Overview

There is a common misconception that fat is only associated with weight issues such as obesity, contrarily, an appropriate quantity of it plays a critical role in maintaining the body’s homeostasis. Fat, also referred to as adipose tissue, is a type of loose connective tissue consisting principally of adipocytes (also called lipocytes or fat cells). Its primary functions include energy storage and insulation, protecting the body from cold or hot environments.

The human body houses different fat depots, each with a distinct structure and function. The main white fat depots include the visceral and subcutaneous adipose tissue (SAT). Visceral adipose tissue (VAT) is hidden, situated within the abdomen, and surrounding internal organs. SAT resides underneath the skin in multiple regions, including the abdominal, thighs, hips, and gluteus areas. In lean individuals, SAT constitutes up to 80% of total adipose tissue. While a normal amount of SAT is essential, an excess of it is correlated with an overabundance of VAT, which contributes to the development of various health implications such as cardiovascular diseases, diabetes and other metabolic disorders.¹

Studies exploring the effects of sex hormones in fat distribution and lipid metabolism revealed a significant discrepancy between men and women. Specifically, premenopausal those assigned female at birth (AFAB) tend to store higher amounts of  SAT, mostly in the hips and glutes, which they metabolise and utilise as fuels notably during pregnancy or lactation. Those assigned male at birth (AMAB) on the other hand, generally have lower overall body fat but are keener in accumulating excessive VAT in the abdomen, rendering them more susceptible to developing metabolic diseases.²

Composition and characteristics of subcutaneous fat

Anatomy and cell distribution

For a comprehensive understanding of SAT’s structure, a concise analysis of the skin layers is essential. The epidermis, dermis, and hypodermis (or subcutaneous tissue), are the three layers that comprise the skin. The latter is the innermost layer of skin and broadly consists of collagen and SAT.³ 

The structure of SAT principally incorporates adipocytes and connective tissue. A specific framework of SAT’s anatomical regions has emerged and includes⁴:

• Adipocyte (fat cells): A singular adipocyte varies in size based on nutritional intake, with a limit to its enlargement. Once this limit is reached, new adipocytes are formed. Age, biological sex, and anatomical region are among the factors influencing the size and number of fat cells.
• Fat lobules: A group of adipocytes forms lobules, which are separated by the fibrous septa. 
• Fat pearls: Fat lobules are encapsulated into fat pearls. These are visible yellow globules observed during SAT intersection.
• Fat section: Multiple fat pearls are arranged together along with fibrous connective tissue septa, which constitute the characteristic white walls within a fat section.
• Fat compartment: Groups of fat sections form fat compartments, such as those found in the hip, buttock, thigh, breast and abdomen regions. Differences in the anatomical skin surface between those AMAB  and AFAB can be attributed to variations in the size and shape of these fat compartments.

Normal functions of subcutaneous fat

A balanced body mass index (BMI) and specifically healthy proportions of body fat are imperative for the human body’s anatomy and normal functions. When it comes to subcutaneous fat, several unique functions have been established, including:

• Energy storing properties: Adipocytes are responsible for storing lipids in the form of triglycerides, consisting of free fatty acids and glycerol. During periods of fasting or exercise, adipocytes serve as reservoirs, undergoing lipolysis, releasing free fatty acids and providing the required energy. They act as buffers for excess lipid intake from a high-calorie diet and maintain energy supply during high-demand periods. Exceeding the storage capacity or the inability to generate new adipocytes leads to ectopic fat accumulation outside of the SAT depot.⁵
• Thermoregulation: SAT functions as an insulator, exhibiting heat resistance, thereby protecting the body from heat loss.¹
• Physical shock absorber: SAT acts as a protective barrier for underlying organs, preventing physical shock and trauma.
• Metabolic properties: Adipose tissue is identified as an endocrine organ, substantial for hormone production and regulation. SAT, in particular, is associated with a reduced risk of various metabolic diseases. Studies have demonstrated that SAT expansion enhances insulin sensitivity, minimising the risk of type 2 diabetes. Furthermore, obesity and metabolic syndrome are primarily linked with visceral fat, rather than SAT, due to the inflammatory state present in it. However, obesity, age and hormonal alterations can compromise SAT’s protective function against abnormal alterations, fostering the progression of metabolic diseases.⁶
• Anchoring properties: Blood vessels and nerves traverse the dermis and SAT, supplying both and establishing a linkage between the skin and the muscles.⁷
• Body sculpting: SAT is soft and pinchable, contributing to the overall body shape and appearance, and affecting the external image of the human body.

