The company that ships them around the world at the rate of about 50 tonnes a month is called Tibet Authentic. But Ms Yangzim and her husband Tsedak, 59, who herd yaks across an 80km stretch of Nyingtri in Tibet were bemused at the sight of the goji berries.

'We have some berries high in the mountains here, but there is nothing like this,' Mr Tsedak said. His reaction is not unusual. After a week in Tibet on the trail of the superfruit, hardly anyone we spoke to had even heard of locally grown goji berries.

The man who bears most of the responsibility for the extraordinary hype surrounding Tibetan goji berries and their introduction to the west is a flamboyant Australian called Antony Jacobson. The founder and president of Tibet Authentic, Mr Jacobson said he 'discovered' the goji berry four years ago when he took a break from his Melbourne-based patenting business and travelled through the Himalayas in search of new health products.

'I saw people in the fields picking a little red fruit,' he said. 'I saw them eating it off the trees, I saw them applying it to their skin and to their hair. I was told it was a berry that is very famous in Tibet, long held in traditional communities from generation to generation. It was a wondrous berry that was not only good for your health and your soul but also very good for your whole character.'

Mr Jacobson returned to the capital of Lhasa and teamed up with the Chinese government-owned Tibetan Medical College whose experts, he said, showed him remarkable evidence of the health benefits of the little red berries.

'I was introduced to a number of people who were very, very old. One woman was 106 or 107 and she told a story about how she ate the goji berry from the first day she could remember. There was another lady who was 109. From what I heard, to live beyond 100 years [in Tibet] is not uncommon, and there is a famous story about a Tibetan scholar who is said to have lived to more than 500 years as a result of consuming the goji berry.'

In reality, Tibetans have a life expectancy of 67 - five years less than the mainland's average. And goji berries have for years been exported to the US from Ningxia province in northwest China, often as wolfberries or Chinese gojis.

What made Tibet Authentic's product different, Mr Jacobson claimed, was that they were grown in the pollution- and pesticide-free Tibetan Himalayas.

By teaming up with the Tibetan Medical College, Tibet Authentic had, he said, tapped previously unavailable sources of wild goji berries, certified by provincial officials as coming from Tibet.

Goji berry production in Tibet, he claimed, had gone from virtually zero to 50 tonnes a month, and his company was now collecting so many berries it was preparing to launch a concentrated goji juice early next year, and later a goji face cream.

Mr Jacobson, a 41-year-old father of two, said that he was living proof of the berry's benefits. 'I used to wear glasses to drive at night,' he said. 'After consuming goji berries for about a year, I put them in a drawer and haven't used them since. When I started taking the berries, it used to take me 30 minutes to do my daily three-mile jog. I now do it in 22 minutes.'

Tibet Authentic claims its berries come from the remote Shannan and Nyingtri regions of Tibet. However, when we flew to Lhasa, the company refused to take us there or say exactly where they grew, claiming the areas were close to the Indian border and off limits to foreigners.

While in Lhasa, we visited 12 shops in the city's Yu Tuo Road, the national centre for traditional Tibetan medicine, and not one had any native goji berries for sale. All instead stocked goji berries from Ningxia, in China, which sell for about 15 yuan for a 250-gram bag.

'We have been in business for six years and we have never heard of Tibetan goji berries,' said Xia Ma, manager of one medicine store.

Richard Zhang, a Seattle-based importer of Chinese goji berries, said the stigma of the mainland's polluted environment made the berries difficult to market, while Tibet's image was a far more potent sales pitch. 'We market our product as Chinese but when we sell them on to sub-distributors, they label them as Tibetan or Himalayan berries,' he said. 'People are much more willing to buy a health food product from Tibet than from China.'

Mr Zhang, who sells 30 tonnes of goji berries a year in the US, said: 'I went to Tibet to look for goji berries myself and the people there asked me: 'What are you doing here? We don't grow goji berries here'. Goji berries grow best at low altitudes and need four months of sunshine. The altitude in most of Tibet is 10,000 to 13,000 feet [3,100 to 4,000 metres]. I believe if someone tells you there are goji berries growing in Tibet, they are probably coming from somewhere in China.'

Tibet Authentic's efforts to convince people of the purity of its product is not helped by the fact that its goji berries are actually processed and packed in Chengdu , 2,000km away in southwest China.

Chengdu is also the main centre for packing and distributing Ningxia berries. Rong Feng, 30, manager of Yi Feng Chinese Medicine Trading, said his company packed 600 tonnes of goji berries a year from Ningxia and sold 100 tonnes of them to traditional medical shops in Tibet.

'If the goji berry is growing in Tibet, why not set up a packing factory there?' he asked. 'And why not sell it in China where there is such a big market for Tibetan traditional medicine?'

It was in Chengdu that we finally met up with Mr Jacobson. He said the factory location was chosen because it was an existing Tibetan Medical College facility and because the effects of altitude on plastic packaging made it impossible to carry out the process in Tibet. Explaining how Tibet Authentic had gone from a standing start to such a scale of exports, Mr Jacobson said the Chinese government had corralled farmers into action and persuaded them to grow the berries.

Mr Jacobson said he had been escorted to some of the growing areas by his Chinese partners, and what he saw 'exceeded my expectations', but had not visited others because of the treacherous journeys involved.

'I have made drivers turn back after up to 12 hours driving and take me back to Lhasa,' he said. 'I found the driving so damn hairy, and at the end of the day I couldn't go on. In those cases, I've sent my staff and seen pictures and video.'

Determined to hunt down the elusive wild goji berries for ourselves, we hired a four-wheel-drive to take us to Nyingtri, one of the two regions where his company says it picks its berries.

At an altitude of about 2,015 metres - nearly half that of Lhasa - Nyingtri is a Chinese military base in a fertile mountain valley near the border with India and is unusually lush and green for Tibet. With no sign of a single goji bush on the spectacular day-long drive through nomad settlements, we finally arrived in Nyingtri's main town, Bayi, and consulted a traditional medicine expert.

Dhundup Tsering, 35, confirmed that goji berries grew in the surrounding mountain valleys, but only in limited quantities. When we told him of the scale of exports, he said: 'That's impossible - there aren't enough around here to send even one tonne overseas.'

After a bone-shaking drive along a fertile mountain valley, we finally found some wild-grown goji berries sprouting haphazardly on bushes along the edge of Banna village, Nyingtri county.

A pig farmer who guided us there was perplexed at our interest. 'Sometimes, if there are many berries, we pick them and sell them in the town,' said Penba Niyama, 42. 'But Tibetan people don't buy them - only Chinese soldiers stationed here who like to put them in their wine. Some old people eat them if they have a headache but usually we just leave them for the birds to eat.'

When I told him people in the west paid the equivalent of 140 yuan for a small bag of the berries, he shook with laughter. 'People there must be very strange,' he said.

It was a view at least partially endorsed by Claire Williamson of the British Nutrition Foundation. 'The UK and the US seem to be obsessed with the latest health fads of the moment and this is feeding into the media frenzy for these latest so-called superfoods,' she said.

There is evidence to suggest fruit and berries have previously unrecognised health benefits, she said, but research on such effects is in its early stages.

Her advice for western consumers fuelling the goji berry craze was blunt. 'Air miles and environmental damage is considerable when fruit travels so far. Why buy goji berries when there are perfectly good strawberries and blackberries available?'