Do you eat meat?' asks João Meirelles Filho, a Brazilian conservationist living in Belém, the last big city on our journey north from São Paulo to the Amazon. As we tuck into filhote ('cub'), the young of a giant Amazonian catfish called the pirahib, it seems no more than a casual question about diet.

'Yes,' I reply, through a mouthful of bones, but Filho surprises me with the venom of his next remark. 'Then you are responsible for the destruction of the Amazon,' he says, 'because 95 per cent of deforestation is caused by cattle ranching. I would love it if every one of your readers boycotted Brazilian beef.'

The tearing down of trees to make way for intensive cattle production has helped destroy an area of rainforest nearly three times the size of Great Britain, Filho says, and his organisation - the Instituto Peabiru - has been monitoring the social and environmental impact. In the next decade, another Great Britain could be lost, along with the animals, birds and plants it supports. Though the message has failed to penetrate Europe, this carving out of pasture is far more pernicious than logging (accounting for three per cent of rainforest loss) or large-scale agriculture, including intensive soya production.

'There are five big companies producing soya in the Amazon, but 420,000 cattle ranches,' says Filho. 'Why is Greenpeace diverting attention to soya?'

As the coffees arrive at our outdoor table on the Estacão das Docas (the old riverfront docks at Belém, converted into a glitzy shopping mall with restaurants), a stench of manure and cattle feed seeps along the quayside from a moored freighter. 'That ship is sailing to the Lebanon,' says Filho, 'and 600,000 head of cattle will leave Belém for the Lebanon and Venezuela this year. There are currently 72 million head of cattle in the Amazon region.'

Since Brazil is such a massive country, I ask why pasture can't be found elsewhere without destroying virgin forest. 'Land is cheap because nobody wants it,' Filho says, 'so it's more cost-effective to rear cattle here. It's crazy.'

Sketching a map on a paper napkin, he divides the rainforest in three and gives me his prognosis. 'Twenty per cent of the Amazon is lost forever, it's deforested, and a further 20 per cent can't be saved,' he says, 'so 40 per cent has gone. Thirty per cent is made up of Indian reserves and national parks, so that is probably safe. The remaining 30 per cent is unknown, but under threat.'

In his view, its future lies in the hands of the two million local people known as ribeirinhos ('river dwellers') whose diverse features, bone structures and skin tones reflect the complex mix of Amazonian Indian, black slave, Portuguese colonist and European migrant that is symptomatic of a fecund colonial past.

'Some have blond hair and blue eyes,' a local guide tells me, 'others look like Africans.' These caboclos, as the genetic mix is called, make a modest living from small-scale and subsistence farming on the banks of the Amazon, using traditional methods passed down through 400 years of riverbank life. 'If we can devise an economy based around them,' says Filho, 'we may be able to save the Amazon. They are the guardians of the rainforest. A third of Amazonia is being guarded by two million people, living in 30,000 communities of between 50 and 300 inhabitants. They need our support.'

At Fruit Towers, the playfully named headquarters of Innocent Drinks in west London, the destruction of the Amazon and the role of the ribeirinhos has not gone unnoticed. One of the company's best-selling fruit smoothies contains a small amount of pulped açai (ass-eye-ee), a berry that grows only within 25 yards of the Amazon's banks. To harvest it, the palm on which it grows does not have to be cut down; better still, it thrives in the shade of other rainforest trees such as rubber, Brazil nut, cabbage palm and miriti palm, encouraging growers to mimic nature rather than plant açai in endless regimented rows - the kind of monoculture that destroys biodiversity.

'We buy 300 tonnes of frozen pulp a year from our supplier, Sambazon,' says Richard Reed, co-founder of Innocent, 'and we're paying them £1,300 a tonne. That puts about £300,000 a year into those riverbank communities.'

In the Amazon, where 90 per cent of ribeirinho families survive on less than US$4 (£1.90) a day, such shared income goes a long way. What's more, Sambazon guarantees to buy all the açai its suppliers produce, eliminating risk to the grower. It calculates the average daily market price, then adds a five per cent premium - which is why Sambazon's pulp (but not Innocent's smoothie) is certified by the US Fair Trade Federation. Sambazon pays cash up-front instead of making farmers wait, and buys direct from the grower to cut out middlemen. Organic, processed in a pristine new factory and fully traceable to the person who grew it, the açai used by Sambazon and Innocent typically fetches 44 reais (£12) a sack compared to 15 reais five years ago.

