A diet which reduces or even excludes meat and animal produce in favour of vegetables, fruit and grains could halve people’s chances of becoming obese, according to new research.

A study carried out in Spain describes the benefits of what researchers call a “pro-vegetarian” diet which does not exclude meat and dairy products but reduces them. It has also been called a “flexitarian” diet – basically vegetarian, with meat and fish consumed occasionally.

Some 16,000 university graduates were tracked from 1999 for 10 years, by which time 584 were obese, according to findings presented at the European Congress on Obesity in Porto, Portugal.

At the beginning of the study, participants completed detailed food questionnaires which were scored to establish how pro-vegetarian their diet was. They got more points for eating from seven plant food groups – vegetables, fruits, grains, nuts, olive oil, legumes (such as peas, beans, and lentils) and potatoes.

Points were then deducted for foods from five animal groups – animal fats, dairy, eggs, fish and other seafood, and meat.

The researchers compared the 20% whose diet included the most animal products with the 20% who ate the least. They found that those whose diet was the most vegetarian were 43% less likely to become obese.

Although fish was included in the animal foods group, the results showed that there was very little difference in the amount of fish that any of the participants ate, so had little impact on obesity rates.

But there was a significant difference in the amount of meat eaten – those who ate most consumed about 200g a day (roughly the weight of a small chicken breast or a seven ounce steak), while those who ate least consumed 142g. There was an even larger difference in the consumption of vegetables – 348g in the meat-eating group and 731g for those whose diet was the most vegetarian. The same applied to fruit consumption – 191g versus 531g.

“Our recommendation is to eat less meat,” said Prof Maira Bes-Rastrollo, one of the authors. “Don’t increase the consumption of animal foods. Prefer plant-based foods to animal foods.”

The study’s weakness is that it is observational – it did not recruit people eating a mainly vegetarian diet in order to compare them with a group of meat eaters, nor did it attempt to change the behaviour of participants.

The pro-vegetarian diet in the study is very similar to the Mediterranean diet. Gaynor Bussell, a dietician and member of the British Dietetic Association said: “We have known for a while that a healthy plant-based diet is associated with less obesity and this new evidence confirms this.

“Other factors could be accounting for the lower obesity in this group; I would also add that although scored negatively, foods such as fish, some meat and dairy are not associated with obesity but it is about the overall balance of the diet. The Mediterranean diet with its reliance on fruit, veg, nuts, beans and little meat is probably an ideal mix and is also associated with lower obesity rates.”

Sarah Toule from the World Cancer Research Fund said: “A mainly plant-based diet not only helps reduce obesity risk, but our own evidence shows it helps reduce your cancer risk too.

“Eating more portions of vegetables and fruit, cooking from scratch and including a wide variety of colours on your plate are all good ways to improve your diet.”