Sure. But 2 months is not long enough. Some unaccounted-for vitamin with a long half-life or a low requirement could give deficiency symptoms after 4 months but not 2.

Also, people on restrictive diets post all the time about how crappy they feel when they reintroduce something. For him to slide comfortably into the explanation "thus my product makes me feel better than restaurant food" is typical of such dieters' enthusiasm.

Although bad restaurant food does exist, much of the digestive upset people experience when going out to eat is simply down to overeating, late eating, or alcohol.

That was also a week he spent travelling. Sleeping away from home, long plane/car rides, irregular schedule, and all the other attendant discomforts are quite enough to throw me off my game, even without dietary shifts.