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"February is the worst month to be born in. Those people live a couple of years less than those born in August." Bad news for winter babies, but Stephen Richards, the Prudential's head of mortality risk, is used to gloom. His job is to research when - and why - we die.

The commercial risk-assessment aspect of the job is vital for Prudential, which has some £20bn of annuity liabilities and pays out income to 750,000 annuity holders a year. But the side-effect of his team's number-crunching is that Richards also has an unique insight into the factors that affect our lifespan. And it's gripping - if grim - stuff.

He explains why February-born babies don't have the best odds: "US researchers have prepared an 18th and 19th century data set and found that month of birth is a useful indicator of life expectancy - August is the best month in the northern hemisphere and the worst in the southern hemisphere. This could be to do with the seasonal availability of nutrients at key developmental stages."

Researching death means that the youthful-looking (March-born) 37-year-old acts on what he finds. "The diet in my household has been transformed since I found that dietary deficiency causes a third of cancers - the same figure as for smoking. We now consume more nuts and seeds, as they are concentrated forms of amino acids and key vitamins."

Over lunch Richards orders plain tomato pasta ("Hold the cheese.") He confirms my suspicion that his two young children aren't allowed McDonald's. But even in his virtuous household the seeds have to be disguised - the only way to get his kids to eat them is to grind them up and hide them under their Weetabix.

And he has calming words for those caught up in the national panic about children's eating habits. Are kids really going to die young and obese? "I don't agree with that idea. Obesity on its own does not cause much extra mortality compared with smoking and dietary deficiency." It's all down to the sort of fat you carry - and this is down to socio-economic factors.

"If you carry a few extra pounds because you got fat drinking red wine and eating a Mediterranean-style diet, that's fine. But if you eat saturated fats and a low-nutrient diet, mortality is a lot higher. Socio-economic group one is [also] a lot less likely to be obese than socio-economic group five."

It doesn't take an actuary to point out that smoking is the easiest way to reduce your chances of a long life. But Richards says that in terms of insurance premiums: "Smoking is [also] an indicator of a different attitude to risk-taking. They take more chances."

Smoking at least is something you can change (and if you give up before the age of 30, you can eliminate most of the "extra mortality" risk).

But Richards' insights about how your longevity is affected by your experience in the womb and early childhood are rather more disquieting. "Exposure to infectious disease in the third trimester of pregnancy, or lack of key nutrients, sets you up for illness in adulthood," he explains. Even in childhood, "Infections - such as influenza - can set you up for heart disease 40 years down the line."

There's been a huge amount of debate about the ethical implications of older mothers having children through IVF. But it's the father's age that affects the child's longevity. "Having a very young or a very old father at conception - less than 20 or older than 55 - can knock two years off your life expectancy. In older men it is to do with damaged DNA," he says. "The effects are only apparent in later life - [these] children are susceptible to cancer and heart disease over 40 years later."

It's easy to get gloomy about things we can't change, but Richards is upbeat. "In my book, life expectancy increases could continue for the next 15 years plus. More than 15 years out, who knows what will happen?"