Malnutrition costs the UK more than £7.3bn a year - more than double the bill for obesity, a report has found.
Malnourished patients require special nutrition
However, the British Association for Parenteral and Enteral Nutrition said malnutrition was given a much lower priority by the government.
The report estimates around 30% of patients in hospitals and nursing homes are clinically malnourished.
In total, more than three million people are thought to be malnourished at any one time.
Effects of malnutrition
Impaired immune responses
Reduced muscle strength and fatigue
Increased difficulties in breathing
Impaired wound healing
Apathy, depression and self-neglect
This includes 10% of people over the age of 65 living in the community.
A report published by the Malnutrition Advisory Group in 2003 warned that many people with malnutrition go undiagnosed.
Malnourished people stay in hospital longer, succumb to infection more often, and visit their GP more frequently.
They also require longer-term care, and more extensive nursing care.
The BAPEN report found it costs £3.8 billion a year to treat malnourished people in hospital, and £2.6 billion to treat them in long-term care facilities.
GP and outpatient visits add another £750 million to the bill.
Professor Marinos Elia, BAPEN chairman, called for effective screening to detect malnutrition, so that appropriate nutritional treatment can be provided.
The association has developed a simple-to-use screening tool to identify those already suffering or at risk of malnutrition.
Professor Elia said: "It is surprising that in England it has not been made mandatory for all patients to be screened for malnutrition on admission to hospital as it has in Scotland.
"It takes a period of time for an individual to become clinically malnourished, often starting in the community, but if at least it is picked up in hospital a great deal of suffering of patients and their families can be avoided.
"Resources must be made available for the training of health and care staff in all settings so that malnourished patients and residents are identified and appropriate treatment and support provided."
A Department of Health spokesperson said measures, backed by new investment, had been put in place to tackle the health inequalities that can lead to poor diet and ill health.
The Better Hospital Food programme had improved quality, access and availability of food to hospital patients.
Minimum standards for meals in care homes had also been introduced.