Fat levels and health implications

Factors affecting subcutaneous fat levels

Genetic predisposition, age, sex, diet, and sedentary lifestyles all play a key role in the distribution and size of SAT. Specifically,

• Genetics: SAT was calculated to be 57% heritable⁸ under specific environmental conditions and within a defined population. Various complex genetic traits are responsible for promoting the storage of excess lipids in SAT. Moreover, distinct genetic variants influence fat mass disposition, showing a preference for either abdominal or lower body fat accumulation.⁹
• Sex: The effects of sex hormones on SAT formation have been extensively documented. Generally, adipocytes contain oestrogen receptors that hold great regulatory functions. People AFAB  tend to have smaller adipocytes in SAT compared to those AMAB. Particularly, premenopausal women exhibit an increased number of small fat cells in the lower body.¹⁰ During the postmenopausal period, characterised by reduced oestrogen levels, there is a redistribution of SAT, favouring its accumulation in the abdomen.¹¹
• Diet: Dietary factors, specifically the type of fatty acid intake, can significantly impact SAT functions. Polyunsaturated fatty acid intake has been associated with a positive impact on the subcutaneous depot. Saturated fatty acids also greatly influence SAT.¹²
• Age: With age, there is a normal redistribution of adipose tissue and a shift from SAT to VAT. The reduction of SAT is responsible for the characteristic aesthetic changes observed in older individuals.¹³

Health risks with excess subcutaneous fat

SAT might not be directly linked to most comorbidities, but an excess of it often results in an increase in VAT levels, which can lead to serious health problems. 

Hormones such as catecholamines stimulate the process of lipolysis, while insulin inhibits it. An imbalance in this mechanism, by either inefficient or increased lipolysis, is associated with diabetes and metabolic syndrome respectively.¹² According to numerous scientific studies, SAT has protective properties against metabolic syndrome, but it is positively associated with high blood pressure and fasting glucose levels.¹⁴ However, excessive lipolysis of fat cells could lead to augmented levels of serum-free fatty acids, potentially contributing to insulin resistance, hypertriglyceridaemia and dyslipidemia.¹⁵

Furthermore, elevated levels of SAT, indicating an overall increase in total fat mass, can contribute to obesity. Obesity (BMI ≥ 30) is characterised not only by an excess of adipocytes but also involves a state of inflammation, along with adipose tissue dysfunction and dysregulated lipid metabolism, leading to the presence of fat ectopically. These aspects of obesity have deleterious effects on health.¹⁶

How to measure subcutaneous fat?

BMI and waist circumference are methods commonly used to estimate total body fat mass and could provide an approximate idea of SAT levels. However, they do not offer detailed insights into fat deposition. Various techniques are utilised for the measurement and assessment SAT levels¹⁷:

• Calipometry: This method estimates SAT depth by measuring skinfold thickness using calipers (pincers). Total body fat can subsequently be calculated using specific equations.
• Ultrasound: The high-resolution images from an ultrasound device make it highly accurate in distinguishing SAT from VAT and in computing SAT depth.
• CT and MRI: These two methods are employed for precise SAT measurement. MRI is highly accurate in determining body fat content and its distribution. However, these methods are not available for routine practice.

How to reduce/manage subcutaneous fat?

Various effective methods exist for reducing SAT levels and overall body fat. Firstly, adopting a healthy low-calorie diet with minimised fatty acid intake can decrease the number of adipocytes responsible for fat storage. Additionally, incorporating an equilibrated exercise routine can effectively utilise excess energy stored in fat cells, further reducing SAT. Aerobic activities and short high-intensity exercise programs like HIIT are particularly efficient in burning most calories and reducing abdominal fat.¹⁶

Furthermore, non-invasive or minimally invasive techniques have been developed in recent years, aiming to reduce SAT and total body fat while improving body image. Cosmetic non-surgical approaches include: low-level laser therapy (LLLT), cryolipolysis, radio frequency (RF) and high-intensity focused ultrasound (HIFU).¹⁸

In conclusion, liposuction is a common invasive solution for reducing SAT, but it is supplemented with numerous adverse effects and a prolonged recovery period.¹⁸

Summary

Body fat, particularly SAT, is crucial when maintained within normal range levels. It serves to provide the body with necessary energy during high requirements and aids in regulating body temperature. An excess of SAT, deriving from surpassing its capacity to store lipids, can lead to storing fat ectopically, causing an overall increase in body fat and potentially resulting in overweight or obesity. As investigated, SAT and VAT levels are interconnected, an excess of the former may contribute to an excess of the latter, leading to various comorbidities, including diabetes, cardiovascular diseases, and metabolic syndrome.

An increase in body weight and body fat is normal with age. It is essential to maintain healthy levels of fat corresponding to age and sex-specific charts. If you are experiencing excessive weight gain or suspect that your SAT levels have increased, consult your physician to develop a personalised exercise regimen and diet plan. Getting professional guidance tailored to your specific needs is essential since each body composition is unique. Additionally, regular health check-ups and a holistic lifestyle hold the criteria for overall well-being.