Jeremy Laurance: Five a day won't stop cancer – so how do we know what to believe?

Junk food lovers of the world, unite! Time to ditch the broccoli, bin the sprouts and forget about peeling an orange. Eating your five a day does not protect against cancer, it turns out – or gives very limited protection at best. We were all, it appears, misled.

Last week's story from the US Journal of the National Cancer Institute was a real shocker. Two decades of research overturned – and millions of people seeking to follow a healthy lifestyle must now rethink their strategy.

While it may delight fruit and veg refuseniks, it is dismaying news for science and for those who, like me, report it. Why believe anything you read about health in the newspapers? The advice will only be contradicted tomorrow. Eat what you like and stop worrying, seems the only sane response.

So first, a caution. The research focused on the protective effects of fruit and veg against cancer, and found it lacking. But the same study showed a 30 per cent reduction in heart disease among those eating their five a day (compared with those eating only one-and-a-half a day), confirming innumerable earlier studies. Those sprouts can stop a heart attack, and that orange can lower blood pressure – so don't bin them just yet.

But the broader charge – if science keeps coming up with contradictory answers, what is the point of acting on them? – is a tricky one. There is no simple response. Scientific know-ledge does not proceed in a linear fashion, and only rarely in giant leaps. Mostly, it inches ahead, taking three steps forward and two back.

This does not lend itself to the news agenda, which deals in extremes – miracle cures, deadly scares – and requires a breakthrough or a disaster to earn a headline. The subtleties of scientific progress get overlooked in the cut and thrust of the newsroom.

Where does that leave us? I am reminded of Larry Gopnik, embattled star of the Coen brothers' latest film, the glorious black comedy A Serious Man, whose search for meaning in the series of accidents that befall him is endlessly frustrated.

That is what science is like – an endless search, constantly renewed, which by degrees may approach the truth but is doomed never quite to reach it. Our task is to follow its twists and turns, aware that it is a road without end – and to be prepared for surprises.

Did Jade Goody die in vain? When the reality TV star succumbed to cervical cancer a year ago, the publicity she attracted led to a 12 per cent surge in the number of women attending for screening. Twelve months on, the cervical cancer charity Jo's Trust reports that the numbers coming forward have fallen dramatically, possibly to below the level before the "Goody effect" took hold. How short memories are.

There is better news on the cervical cancer vaccination programme. It was launched in 2008 and by the end of its first year, 80 per cent of 12 and 13 year old girls had received all three jabs, according to the Health Protection Agency. That is an impressive start.