I would really like to agree with the first poster – but there is no way I could.
We’ve messed up the food chain. The vegan diet right now is the healthiest. What I would say is, follow that as your best choice (access, finances – what is available). If you can’t – be vegetarian. If you can’t – be flexitarian but heavily weighted toward fresh (also darker) greens and fruit.
@Cruiser ‘s post related to carb in/out takes and how your body breaks them down. But I would really – not advice a can of tuna for lunch.
I would recommend Dr. Greger’s Clinical Nutrition reviews. They are easy to watch – and he goes through every article on Clinical Nutrition every year. http://www.drgreger.org/ – or you can at least use it as a starting point and then head over to pub med or other databases to verify – read more details.