A panel of nutritional experts has warned that two million people in the UK are malnourished, and that the condition is under-recognised and under-treated.
By Adam Brimelow
BBC Health Correspondent
A report by the Malnutrition Advisory Group sets out a new system of screening patients to identify those at risk.
The food is piled high at a busy cafe in Egham in Surrey. There is no risk of wasting away here.
But while he waits to be served, 21-year-old student Sirvan Rabbani recalls the misery of malnutrition only too vividly.
"I didn't want to eat any more. I used to lie down most of the time and not think about food, and try not to smell any food or watch peole eat.
"I tried not to eat so I could discourage gonig to the loo because of the pains I would suffer, probably for the next six or seven hours after visiting the loo."
Three years ago Sirvan developed Crohn's disease, an inflammatory bowel disorder.
In the next three months he lost about two and a half stone.
Now he manages his illness with drugs, but he's never fully recovered his weight.
He says the health staff who treated him didn't appear to take the weight-loss that seriously, but it did worry him.
"I'd always taken pride in how I looked and the sports I did to build up what I did and the fact that I lost so much weight caused me to be depressed and if they had been able to affect it even a little bit it would have been a massive help to me."
A report today by a committee of healthcare experts and academics suggests that Sirvan's experience isn't unusual.
According to the Malnutrition Advisory Group about two million people in the UK are malnourished.
Up to 40% of patients admitted to hospital are underweight.
People with chronic disease are particularly at risk, also the elderly, poor or socially isolated. But malnutrition can affect anyone.
The report's main author, Professor Marinos Elia, from the University of Southampton, says this causes unnecessary suffering for patients, and unnecessary expense for the NHS.
"They get tired. They feel weak. They fatigue easily. They can become depressed, anxious.
"And their ability to fight diseases such as infections, their ability to heal after a surgical operation is reduced, and they may have increased complications."
At King's College hospital in London staff have been piloting the first universal screening system to identify the risk of malnutrition.
Patients are weighed and measured, to work out their body mass index.
The information is checked against their disease state to calculate a score, and this helps staff work out the best treatment.
Recent studies have suggested that up to 40% of people are under-weight when they're admitted to hospital.
The director of nutrition at King's, Rick Wilson, says this simple screening check should be carried out every time a patient goes to see a GP - particularly for older people.
"Weight loss and poor nutritional state is not a normal part of aging. And if it's happening we ought to address it and treat it."
He says that means telling some patients to put butter on their vegetables, full cream on their pudding, sugar in their tea - a difficult message amid today's concerns over obesity.
Malnutrition is normally seen as a problem for the developing world, but Rick Wilson argues that here in Britain this costly and demoralising condition has been neglected for too long.
"Hippocrates knew "feed the patient and they'll get better". But today with transplantation and hi-tech staff we seem to have missed out on the basics".