The hypodermis (the subcutaneous layer or superficial fascia) is present directly below the dermis. It connects the skin to the underlying fascia (fibrous tissue) of the bones and muscles. It is not strictly a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well-vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument.
The hypodermis is home to most of the fat that concerns people when trying to control their weight. Adipose tissue present in the hypodermis consists of fat-storing cells called adipocytes. This stored fat can serve as an energy reserve, insulate the body to prevent heat loss and act as a cushion to protect underlying structures from trauma.
Where the fat is deposited and accumulates within the hypodermis depends on hormones (testosterone, estrogen, insulin, glucagon, leptin, and others) and genetic factors. Fat distribution changes as our bodies mature and age. Males tend to accumulate fat in different areas (neck, arms, lower back, and abdomen) than females (breasts, hips, thighs, and buttocks). The body mass index (BMI) is often used as a measure of fat, derived from a mathematical formula that compares body weight (mass) to height. This means that its accuracy as a health indicator is unreliable, particularly in exceptionally physically fit individuals.
In many animals, excess calories are stored as fat to be used when food is not readily available. In much of the developed world, insufficient exercise coupled with the ready availability and consumption of high-calorie foods has resulted in unwanted accumulations of adipose tissue in many people. Although periodic accumulation of excess fat may have provided an evolutionary advantage to our ancestors, who experienced unpredictable bouts of famine, it is now becoming chronic and considered a significant health threat. Changes in lifestyle, specifically in diet and exercise, are the best ways to control body fat accumulation, especially when it reaches levels that increase the risk of heart disease and diabetes.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 5.1: Layers of the Skin
