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The University of Otago study, which has been published in the British Medical Journal Open, looked at the nutritional intake of 54 people at a rest home in Christchurch.
Lead author Wathsala Nanayakkara said she was surprised by the findings, which revealed some concerns.
"The main finding from our study was that the residents at aged care facilities that we studied did not eat enough, despite a variety of food being offered," she said.
The rest home served about 90 percent of the planned nutrients on their menu, but many residents were not eating everything on offer, the study found.
"They are not as active as they used to be so they don't see that they need to be eating a lot of food that is being given."
About 61 percent of the older adults at the residential care facility did not consume the required energy to maintain their body weight.
"When there is an imbalance of nutrients, they are at risk of malnutrition - or they may already be coming into rest homes already malnourished," Nanayakkara said.
"We know that, as a fact, that poor nutrition status is associated with increased risk of pressure sores, falls, infections and longer hospital stays in older adults."
"It ends up costing the health care services but also there is a cost to the individual as well in terms of their quality of life.
Nanayakkara said the rest home had taken the findings on board, and should look to offer smaller meals more frequently to ensure its residents get the nutrients they need.