What did I learn in the end? Lean protein, good fats, healthy carbs. More specifically: modestly sized meals consisting of lots of produce, a bit of lean meat now and then, and grains that haven't been bleached and pulverized into submission. Also, olive oil is good, and snack on nuts and dates.
In broad strokes, that approach works for almost anybody. But broad strokes don't cut it. I also discovered that wheat doesn't cause me problems, that dairy does, and that I should avoid tomatoes. You might be totally different. The Okinawa Program may save your life. The Paleo Diet for Athletes could make you faster. I can't say how you'll react to any single diet.
What I can provide, though, after 12 months alone in the diet-industry wilderness, is a strategy for finding what does work for youmy own take on what is commonly referred to as an elimination diet. You'll have to keep a diary of everything you eat and how it makes you feel, but it won't take a full yearmore like two months.
The first two weeks will be the hardest. Eliminate prepared foods, coffee, dairy, nightshades, wheat, soy, alcohol, corn, eggs, processed grains, processed anything else, added sugar, and all but the most organic, free-range, grass-fed of meats. Relax; this leaves you with a lot of options. You'll find most of them in the produce section. Mix in the occasional serving of fish, turkey, or buffalo, drink herbal tea, discover spelt bread, and learn to cook quinoa. You'll get through.
After that, start methodically experimenting, one at a time, with foods you eliminated and see what happens over the next 72 hours. Did that omelet make you feel nauseated? Any skin issues after tomatoes? Did meat make you feel better? You see where this is going. After two months, you'll have a functioning idea of foods that work for you and ones that work against you. If you can, see your doctor and ask for blood tests at the beginning and end of your two months.
A last bit of advice: Once you've settled on a nutritional approach, cheat. Every now and then, eat whatever you want and wash it down with what's on tap. Knowing you can do this will make it easier to eat well the rest of the time.
That's it. It may not be completely scientific, but I bet it's closer than anything you've tried. I also bet it will work.