It is a cold morning in the depths of winter.

And I feel like I’ve left my sanity as well as my warm clothes in the changing room as I stride out to the edge of a reservoir in my swim shorts.

A brightly-coloured chalk sign informs me that the water temperature today is a chilly 3.9C, as one of the regulars tells me this is not cold water swimming, it’s “ice” water swimming.

How did I end up here?

Well, I’ve become enthralled by the idea of enhancing or boosting the immune system. My body has been the living embodiment of the “quademic” that the NHS spent all winter warning us about. It’s been a relentless stream of colds and coughs and one explosive tummy.

Our immune systems already do a fantastic job fighting viruses and other nasties. If I collected all the air I breathed out over the course of a minute it would contain 100 to 10,000 bacteria, 25,000 viruses and a single fungi, according to Prof John Tregoning, immunologist at Imperial College London.

“You’re breathing these things in all the time, there’s just a swirling mass of pathogens [organisms that cause disease] in the air,” he says.

But there’s plenty of foods, supplements and activities that are touted for their “immune-boosting” properties. Can we dial up our protection?

And that’s why I find myself wading out and pushing off for a chilling blast of breaststroke.

The icy water is like fire on my skin and all my brain can think of is making it to the pontoon without needing help from the lifeguards.

But studies have shown the adrenaline hit of the cold water does flood the blood stream with infection-fighting cells.

White blood cells – which could produce antibodies or attack infected tissue – move out of their normal homes and head out on patrol thinking there could be an infection. So does this mean I’m more protected?

“Within a few hours that all goes back to normal,” says Prof Eleanor Riley, an immunologist at the University of Edinburgh.

“There is no evidence that people who go cold water swimming have fewer colds or fewer infections.”

There might not be the evidence for cold water, but regular exercise may well do the trick.

On average adults get two to three colds per year and children between five and eight, says Dr Margaret McCartney, a GP and expert in evidence-based medicine, at the University of St Andrews.

“[But] people who do moderate amounts of exercise tend to report fewer viral infections,” she says.

There is a lack of conclusive clinical trials, but the data we have is “all pointing in the direction of it being good for you… but not a miracle cure,” according to Dr McCartney.

Laboratory studies have suggested regular exercise may slow the ageing of the immune system. Your body’s defences are in decline from our 20s onwards, but research on 80-year-old cyclists suggested they had the immune systems of people decades younger.

“I know the trials have not really been done in high enough quality, but I’m going to be doing a lot of cycling,” she says.

The first thing that springs to my mind is vitamin C – either by devouring a mountain of oranges or in pill form.

“It’s a no from me,” says Dr McCartney. Being deficient may impair the immune system, but for the vast majority there is little to be gained from having extra. The same goes for multivitamins, which says Dr McCartney just make “expensive urine”.

However, the evidence around vitamin D is hotly debated rather than being a firm no. Vitamin D levels dip in winter as it is made when our skin is exposed to sunlight.

“I think the evidence is pointing towards possible benefits for people who’ve got respiratory disease and who are very low in vitamin D,” says Dr McCartney, but there is “insufficient” evidence that it would help everyone.

And while you’re thinking what you might put in the shopping trolley, the jury is still out on whether prebiotics and probiotics that change the good bacteria in your gut also benefit immunity.

“I do believe that is a really important area of study, but we are lacking the real world data that would enable you to recommend this,” says Dr McCartney.

She also says echinacea, turmeric and ginger shots are not going to give you an immune boost.

Your immune system’s abilities are not fixed throughout the day.

“Our immune system is most effective in the early morning at around the time we wake up, continues to be highly effective through the early part of the day and then later on in the day it starts to wind down,” says Prof Riley.

This is why your cold often feels worse in the morning as your symptoms are the result of an immune system firing on all cylinders.

The decline kicks in “about four or five o’clock in the afternoon”, so you might have better protection if you are vaccinated, or exposed to someone coughing, in the morning.

As the immune system does have this 24-hour cycle, then having “a regular daily rhythm” rather than a mix of late nights and long lie-ins at the weekend “may well help boost your immune system”, says Prof Riley.

While we are thinking of boosting our immune system we should also remember there are things that actually increase our vulnerability to infection.

One of the big ones is smoking because it directly damages the lungs so they are a less effective barrier to viruses.

“If you imagine the lungs like a sieve, smoking will break holes in the sieve, so more can get through,” says Prof Tregoning.

It also dials up inflammation throughout the body. Inflammation is like a thermostat for the immune system and is a normal part of how the body reacts to an infection.

But “uncontrolled inflammation is bad for you” as it disrupts the immune system “so you may respond less well”.

Obesity is another factor that can increase susceptibility to infections as well as the severity by ramping up inflammation in the body.

“Both of those things can be difficult for people to stop or to reverse, but they are probably the most modifiable ones,” says Dr McCartney.

Being constantly stressed raises levels of the hormone cortisol in the body.

Yet, cortisol dampens the immune system which may leave you more prone to infection.

Prof Tregoning says this may be the explanation for why being in nature, going for a walk, spending time with friends – even cold water swimming – may have some beneficial effect.

“You’re less stressed, less cortisol, so therefore your immune system is more fit for the role it is trying to do,” he says.

Prof Riley adds: “There is absolutely no doubt that being happy, being in a positive frame of mind, has a very, very important effect on our bodily functions.”

You have probably seen products in the shop that you can spray up each of your nostrils at the first sign of a cold.

A report in the Lancet showed they do work.

It gave thousands of people either a salt water (saline) spray or a gel-based one. They could use it up to six times a day whenever they felt they were coming down with something.

People who just went about their daily lives without a nasal spray spent an average of eight days being ill during the study.

But that fell to six days in those using a either salt water (saline) or a branded gel-based spray.

Dr McCartney argues that a branded spray would be no better than a normal saline spray.

If you are already doing all the usual things to look after your health – not smoking, eating healthily, exercising regularly – then your immune system is already in the “best possible condition” to respond to an infection, says Prof Riley.

“Can you do anything to boost above being a normal, healthy person? There’s just no real evidence that you can,” she says.

“But there are things you can do to improve your immunity against particular individual infections and that is to be vaccinated.”

And maybe instead of spending your money on the latest immune-boosting craze, she suggests thinking about ways of not getting infected in the first place so “be careful who you socialise with”.

Inside Health was produced by Hannah Robins and the programme airs on BBC Radio 4 on Tuesdays at 09:30 GMT and on BBC Sounds.