Cutting out sugar and increasing exercise have cut tooth decay in Waikato preschool children by a fifth.

The Under 5 Energize programme, which started in 2013 in 121 of the Waikato's 500 early childhood centres, has cut the number of children with visible tooth decay in their before-school health checks from 10.9 per cent to 8.7 per cent.

This is an even bigger reduction, and in a shorter time, than at Yendarra Primary School in South Auckland where a water-only policy adopted in 2007 has reduced tooth decay by a sixth compared with the nine other schools within 3km of Yendarra.

Both initiatives are in stark contrast to a trend towards worsening tooth decay in the other nine South Auckland schools, which researchers blame mainly on sugary drinks such as fizzy drinks, energy drinks and fruit juice.

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The initiatives have had less effect so far on the growing problem of childhood obesity. The obesity rate in the Under 5 Energize preschools has dropped slightly from 4.3 per cent to 3.7 per cent since the programme started, compared with a slight rise from 4.1 per cent to 4.4 per cent in other Waikato preschools, but the differences are not statistically significant.

However the wider Project Energize, which has been running in Waikato primary schools since 2005, reduced the proportion of overweight 7-year-olds by 4 per cent between 2006 and 2011.

"Teeth are an early indicator of better nutrition," said project founder Professor Elaine Rush of AUT University.

"We are finding that, with a year or so of the Energize treatment, the children are running faster than before than before they got the treatment.

"Running is a really nice measure. It's something children love to do. A faster-running child is a healthier child."

Denise Morris, head teacher at Hamilton's Miropiko Kindergarten which has had a water-only policy for more than five years, said removing sweet drinks created "a calmer, more settled atmosphere".

"It's important that children learn from a very young age that drinking water is good for their bodies and good for their brains," she said.

The Energize programmes in both schools and preschools use staff from Sport Waikato to provide workshops, newsletters and other material for children, teachers and parents on healthy food and physical activity - starting with the value of water and milk rather than sugary drinks.

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"It's the first message of Project Energize because it's the easiest one," Rush said.

Under 5 Energize manager Madeleine Kirk said a majority of both preschools and schools in the two programmes have adopted guidelines to let children drink only water or milk.

"Any sugary drinks that come in, including fruit juice, are put on the shelf and given to the family to take home at the end of the day," she said.

"It's a cost saving, so it's a win-win."

She said many parents were shocked to learn about the effects of sugary drinks when the preschool programme started four years ago, but she found most parents were no longer surprised now.

"Over time there has been a lot in the media supporting this," she said. "The repetition of the messages is definitely seeing some behaviour change."

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Even in the Waikato preschools that are not in the Energize programme, children with tooth decay have declined slightly from 11.4 per cent in 2013 to 10.7 per cent.

But the drop to 8.7 per cent in the Energize preschools has been significantly steeper.