By Adam Brimelow
BBC News health correspondent
Malnutrition can lead to a longer hospital stay
The government is being urged to promote the concept of a "healthy weight" to help guard against malnutrition as well as obesity.
A report by an independent panel of advisors says more needs to be done to make sure patients who are malnourished get help eating.
It says official figures significantly understate the extent of the problem.
The health charity, Bapen, says about 3 million people in the UK either have or are at risk of malnutrition.
They are more likely to die or have a prolonged stay if they go into hospital.
The report by the Nutrition Action Plan Delivery Board says official figures are likely to be an underestimate. In 2007, 239 people were reported to have died because of malnutrition in English hospitals.
The advisors suggest the number dying from other causes but with malnutrition is 200 times higher. That would bring the total to nearly 50,000.
The report concludes that healthy eating should not just be about promoting weight loss.
The authors said: "We know that malnutrition predisposes to disease, it delays recovery from illness and it increases mortality.
"It follows that the effect of malnutrition on mortality rates is substantially greater than the number reported to have died because of malnutrition."
Michelle Mitchell, charity director for Age Concern and Help the Aged, said they launched a campaign over three years ago to demand action from the government.
She said: "We know one million older people in the community are malnourished, yet there is still no evidence of any real progress on this problem."
The report was delivered to the government in August 2009 but has only just been released.
Conservative shadow health minister Stephen O'Brien said: "It is horrific that so many patients are dying with malnutrition.
"We have raised this issue many times but Labour ministers have dragged their feet and are doing very little about it."
But Care Service Minister Phil Hope said the Conservatives had failed to properly describe complex statistics and were presenting totally inaccurate figures.
He said: "It completely misleads the public to compare the number of patients dying as a result of malnutrition, which is 239, with the number of people who have some degree of malnourishment, often as a result of ill health."