Fat used to be a feminist issue. Today it has become an insurance risk. According to the reinsurer Swiss Re, life insurance policyholders will see their premiums expand in line with their waistlines in the next few years, as the explosion in obesity begins to offset some of the improvements in life expectancy won by breakthroughs in medical science.

Some insurers are already taking applicants' weight into account when they decide on the price of life policies, according to Swiss Re. The company is urging the insurers to check their sums regularly in order to avoid being caught off guard - and out of pocket - by the fat epidemic, which the government's chief medical officer recently described as a "health time bomb".

"Looking ahead, the life insurance industry must tackle issues associated with increases in obesity by ensuring that the related risks are accurately assessed and rated, and that consumers are charged an appropriate premium to reflect the risk they present," the report says.

Danger zone

For the number-crunchers who have the job of estimating the risk that someone will die before their policy expires, the maths is clear. Studies in the US have shown that a 10% increase in weight is correlated with a 30% rise in the incidence of heart disease. For a 40-year-old, non-smoking woman, being obese knocks eight years off life expectancy. For a 40-year-old man, the deficit is six years.

The problem is increasing, especially among the next generation of potential policyholders. The Department of Health says the rate of obesity among Britain's children has increased by a quarter since 1995 to 17%. In the US, obesity is soon expected to overtake smoking as the leading cause of preventable death.

Insurers are expected to adopt the tried and tested measure used by doctors - body mass index - to signal when potential policyholders have slipped into the obesity danger zone. They divide an applicant's weight in kilograms by the square of their height in metres. A figure of 25 qualifies as overweight, 30 as obese and 40 as morbidly obese.

"Unless the prevalence of obesity is brought under control, consumers will bear the ultimate cost," said Ronald Klein, global head of pricing in Swiss Re's health division. "For those among the population in the developed world who require life insurance cover, as BMI goes up the scale so too will the cost of their premiums." Swiss Re also suggests that insurers stay abreast of new evidence - including whether waist circumference can also be a good indicator of life expectancy.

News that actuaries are eyeing up waistlines could be an extra spur to the government, which has already been shocked into action by the cost of obesity to the NHS - estimated at £500m for 1998, the latest year for which figures are available.

It plans to make tackling obesity a central theme of its forthcoming health white paper, and is working on a cross-departmental food and health action plan. A range of proposals has already been floated, from subsidising gym membership to a "fat tax" that would make unhealthy foods more expensive. With obesity typically concentrated among lower-income groups, there is a socioeconomic dimension to the problem which makes it especially hard to tackle.

While the government wrestles to find the right policy approach, the private sector is responding with entrepreneurial zeal to mounting evidence of the health impact of obesity. McDonald's has begun to sell salads and pieces of fruit alongside its burgers and fries, to cash in on the public's desire to fight the fat (and deflect charges that it should share some of the blame for childhood obesity). Dr Atkins's estate has continued to bank the proceeds from his best selling Diet Revolution books even after leaked medical records revealed that the man himself was obese when he died.

Takeover battle

Pharmaceuticals companies are pouring millions into the search for the holy grail, a fat-busting drug. Phytopharm, which specialises in plant-based medicines, is developing a food supplement based on an African cactus-like plant which it believes will act as an appetite suppressant. French firm Sanofi recently published clinical trial results suggesting its potential blockbuster, rimonabant, helped obese patients to lose weight - and smokers to quit the habit. Indeed, the bitter takeover battle it is fighting with its rival, Aventis, is partly motivated by the need to acquire a sales force to push rimonabant in the US - the really big market for any drug, but especially an obesity-buster.

Insurers are not the first firms to respond to the obesity epidemic, but their business model should force them to be some of the most far sighted - and at the moment they think the world is heading for a future in which higher levels of obesity outweighs the gains of modern medicine and the lower prevalence of smoking.

Yesterday's report called for a concerted effort from the government, consumers and food manufacturers, as forceful as the successful public health battle waged against smoking over the last two decades, to turn the tide. Indeed, Swiss Re suggested higher premiums could help, by giving policyholders an incentive to slim down. "When obesity starts to hit them in the pocket, consumers may start to take more notice," the reinsurer said.

In the short term, insurers' response will be simply to transfer the burden on to the highest-risk individuals - who, because they are also often the poorest, may simply decide not to take out life insurance at all.