong, including a year-long study of almost 19,000 people living in Denmark, which established that any level of physical activity correlated with a person being less likely to need antibiotics.
And a UK study of nearly 100,000 people correlated exercise with halving the risk of dying from an infection over a nine-year period.
Crucially, however, there is a flip side: stress hormones such as cortisol can be produced during exercise, which tend to dampen immune responses. Immune cells also need lots of energy to secrete toxins to attack diseased cells, and multiply in number by dividing.
During exercise, energy is used for muscle activity, which limits that available to immune cells.
Most doctors, then, advise not to exercise heavily during a bout of flu or in the grip of fever.
Adapted from Self Defence by Daniel M. Davis (Bodley Head, £22), published on June 5. © Daniel M Davis 2025. To order a copy for £19.80 (offer valid to 10/06/25; UK P&P free on orders over £25) go to or call 020 3176 2937.
