Purpose: The purpose of this paper is to evaluate an online "teachable moment" intervention to promote healthy eating for overweight and food intolerance symptoms. Design/methodology/approach: The study involves a 2×2 factorial design with two conditions: group (weight loss vs food intolerance) and condition (intervention vs control). The intervention aimed to generate a "teachable moment" by providing knowledge regarding the relationship between food and the problem (overweight or food intolerance), focussing on the negative aspects of the problem, creating a behavioural model, and encouraging hope and reinvention. Participants receiving the intervention (n = 22) completed measures of dietary behaviour and either weight or food intolerance symptoms before receiving the intervention and again one month later. Control participants (n = 20) provided measures but did not receive the intervention. Findings: There were no significant reductions in weight or food intolerance symptoms. However, compared to control participants, participants in the intervention conditions reported greater intentions to eat healthily (p = 0.01) and improved healthy eating behaviour over time, following both an intention-to-treat (p = 0.046) and explanatory analysis (p = 0.042). Practical implications: Encouraging individuals to perceive their everyday situation as a time for change and adopt healthier behaviour early on, may prevent future diet-related medical events. This has benefits for both the individual and for health care costs. Originality/value: A quick and easy-to-administer online "teachable moment" intervention improves dietary behaviour and can be minimally adapted to suit individuals with differing health needs.