Irrigated twice a day by the tidal waters of the Amazon, the açai tree requires little maintenance. Prune it occasionally and keep the grove free of weeds and disease, and it will keep producing fruit - eight to 12 baskets in two hours of picking, from a plot no bigger than a large suburban garden. If managed properly, an acre of rainforest will yield 14 tonnes of berries a year.

It is a high-income, low-impact crop for sure, but that is not the only reason it appears in Innocent's portfolio. Açai is also the ultimate superfruit, its reddish skin containing anthocyanins (plant chemicals that neutralise the 'free radicals' associated with disease and ageing) and other antioxidants. Weight for weight, açai contains 60 per cent more antioxidants than the acclaimed pomegranate, 2.7 times more than blueberries and over six times more than strawberries. Beneath its skin is a yellowish fat, making it rich in calories.

So nutritionally perfect is the açai package, Innocent got carried away last October with a claim for its açai, pomegranate and blueberry smoothie that it could not justify scientifically. Full-page advertisements in magazines and newspapers described it as a 'natural detox superfoods smoothie' - playing into the hands of the Advertising Standards Authority (ASA), which said Innocent had not proved that the drink eliminated toxins from the body. The adverts also claimed the smoothie contained more antioxidants than the five portions of fruit and vegetables a day recommended by the government. Nutritionists say a smoothie is equivalent to one portion of fresh fruit, not five.

'If we've done something wrong, we should be the first to stand up and apologise,' says Reed, acknowledging that his company markets itself almost entirely on its ethics. 'We honestly thought a detox was just something that is good for the body - so for some people it's drinking nothing but water, and for others it's eating nothing but fruit. The ASA thought otherwise, so fair enough.' Innocent promised not to make that claim in the future.

As penance, perhaps, Reed is about to join us on a boat trip up the Amazon to see the açai business first-hand. It is João Meirelles Filho's hope that açai can form part of a low-impact, traditional economy (the other elements being ecotourism, sustainable forestry, heart-of-palm production and honey) which will make life so economically stable that the ribeirinhos will stay put, rather than abandon their land to less sympathetic use. In the hands of intensive açai growers, charcoal burners or even small-scale subsistence farmers cultivating banana, manioc, maize and rice, deforestation could run amok.

'This cornucopia of low-impact produce can save the rainforest,' João Meirelles Filho insists. 'Young people will otherwise move to the cities and be exposed to the violence and crime of the favelas [slums]. Açai could encourage them to keep their traditional way of life, and even lure young people back. If just one of them returned, it would be worth an awful lot; it would make a big difference.'

From the air, the Amazon is every bit as awesome as the figures suggest. As our plane skips the last 200 miles from Belém to Macapá, a no-hope town at the mouth of the delta, the scarred agricultural landscape of Pará state (where most deforestation has occurred) morphs suddenly into a vast, coffee-coloured expanse of water seven miles wider than the English Channel.

Our Airbus 320 banks right over Macapá, and you can see the brackish discharge of the delta stretching way out to sea. A staggering 4,000 miles long and 28 miles wide at its broadest point, the Amazon holds one-fifth of the world's fresh water and has a flow greater than that of all the other top 10 largest rivers combined. So powerful is its output, sailors would have to travel 30 nautical miles out into the Atlantic to get their first taste of salt water.

This, effectively, is our high road - the route that will take us into the heart of the archipelago of Marajó, a cluster of islands in the Amazon where the ribeirinho communities are most concentrated. For the river dwellers, too, the 'big snake' (as they call it) is the only means of connection with the outside world. So remote and steamy is Macapá (the equator passes right through it), few people want to live or work there and nobody has quite got round to building a highway south to connect it with the rest of Brazil; besides, the swampy, inhospitable Ilha de Marajó stands in the way. The only roads run north to Suriname, Guyana and French Guiana, meaning everything and everyone leaves Macapá by ship - or, for those who can afford it, by plane.

