We all love a bit of heroin, don’t we? Sorry, I meant Thai food! We all love a bit of Thai food. But Thailand used to be more famous for heroin than for a delicious green curry. Years ago, the lush and mountainous north was the “gateway into the Golden Triangle, one of the most productive opium growing areas in the world,” said Dr Polly Russell on Thailand: A Royal Food Legacy (Radio 4, 26 February, 12.30pm).

Thai opium farmers yielded 200 tonnes of sticky opium in 1970, some 8% of the global output. It was a popular profession for the farmers, even though they got paid 1,000 times less than the price on the street once their raw opium had been refined into heroin.

Russell played a clip from Radio 4’s Woman’s Hour, which visited Thailand in 1979. “It’s difficult to believe that out of such breathtaking countryside comes one of the world’s grimmest commodities,” said presenter Anne Catchpole.

Fortunately, Dr Russell wasn’t visiting to score. The same opium fields have been transformed into colourful farmland that produces 20,000 tonnes of food a year, reducing reliance on imports and giving profits to the farmers instead of drug-dealing middlemen.

This “simple but ingenious” idea was born 50 years ago by the late King of Thailand, Bhumibol Adulyadej. Today, 300,000 villagers profit from growing food instead of opium, including British classics like beetroot. “This village was very poor,” said Russell. “Now all the children are literate whereas 40% were before. It’s amazing that’s been achieved through carrots and lettuce.”

This was a typically sensitive episode from the Food Programme, and another fine example of its ambition to report on the “power of food to effect transformational change on communities, economies and lives”.