Although the precise definition of vegetarianism is open to debate, the number of people choosing to exclude meat from their diets seems to have followed a steady upward curve over the past decade.
A 2002 Datamonitor report estimated that there are around 12 million vegetarians in Europe, and a Time/CNN poll of 10,000 American adults the same year found 4 percent of the population (more than 11 million people) to be self proclaimed vegetarians.
The scientific review, published in the April issue of Nutrition Reviews (Vol. 64), compiled data from 87 observational and clinical studies, and showed that the weight-loss did not appear to depend on exercise or calorie count, and could occur at a rate of 1 pound per week.
Vegans and vegetarians are reported to consume diets that are higher in carbohydrate and dietary fibre, lower in energy, protein, total fat, cholesterol and saturated fat, compared to people on eating a non-vegetarian diet.
"Typically, vegetarian diets derive more than 50% of energy from carbohydrate from fruits, vegetables, legumes, nuts and whole-grain breads and cereals," explained reviewers Susan Berkow and Neal Barnard from the US Physician's Committee for Responsible Medicine.
"There is evidence that a [vegetarian] diet causes an increased calorie burn after meals, meaning plant-based foods are being used more efficiently as fuel for the body, as opposed to being stored as fat," said Barnard.
The reviewers also reported that insulin sensitivity increased for vegetarians, easing the absorption of nutrients into cells.
Both fat and protein intakes were also lower for people consuming a vegetarian diet, compared to non-vegetarians, and even though vegetarians were not consuming the 'high-quality' protein found in animal meat, all essential and non-essential amino acids are available from plant sources.
"If such diets cause weight loss when adopted by overweight individuals, they may be of substantial clinical value because they also are associated with other health benefits, including improved control of blood lipids, blood pressure and diabetes," said Berkow and Barnard.
The rising rate of obesity is seen as one of the major health challenges of the 21st century. Europe is said to be rapidly catching up the USA where the metabolic syndrome associated with obesity has risen dramatically in the past 10 years and now affects up to 32 per cent of adults - an estimated 50 million people, according to the International Obesity Task Force.
Of particular concern is the rising number of overweight and obese children, said to be growing by 1.3 m children a year. By 2010 it is estimated 26 m children will be overweight, including 6.4m obese, in EU countries.