For many years, there was little interest in the biochemistry or physiology of adipose tissue. The knowledge of adipocyte biology has been considerably expanded during the last 50 years and it is now well established that they play an important, dynamic role in metabolic regulation.
The fat cell is made up of connective tissue (cells, fibres, fluid) with adipocytes containing nuclei, receptors and lipid droplets of fat. Approximately 90% of the adipocyte is storage of triglycerides. The remaining 10% consists of cytoplasm, mitochondria, nucleus, and other organelles.
A normal fat cell is 0.1mm but can fill or shrink depending on water and fat levels. In humans, fat cell numbers can vary drastically.
Lean individuals may have anywhere from 41 to 65 billion fat cells while obese individuals can have upwards of 200 billion fat cells.
Essential Body Fat refers to the fat around your organs (where it provides an important cushioning role), in the nervous system (the nerves are surrounded by sheaths of fat), and in the brain. In males, essential fat typically makes up around 3% of total body fat. In females, essential fat is higher, estimated at 9-12%.
Subcutaneous White Adipose Tissue (WAT) store ~80-90% of total body fat, mainly in the abdominal (around the waist), subscapular (on the upper back), gluteal and femoral (thigh) areas. These subcutaneous adipose tissues have distinct morphological and metabolic profiles and exhibit sex-specific differences in size and function.
WAT represents around 15-30% of body weight, and in obese individuals it increases up to 50%. WAT contain a single large droplet, with few mitochondria. Subcutaneous fat is found directly under the skin.
Visceral Adipose Tissue (VAT) is found underneath abdominal muscles, around organs, heart, liver and pancreas. It is common in males, however, it can also happen in females under certain conditions such as obesity after menopause, and PCOS (due to higher testosterone levels).
Excess VAT is also referred to as central obesity. Metabolic activity is highest in this region when there is a need for rapid energy supply, such as during physical activity or lactation.
Brown Adipose Tissue (BAT) is located mainly in the supraclavicular/dorsal cervical area. They contain multiple small lipid droplets, rich in mitochondria. BAT dissipates energy (mainly from fatty acids) to generate heat.
Human BAT is activated by acute cold exposure, being positively correlated to cold-induced increases in energy expenditure (adaptive thermogenesis) and is stimulated via the sympathetic nervous system. The metabolic activity of BAT differs among individuals, being lower in older and obese individuals. BAT is recognised as a regulator of whole-body energy expenditure and body fat.
Another type of fat, ‘beige fat’ is genetically distinct from white or brown and behaves like a hybrid of the two cell types, being able both to store and burn calories.
The newly discovered hormone that plays a significant role in this conversion is known as irisin. In 2012 researchers at Harvard Medical School first identified irisin as a hormone released from human muscle during physical activity.
Irisin induces the browning of white fat, turning fat from inert to metabolically active.
Continuing research has revealed that irisin is capable of reprogramming the body’s fat cells to burn energy instead of storing it.
Raising levels of irisin also stimulates the growth of neurons and improves cognition through the expression of BDNF (brain-derived neurotrophic factor) and activated genes involved in learning and memory.
The concentration of irisin in the blood is similar to that of other important hormones, such as insulin and leptin. Irisin levels are increased with exercise, which has helped to explain why exercise is healthy and how exercise ‘talks to’ various tissues in the body.
Research on the gut microbiota has also begun to unravel it’s tremendous role in our body.
The results of many new studies have provided insight into the microbiota-fat singling axis and beige-fat development in health and metabolic disease. In particular, on the way calories are absorbed and how fat develops.
A healthy gut microbiome is essential for regulation of multiple metabolic pathways, interconnecting the gastrointestinal tract, skin, liver, brain, and other organs.
The cell biology of adipocytes has led to the discovery of a number of promising concepts, and pathways for kinesiology application.