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Why is Flu Season in Winter?

Updated: Oct 1, 2020

There is a fair bit of interest at the moment in whether there is anything that you can do or eat or take which will ward off colds, 'flus and of course the very topical corona virus. The short answer is no there is no magic bullet, but nevertheless the fact remains that we do seem to have more respiratory tract infections (RTI's) in winter than summer.

First of all though let's make it clear; you cannot 'boost' your immune system and see results tomorrow. The immune system is complex and a concept such as a pill or a food to make it stronger is simplistic. There are many many organs and factors to consider. Besides, an overly reactive immune system is also a bad thing - the immune response to a perceived threat is short term and involves production of some toxic chemicals called cytokines designed to kill the threat (a bacteria, virus or whatever). If these are too easily triggered by an immune system in overdrive then the effect is an increased risk of several chronic diseases such as type II diabetes, cardiovascular disease, asthma and rheumatoid arthritis.

So, the best way to avoid an RTI is to practise good personal hygiene- wash your hands after contact with others and with potentially infected surfaces, and distancing- stay away from people who have symptoms.

Secondly, the best diet for a robust immune system is the same best diet for good health generally - one high in a variety of unprocessed foods - fruits and vegetables, whole grains, protein foods such as eggs, meat, soy products, legumes and nuts; healthy fats and lots of fibre.

Add to that the other habits for a healthy life - regular exercise, good quality and quantity of sleep, happy relationships with friends and family and of course no smoking or too much alcohol and you'll be on the road to having an immune system which can give an RTI its best shot.

Which brings us back to the winter/summer conundrum, and to the fact that Australia and New Zealand are faring so much better in this pandemic than the Northern European countries. Yes, we have less dense populations and yes we got on to testing much earlier. But we are also coming out of Summer, and that just may be a factor.

In a meta analysis of vitamin D supplementation to prevent acute respiratory tract infections, 25 random controlled trials, involving a total of 11,321 participants aged 0-95 years were considered.(1) Results were mixed, however, a weekly or daily supplement of vitamin D was seen to protect against RTI's and this effect was stronger in those who had the greatest deficiency in vitamin D at the start.

Vitamin D is an important contributor to the immune system. It is known to enhance the action of cells to kill off viruses, but also has other roles.

We can make vitamin D in our bodies from a precursor. When the skin is exposed to ultra violet light from the sun, the precursor is converted to active vitamin D. Therefore, in theory, we need never eat foods containing vitamin D nor take supplements.

However, surprisingly, despite the legendary sunny climate of Australia, almost a quarter of Australian adults are vitamin D deficient. (2)There are wide personal and regional differences. Regionally we see that the warmer states such as Queensland have much lower deficiency rates, although even there in winter 15% of adults are deficient. In more southerly Victoria this rises from a summer rate of 16% to 49% in winter. There are also personal differences such as where you were born - dark skinned people need more sun exposure to make vitamin D than white people. Add to this our modern lifestyle - long hours indoors at work, the cultural traditions of some, such as women covering up completely for modesty and also our skin cancer awareness. we have all become just so good at the 'slip, slop, slap' message that we do not give our skin as much sun exposure as it needs to make adequate vitamin D.

Older people are particularly at risk as the ability to convert vitamin D's precursor into its active form declines with age. As people feel less confident to spend time outside for risk of falling, or if confined to the routine of a nursing home, so their sun exposure becomes less and less.

The amount of vitamin D you need varies with your age: (3)

  • Everybody under the age of 50 needs 5 micrograms each day (µg/day). A microgram is one millionth of a gram.

  • People aged 51 to 70 need 10 µg/day.

  • People 71 and over need 15.0 µg /day.

There is some thought now that these Australian guidelines are too low and that a higher recommended daily intake would be advisable.

So, what is the solution? There are several things:

The first is to try to spend 2-3 hours outside with face, arms and hands exposed to sun, without sunscreen, every week. To reduce skin cancer risk this is better done in small amounts of time each day.

Secondly, eat more foods rich in vitamin D. There are not many, but some foods are fortified. The richest natural sources are eggs, oily fish, cod liver oil and strangely mushrooms- but only of they have been exposed to UV light. So, put your mushrooms out in the sun for half an hour before using them! A 75g portion of canned salmon provides more than 100% of the recommended daily intake of vitamin D*. Two eggs provide 82% of the RDI.* Foods such as margarine, some breakfast cereal, some milks, soy milk and some bread are fortified with vitamin D. The upside of a food source is that it also comes 'packaged' together with other important nutrients. Foods are always preferable to supplements in the first instance.

Thirdly, you may need a supplement, particularly in winter. It is possible to 'overdose' and cause other health problems, so check with your doctor.

A cure for corona virus? That it's not. A possible way to avoid corona and other viruses? Well, maybe. Not a sure thing, but a valuable part of an overall healthy immune system and one that seems to correlate with the increase in RTI's in the winter months.

Eat well, stay active, sleep tight and keep your vitamin D up!

*Based on the RDI of an adult aged 19-70 years.


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