SAN ANSELMO, Calif. — Brian Maxwell, founder of the multimillion-dollar PowerBar empire and a former world-class marathon runner, has died of a heart attack, friends said. He was 51.

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Maxwell collapsed Friday at a post office, and emergency personnel were unable to resuscitate him after workers called 911. He was taken to a local hospital where he was pronounced dead.

Maxwell and his wife Jennifer, a nutritionist, co-founded the popular energy bar company in 1986 and began selling PowerBars out of their kitchen.

Over the next decade, the Berkeley, Calif.-based firm grew to $150 million in sales and 300 employees. In March 2000, the couple sold the company to Nestle SA for a reported $375 million.

Maxwell, who was born in London but grew up in Toronto, represented Canada in many international competitions as a long-distance runner. He was part of the 1980 Olympic team that boycotted the games in Moscow.

In 1977 Maxwell was ranked the No. 3 marathon runner in the world by Track and Field News.

Maxwell came up with the idea of an energy bar after he had to drop out of a 26.2-mile marathon race at the 21-mile mark _ about the point where experts say the body ceases burning carbohydrates and begins burning muscle tissue.

Maxwell is survived by his wife and five children.