TL;DR - Eating chills you the fuck out.
I believe, and trust me I’m no neurologist here… but I believe that eating engages the digestive system, which in turn activates the vagus nerve, which in turn helps keep your body in rest/digest mode instead of fight/flight mode… and when you are getting a tattoo or injection or whatever, if you are in fight/flight mode, there is a large chance that sustaining damage which activates pain sensors will tell your brain to make rapid changes in your physiology that would normally aid you during combat or flight… but because you’re sitting dead still in a chair, these changes are not good and cause vasovagal syncope… also called reflex syncope. I’ve talked about this before in other posts… Search results for 'syncope' - Dangerous Things Forum