What does 'fat mass' primarily consist of?

The primary component of 'fat mass' is subcutaneous adipose tissue, which stores energy in the form of fat and plays a significant role in regulating metabolism and hormonal balance. Subcutaneous adipose tissue is the layer of fat located directly under the skin and is the largest component of total fat mass in the body. It serves important functions, including insulation and protection of underlying muscles and organs.

In contrast, muscle and bone do not contribute to fat mass, as they are composed of different types of tissue involved in movement and structural support. Blood and organs also do not consist of fat; they have other functions essential to the body's operation. Similarly, skin and hair do not form part of fat mass, as they primarily serve protective and aesthetic roles. Therefore, recognizing that fat mass primarily comprises subcutaneous adipose tissue is key to understanding body composition and the physiological roles of fat.