Staying Well in Winter – Optimising Immune Function

immuneWith winter around the corner, now is a perfect time to get yours and your family’s immune systems in tip-top shape to fight off any bugs coming your way. While researchers aren’t entirely sure why humans are more susceptible to respiratory infections in winter, there are some factors we know can increase or decrease your risk of catching a cold.

For example, Rhinoviruses, the types of virus responsible for the common cold, replicate most efficiently at around 33°C. While our core body temperature is around 37°C, the temperature inside the nose is significantly lower, especially in colder weather. This means when we breathe in cold winter air, we might be providing the ideal environment inside our nose for the cold virus to replicate. Whilst we don’t recommend knitting a small beanie for your nose, there are some other steps you can take to reduce your risk. We’ve discussed some of these below.

Balance those Blood Sugars!

Keeping your blood sugars stable is something we talk to our patients about all the time, as it is important for weight management, stable moods, energy levels, concentration and you guessed it, immune function! Neutrophils are a type of white blood cell that ‘phagocytise’ or ingest micro-organisms that cause infection. When we eat simple sugars (such as in white sugar, fruit juice or honey), the rise in our blood sugar levels significantly impairs the ability of these neutrophils to ingest micro-organisms for up to 5 hours after a meal! This means if you have a ‘sugary’ breakfast (such as cereal or jam on toast), and throughout the day you consume some fruit juice, a sugar in your tea, a fruity muesli bar or a cookie from the work kitchenette, you are continually impairing your body’s ability to deal with the microbes we are exposed to on a daily basis. This is why it is especially important to keep your blood sugars stable, by eating foods high in protein, fibre and healthy fats, and ensure your carbohydrates are largely coming from unrefined sources, such as sweet potato, oats, brown rice and beans.


Sleep is a wonderful restorative activity that enables your body to rest and repair, so you can feel rejuvenated and refreshed in the morning. Unfortunately, many of us are getting inadequate sleep, which can have disastrous consequences for our immune function (along with our moods, energy levels and ability to regulate our appetite!). Researchers have found that insufficient sleep reduces both the number and activity of important immune cells, such as our T-helper cells and natural killer cells, leaving you more susceptible to infection. Winter is therefore the perfect time to snuggle up in bed early, so you can stay warm and cosy, whilst giving your body a fighting chance against infection. For adults, we recommend at least 8 hours of sleep a night and encourage our patients to be in bed by 9:30pm. Children need more sleep, and teenagers need at least 9-10 hours a night, as their bodies are busy growing and developing! If you’d like to know more about the consequences of poor sleep, and how you can improve your sleeping habits, check out our post HERE.

Immune Boosting Nutrients: Vitamin C and Zinc

No article on immunity would be complete without a mention of vitamin C and zinc. Vitamin C supports immune cell activity and helps neutrophils (the type of white blood cell I mentioned before) ‘ingest’ and destroy micro-organisms. It is also required for the synthesis of collagen. Collagen is a structural component of the mucous membranes lining your nose and respiratory tract. Without sufficient vitamin C, the integrity of these mucous membranes can weaken, increasing your susceptibility to infection. Vitamin C helps regulate the body’s inflammatory response, and helps counter the oxidative stress that occurs during infection. Vitamin C supplementation has also been shown to reduce the duration of the common cold. The major dietary sources of vitamin C are brightly coloured fruits and vegetables, such as red capsicums, strawberries, kiwis, papaya, broccoli and guava.

On the other hand, zinc deficiency is one of the primary causes of sub-optimal immune function. Zinc deficiency impairs key immune cell functions, meaning your body’s ability to fight off infections is reduced. In children especially, zinc deficiency is a major cause of recurrent respiratory tract infections. Interestingly, test tube studies suggest zinc may directly inhibit replication of rhinoviruses, the type of virus responsible for the common cold. Foods rich in this essential nutrient include beef, pork, oysters, pumpkin seeds, edamame and chickpeas. Populations at risk of deficiency include children, pregnant women, the elderly, vegans and vegetarians, and those with certain health conditions, such as inflammatory bowel disease or Coeliac disease.

Protect Your Mucous Membranes

In colder weather, air is typically drier and humidity is lower. Researchers have suggested this lower humidity can dry out the mucous membranes in your nose, making it easier for virus particles to breach the body’s first line of defence. Although there’s not much you can do about the low humidity, you can help protect yourself by ensuring you are consuming adequate nutrients to support mucosal membrane integrity. Two of the key nutrients that support mucus membrane health are vitamin A and zinc. Vitamin A (retinol) is found in animal foods such as egg yolks, liver and grass-fed butter. Plant sources of vitamin A are called ‘pro-vitamin A’ or carotenoids, as the body must convert these forms into retinol. Foods rich in carotenoids include orange and dark green vegetables, such as sweet potatoes, carrots, broccoli and spinach. As many people are poor converters of pro-vitamin A, some vegans may need to supplement with vitamin A to ensure they are receiving adequate levels to support mucosal membrane integrity. As always, if you are considering supplementation, we recommend discussing it with your naturopath at your next visit.

Hand Hygiene

In winter, we also spend more time indoors, in closer contact with other people. This increases our risk of sharing their germs. For example, many people who bike or walk to work in summer, might choose to take the train or bus in winter. This means they’re spending more time breathing the same air as other people, and touching objects they may have touched (e.g. hand-rails, train seats etc). As such, one of the best ways to reduce your risk of getting sick is to wash your hands regularly, especially after getting off public transport, once home and before eating. If you’re going to be out and about for prolonged periods, we recommend carrying a natural hand sanitiser with you, such as Dr Bronner’s Organic Hand Sanitiser or a Squeakie Hand Sanitiser. Encouraging your kids to wash their hands frequently will also go a long way to help reduce the spread of germs!

