As a nation we are getting fatter to the point of crisis. But why? And what are the implications? For starters, it's hard to treat after the age of five and is bankrupting the NHS

1. Nearly two-thirds of the UK population is either overweight or obese

Fat, not thin, is today's norm. But studies show that we don't notice because it has happened gradually and we have got used to seeing people who are overweight. Kids in pictures taken on the beach in the 1950s, with ribs showing, look famished to modern eyes. They are of normal weight. A quarter of us are actually obese, defined as a body mass index (weight in kg divided by the square of your height in metres) of 30 or above. BMI is not a brilliant tool for every individual – biceps packed with muscle weigh as much as flab – but it is satisfactory at a population level.

Men are fatter than women (67% of men and 57% of women are overweight or obese in the UK, according to the Global Burden of Disease study from the Institute of Health Metrics and Evaluation in Seattle). Socio-economically deprived areas tend to have higher rates of people who are overweight, but no income group is immune. There is a community effect: you are more likely to be overweight if your friends and neighbours are and you see it as the norm.

Skinny? No, normal. Boys running on the beach in the 1950s. Photograph: Orlando

2. Obesity is shortening our lives

Moderate obesity (BMI 30-35) cuts life expectancy by two to four years and severe obesity (BMI 40-45) by an entire decade, according to a major study in the Lancet in 2009. This is most likely to affect today's children; more than a fifth of five-year-olds and a third of 11-year-olds are overweight or obese. "Obesity is such that this generation of children could be the first in the history of the United States to live less healthful and shorter lives than their parents," said Dr David S Ludwig, director of the obesity programme at the Children's Hospital Boston and one of the authors of a paper that, in 2005, came to similar conclusions in the New England Journal of Medicine . The proportion of overweight children in the US and the UK is similar.

3. Obesity could bankrupt the NHS

The NHS spends £5bn a year on diseases such as strokes and diabetes that are linked to obesity. Within a few decades, that is predicted to climb to £15bn. Type 2 diabetes is a huge problem: 10% of the NHS budget already goes on that alone. Being overweight is the chief cause and the numbers are soaring, from 1.4 million in 1996 to more than three million today, with a predicted rise to five million by 2025.

A study this month revealed that one-third of the population is on the verge of type 2 diabetes, having high blood glucose levels classified as prediabetes. "If this increase in prediabetes and diabetes isn't tackled now, it will destroy the health service," said Barbara Young, chief executive of Diabetes UK. "Many of the problems the secretary of state is trying to tackle, such as too many people coming in as emergencies to hospitals, are about the one in six people in any hospital at any time who've got diabetes. So it's a massive impact on the NHS and it's going to get even bigger."

Type 2 diabetes is costly in every sense – apart from complications such as blindness and amputation, it makes you five times more likely to have a heart attack or stroke.

The food industry encourages us to buy fattening foods. Photograph: 64/Ocean/Corbis

4. It's an unfair fight

The government spends £14m a year on its anti-obesity social marketing programme Change4Life. The food industry spends more than £1bn a year on marketing in the UK. Guess who has the subtler operation?

Big Food is watching you. Technology has allowed its scientists to track shoppers' eye movements, logging precisely which supermarket shelves we glance at – and which keep our attention. It's not just the in-your-face bright packaging with happy slogans, but which aisle the product is in. Food companies pay a premium to have their merchandise on end-displays, which account for 30% of supermarket sales. We are not as in control of our shopping as we like to believe. We go in with good intentions – we come out with large bottles of fizzy drinks and packets of biscuits.

5. Obesity took off in the have-it-all 80s

But it was unregistered by the government in power. McDonald's moved its headquarters into Margaret Thatcher's Finchley constituency in 1982, three years after she became prime minister. She opened the building in 1983 and visited again in 1989, on the 10th anniversary of her prime ministership, when she congratulated the company on the jobs it had created and its economic success.

Margaret Thatcher at McDonald's HQ, 1983. Is she really going to eat that Big Mac? Photograph: Neville Marriner/Associated /REX

6. Snacking is "a newly created behaviour"

It was virtually unknown before the second world war, according to Barry Popkin, professor of nutrition at the University of North Carolina at Chapel Hill School of Public Health. It is now a big cause of obesity and considered a major growth sector for the food and drink industry. Popkin published a study showing US children were eating almost continuously, with three snacks a day as well as their ordinary meals. "Our children are moving towards constant eating," he said.

