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Dr Sue MacDonell, who is the director of dietetics at the University of Otago human nutrition department, undertook the study of more than 300 people in 16 aged-care facilities throughout the country.
The research highlighted a ''serious'' situation.
''Well-nourished older adults have fewer illnesses and better physical function,'' she said.
Rest-homes generally did a good job of checking for malnutrition risk by usually weighing residents once a month, but she favoured introducing mandatory malnutrition screening, as recommended by international best practice guidelines.
This could be done by doctors and all health professionals ''to identify residents or patients who might benefit from seeing a dietitian''.
''Get rid of the myth that weight loss and wasting is a normal part of getting older - we lose weight when we don't eat enough to meet our needs,'' she added.
As people aged they tended to eat less for several reasons, but the key was to ''maximise the nutrient content'' of the food eaten and ''make every mouthful count''.
Rest-home dinners were usually of good quality and met nutritional requirements, but good protein foods, such as nuts, meat, fish, dairy products and eggs, could be offered as snacks or in evening meals.
Many rest-homes did a ''very good job of ensuring their residents get the best possible nutrition'', but from her experience in the sector, people were often malnourished when they arrived because they had been unwell or unable to cook for themselves.
With regular meals and snacks, many started to gain weight.
Compared with the international literature, New Zealand's rates of malnutrition risk in aged-care were lower, but having half the New Zealand study group at risk of malnutrition was ''worrying'', she said.
The study was undertaken for Dr MacDonell's recent doctorate in human nutrition.
This was the first study in New Zealand to investigate the nutritional status of older adults using biochemical (blood) markers, and the only study in this country to have weighed all the food residents ate for three days.