LivingSocial's Whole Foods offer a hit

The offer let shoppers buy $20 worth of products at Whole Foods for $10. LivingSocial said it was selling roughly 115,000 an hour, which company spokesman Andrew Weinstein said is its fastest-selling offer ever. That's about 30 deals sold per second.

"It is a staggering rate," he said.

The company planned to sell up to 1 million of the deals. They went up for sale at 5 a.m. ET and was scheduled to run until the same time Wednesday. However, by midday Tuesday about half were gone, and the deal was expected to sell out early.

The offer is available to anyone who registers for LivingSocial or is an existing member of the site.

Austin, Texas-based Whole Foods, which already is seeing higher traffic and sales trends in its stores, was not immediately available to comment. The company plans to donate 5% of the sale price to its Whole Kids Foundation, which supports children's nutrition and wellness.

LivingSocial, based in Washington, D.C. and the second-largest daily deal company behind Groupon, has run grocery offers before for regional or specialty retailers. But large supermarkets and grocery stores typically shy away from such deep discounts because they already operate on thin profit margins and offer a host of other promotions already.