On the map, the archipelago of Marajó looks a stone's throw from Macapá but our 50ft vessel will sail all night to get there. We pack hammocks, insect repellent and basic provisions, then step aboard the Brisa do Mar, a classic Amazon riverboat (think Heart of Darkness) with a thumping engine, quarters below deck for the crew and a roofed-over area for passengers where we will sling our hammocks. As the vessel leaves the jetty at Santana, a short drive out of Macapá, caboclo families gather for their evening wash and swim, diving off the half-sunken wrecks of riverboats into the glassy green water.

We cruise upriver for an hour or two, the lights of Macapá disappearing as we hug the northern shore. Then, using a searchlight mounted on the bridge, the skipper begins to navigate the ink-black backwaters of the archipelago, passing the occasional stilted village or the hulk of an abandoned freighter. The night sky is phosphorescent with stars and the riverbank comes alive with jungle noises. We fall into our hammocks and sleep lightly, anticipating dawn.

At 5am, we are woken by the crowing of cockerels in a ribeirinho community tucked away up the Picanço river. Moored to a jetty, our boat rocks gently on the outgoing tide and we groggily observe a stilted shack and outhouses, all linked by wooden walkways raised on stilts to escape the twice-daily flooding of the alluvial plain. Immediately I can see why açai thrives here, lapped by the nourishing Amazon, its roots buried deep in mineral-rich mud. On the other side of the river is a neat village school, plus another cluster of shacks. Even at this hour, caboclo boys are paddling to and fro in their dugout canoes.

'Welcome to our community,' says Rivadavia Cesar Braga, 63, açai grower, head of the family and self-appointed guardian of the rainforest. On the verandah of his house, shaded from the blinding equatorial sun (it is 84 degrees, with 95 per cent humidity), he tells me about a long, hard life made sweeter by açai. These days, he lives with his wife, Lurdiel DeSouza Maia, who is 30 years his junior. Together they have five of their own children and one adopted daughter. The youngest boy, Pedro Paulo Maia Braga, is one-and-a-half, an achievement Rivadavia attributes to the açai in his diet. By a previous marriage, he has 10 children - some of them older than his wife.

Born to rubber-industry workers in 1945, Rivadavia did not wish to follow in their footsteps. 'It was an incredibly hard life for them, collecting latex from the trees at night wearing head torches,' he explains, 'and they would have to work all night and every night just to get enough food to eat. My parents could only sell their rubber every 15 days - with açai, we sell it every day - so we would go for a long time without a meal. We lived off fruits from the forest.'

On the river, the quality of life was poor. 'There were no doctors, no schools, no radios, no TVs,' Rivadavia recalls, 'and even in the 1960s there was only one guy with a radio. People would row in a little boat to see him, to listen to any broadcast messages - "Please tell Joe on the river that his mother is sick"; "Go to the city because your uncle has died". Everyone gathered round the radio for messages, then they would row another hour to give the news.'

To make ends meet, Rivadavia took a job in his teens - as a hired hand on freighters, carrying mostly timber, which plied the waters linking Macapá with Belém and points in between. 'It was hard work, loading and unloading the boat, but I didn't get paid a single real,' Rivadavia reveals. 'I'd eat on the boat, shower and sleep on the boat, take clothing from it - but money I never saw.'

By now, he was married and had mouths to feed, so his family lived off the land as subsistence farmers while he went away to sea. Throughout the 1970s and early 1980s, that was his way of life. The pressures were too much, so Rivadavia divorced his first wife and met Lurdiel. It was she who suggested they grow açai, because she had heard there were good returns.

In 1984 they began preparing a grove here on the Picanço river, just behind the house. To begin with, they earned very little, selling açai at the local market where it would be bought by one of the thousands of vendors who churned the crushed berries by hand to make a nutritious pulp popular with locals; they in turn would sell it from the 5,000 or so 'açai points' on highways and street corners in Belém and Macapá, marked by a red flag outside. Even then, the açai industry in Belém alone probably employed 30,000 people.

