。
She's interested in health food.
Literal
She [topic-は] health-food [target-に] interest [object-を] is-holding.
関心を持つ ('to hold an interest, have a concern') is the more formal cousin of 興味を持つ — 関心 leans intellectual or socially-aware ('an issue I care about, a topic I follow'), while 興味 leans personal or recreational ('something that piques my curiosity'). The target of interest takes に. The ~ている here is the stative 'has come to hold and continues to hold' reading. 健康食品 ('health food') is a productive 健康~ compound — 健康診断 ('medical checkup'), 健康保険 ('health insurance'), 健康志向 ('health-conscious').