Followers have been giving monks foods loaded with sugar, fat and oil, contributing to a health crisis.
Every morning monk Pipit Sarakitwinon takes walks around his temple and does hundreds of arm exercises, part of a new regimen aimed at losing weight as health improvement for monks gathers pace in Thailand.
"Before I dieted I could barely walk 100m without getting tired", Phra Pipit, 63, told AFP during a medical check-up at a hospital for monks in Bangkok, adding that he used to weigh 180kg.
With cases of diabetes, hypertension and knee problems skyrocketing, health and religious officials last December published a "Monk Health Charter", instructing monks to watch what they eat.
Thais seeking to earn merit and honour their ancestors often give alms to monks on their daily rounds but some of the food is unhealthy, such as heavy curries, sugary sweets, sodas and salty snacks. Some even give cigarettes.
Thais have some of the highest rates of obesity in Asia, according to the Asian Development Bank.
Buddhist precepts forbid monks from eating after noon and Buddha himself was said to have told his followers to refrain from consuming meals later in the day.
But certain juices are allowed after midday, which are often full of sugar.
Pre-packed alms that include processed foods are also easily available at Thai stores, making the problem worse.
Although the new guidelines call on monks to look after their health so they can be "ready to deliver Buddha's teachings", refusing offerings is a delicate subject.
"According to Lord Buddha's teaching, whatever they offer, we have to accept. We can't deny, we can't reject," said Phra Rajvoramuni, the assistant abbot at Sungvej temple in Bangkok who helped author the new charter.
Phra Rajvoramuni hopes instead that health education -- including training monks to run basic medical check-ups -- will bring about change.
"The monks also should do something, like exercise... like walking meditation, cleaning the temple in the morning, sweeping the grounds," he said.
