In a 3-part Horizon special, BBC Science, together with Britain’s leading obesity scientists, will investigate the latest weight loss theory: that the best way to lose weight successfully – and keep it off - is a personalised diet that addresses individual causes of overeating.

In Britain today, 11 million people are on a diet. Yet the vast majority of them will fail.

Most diets are a one-size-fits-all approach. But each of us puts on weight for different reasons and scientists are currently investigating how differences in our biochemistry, psychology and our genes affect our weight.

In a ground-breaking national experiment - the first of its kind - Dr Chris Van Tulleken and clinical psychologist Professor Tanya Byron, together with Britain’s foremost nutrition and weight loss scientists from Oxford and Cambridge universities, put the latest theory to the test. They’ve selected 75 overweight volunteers from across the UK who will be put on personalised diets to explore three particular causes of overeating: genes, gut hormones, and emotion-related eating.

Can science succeed, where other diets have failed?

Packed full of science tips and general diet advice, the programme offers credible, useful information for everyone.

Viewers at home can also find out which diet might be right for them by trying the online diet test, available from January 12 at bbc.co.uk/rightdiet.

This unique television experiment is led by a team of world class scientists: from Oxford University, Professor of diet and population Susan Jebb, and Professor of behavioural medicine Paul Aveyard; and from Cambridge University, geneticist Dr Giles Yeo; and gut hormone specialist Professor Fiona Gribble.

The Experiment:

Scientists put 75 overweight volunteers through a series of tests to find out why they eat too much and are overweight.

They are divided into three groups, each based on different reasons for over eating:

Feasters - produce less of a gut hormone which tells them when to stop eating so they don’t realise when they are full during meals.

Constant Cravers – have ‘hungry’ genes that increase their risk of obesity and can make them feel hungry all the time.

Emotional Eaters - eat in response to negative feelings, such as unhappiness or stress.

The volunteers are then allocated one of three tailored diets, designed to address the causes of their overeating and the scientists track their progress over three months.

Episode 1

In episode 1, we follow the hopeful dieters as they go through an intensive 5-day analysis at a residential diet lab at Liverpool Hope University. They undergo a series of experiments run by the obesity scientists to look at whether the volunteers’ eating behaviour matches biological and psychological test results.

First up, the volunteers are secretly filmed at a non-stop sushi banquet. From a nerve centre, they’re closely observed by gut hormone specialist, Professor Fiona Gribble. She wants to test the theory that people who produce less of a gut hormone that signals to the brain when we’re full will eat more at a meal times than other people.

Next, behavioural scientist, Professor Paul Aveyard, tests the power of emotional eating. He assesses how individuals in the Emotional Eaters group - those who turn to food when anxious, depressed or stressed – react to a stressful experience compared to those in a different group.

Finally, the third group – the Constant Cravers – are put to the test by geneticist Dr Giles Yeo. The latest genetic research into eating behaviour shows that within the human body, there a number of key obesity genes, which - if you have them - can make you want to eat constantly.

After confirming that the three groups match the eating behaviour they anticipated, they reveal which groups the volunteers are in and the personalised diets which will best address their overeating.

Episode 2

In episode 2, we find out what happens when the dieters leave the lab and take their diets home. There they face the challenge of sticking to their weight loss plans with all the stresses and temptations of their real lives.

Geneticist, Professor Giles Yeo uses eye-tracking technology to investigate how the Constant Cravers are affected by their ‘hungry genes’, especially when it comes to their behavior around food advertising.

Professor Tanya Byron visits Cambridge University to discover what happens in the brains of the Emotional Eaters that compels them to comfort eat.

The Feasters are brought together for a test by Professor Fiona Gribble, who wants to discover why soup is such a great dieting tool.

We’ll also hear from members of all our dieting groups as they go past the honeymoon stage, and run an experiment at the supermarket to investigate what effect tiredness has on your food choices.

Episode 3

In the final episode, we find out how personalised dieting can work for the 75 volunteers in the long term.

We look into how physiological processes in the body and the brain can make the final weeks and months of a diet difficult. Professor Susan Jebb tests how changes in metabolic rate may be slowing down the dieter’s weight loss and we learn what tweaks they may need to make to their diet plans to keep up their rate of weight loss.

Professor Fiona Gribble runs an experiment to show that how fast the Feasters eat their food will affect their hormones and ultimately, how full they feel.

Dr Giles Yeo demonstrates how eating breakfast can help the Constant Cravers in their weight loss by helping their hungry-brains desire less fatty and sugary foods throughout the day.

Through an experiment with the Emotional Eaters, behavioural scientist, Professor Paul Aveyard demonstrates ways that will help them learn how to deal with occasional indulgences and avoid further relapse.

Finally, we check in on the final results of this unique study to see how successful the diet personalisation approach has been for the group and how many of the volunteers made the 5% weight loss target.

There is also a free interactive eBook to accompany the series, What’s the Right Diet for You?, with the latest scientific advice on how to lose weight and keep it off for good, meal suggestions and delicious recipes and video interviews with people who have tried and tested the diets. The ebook will be available for iPads, Android and Kindle tablets from the January 12 via bbc.co.uk/rightdiet.

HF