We present a longitudinal online experiment (n=1,240) testing the effect of differently framed Implementation Intention (II) interventions on vegetarian food choices. Participants are randomly allocated to either a control group, or one of three different II interventions where the benefits of a vegetarian diet are presented in terms of either personal wellbeing, or planetary wellbeing, or both personal and planetary wellbeing. Participants make choices between vegetarian and non-vegetarian food items in an incentive-compatible task. After the food choice task, they have also the opportunity to donate money to a charity. Finally, participants answer a follow-up survey three days after the food task. We find that, compared to the control group, participants are twice as likely to choose a vegetarian option when exposed to any of the three II interventions. There are no statistically significant differences in the proportions of vegetarian options across the three II conditions. However, in the follow-up survey participants in the II intervention where the benefits of a vegetarian diet are framed in terms of both personal and planetary wellbeing report consuming vegetarian food more frequently than in the control group. Moreover, we do not find any evidence of negative spillover effects on subsequent charitable donations. If anything, participants in the joint personal and planetary condition are more likely to donate to a charity following the food choice task, indicating possible positive spillover effects. Taken together, these results point to the promise of combining the information about the personal and planetary benefits of a vegetarian diet when designing behavioural interventions to nudge diet change.