I shall resist the temptation to make fatcat jokes this week. But I don't think I'm sticking my neck out in predicting that Alan Johnson's trouble over that £3,000 donation to his deputy leadership campaign will not lead to the health secretary's resignation.

Not unless something sinister turns up to explain why one Labour supporter got another one to write the cheque. Peter Hain might have been vain in his ill-judged ambition, but Mr Johnson, who spent£50,000 to come a close second - a quarter of what Hain spent to come fifth - was not. He strikes me as a modest man and says he stuck to the rules.

No damage to Johnson doesn't mean that politics escapes so lightly. Labour ministers enacted laws to ensure greater openness in political funding, then found it hard to abide by them, a bit like those chief constables who get done for speeding.

The donor scandals come at a bad time, when bankers are messing up their own business and worldwide government regulators have struggled to cope. We may soon have more to fret about than that£3,000.

A more worrying challenge of last week was on the other fat front: the sight of ministers moving the goalposts on previous obesity targets, to reverse the fat tide by 2020 instead of 2010.

Shadow health secretary Andrew Lansley had some easy sport digging out past Labour pledges and reminding voters that John Major's government first flagged up overweight problems in the early 1990s. The Blair-Brown regime made public health an early priority, but let it slip as public health budgets were trimmed under financial pressure: they were stronger on exhortation than effective action.

That said, it's not hard to sympathise with ministers' frustration. Changing the way people think and behave is one of the hardest things any government has to do. Unhealthy eating is harder to tackle than smoking, as food is a more complex issue.

When I caught up with Dawn Primarolo, now public health minister after a decade in the Brown Treasury team, she sounded full of determination to see an agreed food labelling system in place (at last) by the end of the year.

"Our preferred option is the traffic light system," she said, after dispensing with my surprise that many headlines had been devoted to the idea that people could be paid to eat well. "That was very early thinking," she said.

For a former left-winger she seems surprisingly modest in acknowledging the limited role of government and talks of the necessary balance between the state and what the individual can only do for themselves.

"Ultimately individuals must decide what they eat and what exercise they take," she conceded, having admitted to The Times mid-week that she finds both hard (the minister is a keen cook who likes chocolate).

The phrase she ultimately settles on is not partnership but "duty of care". It's not about nannying people, "but if the choice is between a neglectful state and a nannying state, I think it's better not to be accused of being neglectful".

The workplace is a crucial forum, as the government's white paper Healthy Weight, Healthy Lives makes clear. Employees who are encouraged to get the work-life balance in good order are more productive, better motivated workers, says Ms Primarolo - a very Brownite view.

That means nurseries as well as exercise, of course. When I reminded her of a remark I heard years ago from Howard Stoate, the Labour MP-GP, that poor people often can't afford better food, the minister insisted "to eat healthily does not have to be expensive".

I'm sure she's right, but it is daunting. I remain sceptical about ministerial determination to knock supermarket heads together, let alone about the scale of the task ahead. So many children now eat so much fast food and take less exercise because their parents are fearful of muggers, cars and rapists. That takes policy making far beyond the Johnson-Primarolo remit where voters are deeply irrational. That stimulating book, Freakonomics, asks which is safer: letting the kids play in a friend's house where there are handguns - or a swimming pool? Answer: guns are safer, statistically the kid are more likely to drown.