When you start physically exerting yourself, you start to pump up your sympathetic nervous system and tone down the parasympathetic. The PNS regulates resting physiological activities such as food digestion. When the SNS kicks in due to hopping on your bike - assuming you are not riding so slow that it literally takes no more effort than walking - then neurotransmitters will be passed through to the smooth muscle neurojunctions which will cause vasoconstriction; that blood will then be shuttled to your working muscles and lung tissues for higher oxygen exchange rates due to vasodilation. I.e., it might be uncomfortable, but the physiology of it is such that eating before a bike ride will probably not affect your body temperature or O2 saturation of working muscles and lung tissues.