From Dr. Fuhrman's book Eat to Live:
Our bodies need carbohydrates more than any other substance. Our muscle cells and brains are designed to run on carbohydrates. Carbohydrate-rich foods, when consumed in their natural state, are low in calories and high in fiber compared with fatty foods, processed foods, or animal products.
Fat contains about nine calories per gram, but protein and carbohydrates contain approximately four calories per gram. So when you eat high-carbohydrate foods, such as fresh fruits and beans, you eat more food and still keep your caloric intake relatively low. The high fiber content of (unrefined) carbohydrate-rich food is another crucial reason you will feel more satisfied and not crave more food when you make unrefined carbohydrates the main source of calories in your diet.
It is usually the small amount of added refined fat or oils that makes natural carbohydrates so fattening. For example, one cup of mashed potatoes is only 130 calories. Put just one tablespoon of butter on top and you have added another 100 calories.
Protein, fat, and carbohydrates are called macronutrients. Vitamins and minerals are referred to as micronutrients. All plant foods are a mixture of protein, fat, and carbohydrate (the macronutrients). Even a banana contains about 3.5 percent protein, almost the same as mother's milk. Fruit and starchy vegetables, such as sweet potatoes, corn, carrots, and butternut squash, are predominantly carbohydrate but also contain some fat and protein. Green vegetables are about half protein, a quarter carbohydrate, and a quarter fat. Legumes and beans are about half carbohydrate, a quarter protein, and a quarter fat.
One of the principles behind the health and weight-loss formula in this book is not to be overly concerned about the macronutrient balance; if you eat healthful foods, you will automatically get enough of all three macronutrients as long as you do not consume too many calories from white flour, sugar, and oil. So don't fear eating foods rich in carbohydrates and don't be afraid of eating fruit because it contains sugar. Even the plant foods that are high in carbohydrate contain sufficient fiber and nutrients and are low enough in calories to be considered nutritious. As long as they are unrefined, they should not be excluded from your diet. In fact, it is impossible to glean all the nutrients needed for optimal health if your diet does no contain lots of carbohydrate-rich food.
Fresh fruits, beans and legumes, whole grains, and root vegetables are all examples of foods whose calories come mainly from carbohydrate. It is the nutrient-per-calorie ratio of these foods that determines their food value. There is nothing wrong with carbohydrates; it is the empty-calorie, or refined, carbohydrates that are responsible for the bad reputation of carbs.