I'm not a nutritionist. However, for the last 5 years or so, I've successfully based my diet on the following suspicions:
  • Popular nutrition advice is probably misleading and puts exaggerated emphasis on fresh fruit, vegetables, and "natural" ingredients.
  • While some people are highly sensitive to additives, and others to a lesser extent, most people aren't.
  • For most people, what matters in diet is, by far, calorie count. It's no use to be eating a vegetable rich diet if you eat so much you're overweight. Conversely, a restricted calorie diet will lose you weight, even if it consists of sugar and fat. As long as your diet gives you enough vitamins and protein, not being overweight is what matters most for health.
There's no such thing as a healthy food. There are only healthy diets. What matters is how the numbers add up, not the individual things you eat.

In this CNN article, a KSU nutrition professor lost 27 pounds over two months on a diet consisting largely of packaged sugary snacks. He also took a multivitamin pill and drank a protein shake each day, but restricted his total calorie intake. After two months, he lost weight, and his blood pressure and cholesterol levels were better.

As long as it's not poison, and you don't have a specific medical condition, your body can process what you put in your mouth. The trick is not to put too much, and make sure that you're getting vitamins and protein from somewhere, and it won't matter if your diet is based on garden greens or hamburgers.

Our species has been on Earth for hundreds of thousands of years, surviving on all types of diets in all sorts of environments. This wouldn't have been possible if our biology required a steady intake of fresh vegetables to live.