Are you the type who is concerned about the way you look? Thin? Fat? Big? Petite? The way you look could be a function of more than just lifestyle. Human body structure is broadly divided into three groups. The endomorph which is characterised by a preponderance of body fat. The mesomorph which is characterised by a muscular look. The ectomorph which is typified by an absence of either muscle or fat. Clearly you would want to tend to a mesomorph. This holds true for both sexes.
The ectomorph finds it hard to gain weight no matter what they eat. They lack body mass as they have a thin frame, poor muscle development and low body fat. If you are one, your diet has to be suited to weight gain. This has to be accompanied by muscle development through strength training and a suitable protein rich diet.
The endomorph has a round and soft body. They gain fat easily even if they watch what they eat. They usually have poor muscle definition and a slow metabolism. If you are one, people might call you curvy instead of over weight. Its not your fault if you gain weight. Your body type is pre-disposed toward weight gain. If you are one, you want to watch what you are eating. Also strength training will help build muscle definition.
The mesomorph has athletic looks. They respond quickly to weight training and muscle definition starts to show quickly. If you are a female mesomorph you may ending up looking bulky with out intending to. They tend to have a good metabolism.
Your body type can determine how you need to adapt your diet. Also, the type and nature of exercise can vary by body type. Understanding your need will help you get the best results. Have you looked in the mirror lately?