Tun Dr Mahathir Mohamad turns 92 in July, but the sprightly grand old man of Malaysian politics shows little sign of ageing, let alone of slowing down.

He is more active than ever, criss- crossing the country to campaign and speak against the government he once led, and even attending demonstrations and candlelight vigils.

Despite stepping down as prime minister in 2003, he says: "I've never actually retired."

This constant activity helps him stay mentally alert, and maintain his weight at between 62kg and 64kg for the last three decades.

His secret? Not overeating.

He tells The Sunday Times about a recent study which found feeding monkeys a low-calorie diet led to a longer lifespan.

"Maybe, I am like a monkey," he says. "I am not completely healthy. I had heart problems, I had some pneumonia at one time and I have periods when I get bad coughs when the lungs get infected."

The former premier points out: "I don't smoke, I don't drink, I don't overeat. I eat just enough food to keep me going.

"There is a tendency for people over a certain age to become obese. They have a big stomach and to satisfy themselves, they drink and they overeat, which places a burden on their heart.

"I have been about 62-64 (kg) for years. I can wear my clothing made 30 years ago."

Although he left active politics some 13 years ago, he still goes to the office every day.

"I happen to be healthy, I can do this work.

"For me, to say I want to go to sleep and retire and prepare for my afterlife, I think that is very selfish," Dr Mahathir added.