The food we eat can generally be broken down into three categories. Proteins, carbohydrates, and fats our bodies use these food sources to produce external (what we see) and internal (the microscopic work of our cells) energy. Examples of external energy would be exercise, cooking, walking, vacuuming, and playing guitar... you get the idea. The cellular activity includes production of hormones, neurotransmitters, muscle and bone formation, digesting food, and other things we hardly think about our bodies doing, yet activity that is constantly occurring within us. Proteins are classified as building blocks for muscle, hormones, immune response, and cellular repair. Carbohydrates are considered our bodies main energy source. Fats are used for energy, organ protection, hormone production, and skin health.
Diets are made up of various combinations of these macronutrients. With science, research, public policy, FDA (food and drug administration), and the internet guiding peoples perception of what is good for them and what they should avoid. Here are some examples of
Protein focused diets: Paleo and Atkins
Fat focused diet: Ketogenic
Carbohydrate focused diets: Vegan, Ornish, and Mediterranean diet
So which one is the best and will yield desirable results, with fewer symptoms, and longevity? This is a tricky question and you will find all kinds of differing evidence. Import questions to consider are do you have food sensitivities, what are your personal goals, what is your belief systems, do you want a lifestyle diet or a quick fix, and what do you eat now. What I can say for sure is that vegetables and fruits yield a good bang for the buck. Fewer calories per gram of food, loaded with vitamins, minerals, and phytonutrients, and can be grown in your own backyard. There is little to no evidence suggesting that vegetables and fruits are bad for you. So eat the rainbow often and in good company!
I like to follow Michael Pollan's advice here "Eat food, Not to much, Mostly plants."