By the late 1980s, pulping machines were commonplace and the demand for açai grew; at the same time, Amapá (the area we are in) was declared a state, bringing the remote region political kudos and an influx of visitors. Rivadavia and Lurdiel saw their business grow, but their açai - now collected from their jetty by riverboat - still fetched low prices and a market was not guaranteed. 'It was always a gamble,' Rivadavia says, 'because we never knew whether we were going to sell it or not - and we often threw it away.'

Three years ago, Sambazon arrived in Amapá state and built a gleaming new factory in Santana. The American company, founded by two Californian surf dudes (roughly in the manner of Innocent), pledged to train and employ mainly local people, fund community projects and source açai sustainably. In one project, Sambazon gives all its bead-like açai stones - churned out by the factory at a rate of seven million an hour - to the Brazilian Women's Group, a cooperative in downtown Belém, so they can make jewellery. 'The crime rate is so bad, nobody wants to wear expensive jewellery,' says Miguel Jorge Hauat, operations director at Sambazon, 'so they are tapping into a strong market.'

The name Sambazon is an abbreviation of 'Save and Manage the Brazilian Amazon' - the company's mission in Amapá. Operating to fair trade and organic standards, it ships sachets of frozen açai pulp to juice bars through the United States and, as well as Innocent, supplies the British companies Happy Monkey drinks (available at Waitrose, Holland & Barrett, Fresh & Wild, Planet Organic and Harrods) and The Berry Company (most major supermarkets, food halls and health-food stores, including the above). Access to this burgeoning global market transformed Rivadavia's fortunes overnight.

'Now, I never lose money on açai,' he says. 'During the harvest period, I sell it for 20-25 reais [£6-£7] a sack and off-season it may fetch 50 reais - and that is a sure thing. I've ended up with so much money, I've had to start thinking about something to invest in - so I've bought nine head of cattle.'

It strikes me as ironic that he is tending the very animal that has brought the Amazon rainforest crashing down, but one glimpse of his humped zebu (or Asian-style cattle) wandering freely among the rubber trees and açai palms makes me realise the two can happily co-exist without environmental cost.

As we talk, João Batista Maia Braga, aged six, explodes out of the rainforest with a family of squealing pigs and half-a-dozen chickens. 'We have 40 hogs, some poultry and some ducks - and they all have to be fed corn,' Rivadavia says. 'Nearly all the money for that comes from açai. I've been able to buy a better boat and engine, a second small speedboat, a TV, an electricity generator and appliances for the house, such as a fridge freezer. We now have four cell phones for the kids... Well, the girls are teenagers after all.'

One of the girls, Maria Carolina Maia Braga, 14, shins up a 60ft açai palm behind the house to demonstrate how the fruit is harvested. With hemp tape looped around her ankles, she gets enough purchase on the tree's smooth, narrow trunk to scale it in less than 60 seconds and slither back down in 10.

In her hands is a sprig of açai, cut from the crown of the palm, with long fronds beaded like necklaces with 50 or 60 rock-hard, burgundy-red berries. These she plucks and collects on a large blue tarpaulin spread out in the shade. As she works, her younger brothers dart in to steal the berries and chew on them, retreating with purple-stained lips and dye all over their faces.

Invited to try some açai the ribeirinho way, I can't understand the fuss. The bulk of the berry is a large inedible stone, coated in a papery red skin that contains the vital antioxidants and, underneath it, a layer of creamy fat. 'Hunger is the number-one problem in the Amazon,' João Meirelles Filho had told me, back in Belém - so for river-dwellers, açai is a rare source of calories as well as vitamins. Mixed up in a bowl with a sprinkling of sugar, each mouthful of berries is followed by a spoonful of coarse manioc flour - and the mixture is moved around in the mouth, before spitting out the stones. To me, it is like chewing on a bag of marbles sprinkled with sawdust, but João Batista, Pedro Paul and their brother Manoel Mateus, four, just can't get enough.

