A government-funded project is being launched to combat malnourishment among the elderly.
One million people over the age of 65 in England suffer from or are at risk of malnutrition, according to charity Age UK, making them susceptible to poor health.
The project aims to increase diagnosis and treatment of the condition, and improve care and support for older people to stop them "slipping through the net".
The Malnutrition Prevention Project is being piloted in five areas of the country, two of which, in Lambeth and Southwark in London, and Salford, will begin in January.
Hospitals, GP practices and care homes will work with community groups to reach out to older people.
The project will focus on older people who have a low body mass index and are underweight or those who are losing weight without trying. It will also target dehydration, which is often linked to malnutrition.
Support services will raise awareness of the symptoms among older people and help them to take action, and volunteers from local Age UK groups will provide extra support.
The project is funded by the Department of Health and run by the Malnutrition Task Force, a group of experts - which includes Age UK - set up last year to address preventable malnutrition and dehydration in older people.
It is part of the Government's response to the Francis Report into the events at Mid Staffs hospital, which revealed that patients, many of them older, were unable able to eat or drink properly and that nutrition was not treated as a priority.
Research has shown that 93% of malnourished older people are living in the community, Age UK said.
Yet the condition often goes undiagnosed or untreated because of the incorrect belief that a smaller appetite and losing weight is a normal part of ageing.
People who are malnourished experience increased ill-health, hospital admissions and risk of infections. It also takes them longer to recover from surgery, the experts behind the project say.
When compared with well-nourished people, those who were malnourished saw their GP twice as often, had three times the number of hospital admissions and stayed in hospital more than three days longer.
One in three older people are already at risk of malnutrition when they arrive at hospital, making speedy recovery from other conditions more difficult.
In addition, 37% of older people who have recently moved into care homes are at risk of malnutrition.
Dianne Jeffrey, chairman of the Malnutrition Task Force and of Age UK, said: "Eating and drinking properly is critical to being healthy and remaining independent. Yet malnutrition goes untreated and diagnosed in nearly one million older people in England.
"We hope that by working closely with hospitals, GP surgeries and care homes we can tackle this hidden problem and help ensure that older people do not slip through the net."
Health minister Dan Poulter added: "Many people think of malnutrition as a problem that only affects the Third World. But the reality is that over three million people in the UK have the potential to become malnourished - many of whom are frail and elderly.
"Action must be taken, and that is why we're teaming up with Age UK and funding the first stage of its campaign. Carers and NHS staff will be better equipped to identify, tackle and prevent malnutrition - so older people get better care and receive more support in their own homes and communities.
"Around 13,000 people a year are admitted to hospital while suffering from malnutrition, and that is something that needs to change."
It is hoped the scheme will be expanded to tackle malnutrition nationally.
Lenny Kinnear, of Age UK Lambeth, said the charity believes that one in 10 people over 65 suffer from malnutrition, meaning about 2,500 people in Lambeth.
Mr Kinnear told BBC Breakfast the project can work if the health side of care is integrated with community-based support, which would help protect the health service from increased emergency admissions from malnutrition.
He said the Lambeth pilot might involve a "social prescription" that would involve advice on exercise and healthy eating.
"Older people go for regular health checks and the idea is that when they get the prescription they get some information on nutrition and exercise," Mr Kinnear said.
"We would like the GPs to refer them to us. We can give them an adviser who will pair them up with people locally who will befriend them and volunteer to go shopping and things like that."