Keep your Extremities Warm!

Don’t let your kids run around in shorts and T-shirts when it is cold! When we lose too much heat from our head, hands and feet, the blood vessels in our skin constrict and blood is shunted towards our vital organs (like our heart and lungs) to keep them warm. As blood carries our infection-fighting white blood cells, less blood flow to the nose and mouth can mean greater susceptibility to infection. When our nose is cold, the little hairs inside it (known as cilia) also move more slowly. In normal circumstances, these cilia work with mucus in the nose to trap pathogens, such as bacteria and viruses. When their movement is slowed, their function can be compromised, increasing your risk of infection. So bundle up and stay warm, remember “Cold Feet Aren’t Cool!”

Immune Superfoods: Garlic and Green Tea

While consuming a variety of wholefoods is important for your general health, there are two key stand outs in terms of their effects on immune function: Garlic and green tea.

Fresh garlic contains a compound called allicin, which is responsible for its pungent odour. Allicin actually serves as a defence mechanism for the garlic plant, protecting it from being eaten by pests. Allicin has anti-viral and anti-microbial effects and has been shown to inhibit replication of rhinoviruses, the type of virus responsible for the common cold. In a small study of 146 volunteers, participants were randomised to receive a high-dose allicin supplement or placebo. Over the 12-week period, those receiving the allicin supplement suffered 64% fewer colds and experienced a shorter duration of symptoms if infected. It is important to note allicin is only released when the cell wall of the garlic is cut (such as when you dice or crush it), so don’t swallow the clove whole. It’s also deactivated by cooking, so try to enjoy it fresh (like in our Hummus or Magic Dressing) or add right at the end of cooking, just before serving.

Green tea, from the Camellia sinensis plant, contains compounds called catechins, which have been shown to have anti-viral properties. These catechins attach to the hemagglutinin molecule of the influenza virus, preventing it binding to cells in the upper respiratory tract. Green tea also contains a compound called theanine, which enhances the function of a type of immune cell called gamma-delta T cells. In a study of more than 2000 Japanese school-children, consumption of green tea was inversely associated with the incidence of influenza; this means those who drank more green tea (1-5 cups a day, most days of the week) were significantly less likely to suffer the flu. In a smaller study, healthcare workers were randomised to receive a capsule containing catechins and theanine from green tea, or a placebo. Over the 5-month study, only 4% of those receiving the green tea capsule were diagnosed with influenza, compared to 13% in the placebo group, further supporting the hypothesis that green tea consumption may help protect against the flu. One thing you may notice in clinic is that our naturopaths are often drinking green tea in consultation as we treat so many patients with colds and coughs and we need to stay healthy to stay on task!


Whilst excessive and prolonged exercise may actually diminish immune function, studies suggest regular, moderate intensity exercise may reduce your susceptibility to upper respiratory tract infections. For example, in 2011, researchers followed 1000 adults for a 12-week period, and found that those who engaged in aerobic exercise at least 5 times weekly had 43% fewer days suffering upper respiratory tract infections, compared to those who exercised only once a week or less. Of those who did get sick, more regular exercisers experienced less severe symptoms. These beneficial effects on immune function are yet another wonderful benefit of regular exercise, along with weight management, greater bone density, better sleep and improved moods.

Vitamin D

More time indoors also means less sunshine. When our skin is exposed to ultraviolet B (UVB) from the sun, the body produces vitamin D. At Perth’s latitude, we typically only get sufficient UVB from the sun between November and April. This means our vitamin D levels start to drop throughout the winter, and if they get too low, so too can our immune function. In one study, maintaining adequate vitamin D status throughout winter was associated with a two-fold reduction in the risk of developing an acute respiratory tract infection (check out the study linked HERE if you’d like to know more). Adequate vitamin D is also needed by the body to synthesise an antimicrobial compound called cathelicidin, which may help protect the respiratory lining against infections. This reinforces the need to get enough sunshine in summer (without getting burnt!), consume vitamin D rich foods (such as egg yolks and salmon) and for some susceptible individuals, supplement vitamin D3 with the guidance of your naturopath.

Key Points
  1. Keep your blood sugar levels stable (see our post on stabilising blood sugars HERE).
  2. Aim for at least 8 hours of sleep a night, to keep your immune system functioning at its best.
  3. Consume foods rich in vitamin C, zinc, vitamin A and vitamin D (see the article above for suggestions).
  4. Wash your hands regularly (especially after getting off public transport, once home and before eating or touching your face!).
  5. Bundle up! Wear a hat, a beanie, ear muffs, socks, mittens, scarves or whatever it takes to keep your body warm!
  6. Consume green tea and fresh garlic daily, for their anti-viral and immune-boosting effects.
  7. Engage in regular exercise, including both aerobic activity (e.g. walking, swimming or bike-riding) and strength training (e.g. lifting weights, rock-climbing, Pilates etc).
  8. Get adequate sunshine in summer (without getting burnt) to optimise your vitamin D levels for winter.
  9. Discuss with your naturopath whether you should be supplementing specific nutrients, such as zinc, vitamin A and/or vitamin D.