We live, literally, off the fat of the land. We've never had it so good. Life has never been so easy. And it's killing us. We used to die of disease. In 2020 we will die of life - or more specifically, of the way we live. Modern life is storing up trouble for us. Our health is paying for our convenience.

Do you want to know how bad it could be? "Should the gloomier scenarios relating to obesity turn out to be true, the sight of amputees will become much more familiar in the streets of Britain. There will be many more blind people. There will be huge demand for kidney dialysis. The positive trends of recent decades in combating heart disease, partly the consequence of the decline in smoking, will be reversed. Indeed, this will be the first generation where children die before their parents as a consequence of childhood obesity."

That comes not from the Daily Mail, but from the House of Commons' health select committee, which issued its report on obesity in May.

It all seemed to be going so well in the late 20th century. Life expectancy has been steadily increasing. Medicine has conquered diseases such as pneumonia, typhus and cholera, which used to kill young as well as old. But the truth is that many of us will not enjoy healthy, long life. We will be burdened for years by chronic, debilitating illnesses brought on by our lifestyle, which will also be cause our death. Men born in 2001, who will be 19 in 2020, can expect to live to 76, but will be healthy only until 67. Women will live to 81, but healthy life expectancy is 69 years. That gap between the end of good health and the end of life is increasing.

How do we live in 2004? We live fast. Our food is fast. Our communications are fast, by phone and internet, allowing us to work fast without leaving our desks. Our supermarkets allow us a one-stop fast shop, which we load into our car and drive home quickly. Most of the effort we make and will continue to make involves pressing keys and pushing buttons, not digging, hewing or heaving as we did just a few decades ago.

Today's children are the NHS chronic disease patients of the future because of the way we live: the boy who goes into a burger bar for breakfast before school, the girl who is driven from door to door because her parents are afraid she will be knocked down by a car while crossing the road. They eat crisps at break and a chocolate bar at lunch. She sneaks a cigarette with friends behind the bike shed. They go home after school and play on the computer for two hours or watch cartoons while snacking on biscuits and more crisps. Dinner is a microwave meal.

This big-food, little-effort lifestyle is a recipe for obesity, with its attendant risks of heart disease and diabetes. Obesity links directly into diabetes - a chronic condition that takes two forms. Type one can set in young for genetic reasons and require lifelong insulin, while type two can develop later in life, usually because of weight problems. There can be distressing complications, involving blindness, kidney problems and foot amputation. Our lifestyles also put us at risk of stroke and cancers, a risk exacerbated if we smoke, as girls and young women are doing in increasing numbers. Heart disease and strokes are the leading causes of death in the UK today, accounting for 39% of fatalities, followed by cancers at 26% and respiratory disease at 13%, according to the Office for National Statistics.

In all regions of the world except Africa, where HIV is wreaking such devastation, these chronic non-communicable diseases are the biggest killers, says Robert Beaglehole, the director of chronic diseases prevention and health promotion at the World Health Organisation. "The situation in 2020 will remain the same, even if we get serious," he predicts.

We know what to do to fight the problem, he says, but we're not doing enough of it. "Obesity is one risk factor for these conditions. The most important and the most preventable one is tobacco smoking. We have known about the deleterious effects for 50 years."

We have made an effort with tobacco control in the UK and smoking rates are going down. Lung disease and the cancers and heart disease and strokes associated with cigarettes should also decline, but the improvements are much more marked among the best educated, most affluent groups in the country, who find it easiest to quit. "For a single mother living under terrible conditions on benefit, smoking might be her only succour," says Beaglehole.

And so with diet. Obesity is more prevalent in poorer parts of the country. Junk food is cheap and filling. "The availability of fruit and vegetables is socially patterned," says Beaglehole. "If you go to a poorer neighbourhood, the supermarkets will have a different approach to what they are flogging. There might even be some price differentials. Hence the important role of governments in setting the environment. The government has a fundamental obligation to protect the health of its population. You can call it a nanny state if you like. I would say it is setting the conditions which allow individuals to make healthy choices."

