Google Trends is a free online tool that provides information on the interest on a topic measured as Google searches in the big data era. In this work, we present methods for detection of trends and seasonal effects in online interest for health topics based on Google Trends data. We present data for the term obesity worldwide and in a convenience sample of countries (Denmark, United Kingdom, USA, Japan) over the last five years. Our analysis shows that, despite obesity being one of the global health challenges, there is a decreasing trend in online interest in the topic. We also observed seasonal effects, with less interest for the topic during December and the summer months. Identifying trends and seasonal patterns in online interest can support resource allocation and planning and allows one to evaluate the effectiveness of global and local outreach efforts on public health.