A plant used to stem the hunger pangs of African tribes could soon be added to the meals of obese westerners desperate to lose their excess pounds.

The Anglo-Dutch multinational Unilever is to develop a new range of dieting aids, similar to its Slim-Fast range, that will contain extracts of the hoodia plant, a rare cactus found in southern Africa.

Last month it struck a $38m deal with the British botanical firm, Phytopharm, for the commercial rights to use the plant. The deal was greeted with enthusiasm by City analysts and the Phytopharm share price shot up 11% on the news, valuing the business at about $190m and chief executive Dr Richard Dixey's personal stake at about $36m.

For generations the San bushmen of southern Africa have used hoodia in a different way to suppress hunger when food is scarce or when hunting. "When children are feeling hungry the San feed them hoodia so they don't feel hungry all the time," said Axel Thoma, a trustee of the South African San Council, which represents the tribes. "There is a general problem in regional areas of food shortages.

The food handouts are powdered milk and enriched [maize] meal, and sometimes tins of fish, but that's it." The problem in the developed world, where more than 20% of the population is obese, is at the other end of the scale.

The United States is the worst affected: obesity is estimated to cost the economy $120bn a year. Drugs firms are desperately seeking a chemical solution to the problem, and therefore access to a multibillion-dollar market.

The Phytopharm chief executive, a biochemist turned Buddhist entrepreneur, has had his eye on the market for some time. The firm looks to plants to provide treatments for diseases as diverse as Alzheimer's in humans and arthritis in dogs.

He had been developing an appetite-suppressing drug based on hoodia in a joint venture with Pfizer, the largest drugs firm in the world. When it was given to overweight men for two weeks in clinical trials, they ate fewer calories and their body fat reduced. That deal fell through last year, however, and so Dr Dixey turned to Unilever, the owner of Ben & Jerry's ice-cream and Dove soap as well as Slim-Fast, to see if the plant could be added to food to help people lose weight.

Unilever has considerable experience in the market, selling hundreds of millions of dollars worth of Slim-Fast products last year, but sales have slumped as slimmers have turned to the Atkins diet.

Unilever said it would invest up to $40m to test the plant extract for safety and develop a new range of food and drink products over the next three years. Phytopharm will receive an undisclosed portion of the money and a fraction of the sales in royalties if the products go on sale.

If the project gets this far the rewards could be enormous. "Some people are talking about $600m a year in sales; my suspicion is it will be more than $1bn or $2bn," said Erling Refsum, an analyst at Japanese bank Nomura. "Up to 50% of the western population is overweight.

It's the biggest epidemic we are facing." The deal could also benefit the finances of the San, although on a much smaller scale. Phytopharm does not reveal how much of its income from hoodia will be given to the South African research agency CSIR, from which it licensed the rights to the plant in 1997, but the San will only get between 6% and 8% of it. Phytopharm paid money to the CSIR some time ago due to its multimillion-dollar deal with Pfizer, but the trust that will take the bushmen's share is still being set up.

There is 260,000 rand ($45,800) waiting in government coffers that should be given to the tribes. This has been earmarked for offices and infrastructure for the South African San Council, Mr Thoma said.

The bushmen should also receive a small portion of the deal signed last month and a tiny fraction of product sales in the future.

These funds would be paid into the trust used for education and buying back land that historically belonged to the San. "It will pay for the younger generation to help them get into higher education so we can rely on our own people in future," said Mr Thoma. "If there is a lot of money we will start buying land back that was once taken from us, so we become landowners and secured."

There is an irony in the fact that the hoodia plant is becoming rarer, particularly as tourists and unofficial peddlers try to exploit it. Unilever will be setting up vast farms to produce enough of it.

The contrast between those with plenty and those without is clear, but the billions to be made could provide the thousands that could make an enormous difference to the San tribes.

Fat westerners may be last hope for broken bushmen Rory Carroll in Johannesburg A drive through the windblown San settlements of grass and wood scattered across southern Africa is a depressing experience: families ravaged by poverty, unemployment and alcoholism their hunter-gatherer tradition a fading memory.

For the past few years their leaders have spoken of a bonanza from western pharmaceutical companies in payment for the San Bushmen's knowledge of a plant that might one day cure obesity. They have yet to see a penny. "They are frustrated by the delay, but these are people who take things as they are.

There is not huge agitation yet," said Richard Wicksteed, a documentary maker who has filmed them for two decades. "The average San knows there is value in their medicinal knowledge.

Having used hoodia when they were starving, the irony of obese westerners using it to lose weight is not lost on them." Roger Chennels, a lawyer and spokesman for the San Council, welcomed the chance to put the commercial development of hoodia back on track in the wake of the aborted Pfizer deal. "We've been expecting it.

We're very pleased and excited about Unilever being the licensee. It's the end of the period of waiting since the Pfizer fall-out." If a portion of Unilever's millions does reach the San it would buck a long, sad history of dispossession.

These indigenous people of southern Africa have a culture dating back 20,000 years. The more warrior-like Bantu tribes from the north, then European colonialists and finally the apartheid regime swept through their land, pushing the San into dwindling pockets of territory. The three San groups most closely associated with hoodia the Khwe, Xu and Khomani number only a few thousand.

The total number of San across the Kalahari is estimated at 100,000. Post-apartheid South Africa tried to redress some of the injustice by granting them ownership of more than 40,000 hectares. But their communities are broken. Only a handful remain the hunter-gatherers of western imagination.

The rest scrabble what living they can in bleak settlements which are often hundreds of kilometres from decent roads, schools and clinics. "This is the last stand of the San the last generation that still retains some of their culture and heritage.

They are a tiny people with a tiny voice," said Mr Wicksteed. Some firms have already tried to pirate the San's knowledge of the hoodia, and Unilever and Phytopharm are expected to try to stop such practices. Chris De Plessis, a lawyer representing the San in Botswana, said that few San understood the ramifications of intellectual property rights, and mistrust was growing that their leaders might pocket the revenue if it ever comes.