**2. Adipose tissues: origin and development**

Adipose tissue is fundamentally fabricated from adipocytes as well as pre-adipocytes, macrophages, endothelial cells, fibroblasts, and leucocytes that are considered as a major player of systemically metabolic regulation [3]. The adipose tissue acts as a central metabolic organ for systematic energy homeostasis by acting as a caloric reservoir [4]. It is characterized as an important endocrine organ that is responsible for the secretion of many molecules like proteins, lipids and, miRNA (microRNA) [5]. These elements act as paracrine and endocrine signals that are critical for the function of adipose tissue as well as for non-adipose tissues that are required for the regulation of the body's metabolism and insulin sensitivity [4, 5].

Broadly, adipocytes are classified into two main categories i.e., white or brown adipocytes depending upon their morphology and nature of work/function. Adipose tissues also act as endocrine organs that are responsible for the secretion of multiple hormones. WAT plays a role in fatty acid biosynthesis by storing lipids in form of triglycerides as its cell have large vacuoles and fewer mitochondria. Similarly, BAT plays a role in glucose uptake and fatty acid breakdown, leading to energy dissipation and heat production. Its cells are multilocular with central nuclei and mitochondria rich in the expression of uncoupling protein-1 (UCP-1) that mediates the uncoupling of electron transport that leads to a decrement in the generation of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) with subsequent heat generation [5, 6]. The principal function of BAT is non-shivering thermogenesis; an energy-intensive process in which chemical energy is transformed into physical heat [5]. Further, the

### **Figure 1.**

*Physiological, morphological, cell composition, and function of adipocytes. The figure was modified from the following research paper by El Hadi et al., 2019.*

*Mechanism and Impact of Food Components in Burning Calories from White-to-Brown Adipose… DOI: http://dx.doi.org/10.5772/intechopen.104616*

process of thermogenesis is also performed by the third type of adipose tissue cell known as beige adipocytes. Beige adipocytes have many properties similar to brown adipocytes i.e., the presence of multilocular lipid droplets and numerous mitochondria expressing UCP-1 [5]. However, the process of thermogenesis is not restricted to brown adipocytes but it may be done by beige adipocytes that may emerge within WAT depots in a process known as "WAT browning" [5–7]. The physiological, morphological, cell composition, and function of adipocytes are shown in **Figure 1**.
