Dietitians have called for malnutrition screening to be mandatory at hospitals, after research revealed malnutrition often goes undiagnosed and untreated.

Dietitians Association of Australia CEO Claire Hewat said malnutrition screening was "cheap, simple and quick".

"Malnutrition screening should be mandatory for all admissions to pick up those who are already malnourished and refer early to a dietitian or to provide baseline for others who may be at risk and can be monitored and supported during their stay," she said.

Accredited practising dietitian Dr Karen Charlton recently revealed many elderly Australians are either admitted to hospital suffering malnutrition or become malnourished while in hospital, increasing their length of stay in hospital and health care costs.

Dr Charlton said malnutrition was not considered a clinical priority in hospitals and aged care settings despite figures showing more than one in three hospital patients in Australia are malnourished, with the rates as high as 70 percent in residential aged care.

A study of 275 patients in one Melbourne hospital found 90 percent were malnourished or at risk of malnutrition, based on measures such as weight and appetite.

The study found only 15 percent of malnourished patients were correctly picked up by hospital staff as being malnourished, while close to 60 percent were actually malnourished.

Ms Hewat said good nutrition was essential to the healing process and in reducing the risk of future disability.

She said hospitals "one size fits all" food system needed more flexibility to cater to clinical needs, not just special diets.

"Until the system stops considering the provision of food as a hotel service and starts to recognise that it is part of clinical care we will get nowhere," she said.