LONDON (Reuters) - Social media ads are harming efforts to curb obesity in the UK, with casual trolls in particular undermining genuine attempts to encourage healthier eating, a study has found.

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Working with a panel of individuals who eat less than 5,000 calories a day, researchers focused on ads broadcast on sites such as Pinterest, Facebook and Instagram. The majority of those targeted have built a social media following by espousing healthy lifestyles.

Eighty percent said they were exposed to the ads, which the researchers said were sometimes public health messages and other times not.

One of the most harmful ads, given to participants while they were on Instagram, is a diet reality television show whose host speaks directly to the camera about thin bodies being the “norm”. In an alternative ad, a participant responds that it is not the norm.

“The anonymity and ability of these ads to go viral in online communities means they are spreading unhealthy messages,” said Lars Olofsson, head of the Metabolic and Molecular Diseases Program at the University of Helsinki, who led the study.

Social media is a key forum for spreading unhealthy messages about weight, in particular the “gold standard” dieting website My Shape and its “eat yourself thin” mantra, researchers said.

In his version of the video, the author’s Snapchat handle “DrRoom” remarks that it is a common problem for obese people to exercise for only one hour a day, and then revert to a saturated fat diet.

In a follow-up study, the researchers tracked volunteers who had advertised the same content. Of those who got the ads, 55 percent decided to stop advertising it.

The study was published in the Lancet, a leading medical journal.