Final answer:
John's diminished food enjoyment can be attributed to the natural aging process affecting his digestive system and sense of taste and smell, leading to a decrease in the sensitivity of taste buds and olfactory function. This change in sensory perception often alters his eating experience and habits.
Step-by-step explanation:
John's decreased enjoyment of food as he ages is most likely due to the natural age-related changes that occur within the digestive system. As we get older, our taste buds become less sensitive, which makes food taste less appetizing. Additionally, an elderly person may also experience a decrease in saliva production, difficulty swallowing, and a slower movement of food through the alimentary canal. These factors combined with a potential reduction in olfactory neuron replacement can lessen the enjoyment of eating and flavor perception, often leading individuals to alter their eating habits such as adding more salt for taste, which in turn can pose health risks.