But first recognise your enemy. In 2004, it appears that obesity has crept up on us almost without our noticing. Our lives have been transformed over just a few decades - an extraordinary pace in evolutionary terms - and the consequences have taken until now to be realised. But according to Weight Concern, a charity that is trialling programmes to help overweight families, the perceptions of many of us have also altered so we do not recognise obesity when we see it, because in some parts of the UK it is all-pervasive. Jude Cohen, the charity's executive director, says it has had angry calls from parents or grandparents who have calculated a child's body mass index (BMI - a formula based on height and weight) on the website and refused to believe there is a problem. "They are really upset. They say, 'According to your website my grandson is obese but he's not - he's just chunky.' It is more likely the child goes to a school where so many children are obese or overweight that they are beginning to think that what their child looks like is normal."

Jane Wardle, a professor of psychology who works in the Cancer Research UK health behaviour unit at University College London, and who also works with Weight Concern, thinks the prognosis for 2020 is bleak. "Nothing yet suggests this inexorable rise is going to plateau, because we're running a few years behind the US," she says. The situation across the Atlantic is even worse. Will obesity-related diseases increase in the UK as well? "I'd say that will happen and they will be coming on progressively earlier in people's lives."

We can try to modify our culture, she says. "There's quite a strong political will to do this, motivated by the potential health costs, but not a very strong public will at the moment except in relation to children."

The problem is that the changes we would have to make would unpick the advances of the convenience society. Do we want to have to walk further than the fridge, take hours instead of minutes preparing food, eat less and cut out the high-fat tastes we have come to like? "I think it is going to be quite difficult to make changes in the world because the things that have caused obesity are more convenient homes and workplaces. We have designed physical activity out of modern life and it is difficult to design routine physical activity back in. We have designed convenience into our food supply," she said.

The alternative, she suggests, is to help people to take deliberate physical exercise and limit themselves in the face of food temptation, so they say: "I'd like this but I won't have it; I'd like more but I won't have it; I'd like it now but I'll have it this evening."

But that route depends on a cultural shift. She notes the numbers of young girls in cropped tops, showing off rolls of fat around their middles in a way that used to be unusual. She wonders whether a concern about anorexia has gone too far the other way. Parents, and she includes herself, she says, have become worried about suggesting to their daughters any other relationship to food than a free-for-all. "A spectre has hung over us and while we weren't looking, the world changed," she says.

Professor Sir Charles George, the medical director of the British Heart Foundation, is not so pessimistic as the MPs of the health select committee. Although overeating and lack of exercise will push heart disease rates up, he thinks we can stave off the worst of the problems with medical interventions, such as widespread use of the class of drugs called statins, even if we face an uphill struggle convincing people to change their lifestyles. "We will have more and more people over the age of 65 and a still greater proportion over the age of 75. But we're going to see postponement of coronary heart disease events by quite a long period. There's going to be a still further decline in death rates in the under-65s and some deferment of those aged 65-74 into a later age," he says. Prevention is key, though, he says, whether by better diet and more exercise or through preventive drug treatment.

Other chronic diseases that will become more prevalent for the foreseeable future are harder to prevent. Asthma, for instance, has a whole panoply of possible underlying causes and triggers. But it is undoubtedly a disease of our times and our lifestyle: it is possibly associated with diet, possibly with modern hygiene and possibly with the quality of the air we breathe. Nothing looks likely to derail the upward trend in the incidence of asthma.

Cancers, too, are linked to our lifestyle in complex ways that are not fully understood. The numbers of cancer cases will continue to rise, although the mortality rates will probably also continue to fall with ever better treatment, as has been the pattern for some years now. Breast cancer is linked to the decline in breast-feeding, as we have fewer children, later in life, and hurry to wean them, but diet may also play a part, as it does in many cancers. Some chemicals in general use are known carcinogens.

We can do something to improve our environment and our lifestyle, but we can't do anything about the genes we inherit, and these can play a big part in our health expectancy. However, by 2020 we may choose to stop ourselves passing them on. Fertility clinics can already test and select embryos, created in the laboratory, that carry certain genes, such as the gene for early-onset Alzheimer's disease and for some cancers. These could be replaced in the womb with disease-free embryos. Only a few of the "problem" genes can be identified as yet, but within 16 years we may well all have the option to choose our future child's health and "breed out" certain genetically determined conditions.

But even genetically perfect babies, free of all the disease genes we can identify, can become couch potatoes doomed to a life of medication and chronic disease clinics. We will undoubtedly have a longer life expectancy in 2020, but how good a life may depend on how much we are willing to scrutinise our lifestyle and reform it in the years to come. It may be that life will have to become just that little bit less easy if we are to enjoy it to the full.

Sarah Boseley is the Guardian's health editor