All of my life, when I have heard people saying that they no longer enjoy a certain food, the phrase "my taste buds changed" is used. Is that really what causes changes in what people like? Is liking particular foods purely psychological, or is it actually defined by taste buds?
The question is a bit vague and depending on interpretation either fairly trivial or very likely not answerable with current knowledge.
Is a change in taste buds a possible cause of change in taste? Sure, it's possible for a change in taste to be a result of actual changes in taste buds, as in their density... but many other things can lead to the same effect: saliva issues, loss of smell, neurological, drug-related, etc. Aging in particular causes it directly or indirectly.
On the other hand studies on diet changes found for instance that
Prescription of diets that promoted restriction of specific types of foods resulted in decreased cravings and preferences for the foods that were targeted for restriction.
But this doesn't say anything as to what the mediating pathways are in the body (including in the brain). If you want to ask what is the most common pathway mediating food preference change... it's a good question, but it doesn't look like researchers have an answer for this yet. Differences in brain activation as they correlate with food preferences across individuals have been studied, e.g. using fMRI, but longitudinal monitoring of changes in taste and their pathways would be a pretty expensive proposition (given the limited benefits of the resulting info).