As anyone whose knees get stiffer as winter draws on will know, some conditions worsen at certain times of the year. And some, such as diabetes, can emerge for the first time as temperatures drop.

But thanks to new research, we now know why - blame your genes, which alter their behaviour to suit the season.

This, say scientists, could explain all kinds of seasonal variation in our health - from why inflammatory conditions such as arthritis worsen when it's cold to why deaths from heart disease peak in winter months, and why we put on weight at this time.


It appears that our immune system genes are switched on and off on an annual cycle in an attempt to prepare our bodies' defences to counter the health threats most likely to arise at different times of the year. This could have implications for treatments in winter - and even the best times to have jabs for seasonal illnesses, such as flu.
One of those pioneering the new research is John Todd, a professor of medical genetics at Cambridge University. One of his PhD students made the chance discovery that immunity genes were more active in white blood cells, our infection-fighting cells, during winter.

Professor Todd then led a team who analysed blood and tissue samples from more than 16,000 people worldwide.

They examined 22,000 genes - nearly all the genes humans possess - and discovered that a quarter of them show clear signs of changing with the seasons, which they reported in the authoritative journal Nature Communications.

And in climates such as Britain's, where the weather changes significantly with the seasons, the researchers found that our immune system genes get more active in the winter months.

In Iceland, where it is cold most of the time, they found fewer seasonal changes. And around the equator, the scientists discovered that the immune genes rev up in the rainy season, when diseases such as malaria are most active.

It's not clear why this happens, although it's thought that our genes may take their cue from environmental signs, such as daylight or temperature. The researchers suggest this seasonal gene switch would have been useful in prehistoric times, when infection was more likely as people huddled together in the colder months.

There is, however, a cost to revving up our immune systems: they are more likely to go awry, mistakenly attacking our own bodies, explains Professor Todd.

Our immune genes control our immune cells and trigger inflammation - the body's way of fighting off infection by releasing chemicals that cause swelling as a defence against invaders.

But too much inflammation is also dangerous: chronic inflammation causes damage throughout the body and plays a significant role in conditions such as heart disease, type 1 diabetes and arthritis.
So the seasonal changes to our immune system genes, and the subsequent ramping up of our inflammatory response, may explain why symptoms of certain conditions peak in wintertime in Britain.

'We see a rise in new cases of type 1 diabetes in January, February and March,' says Professor Todd. 'And heart disease is much worse in the winter months.'

Figures from the British Heart Foundation show that deaths from coronary heart disease are significantly higher in winter.

In England in 2012, there were about 15 per cent more male deaths and 17 per cent more female deaths in winter than in summer.

The new findings may help doctors to treat patients more effectively during the winter months, using drugs to reduce inflammation, as well as standard medication for conditions such as diabetes and heart disease.

There may also be implications for the timing of annual vaccination campaigns. For example, the NHS currently advises that the best time to have a flu jab is in the autumn, from the beginning of October to early November.

However, Professor Todd's study found that a set of genes associated with our response to vaccination is more active in winter, suggesting that some vaccination programmes may be more effective if moved deeper into the winter months, when our defences are already 'primed', meaning they may produce more disease-fighting antibodies in response to a jab.

The study also found a possible explanation for why, during winter, Britons gain an average of between 2lb and 4lb.

It discovered that those genes linked to controlling our metabolism also change with the seasons, slowing down our metabolic rate in winter so that our bodies burn less energy.

'Presumably, these would help with conserving energy to survive when there is little food and shelter,' says Professor Todd.

Now that we live in centrally heated homes with no risk of winter famine, this survival mechanism can instead create unhealthy weight gain.

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