Apple Inc., seeking to make further inroads in health care, tested an app this year to help people with prediabetes manage their food intake and make lifestyle changes, according to people with knowledge of the matter. The company tested the service on select employees earlier this year, part of its broader push into blood-sugar features, according to the people, who asked not to be identified because the work is secret. Though Apple doesn’t have plans to release the app, the company may eventua...