Your company already knows whether you have been taking your meds, getting your teeth cleaned and going for regular medical checkups. Now some employers or their insurance companies are tracking what staffers eat, where they shop and how much weight they are putting on—and taking action to keep them in line.

The goal, say employers, is to lower health-care and insurance costs while also helping workers. Last month, 1,600 employees at four U.S. workplaces, including the City of Houston, strapped on armbands that track exercise habits, calories burned and vital signs, part of a diabetes-prevention program run by insurer Cigna. Some diabetic AT&T employees also use mobile monitors; in September, AT&T also started selling to employers its blood-pressure cuffs and other devices to track wearers 24/7.

But companies also have started scrutinizing employees' other behavior more discreetly. Blue Cross and Blue Shield of North Carolina recently began buying spending data on more than 3 million people in its employer group plans. If someone, say, purchases plus-size clothing, the health plan could flag him for potential obesity—and then call or send mailings offering weight-loss solutions.

Marketing firms have sold this data to retailers and credit-card companies for years, and health plans have recently discovered they can use it to augment claims data. "Everybody is using these databases to sell you stuff," says Daryl Wansink, director of health economics for the Blue Cross unit. "We happen to be trying to sell you something that can get you healthier."

Some critics worry that the methods cross the line between protective and invasive—and could lead to job discrimination. "It's a slippery-slope deal," says Dr. Deborah Peel, founder of Patient Privacy Rights, which advocates for medical-data confidentiality. She worries employers could conceivably make other conclusions about people who load up the cart with butter and sugar.