Society has "gone too far" with hygiene, and parents should expose children to bugs and different foods as babies to prevent allergies in later life, a leading scientist says.

Exposing children as young as 6 months to new foods, germs and the outside world would benefit their health in the long run, Professor Mike Berridge of the Malaghan Institute of Medical Research, said.

This needs to be done before they reach the age of 2, he said, or it's too late.Speaking on TV3's The Nation to promote his new book, The Edge of Life: Controversies and Challenges in Human Health, the cancer researcher said he penned the book as a way to communicate developments in science to the general public.

Science had now proven that being too clean was bad for you, he said, saying that society had "certainly gone too far" in the cleanliness stakes. "We need to be exposed to our environment," he told the programme.

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"If we clean things too much, if we don't expose kids to the environment, if we don't expose them to foods that perhaps we're allergic to, then those kids will carry on with the health problems that we have. "So we've got to rethink the way in which we are addressing some of these health issues.

Exposure to a wide range of environmental things, to food, are going to have massive effects on allergies, respiratory allergic problems, and food allergies." Children should begin to be exposed - but not necessarily eat - new and different foods from around 6 to 9 months old, he said, "and certainly between 1 and 2 years".

That age bracket was "one of the periods in child development when the immune system is developing massively", he said."We need to ensure that we make the maximum use of that early lifehood exposure, because that stays with us for life - by the time you're 2 you're locked in."

Bacteria and germs in the outside world, and the things that we ingest, were working to make us resistant to illness and infection, he said. "Microbial exposure is extremely important," Professor Berridge said.

"If you raise, for example, a mouse in a sterile environment that mouse has enormous problems, it just will not survive. You cannot survive without your microbes."Our bodies were covered in millions of bacteria, he said, and these were important for health.

"Those bugs are important in virtually every aspect of our health and wellbeing - it's the development of our immune system, our metabolism, and the functioning of our brain.

They all involve our getting on with, and nurturing, our microbes as well as our human being."
While we needed a certain level of cleanliness, he advised: "Don't over do it." Professor Berridge's book covers a number of hotly contested public health issues, including cancer treatment, sugar consumption, and water flouridation.

He also spoke to The Nation about the over prescription of antibiotics, a rise in births by caesarean section - which he linked to an increase in allergies in countries with high rates of the procedure - and his support of a 20 per cent sugar tax.

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Evidence suggested New Zealand could see around $30 million of health benefits if it implemented such a tax, he said. "I think ... there is less negative feeling about a tax on something we now know conclusively is bad for our health because it's just a rush of sugar into our blood ... and predisposing us, if we do it again and again, to diseases like Type 2 diabetes."