Medicare, which covers some 42 million American seniors, will pay for obesity screening and behavioral therapy as part of its portfolio of preventive services. About 72 million American adults, including about a third of Medicare recipients, are obese, and the related health-care costs total some $325 billion a year. The federal decision will now allow Medicare beneficiaries to see their doctors for regular weight-loss counseling?a low-cost, low-tech solution?and may spur private insurers to do the same.