Hippocrates' quote (circa 400 BCE) remains true. The best way to improve one's health is to improve one's diet. At least, it's the first step; the base on which to build.
However, the world of food is no longer the same. The landscape has changed vastly, even from as recent as our grandparents' childhood dinner tables.
These days, food is made in larger quantity, at cheaper cost, and with longer shelf life. Plus, the further it gets from being actual food, the more it screams, "Healthy!" "Natural" "Nutritious!"
Think about it! What's more processed than soy cheese or a protein bar?
Sugar (in all forms, even fructose) is more accessable than any other time in human history. New studies are linking it to chronic inflammation, and through it, to increased allergies, autoimmune disorders and even cancer.
Even changes we thought were beneficial (like fresh fruits and vegetables being available all year round) has opened our eyes. Turns out, preserving food for winter allowed for fermentation which provided essential probiotics. Not only that, certain probiotics are found in the earth, and as fewer back yard gardens exist, fewer guts benefit.
Fear not, there are solutions, and they're still fairly simple.
HERE ARE SOME GENERAL RULES TO FOLLOW:
Avoid processed foods, refined sugars & artificial colors. (Generally these are foods found in a box or a bag), and as a nonchemist, if you don't recognize all the ingredients, skip it.
Snack! Eating small meals every 2-3 hours helps to keep blood sugar levels stable. This provides improved and more sustained energy, stable mood, and less stress on the pancreas and liver.
Eat more vegetables, especially leaves, in a variety of ways. Studies have shown that certain nutrients are released when a vegetable is eaten raw and others are released when they are cooked. (Cruciferus vegetables are the exception. They are hard to digest in their raw form, and in sufficient quantity, can be harmful to the delicate thyroid gland. It isn't necessary to cook them until they're soft, just enough to break the outter layer of cellulose.)
East meat that itself ate well. (If we are what we eat, wouldn't we rather be happy, justly treated animals rather than the tortured, injected variety?)
Drink water! I know how overwhelming choosing a water source can feel. My general rule is to stick to glass bottles over plastic. Find a filter or a water source that instills confidence, and then drink half your body weight in ounces.
If you crave junk food, make it at home. You'll use more quality ingredients than your local fast food establishment will.
Don't fear healthy fats. More and more studies are linking heart disease to chronic inflammation. Good fats can actually benefit the body by raising good cholesterol levels.