What makes the most difference to a nation's health? Is it spending money on hospitals, medical equipment, new drugs? All are quite rightly expected of any country. But despite the huge medical and scientific advances made at the end of the 20th century, it was a simple public-health campaign – the sort of thing politicians too often see as an adjunct to health-system spending – that led to dramatic falls in late-life death, and a notable increase in UK male life expectancy.

In the 1980s, 11,000 men in their early 60s were dying each year from heart disease. By 2000 this had halved. Furthermore, male deaths in these ages from cancer had gone down by one-third. With stroke it was two-thirds. What was behind these dramatic changes? Simply, the arrival of the first generation of male non-smokers at old age. In 1971, half the UK male population was dead by their mid-70s; now nearly half are still alive in their mid-80s. It is estimated that up to two-thirds of the increase in male life expectancy can be attributed to the simple decision to quit smoking.

If smoking was the unhealthy practice of the 20th century, then over-consumption of food and alcohol has become the threat in the 21st, leading to dramatic increases in obesity in the western world. The US leads the way, with more than one-third of adults obese. But the UK is not far behind. One-third of British women over 50 are now classified as clinically obese, and just under one-third of men. Obesity rarely kills in itself, but it does lead to increases in diabetes and other chronic disease such as cancer and cardiovascular disease. Yet despite continued increases in OECD obesity rates, expected deaths from obesity-related causes appear to have gone down over the last decade. One plausible explanation is that increased use of drug therapies are already having an impact, compensating for our unhealthy lifestyles and reducing mortality rates.

This may be leading us to a future where preventable chronic disease will not be addressed by tackling lifestyles, but by drug therapies to control and reduce the symptoms. We have to ask if we wish our future to be one where, at increasingly younger ages, we pop pills rather than eat healthily, stop smoking, reduce alcohol and take up exercise.

This is not to deny the relief of suffering that modern, increasingly sophisticated, pharmaceuticals can bring. Yet in a world that will see increasing longevity and potentially disabling conditions, it is prudent to ask whether such therapies are best used to alleviate conditions that might also be tackled by individuals changing the way they live.

While public health initiatives may not always be welcomed, there is growing evidence of the real difference they can make, even in a modern, scientifically orientated society. Oxford's recent study of 2m pension records, including 500,000 deaths, revealed that healthy lifestyles contributed as much to extending lives as did high income.

There is also evidence at a national level of the gains to life expectancy from investment in public health. While Japan's proportion of older people, aged over 65, has increased from 6% to 23% over the last 50 years, its healthcare expenditure – on a system that incorporates a large amount of social care and the promotion of healthy lifestyles – has only risen from 3% to 8% of its GDP.

The US, with a much slower growth in its older population, rising from 9% to 13%, has seen its health expenditure in a highly medicalised system more than treble, from 5% to 17% of GDP over the same time period.

In most OECD countries, it is not the ageing of the population per se that is pushing up healthcare costs but the increasing costs of health provision, particularly through advances in medical techniques and drug therapy. Just as important, it is the growing demand for such therapies by a population which is learning that it can eat poorly, drink a lot and pop a pill.

A simple lifestyle change had a dramatic effect on life expectancy last century. It can happen again.

Professor Sarah Harper will deliver the Oxford London lecture on Tuesday 13 March at 18:45 in Church House, Westminster. The twitter hashtag for the lecture is #oll2012. The lecture webpage is www.ox.ac.uk/oxfordlondonlecture