7. The food industry is behaving as the tobacco industry did

Critics say it pays experts, funds scientific papers that support its case, rubbishes the evidence that goes against it and declares, as part of its participation in the government's Responsibility Deal, that it is making its products more healthy.

Big Food and the politicians who support the industry say there is no such thing as bad food. There is an element of truth in that. One Mars bar (once marketed as a healthy, energy-giving snack with the slogan: "A Mars a day helps you work, rest and play") won't in itself do you any harm. Daily sweet snacks, washed down with sugary drinks and supplemented with crisps, prior to a cheeseburger with chips, are highly likely to contribute to heart disease, however.

Large numbers of scientists advise the food industry and take funding for research because they are focused on the micro, not the macro picture. The "sustaining members" of the British Nutrition Foundation include Coca- Cola, Kellogg's, Mondelez (owner of Cadbury), Nestlé, PepsiCo, Tate & Lyle, Associated British Foods and Unilever. The chair of the government's nutritional advisory committee investigating carbohydrates, including sugar, is Professor Ian Macdonald from Nottingham University, who has been an adviser to Coca-Cola and Mars.

8. Your brain, not your stomach, tells you when to stop eating

Hunger is in the mind. Dr Suzanne Higgs at Birmingham University carried out a remarkable experiment to prove it. Her team gave a group of amnesiacs a lunch of sandwiches and cakes. When everybody had finished eating, they cleared away and brought in a fresh lunch 10 minutes later. A control group of people with no memory problems groaned and refused any more food. The amnesiac group tucked in and ate the same again.

When we eat in front of the television or while looking at our computer screen at work, we are not giving lunch or dinner our full attention. Our brain is not registering how much we have eaten and we may well feel we haven't had enough. Higgs is working on a phone app so that people can take pictures of their meals and snacks as a reminder that they've actually had enough.

9. By the age of five, it is almost too late to intervene

More and more very young children are putting on weight they will find hard to shed. Photograph: Najlah Feanny/Corbis

The EarlyBird diabetes study of 300 children in Devon showed that they had already gained 70–90% of their excess weight before primary school. It is far harder to get rid of weight than to put it on, even as a child. Some experts think that if we want to prevent obesity, we're going to have to find ways to help parents from, or even before, the birth of their baby.

We think obesity is about adults eating fried chicken and chips. But most babies in the UK are overfed – 75% of those aged four to 18 months in the government-commissioned Diet and Nutrition Survey of Infants and Young Children, published in 2013, were getting more calories than they needed from formula milk and solid foods. Breastfed babies, who can look skinny compared with their bottle-fed friends, are in fact usually the right weight. Big, bouncing babies, contrary to the old wisdom, are not healthier babies. Slow growth is best. Low birthweight babies, in particular, should not be overfed in a bid to help them catch up.

Those who think children are getting fat because they sit in front of the television too much may also be wrong. Another finding from EarlyBird was that inactivity does not lead to obesity – obesity leads to inactivity. Overweight children feel less like running about. Shockingly, out of 300 children monitored from the age of five for 12 years, three had developed diabetes by the end and 55 had high blood glucose levels that suggested they were on the verge of it too.

10. Obese children are increasingly being taken into care

Watching too much TV doesn't cause obesity – it is obesity that makes kids become couch potatoes. Photograph: RayArt Graphics/Alamy

At least 74 in the past five years, according to a Freedom of Information request from the Daily Mirror, which got usable responses from only 128 out of 206 local authorities in England, Scotland and Wales, suggesting that the numbers are even higher. Their request followed the revelation that a five-year-old girl weighing 10st 5lb had been taken away from her family in Newport, south Wales, in August 2012. Earlier this month it was revealed that parents in King's Lynn, Norfolk, had been arrested on suspicion of neglect and child cruelty because their 11-year-old son weighed 15 stone.

Doctors and social workers have a dilemma, however. Obese children may have caring (possibly also obese) parents who may not succeed in getting their child's (or their own) weight down. "As obesity remains extremely difficult for professionals to treat, it is untenable to criticise parents for failing to treat it successfully if they engage adequately with treatment," said Dr Russell Viner of the Institute of Child Health, who along with colleagues proposed a framework for action in the British Medical Journal in 2010.

The Shape We're In by Sarah Boseley is published by Guardian Faber on 26 June. To order a copy for £8.99 (RRP £12.99), visit theguardian.com/bookshop or call 0330 333 6846.