A new study by US researchers has found that dieting alone is just as effective as dieting plus exercise when it comes to weight loss.
However, Irish experts in nutrition and exercise are warning that a combination of a balanced diet and an active lifestyle is the healthiest and most sustainable way of shedding the pounds.
Researchers at the Pennington Biomedical Research Center in Baton Rouge, Louisiana, found that people who chose to diet without increasing their level of physical activity lost a similar amount of weight to people who reduced their calorie intake and exercised more.
They monitored 35 overweight but otherwise healthy people over a period of six months.
Twelve of these people were assigned to a diet-only group, and reduced their calorie intake by 25 per cent; 12 others reduced their calorie intake by 12.5 per cent and increased their exercise by 12.5 per cent; while the remaining made no significant changes to their lifestyle.
The result was that the diet-only group and the diet and exercise group lost roughly the same amount of weight - about 10 per cent of their body weight, 24 per cent of their fat mass and 27 per cent of their abdominal visceral fat - which is the deep internal fat thought to increase the risk of heart disease.
"Weight loss will occur where energy expenditure is greater than energy intake," according to Margaret Donnelly, consultant nutritionist with Calista Nutrition and senior dietitian at the Mater Private Hospital in Dublin.
"The study points out that if you are interested only in shedding weight then dieting alone works, but as a health professional we would look at the overall picture of risk reduction and healthy weight reduction and, in this case, you need to include exercise," Ms Donnelly said.
The study, which was printed in the Journal of Clinical Endocrinology and Metabolism 2007, also found that exercise did little to tone specific areas of the body.
Fat was reduced consistently across the whole body and not more in any one trouble spot.
"This is true in the case of cardiovascular or aerobic exercise," said Ms Donnelly. "But you can do specific exercises to improve muscle tone in certain areas.
"However, it's important to know that you can't tone fat.
"As the study states, genetics has a certain role to play in determining your body shape, but age and hormones also contribute," she said.
Dr Philip Carolan, a sports exercise physician and honorary secretary of the Department of Sport Science at the Royal College of Surgeons of Ireland (RCSI), agrees that weight loss can occur through dieting alone, but maintains that an active lifestyle is also important.
"Certainly the study is half right. Based on my own experience with an overweight wheelchair-bound patient who couldn't physically exercise, it is possible to lose weight solely by altering your calorie intake, and diet can certainly help with weight loss," Dr Carolan said.
"However, the benefits of exercise, from a cardiovascular point of view, are significant.
"Exercise can improve your blood pressure, as well as your heart rate and can help tackle diabetes by using up the glucose in your body," he said.
"There is a gold standard of what we should be eating . . . and many people will find they're at the upper end of the recommended daily allowance already and can start by reducing that intake.
"You can lose weight by doing this alone - but it would be more beneficial to do a bit of dieting along with some exercise.
"Improving your overall health far outweighs the benefits of just losing weight," Dr Carolan said.