At mealtimes, açai pulp (the nutritious red parts, bulked up with water) is traditionally poured over shrimp or jerk chicken as a sauce; again, kids go crazy for it. 'My favourite dish is fried fish with açai,' says Lenilson Oliveira Moraes, aged 10, who has paddled his canoe from across the water to see what all the commotion is about. He tells me how he and his seven brothers hike into the interior to catch fish, which have been stranded by the retreating tide in puddles and small canals; he also lands fish by line from a dugout, in traps and in nets, and he can catch shrimp. 'I learned to paddle a canoe when I was six,' he tells me proudly, 'and I learned to swim when I was seven.'

In Belém, the boys selling fish in the Ver-o-Peso market, where vultures circle, have forgotten how to catch it. 'They don't know how to go 50 yards off the quayside to fish,' says Miguel Jorge Hauat of Sambazon, 'because as soon as people move to the city, they lose that resource. Nature is abundant, but they have lost the code to harness it.' Here on the Picanço, such skills are being kept alive - by pursuing a traditional way of life built largely around açai.

As we chat, Lenilson Oliveira darts into the forest to fetch me inga, a dozen khaki pods about 1ft long, each containing a handful of furry white, gummy fruits with black, glossy stones. They taste like lychee. With his conhecimento tradiçional ('traditional knowledge') and love of the outdoor life, Lenilson looks set to become the next guardian of the rainforest. Though he, like all the other children, goes to school and is probably bright enough to become a lawyer, he seems more likely than Rivadavia's offspring to follow in his footsteps.

In the manner of most self-improved parents, the Bragas want their children to escape the endless loop of agricultural toil and become qualified professionals. 'I'd love to have a son who was a doctor or a naval officer,' Lurdiel tells me, while Rivadavia says his children 'don't seem that interested' in growing açai because 'they don't have a perception of the business yet'.

From across the river, where the fruits of the couple's Sambazon labours have not gone unnoticed by Lenilson Moraes, açai looks like an attractive option. 'My father is an açai grower and all my brothers help with the harvest,' he says. 'It's the only job I really want to do - apart from playing football.'

It endorses what Rivadavia told me when I asked if he saw himself as a guardian of the rainforest and its traditional ways. 'I am the custodian of that knowledge,' he says, 'simply on account of my age. The way I do things serves as an example to others. When new neighbours come here, they learn from me - and if children want to know about açai, I teach them. Traditional knowledge isn't always passed on through the family; there are other ways.'

Among his regrets is that his own children have lost some of that traditional knowledge. 'My daughters no longer know how to cook game,' he says, and his sons know little about birds and the way they perpetuate the açai crop: toucans spread the açai seed broadly by regurgitating it as they fly; parrots regurgitate seeds while sitting in the tree, sowing only the immediate area.

If the ribeirinho way of life holds, this vast wilderness of islands, backwaters, palm groves and virgin forest will be ring-fenced against more destructive land use - whether slash-and-burn subsistence farming, small-scale cattle grazing or illegal logging. While Amapá is largely owned by the Brazilian government and a great deal more protected than neighbouring Pará (the most ravaged state in Amazonia), a 'new economy' based on ecotourism, heart-of-palm, açai and the collection of rainforest honey would make its future more certain.

'It's an insignificant slice of the Amazon's economy,' João Filho of Peabiru admits, 'probably nine per cent. The remaining 91 per cent is mining, timber and cattle ranching. Of that nine per cent, açai accounts for one per cent - but although it is small, it's an inspiration and a reminder that the consumer holds the key. Until now, these ribeirinhos have lived in medieval times; they never entered the market, the capitalist system - but by selling açai abroad, which is new, they have a chance to enter the global economy, not just the local economy.'

It's a mixed blessing, since unprecedented wealth brings unprecedented social problems. 'The money they have now, they have never seen in their lives,' says Filho, 'so one thing they need is financial education. We see people buying a fridge when they have no electricity, just using it as furniture. As part of the Sustainable Açai Project which Peabiru runs, we will be monitoring the social and environmental impacts before and after Sambazon.'

I ask if buying an açai smoothie can really save the Amazon rainforest, and Filho is adamant it can. 'If there is a 20-year commitment to sourcing açai organically, sustainably and responsibly, then we can achieve that objective,' he says. 'With Sambazon, Innocent and others, we can create a revolution.'