It may be meant as friendly advice, but telling someone they are piling on the pounds just makes them delve further into the biscuit tin.
Those who are made to feel ashamed about their size are six times as likely to become obese, a study has found.
Experts say that being criticised for their weight drives those who are already sensitive about it to comfort eat. Fear of ridicule may also mean they avoid exercise.
Researchers from University College London said that the obesity crisis could be eased by teaching people - including doctors - that it is counterproductive to discriminate against others because they are overweight.
They said that public health campaigns should also avoid making people feel bad about their weight.
Lead author Sarah Jackson advises medics to avoid using the word "fat" where possible and replace it with the terms overweight or obese.
The study involved almost 3,000 English men and women aged 50-plus who were weighed twice, four years apart. They were also asked if they had been discriminated against because of their weight.
Examples of this included being treated with disrespect - including being the butt of jokes - receiving poorer service in shops, restaurants, hospitals and doctors' surgeries, being threatened and being assumed to be stupid.
About 5 per cent said they had been treated differently because of their weight - with the figure rising to 36 per cent among those who were the most overweight.
The study, published in the journal Obesity, found that those who were victims of "fat shaming" put on just over 1kg on average over the course of the study. They were also six times as likely to become obese.
In contrast, those who were not criticised for their weight actually became slimmer, albeit by a small amount. Dr Jackson said: 'Our results show that weight discrimination does not encourage weight loss and suggest that it may even exacerbate weight gain.
Stress responses to discrimination can increase appetite, particularly for unhealthy, energy-dense food. Weight discrimination has also been shown to make people feel less confident about taking part in physical activity, so they tend to avoid it.
People may not feel comfortable going to the gym if they think they are being judged because of their weight.
Previous research has shown that it is not just the general public who are guilty of making the overweight feel ashamed of their size.
Dr Jackson said: "Studies have shown that quite a wide range of health professionals hold negative attitudes about obese people and patients pick up on that bias.
"Doctors tend to spend less time with obese patients. They feel that treating obesity is a futile task and many avoid doing it.
"We need to find a way of addressing this during training, highlighting that blaming and shaming isn't going to help resolve the problem."
University College London Professor Jane Wardle said: "Our study clearly shows that weight discrimination is part of the obesity problem and not the solution."
Tam Fry, from the National Obesity Forum, said: "If you are aggressive with the way you put the message across and denigrate and tease, you are not going to get anywhere at all. What you need is tough love."
He suggested that instead of talking about fat, medics should warn about the consequences of obesity, such as risk factors associated with type 2 diabetes - blindness, amputation and early death.
In 2008, the word "obese" was banned on official letters to parents of overweight children.
It was replaced by the term "very overweight" on warning letters sent out as part of a national child measurement programme.
- Daily Mail