A major survey is to try to establish how many people are malnourished when they enter hospitals and care homes.
The sick and elderly are particularly prone to malnutrition
The three-day investigation by nutrition charity Bapen - assessing more than 500 institutions across the UK - is the first of its kind.
Based on studies carried out 10 years ago, an estimated 30% of patients in hospitals and care homes are clinically malnourished - a total of 3m people.
Malnourished people stay in hospital longer and are more prone to infection.
The British Association for Parenteral and Enteral Nutrition (Bapen) hopes its Department of Health-backed survey will give hospitals and care homes a clearer idea of the scale of malnourishment in the community.
Over the next three days, all patients admitted to a participating hospital will be screened.
All those who have entered a participating care home in the past six months will also be screened.
Three-quarters of the institutions are in England, 12% in Scotland and the rest in Wales and Northern Ireland.
There has been fresh focus on malnutrition in recent years.
In 2004, the Department of Health issued core standards for the NHS which commit trusts to providing patients with a balanced and nutritional diet.
Impaired immune responses
Reduced muscle strength and fatigue
Increased difficulties in breathing
Impaired wound healing
Apathy, depression and self-neglect
Two years later, the National Institute for Clinical Excellence issued guidelines recommending that all new patients be routinely screened on admission and offered specialist nutritional support if necessary.
But charities such as Age Concern complain malnutrition remains prevalent because policy is not necessarily being put into practice.
Nurses, it is often argued, simply do not have the time to ensure patients are eating properly.
The problem is an expensive one, thought to cost the NHS more than £7bn every year.
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 intensive nursing care.
However, the symptoms may not be immediately obvious.
Malnutrition is a particular problem for the elderly. As many as 10% of people aged over 65 are malnourished.
That figure rises to 60% when it comes to elderly people in hospital.
A spokeswoman for Help The Aged said: "Even though we know malnutrition in older people is a huge problem in the community, the evidence is out of date and patchy.
"This screening week is therefore extremely important.
"Until we know the extent of the problem, hospitals and care homes won't have the evidence they need to care for malnourished older patients that are admitted."