Doctors who spend 30 seconds telling patients they need to lose weight can have a dramatic impact, a study shows.
Some people, who had no intention of shedding the pounds, lost 10% of their body weight after being offered a free weight-loss programme.
The University of Oxford researchers said the 30-second chat would have a huge effect if every GP did it.
The findings, published in the Lancet, showed that patients were not offended by the advice.
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"More than any other person, your doctor can bring up the question of weight, it is not like telling your wife she's overweight," said Paul Cooper who took part in the trial.
He was going for a normal appointment in Northampton, when his GP told him that weighing in at 98kg (15st 6lb) was unhealthy.
"Looking down, I couldn't see my feet," the 69-year-old said.
'While you're here....'
Paul was one of 2,728 severely overweight people who were involved in the trial.
All were seeing their doctor for another health condition when the GP said something like: "While you're here, I just want to talk about your weight."
Half were offered a free place on a weight-loss programme like Slimming World or Rosemary Conley.
Only four in 10 actually turned up, but nonetheless a quarter of patients initially offered a place lost 5% of their original weight and around one-tenth had lost 10% of their weight after a year.
Overall, their weight loss averaged 2.4kg (5.3lb).
The other half were told they could do with losing weight, but were given no more support. They lost 1kg (2.2lb) after 12 months.
Prof Paul Aveyard, from the University of Oxford, told the BBC News website: "The impact is pretty substantial given the effort - 30 seconds - that went into it.
"If we were year-on-year to knock 2.4kg off the heaviest people in society then that would have a very big effect in health terms."
Prof Aveyard, who is also a GP, said: "This should be added into the repertoire of things we all attend to in appointments like flu jabs, blood pressure and stopping smoking."
Paul Cooper says he "went on to a pretty strict diet" and now weighs in at 84kg (13st 3lb).
He told the BBC News website: "I can now see my feet!
"It wasn't until I lost the weight that I realised you didn't have to feel like that [in your late 60s], I can still do a 15-mile walk in the Peak District or play cricket in the summer."
His advice to people trying to lose weight is: "You don't have to become a party pooper.
"Have a small piece of birthday cake, eat it very slowly and don't have a second one.
"Before, I might have had half a dozen chocolates in front of the TV, really take your time and tell yourself it's a real treat and make it last."
Dr Imran Rafi, from the Royal College of GPs, said: "Levels of obesity are a growing concern in the UK and can lead to a number of debilitating and serious conditions.
"If this scheme is low-cost and effective, which this research claims it is, it makes sense to consider it on a wider scale.
"We must understand that while some patients in this study did benefit from a referral to a weight-loss programme, it won't work for everyone and shouldn't be considered as a blanket solution to curb growing levels of obesity."
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