Pressure from Wal-Mart and Costco at the bottom and Whole Foods and Wild Oats Markets at the top leave midlevel chains vulnerable.

FORBES

The second-ranked organic food chain, sad-sack Wild Oats Markets, often loses money.

FORBES: Not So Wholesome

United Natural of Dayville, Conn. is the primary distributor to Whole Foods and Wild Oats Markets, the nation's first- and second-largest hyperhealthy chains.

FORBES: Food Markets