Rio de Janeiro is the city that worships health and beauty and where the healthy and the beautiful drink açaí. Pronounced ah-sah-yee, açaí is more of a lifestyle option than a foodstuff; a magic fruit potion that fuels the hedonistic energy of Brazilian beach life.

Shortly after I moved to Rio, I was told about the açaís berry's amazing nutritional properties: Brazilians believe it gives you strength, energy and is great for sex. A friend told me that when he was having difficulty in fathering a child, the first thing his doctor recommended was 'drink lots of açaí'. And it worked!'

The way it looks is integral to its appeal. It is made from dark violet berries about the size of a raspberry; a deep, dense colour that seems weighted down by its nutritional secrets. It reflects no light and has the texture of mud. I wasn't immediately sure about the taste, which was very sweet and medicinal. But by the end of the cup I was hooked. It is fruity with a chocolatey kick.

The nutritional breakdown of açaí is prodigious. It has high levels of iron, calcium, carbohydrates, fibre and antioxidants. And energy. A small 100g cup has almost 300 calories. Combined with the mystique of its Amazonian origins, açaí's contents have made it the beverage of choice for Rio's sporty elite.

Açaí is indigenous to the flood plains of the Amazon estuary. The açaí palm regenerates with ease and in areas where human development has destroyed natural vegetation the first tree that grows in its place is açaí. (Açaí palms cover an area equivalent to half the size of Switzerland.) In this region, its abundance and role as primary nutritional resource cannot be over-estimated: it is literally the fruit that has saved many poor families from starvation.

'Açaí is the main food staple of river communities in the Amazon estuary,' says the agronomist Oscar Nogueira. It is drunk for every meal - in much the same way as bread or rice is eaten in other cultures.

Having become an açaí fan in Rio I was keen to visit Belém, the main city in the Amazon estuary and world centre of açaí. If ever a city was so strongly defined by a single fruit, it's Belém. There is a local saying: 'Who arrives here and stops, drinks açaí and stays.' In Belém more of the fruit is drunk than milk. An estimated 200,000 litres of the purple liquid is consumed per day among a population of 1.3 million.

Açaí is highly perishable and the only way it gets to Rio is in frozen packages. In Belém, the fruit is always consumed fresh. Since it goes off within 24 hours, in order to service the population with fresh açaí on a daily basis an enormous infrastructure has grown in Belém that employs an estimated 30,000 people.

The cycle starts in the rainforest. The açaí palm has a long thin trunk up to 25m high and a clutch of branches at the top from which hang ribbon-like leaves. Hundreds of açaí fruits dangle from branches in clusters that look like nests of bluebottles.

The fruit picking is done by hand. In the afternoons, river-dwellers scramble up the trees, cut off the branches and climb back down again exactly as they have done for hundreds of years. In the evening, boats containing baskets of açaí leave the rainforest heading for Belém's market, where they arrive in the middle of the night.

The açaí market is a dockside next to the city market. By the early hours small boats have started arriving with baskets of the fruit which quickly fill the quay. By 3am men like Armando Ribeiro arrive.

Armando owns the Casa do Açaí, one of Belém's 3,000 açaí points, where the fruit is pulped,into juice. Armando buys several baskets of the best açai and takes it back to his premises, a small patio in a backstreet. When I arrive, shortly after 11am, Armando has been pulping the fruit for an hour. Customer demand for açaí is at lunchtime, and they prepare it fresh. He pours the fruit into the pulping machine and keeps on re-pouring the discharge until the blend is perfect. He sells three versions; thick (£1), medium (60p) and dilute (40p).

In Belém, you are never more than a block away from an açaí point. Wherever you look, your eye always finds a red açaí sign. I find a bar and order a bowl. It is served like soup. The taste is almost unrecognisable from what I have become used to in Rio. The exotic sharpness and zesty kick is not there. The sensation is of a simple, neutered, bitter freshness. Açái is not a versatile fruit since it can only be stored frozen and cannot be cooked, so for the most part, it continues to be drunk just as the indians have drunk it for centuries.

For açaí to catch on outside the Amazon, it needed a pioneer. That man was Carlos Gracie, the great-grandson of Scottish immigrants from Dumfries, who was born in Belém in 1902. In his early teens, a chance meeting with a Japanese immigrant led to his obsession with the martial art jiujitsu. In 1922 the Gracies moved to Rio and Carlos opened Brazil's first jiujitsu academy.

When a shop near his Copacabana home specialising in obscure foods started to import frozen açaí, he began to incorporate it into his diet and also to encourage all his jujitsu students to drink it. The jujitsu boys were pin-ups with the best bodies: everyone wanted to know what 'miracle' potion they were drinking. Soon Rio's surfers became fans, and gradually the drink crossed over to become part of beach culture. By the early 1990s, no juice bar could exist without selling it.

The boom in açaí over the last decade has had more effects than changing the eating habits of Rio's body-obsessed men (and women). Scientists have discovered that açaí is rich in anthocyanins, the group of chemicals in red wine that are believed to lower the risk of heart disease. Swig per swig, açaí contains over 10 times more of them than red wine. It is also rich in essential fatty acids, calcium and vitamins. Açaí's recent success is also changing the nature of agriculture in the Amazon estuary. Agronomists have been successful in developing ways of cultivating açaí sustainably with high yield. In the last five years açaí production has tripled and brought work to poor rural areas. Belém, now has more than 60 factories that export. 'Açaí is the most promising product we have here for development,' says de Jesus.

Açaí was an Amazonian secret that conquered Brazil. Whenever friends visit Rio they fall in love with the taste. I have lost count of the number of excited conversations about how we could export it around the world. I discovered recently that I've been beaten to it. A company in California now imports it to the US and next month Selfridges will introduce it to British palates. It may not be the same as sipping it fresh in Rio, but make no mistake, one day açaí will conquer the globe.