Half the babies born this year will live to 100, it has been reported. But how does that square with predictions about the health effects of obesity and other modern-day perils, asks Michael Blastland in his regular column.

Are you a lifespan optimist or pessimist? Is immortality just around the medical corner, or will we be so fat we'll have to be rolled into super-sized early graves?

Average lifespan is a number and numbers often claim a basis in calculation and objectivity. And there are some impressive numbers to hand. They tell us authoritatively what has happened recently, they tell us roughly why, and the answers are astonishing.

Here's the data (see graph right). This is life expectancy at birth based on the mortality rates each year. Much of the improvement has been due to falling rates of coronary heart disease - Britain's biggest killer - in large part a result of less smoking. We're gaining years at the end of life and not only by reducing infant mortality - the cause of the biggest gains in the past.

A striking fact is what's happened to men. They're catching women, but more interesting is the rate at which they are adding years. It's accelerating. Every year they gain about another three months. This trend shows no sign of changing. The rate of gain now is twice what it was in the 1980s.

If this trend continues indefinitely, men will eventually add another year before they've lived the previous one, if you see what I mean, and so with every passing year will have longer to live. Er, yes. All of which shows the problem with extrapolation.

Imagination

So the good numbers now are not so good for the future. If the trend is going to change, when? In 20 years? In 50? And in which direction? Up but less quickly, flat, or down? And if the past isn't a reliable guide, what is?

This is where optimism and pessimism and you come in. Make your selection of the options below:

A. Lifespan has always grown so why should it stop?
B. But lifespan has been growing so long that it must be near the limit.

A. Obesity and other lifestyle factors will kill us younger.
B. But health obsession will have us all in the gym eating five a day and dancing into the twilight.

A. Medicine must reach its limits. We have to die of something.
B. But medicine has limitless potential and might even conquer death one day. It could certainly cure cancer.

A. There'll be a catastrophic flu.
B. No there won't. And if there is, we'll sort it.

A/B. There'll be a crisis of food scarcity due to global warming meaning no obesity but a restricted diet.

The list could go on. This is much the same kind of argument that experts on lifespan also have - arguments based partly on the quality of their imagination, partly on more numbers. (How many Americans are obese and what effect does that have on their lifespan now?)

Experts too can be grouped into optimists and pessimists, some of them at the extremes - though both sides fight for the label "realist". Our own official forecasters offer a range of possibilities that differ by about two or three years.

'Dribbling into All-Bran'

For all these reasons, it's been said that you should never forecast anything, especially about the future. The Government Actuary's Department has had to revise its own estimates for longevity with breathless frequency. Numbers of this kind are educated guesses, but such are the uncertainties that an ill-educated guess might turn out more accurate.

Half of babies 'will live to 100'

So how should we think of the forecast that about half of today's babies will live to be 100? Go Figure suggests treating it more as a "what if". All such predictions are better understood as thought experiments or scenarios - more or less plausible - to help concentrate minds.

What will we do if half of all people live to 100? Will rich countries be old in their behaviour as well as old economically? It's worth thinking about even if it never happens.

Worst-case obesity scenarios are similarly impossible to state with confidence, not least because people will react to the hypothesis, perhaps to forestall it. So even the prediction itself can help ensure that it doesn't come true.

Let's say that lifespan will one day average 100 for all. Where does that get us? To another uncertainty. Then we start arguing, or rather speculating, about the consequences. Optimists look forward to a young old age. Pessimists fear "dribbling into their All-Bran" as one doctor put it. Whatever the outcome, only you can decide if you like the sound of it.

At the Understanding Uncertainty website you can begin to play with this by putting one or two of your own details into an interactive graphic and seeing your chances of surviving each year ahead.

To end, here's a neat animated graphic that helps concentrate the mind about the future shape of the population, based on forecasts of course, from National Statistics. It shows at what age half the population will be older and half younger, and presents the results by area on a map of the UK.

Below is a selection of your comments.

Or c) Obesity isn't as big a risk factor for health problems as is reported, it's more the lifestyles that we lead that can contribute to obesity but does not guarantee it. A healthy lifestyle also does not guarantee thinness, and the more we realise this, the healthier we will be. A restricted diet enforced by world food shortages is the first way we will see a reverse in our life expectancy, not an extension. Nutrition has never been so good for us. We are taller and wider, and stronger than our previous generations, because malnutrition stunts growth and damages health and we in the western world see remarkably little of that.
Maria Bovor, Evesham, UK

Nothing is ever simple. Genetics also play their part. My mother's side were historically large and lived to great ages. My father's side were thin and lived shorter lives. When my mother was approaching 90 a young doctor told her to lose some weight and she might live a good deal longer. My mother told him to watch his stress levels or he might not make it to 40. Ah well, I've had a good life and that's the main thing.
Phil, Redbourn, UK

Obesity is a bell curve: most people are, by definition, of average size and in excellent health. Most people in the overweight-but-not-obese category can expect to resist disease and accident more robustly than their frailer ancestors and thus live longer. I think the maximum life expectancy will likely be 120 years, with few reaching that age even in the future. The rate of acceleration in life expectancy will likely flatten out in time.
Jo, Cambridgeshire

The claim that there is an obesity epidemic seems over hyped. Most teenagers and those in their early 20s seem to be quite thin; many people of my generation (50-something) have clearly taken care over their diet and exercise regularly. Those that live on a diet of fried food and pizza and lead a sedentary lifestyle will be more prone to obesity; but any claims regarding obesity and life expectancy also have to take into account which class and educational background you belong to.
Jay Furneaux, UK

My grandmother lived to the ripe old age of 88, despite having smoked and drank all her life. What did she die of? Old age, nothing more. There was no evidence of illness; her body had just had enough and she slowed down and shortly stopped. There's no amount of medical science that can keep us going once our bodies call time.
Maria, Farnborough, UK

We are an ageing population. With that will come more diseases associated with age - diabetes, kidney failure, heart disease and cancer. Many of my generation may well live into their eighties or nineties but unless more focus is put on healthy living and prevention as opposed to cure, we will end up in a hospital society. I'm not sure I would want to live 10-15 years more if it means I have no quality of life.
Nick, Exeter

It may well be so that more people will reach the age of 100 years. That is already happening. But things like obesity, cancer etc may have a counter effect on longevity. We have to die one day. Research also shows us that people live longer but the amount of healthy life years is not increasing by the same amount. So if you live to the lovely age of, say 86, the number of years with a bad quality of live is an average of 14 years. Do you mind if I then say I rather live shorter but enjoy life than longer with the last years of my life a misery. The older you get the more difficult it becomes and the more health and mobility problems you get. No I do not want to become 100 or live for ever if that means living with poor health, poor mobility. Enjoy life eat and drink and exercise as much as you want.
Mariken, Fleet

I believe you can add a few years by looking after your health and exercising regularly, but I also believe that a date, time and place has already been decided by God for every individual and no matter what you do death will come at that point which has already be destined.
Yasmeen, Edinburgh