Loss of appetite is among the most common side effects of stimulant for ADHD. Across studies, approximately 20% of patients with ADHD who were treated with stimulants reported a loss of appetite [1,2]. Weight loss is also quite common, as are digestive problems . Together, such side effects are often referred to as “gastro-intestinal adverse events”. But why do stimulants change the way we go about eating? And what could this tell us about ADHD itself?
Appetite can arise in response to physical cues, such as an empty stomach or low blood sugar. Psychological cues can also influence our appetite; for instance, we may get hungry when we watch other people eat, or when we are bored. For most people, eating is a pleasant and rewarding activity. In the human brain, pleasure, reward, craving and, thus, appetite, have everything to do with dopamine. More specifically, with dopamine levels in the striatum, a cluster of neurons at the very base of the forebrain. The striatum is strongly connected with the prefrontal cortex. The prefrontal cortex exercises cognitive control over the urges of the striatum: when we’re hungry, the striatum makes us crave high-caloric, high-fat, or sweet foods; at the same time, our more rational prefrontal cortex helps us make responsible food choices.
Interestingly, ADHD also has everything to do with dopamine and the striatum. Dopamine levels in the striatum are slightly ‘off’ in individuals with ADHD. As a result, people with ADHD feel a higher urge to seek pleasant experiences, and less prefrontal control over this urge. Impulsivity, a prominent feature of ADHD, can be viewed as a failure to sufficiently activate the prefrontal cortex. Finding a balance between pleasure-seeking on the one hand, and rational decision-making on the other, can be difficult for all of us. However, for people with ADHD whose dopamine balance is slightly off, making healthy, non-impulsive decisions about what to eat may be even more challenging. Indeed, overweight, obesity and diabetes seem to be more common in people with ADHD compared to people without ADHD .
Stimulants such as methylphenidate and dexamphetamine can restore the dopamine balance in the brain. This may result in less craving for food (as well as for other pleasant activities) and more control over impulsive urges. It is thus not very surprising that stimulant medications may cause a loss of appetite or even weight loss. Interestingly, stimulants are sometimes used to treat obesity and certain eating disorders as well. Especially for eating disorders involving impulsive eating, such as bulimia nervosa and binge-eating disorder, stimulant treatment could be promising. 
There is one other interesting angle on stimulants, dopamine, and eating. Did you know that most of the dopamine in your body is not located in the brain? In fact, a substantial proportion of all dopamine-related processes in the human body take place in the gut. Throughout the gastro-intestinal tract, dopamine receptors are abundant. Therefore, in addition to the indirect effects described above (i.e., via craving and/or impulse control), stimulants may have direct effects on eating behaviours as well. Unfortunately, we know very little about such direct effects.
 Storebø, Ramstad, Krogh, Nilausen, Skoog, Holmskov et al. (2015). Methylphenidate for attention-deficit/hyperactivity disorder in children and adolescents: Cochrane systematic review with meta-analyses and trial sequential analyses of randomised clinical trials. Cochrane Database Syst Rev (11):CD009885. doi: 10.1002/14651858.CD009885.pub2
 Storebø, Pedersen, Ramstad, Kielsholm, Nielsen, Krogh et al. (2018) Methylphenidate for attention deficit hyperactivity disorder (ADHD) in children and adolescents – assessment of adverse events in non-randomised studies. Cochrane Database Syst Rev 5:CD012069. doi: 10.1002/14651858.CD012069.pub2
 Holmskov, Storebø, Moreira-Maia, Ramstad, Magnusson, Krogh et al. (2017) Gastrointestinal adverse events during methylphenidate treatment of children and adolescents with attention deficit hyperactivity disorder: A systematic review with meta-analysis and Trial Sequential Analysis of randomised clinical trials. PLoS One 12(6):e0178187. doi: 10.1371/journal.pone.0178187
 Cortese, Moreira-Maia, St Fleur, Morcillo-Peñalver, Rohde & Faraone (2016). Association Between ADHD and Obesity: A Systematic Review and Meta-Analysis. Am J Psychiatry 173(1):34-43. doi: 10.1176/appi.ajp.2015.15020266
 Himmerich & Treasure (2018). Psychopharmacological advances in eating disorders. Expert Rev Clin Pharmacol, 11(1):95-108. doi: 10.1080/17512433.2018.1383895