Dame Barbara flattered the then Prime Minister by telling her she “looked 25”, and assured her there were “no side effects” to the treatments she was recommending.
The letters were written in an era when Mrs Thatcher was secretly trying to press ahead with a plan to dismantle the health service, despite what her former Chancellor Nigel Lawson referred to in his memoirs as “the nearest thing to a Cabinet riot”.
An earlier release of Treasury papers disclosed that the Conservative leader, who had previously declared the NHS to be “safe with us”, privately favoured a system of compulsory private health insurance.
The latest papers to be published as part of a project by the Margaret Thatcher Foundation to make available her private files through the Churchill Archives Centre in Cambridge, reveal that the Iron Lady’s sobriquet might have owed as much to her dietary supplements as to her resolve.
She received “nutrimental capsules” from Dame Barbara and a pill which, the author assured her, “takes oxygen to every part of the body, including the brain”.
Mrs Thatcher, who famously slept for only four hours a night, was sent a package June 1989 with a note that read: “My dear Prime Minister, You were wonderful last night, as usual.
“It is incredible, with all you do, how you can still look as though you were 25.
“In case you ever feel tired, I am enclosing the very latest product we have in the Health Movement. My son, aged 51, says that he wakes up in the morning and feels like a boy of 16, and at nearly 88 I find it fantastic.”
Dame Barbara wrote to Mrs Thatcher’s diary secretary the following month with further supplements, ahead of a prime ministerial visit to the Far East.
“I hope that there are enough because it is a very long trip. I did it myself and it does feel ghastly when you get home,” Dame Barbara wrote.
She added: “As far as I know there are no side-effects at all, and they are not soporific, so that you feel you must go to sleep.
“It just stops that awful feeling in the head and ears.”
The millionaire industrialist Sir Emmanuel Kaye, a strong supporter of the Conservative Party, is revealed to have been another supplier.
He wrote to Mrs Thatcher after seeing her at the Glyndebourne opera, offering advice about her supplements.
He said he could “sort out vitamins, minerals etc and, if you like ... check whether the Vitamin C and the Royal Jelly you are having are of the best variety for you and work out the optimum dosage”.
Sir Emmanuel also mentioned he had evolved “an advanced form of homeopathy called body tuning”, though it is not known whether he did any for Mrs Thatcher.
The same year, a profile of the PM in Vanity Fair claimed she was fond of “electric baths”, in which 0.3 amps of current was run through the water in an attempt to instil youthfulness.
Chris Collins, of the Margaret Thatcher Foundation, said the PM’s press secretary, Yorkshire Post columnist Bernard Ingham, could not countenance “the impression of dottiness, of a woman slightly off her trolley”.
He said that references to health cures in the correspondence were “obscure, perhaps deliberately so”, though he added that he believed her interest to be genuine.
The archive will be published on Monday at www.margaretthatcher.org.