Americans' ardor for frozen food is going cold.

Long at the center of the supermarket and the heart of kitchen convenience, freezer-aisle items are struggling today as Americans shift their tastes to fresh food that they see as healthier.

Frozen-food sales, by dollars, have lagged behind the rate of inflation the past four years, and sales by number of units have fallen. Dollar sales of frozen juice, chicken and pizza all are down since 2009, according to market-research company Nielsen. And sales of frozen meals fell 3% to $8.92 billion from 2009 through 2013, according to Euromonitor International.

The toll has been especially heavy for big brands that target health-conscious eaters. Nestlé SA 's Lean Cuisine low-calorie frozen-meals brand lost more than a quarter of its sales over the past five years to less than $1 billion, says IRI, a Chicago-based market-research firm. ConAgra Foods Inc. has discontinued several of its Healthy Choice dinners, because of lagging sales and changes in consumer tastes.

The struggles of a category that for decades drove innovation show how fast consumer perceptions of health and convenience can turn—forcing companies to catch up. Consumers in their 20s and 30s, in particular, are driving the changes in frozen food, executives say, as they grew up in an era of heightened awareness about nutrition, intense scrutiny of the food industry and the advent of the Internet and social media to fuel that fire.