@Gary2, Like you, I was very concerned about staying healthy when I thru-hiked the AT in 2014. I chose to do my hike without supplemental vitamins (and also without the half-gallon challenge--though there may still be some residual regret at having not participated in that rite of passage, haha). My backpacking menu was intentionally varied--including carrying fresh vegetables most fellow hikers shunned because of weight--specifically for reasons of getting the nutrients I needed directly from food. My method seems to have worked, but for one thing. My vitamin levels were all normal upon completion (even though I was out there for 7 months), BUT my cholesterol became high. So much (shelf-stable) bacon, summer sausage, cheese, an all-you-can-eat buffet or 16, etc., was to blame, I suppose. I don't want to take away from anyone who has a recommendation specifically to what you are asking, but I did want to offer my experience.
Also, for perspective, even a long thru-hike is only a snapshot in time for your nutritional health, especially if you start out healthy; and since that time, even my cholesterol levels have returned to normal without medication. It's great that you are thinking about this stuff ahead of time. Do get some blood work done ahead of your hike, though, if you are at all concerned. You would hate to learn, for example, that you were on the edge for Type 2 diabetes, and all those family-size Snickers bars pushed you over into requiring insulin.
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