Vitamin pills and icy swims: Can you really boost your immune system?

It was a particularly cold morning during the winter season. In search of ways to boost my immunity, I arrived at the reservoir, changed into my swimming trunks, seemingly leaving reason and warm clothing behind in the changing room.

A brightly colored chalk sign indicated that the water temperature today was a frigid 3.9 degrees Celsius. A regular told me that this was no longer cold-water swimming, but "ice-water" swimming. I have recently been drawn to the idea of boosting immunity because my body seems to be a living embodiment of the "twindemic" warned by the UK's National Health Service (NHS), constantly suffering from colds, coughs, and gastrointestinal discomfort.

Our immune systems have done an excellent job of fighting viruses and other harmful substances. Professor John Tregoning, an immunologist at Imperial College London, said that if I were to collect all the air I exhaled in one minute, it would contain 100 to 10,000 bacteria, 25,000 viruses, and a fungus. "You're constantly breathing this stuff in; the air is full of pathogens."

Many foods, supplements, and activities are advertised as having "immunity-boosting" properties. So, can we really enhance our own defenses? Can cold-water swimming really prevent illness?

This is why I found myself wading into the water, beginning a chilling breaststroke. The icy water burned my skin like fire, and the only thing on my mind was reaching the pontoon safely, without needing the help of a lifeguard. Studies have shown that the adrenaline rush from cold water does flood the blood with infection-fighting cells.

White blood cells—which can produce antibodies or attack infected tissue—leave their usual habitats to go on patrol, thinking there might be an infection. So, does this mean I am better protected? Professor Eleanor Riley, an immunologist at the University of Edinburgh, says, "Within a few hours, everything will return to normal. There is no evidence that people who swim in cold water get fewer colds or infections."

While there is no evidence that cold-water swimming is beneficial, regular exercise may indeed be effective. Dr. Margaret McCartney, a general practitioner and expert in evidence-based medicine at the University of St Andrews, says that on average, adults get two to three colds a year, and children aged 5 to 8 get even more. "However, people who do moderate exercise tend to report fewer viral infections."

Although there is a lack of definitive clinical trials, the data we have "all points in a direction that is good for you... but it's not a panacea," Dr. McCartney said. Laboratory studies have shown that regular exercise may slow down the aging of the immune system. From our 20s onward, our body's defenses decline, but studies of 80-year-old cyclists have shown that they have "immune systems of people decades younger than they actually are." "I know the quality of the trials isn't high enough, but I'll cycle more."

My first thought was vitamin C—whether through eating lots of oranges or taking pills. "I disagree," says Dr. McCartney. A lack of vitamin C can harm the immune system, but for the vast majority of people, extra vitamin C does little good. The same goes for multivitamins, Dr. McCartney says, they only produce "expensive urine."

However, there is fierce debate, rather than outright denial, about the evidence for vitamin D. Since our skin produces vitamin D when exposed to sunlight, vitamin D levels drop in the winter. "I think there's evidence that people with respiratory illnesses who are very low in vitamin D may benefit," Dr. McCartney said, but there is "not enough" evidence that it helps everyone.

Whether prebiotics and probiotics can alter the beneficial bacteria in your gut, thereby benefiting immunity, is still inconclusive when you are considering what to put in your shopping cart. "I do believe it's a really important area of research, but we lack the real-world data that would allow you to recommend it," Dr. McCartney said. She also said that echinacea, turmeric, and ginger drinks will not boost your immunity.

Your immune system's capabilities are not constant. "Our immune system is most effective when we wake up in the early morning, continues to be highly efficient in the early part of the day, and then begins to weaken later in the day," Professor Riley said. This is why your cold usually feels worse in the morning, because your symptoms are the result of your immune system going all out.

This decline occurs "around four or five in the afternoon," so you may be better protected if you get vaccinated or come into contact with a coughing person in the morning. Since the immune system does have this 24-hour cycle, having a "regular daily rhythm," rather than a mix of staying up late and sleeping in on weekends, "is likely to help boost your immune system," Professor Riley said.

While considering boosting our immune systems, we should also remember that some things actually increase our risk of infection. One of the main factors is smoking, as it directly damages the lungs, making them a less efficient barrier against viruses. "If you imagine the lungs as a sieve, smoking puts holes in the sieve, so more stuff can get through," Professor Tregoning said.

It also increases inflammation throughout the body. Inflammation is like the immune system's thermostat and is a normal part of the body's response to infection. But "uncontrolled inflammation is bad for you" because it disrupts the immune system, "so your response may not be as good." Obesity is another factor that can increase susceptibility to infection, as well as increase its severity by increasing inflammation in the body. "It can be difficult for people to stop or reverse these two things, but they are probably the easiest to change," Dr. McCartney said.

Sustained stress raises levels of cortisol in the body. However, cortisol suppresses the immune system, which may make you more susceptible to infection. Professor Tregoning said this may explain why being in nature, going for a walk, spending time with friends—or even cold-water swimming—may have some beneficial effects. "You're less stressed, you have less cortisol, so your immune system is better suited to what it's trying to do," he said.

Professor Riley added, "There is absolutely no doubt that a happy, positive mindset has a very, very important impact on how your body functions."

You may have already seen some products in stores that you can spray into each nostril at the first sign of a cold. A report in The Lancet showed that they do work. It gave thousands of people either a saline (saltwater) spray or a gel-type spray. They could use it up to six times a day when they felt they were getting sick.

During the study, people who did not use a nasal spray and just went about their daily lives were sick for an average of eight days. But in people who used a saline (saltwater) or brand-name gel spray, that number dropped to six days. Dr. McCartney believes that brand-name sprays are no better than plain saline sprays.

If you're already doing all the usual things to take care of your health—not smoking, eating healthily, exercising regularly—then your immune system is already in the "best possible state" to fight off infection, Professor Riley said. "Is there anything you can do to boost it above a normal, healthy person? There's no real evidence that you can," she said.

"But there are things you can do to improve your immunity to a specific individual infection, and that is to get vaccinated." Perhaps instead of spending money on the latest immunity-boosting craze, she suggests considering how to avoid infection in the first place, so "be careful who you associate with."