Nooyi giving the 2011 commencement speech at Wake Forest University, Ill.

Until now, we hadn’t thought much about the stock price of soft drinks, and didn’t even know that PepsiCo’s stock was down due to dissatisfaction with Indra Nooyi‘s healthy-products strategy. That strategy is described in a CNBC documentary tonight, Pepsi’s Challenge, which “gives viewers an exclusive look inside the new age test kitchens of the snack and beverage giant as it mounts an intensive effort to move its goods in a healthier direction.” Why do that, if it’s unpopular? The CEO has an explanation.

Below, Nooyi, in an interview with Fortune Magazine executive editor Stephanie Mehta at Fortune‘s Most Powerful Women Summit, tags her vision as “Performance with Purpose,” and lets us in on its taproot: growing up in southern India and thinking about why companies do what they do. Watching it, we felt inspired by her story and by her astute observations about what that question means in today’s queasy economy.