I was reading a book about politicians and their illnesses when news broke that John Prescott has suffered from bulimia, what some newspapers were unkind enough to call a girl's illness.

Bulimia does not appear in the index of In Sickness and in Power, which has just been written by Dr David Owen. Yes, that's Lord Owen, medic, ex-SDP leader and former Labour foreign secretary, now a perky 69.

For HSJ readers I should note that he says being Barbara Castle's number two at health in the 1970s gave him more "personal satisfaction" than any other job he has done. That includes being UN peace negotiator in Yugoslavia in the 1990s, for which the battles with the NHS unions must have been good practice.

Paranoia, depression, hypertension, alcoholism, and allegations of bipolar behaviour - rarely proved to medical standards - are the book's recurring themes. Owen invents a condition not yet recognised by medicine, though invented by Greek playwrights 2,300 years ago. He calls it "hubris syndrome" - an excessive belief in one's own ability which leads a successful leader to abandon good judgement and advice. Hitler (not mad at all) fell into it after 1940: it caused him fatally to declare war on the US in 1941 when he had no need to and was already losing in Russia.

So did Margaret Thatcher at the end. Stalin did not: he learned to trust his generals as Hitler did not. Stalin was paranoid, but that is a personality trait, not an illness. After the absurd "doctors' plot" (a supposed conspiracy to poison Soviet leaders), aides were afraid to call a medic after his last stroke. He died: that will teach him.

Churchill (who mastered his own depression) did not suffer hubris syndrome, says Owen. George Bush and Tony Blair did and it took them into Iraq, the big 80-page obsession of an otherwise balanced volume. Mussolini was bipolar and depressed, Chairman Mao and the manically energetic US President Theodore Roosevelt may have been. Lyndon Johnson came from a bipolar family.

But the most important point Owen makes about illness and politics is relevant to John Prescott. Namely that, generally speaking, candour with the voting public matters: be frank and hope they trust you.

Everyone now knows that President Woodrow Wilson had severe high blood pressure and early signs of dementia - including over-rigid thinking - at the 1918-19 Versailles peace talks that ended the First World War, but set up the Second World War. His secret stroke (unable to work for seven months) probably caused the US not to join the new League of Nations: fatal to its hopes. He should have resigned.

On the other hand, Owen acquits President Franklin Roosevelt of being unfit to negotiate with the wily Stalin at Yalta three months before FDR's death in 1945. FDR's secret, of course, was that he had long been crippled by polio. But the medical evidence is that, though weak and with blood pressure as high as 186/108 in 1944, his brain was fine.

We are talking here of remarkable men. Churchill's doctor, Lord Moran, realised the prime minister had had a heart attack while staying at the White House shortly after Pearl Harbour, the 9/11 of its day, brought the US into the war. He dared not say so (even to the boss) and hoped for self-healing. Churchill's capacity for work, food and buckets of drink went on unchecked.

Churchill's second term (1951-55) was another matter. His massive stroke in 1953 was covered up (with media help). He should have quit but doubted his successor Eden's ability ("I don't think Anthony can do it" - which has a Blair-Brown ring to it), as Howard Brenton's National Theatre play, Never So Good, reminds us.

The contrast with President Eisenhower is striking. Ike let the world know about his heart attack and was re-elected despite it. Yet handsome, young JFK who followed him was ill all his life - and covered it up. So did Nixon, paranoid and drunk, whose medical records are sealed for 75 years. Ronald Reagan was frank about colon cancer and Alzheimer's, Blair was not frank about his heart condition.

If it is any consolation, the French are even more secretive than we are.