Eating more "roughage" from grains could save New Zealand $600 million a year, a new report claims.

The report, by an economist commissioned by cereal maker Kellogg's, says eating the recommended amount of grains could reduce the numbers of New Zealanders with heart disease by 14.5 per cent, and those with type 2 diabetes by 33 per cent.

Those reductions would save $204 million a year in healthcare costs and boost economic output by $403 million a year as healthier people increase their productivity, stay at work longer and live longer.

Health experts greeted the report cautiously. Auckland University Professor of Population Nutrition Boyd Swinburn said eating more fibre was important, but it was not the most important way to reduce heart disease and diabetes.

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"Studies that have been done show a significant effect of fibre on the incidence of those two diseases, but not nearly as much as other aspects of diet," he said.

"For diabetes, sugar is the main one. For cardio-vascular [heart] disease, it's mainly around salt and saturated fat, and fibre has a smallish extra component."

But for one Auckland man, Avan Polo, extra fibre in the form of nuts, seeds and vegetables has been a key element in losing 18kg since he was diagnosed with type 2 diabetes last May.

He cut out cream and sugar, drank less coffee, ate less at every meal, and ate more fibre-heavy nuts, beans and radishes.

"I did use My Food Bag's lite box. It just taught me the value of fibre that I could add to all my meals," he said.

For breakfast, he alternated between eggs, brown toast, avocado and tomato one day, and nuts, beans and yoghurt the next day.

"If I was hungry during the day I'd have a bag of mixed nuts, I call it my roughage bag, and I'd get a handful of nuts and seeds and eat that and have a piece of fruit at the same time," he said.

Fibre is the indigestible part of plants, which is valuable for health because it increases the amount of food that goes through the body into the toilet, reduces cholesterol levels and keeps sugar levels under control.

Swinburn said fibre content in most Western diets had declined with the spread of processed foods, which removed the indigestible parts of plants such as their skins and added in extra fat, sugar and salt to improve the taste.

"Getting people back to eating whole foods is actually really important, rather than consuming more processed food," he said.

The last Ministry of Health survey in 2008-09 found that NZ men averaged only 22 grams of fibre in their daily diet compared with an "adequate" intake of 30g and a "target" intake of 38g.

Women consumed only 18g compared with 25g for an "adequate" diet and a target of 28g.

Bread was the biggest source of fibre in NZ diets in 2008-09 (17 per cent), followed by vegetables (16 per cent), potatoes, kumara and taro (12 per cent), fruit (also 12 per cent), grains and pasta (8 per cent) and breakfast cereals (7 per cent).

Otago University nutritionist Dr Lisa Te Morenga said the easiest way to increase fibre intake would be to eat more grains in high-grain breads and cereals.

"Fruits and vegetables are a good source of fibre, but you have to eat a lot more of those than most people are now to get an adequate level of fibre."