Gastroenterologist Dr Penny Neild, who works at London's St George's Hospital, said training on how to spot and tackle malnutrition was "patchy".
She said medical schools and junior doctor programmes were focusing too much on the science of being a doctor rather than basic care.
But regulators said courses should give medics a "good understanding".
Up to one in four patients may be at risk, research shows.
But the British Association for Parenteral and Enteral Nutrition (Bapen) campaign group has warned that doctors working both in hospitals and as GPs often miss the signs.
To address the issue, the government published a Nutrition Action Plan in 2007 encouraging better screening and staff training.
But Dr Neild, who is an adviser to Bapen, said it was now time to improve doctor training, in an article for the newly-formed Frontline Gastroenterology journal.
Nutrition is a core element in the first year of a junior doctor course, but it is not a mandatory part of medical school curriculums or many specialist training courses in the latter part of the junior doctor training programme.
Dr Neild said as a gastroenterologist she had to deal with the consequences of severe malnutrition which requires tube-feeding to be introduced.
"The problem is that doctors do not recognise it and if it is not picked up the patients cannot be passed on to dieticians to address the problem.
"Doctors are taught a lot about medical interventions, but not how to assess and manage poor nutrition."
'Good understanding'
However, she accepted progress was being made.
The introduction of nutrition in the first year of junior doctor training has only been rolled out in recent years, while the royal colleges are looking at devising a standardised curriculum for medical schools which may well have more emphasis on nutrition.
But Dr Neild said it was important to build on this. In particular, she wanted to see specialist training - the part of the junior doctor course which enables medics to become a particular type of doctor such as GP or surgeon - incorporate tailored courses on nutrition.
The General Medical Council, which from April will take responsibility for all stages of doctors' education and training, said that by the time medical students graduate they should already have a "good understanding" of nutrition.
"The GMC outlines the knowledge, skills and behaviours that UK medical students should learn in its guidance, Tomorrow's Doctors.
"It specifies that newly qualified doctors must be able to make an assessment of a patient's state of nutrition; discuss the role of nutrition in health; and be able to apply to practice the scientific principles on nutrition."
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