The advice on how to keep fit has changed so often that it has left us exercising our minds more than our bodies.

First, we were told that vigorous activity such as jogging and competitive sports was necessary to burn off the calories. Then, about a decade ago, the guidelines changed and moderate exercise was deemed sufficient.

Not surprisingly, the latter proved more acceptable to the British public, who were quite prepared to include everyday activities such as walking, housework, or gardening in their workout programmes.

But is it enough to keep us fitter and alive longer? Not according to the authors of a study published today in the journal Preventive Medicine.

They found that 56% of men and 71% of women now believe moderate activities are the most beneficial, even though many medical studies show that the greatest health benefits are derived from regular participation in vigorous activities.

An expert from Glasgow University said yesterday that the results showed there was a need for clearer advice on how much people should exercise.

One of the study's authors, Exeter University physiologist Dr Gary O'Donovan, said: "Time and time again the largest and most robust studies have shown that vigorously active individuals live longer and enjoy a better quality of life than moderately active individuals and couch potatoes.

"It is extremely worrying that British adults now believe that a brief stroll and a bit of gardening is enough to make them fit and healthy. The challenge now is to amend Britain's physical activity guidelines so that they emphasise the role vigorous activity plays in fighting obesity, type two diabetes and heart disease."

Traditionally, adults were encouraged to take part in 20 to 60 minutes of vigorous exercise three or more times a week. In 1990, research showed that around 90% of British adults believed vigorous exercise was important in maintaining and improving health and fitness.

But since 1995, health officials have instead promoted 30 minutes of moderate exercise five times a week. The Exeter research team believes this shift in attitudes is threatening the nation's health and is calling for evidence-based guidelines.

Dr O'Donovan said: "Brisk walking offers some health benefits, but jogging, running, and other vigorous activities offer maximal protection from disease. Sedentary adults should complete a six to 12-week programme of moderate exercise before beginning a programme of vigorous exercise. Men older than 45 and women older than 55 should consult their GP before taking up vigorous exercise."

The research team believes that 30 minutes of brisk walking per day might be sufficient to reduce the risk of breast cancer, but regular participation in vigorous exercise is probably necessary to reduce the risk of prostate and colorectal cancers.

However, Dr Jason Gill, a senior lecturer in the Institute of Diet, Exercise and Lifestyle at Glasgow University and chairman of the British Association of Sport and Exercise Science division of physical activity for health, said: "The reason for the shift is that in the past when the emphasis was on vigorous activity people weren't doing it because they didn't think they could achieve the goal.

"If we want to improve public health by getting people more active, we have to have this balance with something that people are prepared to do."

He said he believed the government should now be more clear in its guidelines, emphasising that moderate exercise was only an absolute minimum and benefits would always be greater with vigorous activity.

"Clearly moderate exercise is good, but we mustn't think that is the pinnacle; you will get an additional benefit over and above what you would get from moderate activity with vigorous activity.

"Volumes of activity are also minimums. That has always been implicit (in recent government advice) but what happened was the headline moderate activity'."

How to burn the calories Examples of moderate activities in healthy adults include: brisk walking, cycling at less than 10mph, and mowing the lawn with a power mower. Examples of vigorous activities to promote health include: doubles tennis, competitive badminton, circuit training, and jogging and running.