BOLDLY and clearly displaying the calorie content of food and drink 'in big letters' on supermarket packaging and restaurant menus would be more effective in curbing obesity than a sugar tax, according to Scottish medical experts.

Professor Naveed Sattar, an expert in metabolic medicine at Glasgow University whose work has previously been published in the prestigious Lancet journal, said evidence shows people vastly underestimate the calories they consume.

He said printing the calorie content of items "in big letters", including for dishes on restaurant menus, would alter consumers' behaviour.

Prof Sattar said: "Most people are overweight not because they're eating more sugar, but because they are also eating too much fat.

"To me a sugar tax on specific things like sugary drinks may work. It has been tried in other countries and shown benefits therefore we should try it here. Doing nothing is just ludicrous, and we could also use the money that's raised to make fruit and veg cheaper.

"But the biggest problem is calorie density. Maybe an easier way would be to put the total calorie content on as many foods as possible. For instance, if someone goes to buy a latte and a cream bun then sees that the latte is 250 calories and the cream bun is another 350 calories, they start to get educated and say to themselves 'wait a minute, maybe I'll just have a cup of tea and a banana'.

"If you put total calorie contents in big letters it may start to push people in a better direction. If they then start to gravitate towards the healthier foods, the unhealthy foods become less profitable for the companies, and that drives them to make formulations with fewer calories. I think that's the kind of direction of travel we need."

Although smaller items such as a can of Irn-Bru or a Mars bar display their total calories on the packaging in small lettering, with many other products consumers are left to calculate it themselves.

Some items tend to display calories "per portion" or per 100g, which does not explain clearly the full calorie count. For example, 150g "sharing bag" of salt and vinegar Kettle Chips totals 753 calories while a 500ml tub of Ben and Jerry's Cookie Dough ice cream comes in at a belt-loosening 1,350 calories.

Many takeaway items come with no nutritional information at all, except on spreadsheets tucked away online. A Greggs sausage roll averages 349 calories per portion, according to the company's website, while a large stuffed crust pepperoni pizza from Dominos is 286 calories per slice - or 2860 per pizza.

Even typical cinema snacks can be shockingly high in calories. A large tub of sweet popcorn from Cineworld totals 1,180 calories - with salted popcorn not far behind on 811 calories.

Earlier this year, MEPs backed calls for nutritional information to be added to alcohol labelling to make consumers more aware of the calories in beer, wine and spirits.

On average, men are advised to consume no more than 2,500 calories per day and women a maximum of 2000.

Emma Kinrade, a dietitian and lecturer in nutrition at Glasgow Caledonian University, said a "family-based approach" where children learn healthy eating habits in the home which are then reinforced by an emphasis on fruit and vegetable consumption in schools and nurseries would be more effective than a sugar tax.

She said: "I've been a dietitian since 1999 and my job used to be working in school meals and healthy snacks and it can work when the schools do it really well, but equally I think it takes a strong headteacher to implement something like that and to follow it through."

Kinrade also stressed that both sugar and fat consumption needed to be tackled.

"It's very pleasing to eat sugar and your body doesn't have the receptors to say 'I've had enough' - but it's the same with fat. The combination of fat and sugar is the one thing that people can't resist, and the desire to have that nice taste overrides the ability to say 'enough'. They're a lethal combination."

A spokeswoman for the Scottish Government said it had "no current plans to introduce a sugar tax", but were investing £10 million in healthy eating initiatives over a four-year period to 2016. Any new tax would require Westminster approval.

However, she said ministers are awaiting "with interest" the outcome of three reviews on the health effects of taxes on unsaturated fat, sugar and sugar-sweetened beverages. The findings are due next year.

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In 2014, 4000 children aged five to nine in Scotland were admitted to hospital for dental surgery brought on by tooth decay.

Teeth are attacked when sugar is converted into acid by bacteria in the mouth. The more frequently someone consumes sugar, the less time there is for the teeth to recover.

Colwyn Jones, a member of the British Dental Association in Edinburgh and a dentist for more than 30 years, said: “The best drinks for teeth are water and milk, so anything you can do to get away from sugar-sweetened drinks is a good thing.”

However, he added that a sugar tax would be a “blunt instrument” unless accompanied by other health interventions, such as bans on advertising confectionary and sugary drinks to children.