The 77-year-old, who is expected to replace Mary Berry on The Great British Bake Off, hit out at the "foul-smelling sludge" served in hospitals.
In a column for The Oldie magazine, she writes: "I once watched my mother lift a cloche from her plate. A grey lake slowly flowed to the edge.
"Next to this foul-smelling sludge was a pool of watery mash and some khaki peas.
"All the staff do is reheat frozen meals and lay trays with difficult-to-undo packs of cutlery, sweaty squares of cheese, cheap biscuits and dabs of marge."
She slammed a budget of Â£1.49 per meal as "ludicrous", adding: "But that's not an excuse for inedible meals."
The Masterchef Australia judge also quoted a report from The Patients Association which she says found people complained about "tasteless food" and "not being offered anything to eat if they have to miss meals".
She added: "We need to face the fact that a major overhaul is due, driven by two things - money and convictions.
"Health ministers, doctors and administrators need to be convinced that good food is medicine.
"If they believed that healthy food would lead to faster healing, that the pleasure of a good meal would cheer their patients up, that the Holy Grail of fewer bed nights (and therefore reduced cost) was attainable, I bet they'd find the money for better quality ingredients, better training, closer kitchens."
Last month, Leith told the Press Association that replacing Berry would be a dream come true and that she had already had two auditions and lots of meetings about joining the hit show which is moving from the BBC to Channel 4.
* The Oldie magazine is out now.