As Americans expand their waistlines, Silicon Valley venture capitalists are seeing opportunities to expand their wallets.

The amount of venture money injected into anti-obesity companies has gone up tenfold in the past two years. VC firms all over Silicon Valley have their hand in the mix, funding companies that are feverishly -- and more often than not, secretly -- working on solutions like shrinking the stomach or tricking it into feeling full.

The appeal is clear: In a super-sized nation that is getting more so, the market won't shrink anytime soon. Current products are limited or flawed. And at a time when many of Silicon Valley's other sectors seem crowded -- for example, chips, software and telecommunications -- investors are eager to put some money in the fight against fat.

"Everyone is doing stealth deals," said Guy Nohra, a partner at San Francisco venture capital firm Alta Partners, who has invested in four anti-obesity drug companies and one device company. "They all have projects."

Almost a third of American adults are obese. Lehman Brothers predicts the total U.S. obesity market could exceed $5 billion by 2010.

So it's perhaps not surprising that venture capitalists nationwide have increased their investments to fight obesity. The roughly $250 million raised last year is still tiny compared with $1.9 billion for telecommunications companies, for example, but the anti-obesity investments are growing much faster.

The drug option

The easiest option is drugs, which work to reduce fat absorption. But drugs are often limited in their effectiveness. The most successful drug combination, "fen-phen," was withdrawn from the market in 1997 after numerous cases of heart-valve disease were reported.

Only two prescription drugs -- Roche's Xenical and Abbott's Meridia -- have been approved for long-term weight loss. Some consumer groups have called for banning Meridia because of side effects. And Xenical, the industry leader, is so effective at stopping fat absorption that large globs of fat leak out of the body -- effectively making many users incontinent.

That's where Peptimmune comes in. Peptimmune is a start-up in Cambridge, Mass., which has won $61.6 million from Vanguard Ventures of San Francisco, New Enterprise Associates of Menlo Park and other investors. The company has developed a polymer that binds to the droplets of fat and prevents leakage.

Even with such work-arounds, drugs are still limited in the amount of body fat they eliminate.

A more effective way to reduce fat is through bariatric surgery. In the standard method, doctors staple a line across the upper portion of the stomach, effectively cutting it off from the lower part. Then a portion of the bowel is lifted up and attached to the smaller stomach pouch, bypassing part of the intestine.

The radical shift in the internal organs results in up to 70 percent weight loss. The problem is, it can create serious complications, and about one in 200 patients eventually dies from the procedure, according to the University of Chicago Hospital Center.

So two Silicon Valley companies are trying another way. Competitors Satiety of Palo Alto and BaroSense of Menlo Park make a tubed instrument that passes through the mouth and down to the stomach to do the surgery internally.

Robin Bellas of Morgenthaler Ventures, an investor in Satiety, says it's a great time to invest in bariatric surgery. It's widely accepted by patients and the medical industry, the FDA is approving such devices faster, and insurance companies are approving reimbursement of costs now that obesity is accepted as a disease.

"What better time than four big waves to jump on a wave?" he asked.

Both Bellas and Mark Foley, a venture capitalist at RWI in Palo Alto who led a $10 million investment in BaroSense this year, said their companies aren't revealing anything about their exact methods.

Stomach band

Inamed, a Santa Barbara public company, has developed an alternative to bariatric surgery, including a doughnut-like band around the stomach that shrinks it from the outside. It has also developed a balloon that goes inside the stomach to make it feel full.

Finally, a slew of other companies are developing ways to send electrical energy to the stomach or brain to create a feeling of fullness. This method generally helps reduce about 30 percent of excess body fat. For example, New Jersey's Transneuronix and Menlo Park start-up IntraPace target the stomach.

Orexigen, which is based in Princeton, N.J., is attempting to "trick the brain," said Mike Powell of San Francisco's Sofinnova Ventures, which has invested in the company along with Menlo Park's Kleiner Perkins Caufield & Byers. The company's choice of which brain receptors to hit is secret, he said.

Likewise, many anti-obesity companies are staying mum. Powell's group has also invested in a San Francisco company called Diobex, still secretive about its obesity-fighting method. And Lou Bock, of BA Venture Partners in San Francisco, invested two months ago in a small team of four based in San Francisco and San Diego: "We're trying to keep things quiet," he said.

Many investors believe no one method will win out, and that each will target different sorts of patients. And the field is still young enough that companies have a lot of maneuvering to do, noted Alta's Nohra: "It's going to take 10 or 15 years for this to play out."