Prunes (dried plums) have a reputation for giving a helping hand to your daily ‘motions’, but they may deserve a regular role in your muscle-building plan, too. Researchers at Oklahoma State University have shown that regular consumption of prunes increases levels of IGF-1: a protein that regulates the effects of growth hormone. 2 Try a handful as a pre-workout snack and you’ll also get a useful hit of carbohydrates to fuel your session – just be careful not to overdo it, to save yourself some unplanned toilet sprints mid-workout.