The government has announced its ambition to reduce the number of calories in some popular foods by 20 per cent by 2024.

Will this really help the nation lose weight? There’s often confusion and debate about how best to tackle the UK’s obesity epidemic.

Some commentators have blamed obesity on over-consumption of certain food types like carbohydrates, sugar and fat, while others say it’s the total number of calories you consume that matters most.

Should we be counting calories or prioritise cutting down on fat… or sugar? And what about exercise?

Here at FactCheck, we don’t claim to be scientists. So, instead, we spoke to some of the country’s top experts on obesity.

Here’s what they told us:

‘Be calorie conscious’

Professor Susan Jebb of Oxford University and a member of the Public Health England’s Obesity Programme board:

“If your focus is obesity then you’ve got to focus on calories. What is it that makes people overweight? It’s calories.

“All of the other things matter – saturated fat, sugar and salt. But if people eat fewer calories, they will probably also get less saturated fat, less sugar and less salt, because they’re eating less food.

“Once you’ve got calories right, of course we should then be saying the quality of food matters. However many calories you’re eating, what you want to do is to make those the healthiest calories they can be. And those two messages of course have to go alongside each other.

“The huge challenge is how do you get the nation to eat fewer calories? The answer is not saying that everyone has to count every calorie they eat. That doesn’t get us anywhere, partly because it’s too boring and we’re too busy. Also because it’s actually really hard to know how many calories there are in things.

“But you can still be calorie conscious – you need a general awareness. But even better would be if the food industry took some of the unnecessary calories out of food.

“Saying we need to reduce calorie intake does not in any way detract from saying we need to be more active. We need to take action on both.”

‘Calorie counting does not work’

Professor Jimmy Bell of the Research Centre for Optimal Health at the University of Westminster:

“I was slightly surprised by this emphasis on calories. The government is skirting around the edges of the problem by saying we’re going to reduce calorie content in food by 20 per cent. It’s like saying we’re going to have a really nice Brexit: we all wish the same thing, but it doesn’t mean it’s going to happen.

“At the end of the day, we humans like eating food. But most of us find it impossible to tell how many calories are in anything.

“We should try to get the manufacturers to come out with alternatives where the calorie should not be the main drive, but satiety should be the main drive. So you might have a small volume but you feel satisfied.

“Calorie counting has been shown that, long-term, it does not work. The answer to weight loss was given years ago: eat less, walk more. That’s it. I’m really surprised that the government did not say anything about physical activity.”

‘Too little, too late’

Professor Anthony Barnett of the University of Birmingham and a clinical director at the Heart of England NHS Foundation Trust:

“It’s great to see they’re taking some interest, but it really is – in many ways – too little too late. This is a programme that will take us beyond the next election, and who knows what the next government will do.

“It’s much too long term, and I think they need to have a much more holistic approach to the problem, rather than just the focus on calories. To an extent it’s laudable, but there’s so many other things that need to be done, so we need a bit of joined up thinking.

“What we really need in this country is a major public health campaign, amongst other things. Something along the lines that we had with smoking, HIV, cervical cancer and so on, which were actually very successful.

“Reducing calories in certain foods is all very well, but there’s so many other aspects to our overweight and obesity problem. If you actually look at calorie intake in this country over the last thirty years, calorie intake actually hasn’t increased very much. But what has reduced dramatically is the amount of physical activity people are taking.

“We also have a problem at the other end of the spectrum with anorexia, particularly with teenage girls. Calorie restrictions is going to be the last thing they need. It’s something which needs to be thought about – it comes down to balance and individualisation and the care that you’re offering.”

‘It is to do with calories’

Dr Kieran Clarke, professor of physiological biochemistry at the University of Oxford.

“Anything to help is a good thing. Trying to get people to exercise is really difficult, mainly because you have to exercise so long to get rid of those calories. So it’s just much easier to reduce the number of calories.

“Personally, I’d rather see a tax on foods with empty calories, such as biscuits, and a subsidy on fruit and veg.

“But I think it is to do with calories. The easiest thing to do is to know how many calories you are drinking in a latté, for example. Counting calories, weighing yourself every morning, is a good thing to do.

“They don’t have to know to the minutest details about what calories are in what. But they have to have an idea that if they eat a banana, it probably has between 70-100 calories and that’s a good thing.

“The government has to do things that have simple messages – and I think this is a good simple message.”

‘Not all calories are equal’

Professor Nita Forouhi, programme leader of the Nutritional Epidemiology programme at Cambridge University’s MRC Epidemiology Unit:

“It’s a step in the right direction, mainly because it gives a very clear message that it’s not just about individuals and personal responsibility alone. Calorie control and portion size control are definitely an important part of the overall strategy.

“But obesity – and the consequences of obesity – are critical public health burdens, and the managing of this has got to be multi-pronged. Beyond diet, there are other things that feed into it too, not least our increasingly sedentary behaviour.

“Not all calories are equal. Calories can be healthy or unhealthy calories. For example, you get nine calories per gram of fat. But whether you eat saturated fat or unsaturated fat calories, that makes a huge difference.

“As a public health official, I think one has to give bold strategies for action to get the message across. The next step which needs unpicking is what action is required.”

‘Fits with the scientific evidence’

Professor Ian Macdonald, of Sheffield University’s Faculty of Medicine & Health Sciences:

“Counting calories is not wrong, but it’s very crude and you need to know how much you’re burning up in order to know what calorie target you’re aiming for.

“We do research in which we get people to lose weight. If we say to the participants ‘we want to eat a diet that gives you no more than 1,500 calories a day,’ then counting calories is really important to achieving that. We teach them how to do it and give them guidance on how to achieve it.

“But if you say ‘we want you to eat 10 per cent fewer calories than you’re burning up’, then that’s quite difficult because, first of all, we don’t know what we’re burning up.

“This policy fits in with the scientific evidence which has been provided, but it will be hard. It will need health professionals to help people understand what it is they need; it will need the media to help them understand as well.”