Remember when you were a teenager and it seemed like you could eat anything? Why do some people develop food sensitivities over time that lead them to trying restrictive diets? What's changed?
Like most chronic conditions in the body, the changes begin not necessarily with the organ system itself, but with the central control mechanisms of the body - the neuro-endocrine axis. Aging and stress cause changes to how the nervous system behaves, including the Vagus nerve - a cranial nerve running directly from the brain that controls much of the digestive tract, with different branches controlling upper and lower parts of the GI tract. The Vagus nerve functions correctly when the system is in a relaxed state, not in a stress response. If we are chronically stressed, especially if our sleep is disrupted consistently, the functions that only work optimally in a relaxed state will begin to lose their tone and proper activity. This is most clearly seen in individuals who suffer from stress-induced IBS, but lack of proper Vagal nerve activity can contribute to a wide variety of issues in the GI tract like acid reflux, food allergies, constipation and loose stool. The Vagus nerve also inhibits improper inflammation, so when it isn't working right, it can lead to persistent inflammatory conditions in the gut.
Hormones also can play a role. Under prolonged stress, many hormone levels can become dysregulated causing a see-saw effect on their relative numbers. Cortisol increases, which has a suppressive effect on the thyroid (which controls metabolism and helps brain function), pregnenolone, and progesterone (which also do a lot of neurological and immunological regulation). Progesterone levels have to be high enough to counteract negative effects of unopposed estrogens in the system, which if left unchecked, have a number of negative effects especially for women, chiefly in the areas of menstrual disorders and digestive conditions, as it can overly promote tissue-building and is excitotoxic over time, just like cortisol.
When the neuroendocrine system becomes dysregulated, it opens the door to uncheck inflammation: the immune system becomes over-sensitized and can begin to cause inflammatory reactions to different food groups as the digestion becomes compromised.
This is why it appears that many people can tolerate a wide variety of foods when they are younger, but as they age or have serious insults to their health, all of a sudden certain foods do not agree with them or they seemingly develop an allergy or intolerance to them. While there are certainly foods that are problematic for the digestive and immune systems (many people react to GMO corn and soy, wheat, dairy, excessive sugar, etc), in a healthy individual the damage from these foods can be minimized because the rest of the system is robust and can keep inflammation in check, and toxins can be effectively eliminated. As this fails, it appears to the casual observer that sensitivities to foods increases, but it is not necessarily the foods that are the main problem.
Restoring digestive regulation, and counteracting the damaging effects of stress on the neuroendocrine system are just as important, if not more important, than working with someone's diet. This can be accomplished by using adaptogenic and digestive herbs, acupuncture, appropriate nutritional medicine, bodywork that works on the autonomic systems, and other supplements and nutrients like probiotics - that can restore the functioning of all these different components like Vagal nerve tone, immune regulation in the gut, and hormonal balance.