The daily afternoon fasting of Thai Buddhist monks is making them fat, thanks largely to modern sugary drinks, says a leading nutritionist.

The most recent figures show 48 per cent of monks are obese and more than 10 per cent are diabetic.

"Many of them are obese or have problems with their knees or legs," said Phra Sajjayanoe, a 34-year-old, taking a break from his bicycle repair business to become a monk for a few weeks.

"Some monks have diabetes and had their legs amputated, so they can't walk."

Prah Sajjayanoe walks the streets with his alms bowl in which he accepts food from members of the public. ( ABC News: Liam Cochrane )

When researchers studied monks' dietary habits, they were initially puzzled.

They found the total calorie intake of monks (1,350) was about the same as that of the general population of Thai males in Bangkok (1,500).

"When we really do research about this we are surprised … it is the drink," said nutritionist Jongjit Angkatavanich, from Chulalongkorn University's Department of Nutrition and Dietetics.

Monks are forbidden from eating after midday, but many sip sweetened drinks to keep up their energy.

"In Buddhism we call it panna [the] pali terminology for the drink that is allowed for monks to consume after midday," Ms Jongjit said.

"But right now in the present day … the kind of drink that is offered to monks has really changed … it is like soda, it is like sweetened beverage."

She compared the obesity problems of Thai monks to those of American teenagers brought up on fast food and soft drink.

Jongjit Angkatavanich shows the tape measure designed for monks to check their waistlines. ( ABC News: Liam Cochrane )

Brown rice and special belts

Another issue, Ms Jongit said, was the quality of food donated to monks.

Traditionally, monks and novices leave their temples close to dawn and walk the streets accepting food in special alms bowls.

The community's support of their local temple thrives, even in the capital.

"I usually give spicy dip and vegetables, but not curry or any oily food," says Somwong Palakawong, ladling spoonfuls of steamed cauliflower and broccoli into small plastic bags and dropping them in the monks' bowls.

"I won't give sweet deserts because it will make them fat," Ms Somwong said.

A small group of monks gather around her and chant a short blessing.

It's an age-old scene repeated all over the country, feeding more than 120,000 monks across Thailand.

Monks and novices sort out food donations, keeping food to eat for their late morning lunch and drinks for the afternoon fasting. ( ABC News: Liam Cochrane )

But not all food donors are as conscientious as Ms Somwong.

"Some food has got too much MSG which causes disease and obesity," monk Phra Sajjayanoe said.

"Sometimes the rice and curry has already turned bad … or there is mould on the dessert."

One of the explanations for the rotten food is that excess donations are sometimes sold back to the local shops, who may re-sell it the next day, and the next.

"Those who sell the food for this specific purpose, they should realise that they are the main contributor to the monk's health," Ms Jongjit said.

Her team has published easy-to-prepare healthy recipes for Buddhist devotees to make at home and give out on the street.

She suggests meals of brown rice, vegetables and protein.

Prah Samusupan is a 90-year-old monk and says moderation is the key to healthy eating. ( ABC News: Liam Cochrane )

Monks told researchers they often didn't realise they were putting on weight because of their loose-fitting robes.

So the traditional monks' belt was adapted with knots to show monks where their waistline should be — set at 85 centimetres (size 33).

Some monks have managed to avoid the modern trappings of monastic life, remaining lean and living well beyond the average Thai lifespan.

"Monks should eat the right amount of food, not too much," said 90-year-old Phra Samusupan, who avoids spicy food.