Introduction

The adipose tissue is the largest endocrine organ in humans. It is mainly located beneath the skin (subcutaneous adipose tissue) but also in other areas (e.g., visceral adipose tissue and ectopic adipose tissue) [1-5]. It plays a vital role in the survival of humans. In addition to its mechanical and storage properties, adipose tissue has important metabolic and endocrine functions.

Excess adiposity is defined as the expansion of adipose tissue (e.g., hypertrophy and hyperplasia) leading to overweight/obesity and subsequent morbidity and mortality [6-8]. Obesity is a major health problem worldwide inflicting high cost to the society [9,10].

Management of excess adiposity requires multidisciplinary approaches including lifestyle (e.g., diet, exercise, and behavioral change), food supplements, drugs, medical devices, gut microbiome modulation, body contouring, and bariatric surgery [6,11-24].

Normal Adipose Tissue

The adipose tissue is the largest endocrine organ in humans (Figure 1). In normal young adults, the adipose tissue represents 8 to 19% of total body mass in men and 21 to 32% in women. It contains adipocytes, connective tissue, nerve tissue, vascular cells, and immune cells [2,5]. There are three types of adipose tissue, white adipose tissue (the predominant type), brown adipose tissue, and beige adipose tissue (new classification) [1-5].

Figure 1: The adipose tissue is the largest endocrine organ in humans. Copyright katerynakon (Kateryna Kon)/Depositphotos Inc.

Distribution

White Adipose Tissue

The white adipose tissue is located beneath the skin (subcutaneous adipose tissue) and other areas (e.g., visceral adipose tissue and ectopic adipose tissue) [1-5,25]. The subcutaneous white adipose tissue is the most abundant component of adipose tissue in lean subjects, representing approximately 80% of total adipose tissue (Figure 2). The visceral white adipose tissue is localized around several internal organs (e.g., epicardial adipose tissue). The ectopic white adipose tissue is localized within internal organs (e.g., liver and pancreas). These adipose tissues represent 6 to 20% of total adipose tissue.

Figure 2: The subcutaneous white adipose tissue represents approximately 80% of total adipose tissue in lean subjects. Copyright tussiksmail.gmail.com (Victor Josan)/Depositphotos Inc.

Brown Adipose Tissue

The brown adipose tissue is located primarily in the cervical, axillary, and paraspinal regions [1,2,5,25]. Brown adipose tissue represents approximately 1-2% of total adipose tissue [5].

Beige Adipose Tissue

The beige adipose tissue represents a new classification. It has been described as the presence of brown adipocytes within white adipose tissue [4,5].

Regulation

The white, brown, and beige adipose tissues are innervated by the sympathetic nervous system whose activation is necessary for the functions of different adipose tissues [4]. The metabolism of adipose tissue is also influenced by several hormones including growth hormone, leptin, insulin, and cortisol (non-exhaustive list).

Role

The white adipose tissue plays a vital role in the survival of humans. It is involved in heat insulation, dermal infection barrier, mechanical protection (cushion), and storage of excess energy as triglycerides. It is a highly active metabolic and endocrine organ involved in the production of adipocytokines (e.g., leptin, adiponectin, resistin, omentin, interleukin 6, and tumor necrosis factor alpha) which act at both local (autocrine, paracrine) and systemic (endocrine) levels, affecting energy homeostasis, insulin sensitivity, and neuroendocrine, cardiovascular, and immune functions (Figure 3) [1,2,4,5,25].