Background Online health information seeking using social questions and answers (Social Q&A) sites has been increasingly popular in recent years. It calls for better understanding of health information seeking behaviour and interactions between information seekers and information providers. Objectives The study investigates how diet information seekers interact with information providers on WebMD Answers, which is a Social Q&A site devoted to health‐related topics, and examines the factors that constitute a ‘helpful’ answer from an information seeker's perspective. Methods Bales’ interaction process analysis was applied as the framework to analyse 568 diet‐related Q&As from WebMD Answers to identify interaction patterns. Results Most diet information seekers post questions anonymously and without any detailed description. Individual experts or health organisations provide most answers. Overall, answers are positively received and had a high satisfaction rating. It was also found that information seeker‐perceived helpfulness does not depend on who answered the question but to how an information seeker posted the question. Conclusions This study indicates that answers at WebMD Answers are helpful for diet information seekers. It sheds new light on the interactions during the Q&A process, preferred site functions and important factors that contribute to perceived helpful answers.