Each winter, as colds and flus start to spread, so too do a host of well-marketed products claiming to 'boost your immune system'. After a series of particularly brutal and drawn out flu seasons, it's natural that many of us are searching for something that will protect from the dreaded winter cough and runny nose.

But is keeping clear of sickness really about finding a supplement that can 'boost' you? Or are we looking for solutions in the wrong place?

"The problem is the word 'boost'. It implies a dial that can be turned up," says functional medicine doctor and co-founder of Vitality360, Dr Jasmina Dedic-Hagan.

The idea of "boosting" your immune system is not a medically accurate one. In fact, increasing immune activity does not necessarily mean your immune system is strong. Instead, she says that "supporting" your immune system is a much better description. And often, this looks like ritualistic healthy habits rather than quick fixes.

We connected with Dr Dedic-Hagan for a no-nonsense discussion on immune health, being rundown and why the cold can actually make you sick. Below find her advice on what you can actually be doing to support your immune system in winter.

This is what being 'rundown' actually means

Being rundown is quite literally about running yourself down and depleting your stores. This means accruing a sleep debt, low-nutrient diets, under-eating and sitting still. One week won't undo everything, but chronically running down your sleep and nutrient stores means your immune system will be slower to respond to bacterial or viral threats.

The other big killer is stress. "Chronic stress is one of the biggest factors for immune dysregulation," said Dr Dedic-Hagan.

Putting your body under sustained pressure causes cortisol to stay elevated until your immune cells stop listening to it properly. It means you end up with inflammation that won't switch off and along with suppressed antiviral defence.

"Alcohol also deserves a mention," she says."It's so normalised but it impairs the cilia that clear your airways, loosens the gut barrier and blunts your frontline white cells."

But if you are rundown, there is one proven quick fix – sleep. "Immune cells drop sharply with sleep loss, but largely restore within a night or two," she said. Sleep is so important that in studies, people sleeping under six hours were roughly four times more likely to develop a cold after exposure than those getting seven or more.

As for other deficits you may have built, they can take more time to repair. Regulating your cortisol rhythm takes a month, vitamin D levels take two to three months to reach steady levels, and if ferritin stores are depleted, this can take three to six months to restore.

A key takeaway, she said that even if you are rundown, getting a cold every fortnight or so isn't normal. Adults average two to three a year. If you're being hit more often than that, it's worth getting a full check up. As for what to ask for, she said:  "I'd want a full blood count, ferritin, vitamin D, thyroid, glucose and coeliac screening, and immunoglobulins if it's truly recurrent."

Yes, the cold really can make you sick

No, it's not just an old wives' tale. Being cold can make you catch a cold.

It does this by lowering your immune defences and weakening the point of entry to your body.

"When the temperature inside your nose drops by around five degrees, which happens fast in cold air, the lining loses close to half of its innate antiviral response," Dr Dedic-Hagan explained.

"Rhinoviruses also replicate more efficiently at cooler temperatures than at core body temperature, which is part of why they colonise the nose in the first place. Add dry indoor air, close proximity and no sunlight, and risks increase as well."

So there really is a good reason to make sure you bring a jacket – and maybe a scarf too.

A supported immune system is about correcting deficits – and then healthy habits over time

"What you want is regulation; a system that responds effectively, and is able to find resolution," said Dr Dedic-Hagan. To do this, you need to support your immune system by giving it what it needs to do its job – both long and short term.

"If you're short on vitamin D, low on zinc or chronically under-slept, your immune function sits below its capacity and is not able to work properly. Correct that deficit and function improves, sometimes dramatically," she said.

It is important to remember while it does take time to run yourself down, it also takes that same time to build yourself back up in terms of microbial diversity, nutrient stores, muscle mass and blood sugar balance. You need to make a habit of it. "One bad week won't undo you, but one good week won't fix you," she said.

Sleep, fibre and plant variety, moderate exercise and early daylight are more obvious metrics to focus on for immune health. But there are a few key supports that are less obvious. One is eating enough protein. "Your antibodies and cytokines are proteins, and you can't build them from a deficit," she says.

The other less obvious one is keeping up social activity. "Loneliness has a measurable impact on gene expression; inflammatory genes up, antiviral defence genes down."

Your immune system is in your gut – so food plays a bigger role than you think

While every immune cell you have is made in your bone marrow, your gut is where most of them go to work.

"Roughly 70 per cent of your immune cells are positioned around the gut wall because it's your largest interface with the outside world," Dr Dedic-Hagan said.

This is why making sure you have a healthy gut is so important. A gut fed with fibre creates a chain reaction that teaches your immune system tolerance.

It's for this exact reason that when looking for supplementation, Dr Dedic-Hagan said we should look to "food first, always." But if you are deficient in a nutrient that plays a key immune role and you can't get enough from your diet, supplementation can help to correct that deficiency. And this should be something done over time rather than megadosing while you're unwell.

These are the natural remedies that can actually help

So, if you're wading through the claims on social media, are there actually some natural remedies that do prove beneficial? Dr Dedic-Hagan says yes, there are indeed some excellent ones to look out for.

"Zinc is the standout. Zinc deficiency shrinks the thymus, the organ where your T cells are developed," she said. So having enough is extremely important. "Vitamin D also has modest, real evidence, but the benefit is concentrated almost entirely in people who were deficient to begin with." And oddly, a number of people in the Southern regions of Australia actually are deficient in the colder months.

She also says that lozenges started within the first 24 hours can actually shorten a cold by roughly a day or two. Honey is another great one. It outperforms usual care for coughs. Elderberry looks promising for shortening cold symptoms once you're already sick.

As for those cure-all ginger and turmeric shots? She says unfortunately no one has run a trial on their impact on winter illnesses. Fresh ginger does show antiviral activity against respiratory viruses in the lab setting, which is an interesting first step. Also, curcumin, which is found in turmeric, does lower inflammatory markers like CRP and IL-6. But this is as far as science has gotten.

"Drink them if you enjoy them – you're hydrating, flooding your body with antioxidants, and helping to lower inflammation, but I wouldn’t expect them to prevent illness," she said.

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