I am going to start a few separate posts in regards to nutrition to help in understanding. This post will cover the basics of the macro-nutrients.
The main nutrients in a well balanced diet are known as Macro-nutrients. This consists of carbohydrates, proteins, and fats.
-Carbohydrates are your bodies preferred source for energy, and although all the macro-nutrients can be used for energy, consuming carbohydrates will allow the other nutrients to do what they're best used for. So although there is the very popular Atkins diet, which created carbohydrates as an evil fat creating macro-nutrient, it isn't that simple. Consumption of carbs should be complex, such as whole grains which are slower digesting, which caused a more controlled release of insulin (a lot of insulin correlates directly with fat storage)
-Proteins are the building blocks of muscle (actually that would be amino acids, which are the building blocks of protein). Consuming complete protein, such as chicken or soy, in your diet will help you maintain your muscles, or aid in their growth. If the body does not have enough fuel from your diet (say if you were trying the Atkins Diet or fasted exercise), then the body will in turn burn muscle in your body for fuel for more immediate energy.
-Fats are important for your health when they are in the form of mono and polyUNSATURATED fats, which can be found in nuts, fish, and oils. These are critical for proper organ function. It is critical to include these fats in your diet because the body cannot produce them itself, like other fats. Stay away from anything that is a TRANS fat or any of the other code names such as "partially hydrogenated", because these will raise your LDL, "bad" cholesterol levels; which of course can lead to heart disease.
There are always different studies saying one thing is better for you than another thing. The body is very adaptive, but the key to a successful diet and a healthy lifestyle is moderation. Control your portion sizes and make sure you are consuming enough to fuel your lifestyle.