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A British expert is calling on New Zealand to take the lead in reducing violent reoffending by improving prison diets.
Bernard Gesch, of Oxford University's physiology department, gave 231 prisoners at Britain's Aylesbury jail either multivitamins and fish acids or a placebo for up to nine months and found that the supplements reduced violent and other serious disciplinary offences 37 per cent.
Another study in an American institution for juvenile offenders saw violence reduced by 80 per cent in a group who were given extra vitamins and minerals plus education about a healthy diet, compared with a 56 per cent reduction in another group who were given only diet education.
Another test in two Arizona primary schools reduced antisocial behaviour by 47 per cent in children given vitamin and mineral supplements.
The Dutch Government is now conducting a large trial to see if nutritional supplements have the same effects on its prisoners.
Mr Gesch said the studies all showed that "we may have seriously underestimated the importance of diet in social behaviour".
"We have made unprecedented changes to modern diets with little or no systematic evaluation for the effects on the brain," he said by email.
"Several Governments are beginning to question if their existing approaches to crime are working. I hope that Governments wake up to the possibilities, as the only risk to the dietary approach to behaviour is better health.
"To date the Dutch seem to be the most progressive. Maybe New Zealand might take the lead in the Southern Hemisphere. I do hope so."
The director of the Mason Clinic for offenders with mental disorders, Dr Sandy Simpson, said menus at his clinic had already been reviewed by a dietitian and supplemented with omega-3 fish acids.
"In relation to omega-3, there are a number of studies suggesting that depression and psychotic illness may be improved with adequate levels of omega-3."
A doctor at Child, Youth and Family's youth justice centre at Wiri, Dr Glenn Twentyman, said he often prescribed omega-3 fish oil and vitamin supplements for young people who missed out on vitamin D through lack of exposure to sunlight.
"It's the hoodies and the hats and the downward glance of the teenagers, shading your face all the time," he said.
"A lot of these kids stay away from sunshine. They don't hang out at the beach or in the bush. Some are into drugs and alcohol and a lot of it is indoor activity and night-time activity. They sleep during the day. They are wearing those hoods and literally they don't get out in the sun."
He said that young people who were given the supplements showed "a vast improvement in energy and mood".
Auckland University medical school associate professor Robert Scragg said prisoners would also have low vitamin D levels unless they exercised in the sun for at least 30 minutes a day or every other day.
A Christchurch expert on diet and behaviour, dietitian Lea Stening, said New Zealanders were learning to reduce fat intake to deal with obesity and heart problems, but had not learned that a balanced diet was also important for the brain. The brain needed fewer saturated fats and more proteins.
"Saturated fat slows the brain down, affecting learning processes and our ability to make judgments," she said. "New Zealand has a very high saturated fat diet, which makes us more sluggish and makes it harder to remember things and concentrate.
"Protein is a thing that children don't have in regular amounts. They live on carbohydrates. Very small amounts of low-fat protein can regulate blood sugars."
A Ministry of Health survey of prisoner health found that 56 per cent of prisoners were overweight or obese, roughly in line with the overall population.
Prison menu
Breakfast: 30g cornflakes, 3 x toast, 15g margarine, 20g spread, 1tsp bran, 35g sugar, tea.
Lunch: 3 sandwiches - 1 x cheese and onion, 1 x spaghetti, 1 x vegemite; 1 piece fruit; tea.
Dinner: Italian saveloys, spaghetti noodles, 2 x vegetables, tomato and garlic sauce, 1 piece fruit, 300ml milk, tea.
Supper (provided with dinner): 2 slices bread, 15g margarine.
Source: Corrections Department, for males. Menus vary slightly over a four-week cycle.
