The accumulation of fat tissue just above the buttocks is a common biological phenomenon. This area of fat storage often seems resistant to diet and exercise, leading to a perception that it is different from other body fat. The appearance of this fat is rooted in a complex interplay of genetics, hormones, and the unique metabolic characteristics of the fat cells in this location. Understanding the biology behind this accumulation provides clarity on why it forms and how it relates to overall health and body shape.
Identifying the Area
The fat located on the lower back, above the curve of the glutes and around the sides of the waist, has both informal and formal anatomical names. Colloquially, this region is often referred to as the “flanks” or “love handles.” This terminology refers to the excess tissue that extends outward from the waistline, particularly noticeable when wearing fitted clothing.
From an anatomical perspective, the tissue is broadly classified as the lumbo-sacral fat pads. The accumulation can occur around the iliac crest—the curved upper border of the hip bone—and sometimes extends into the corpus adiposum trigoni lumbalis, or the fat pad of the lumbar trigone. This fat is predominantly subcutaneous, meaning it lies just beneath the skin, contributing significantly to the overall contour of the lower torso.
Biological Drivers of Fat Storage in the Flank Region
Fat is preferentially stored in the flank region due to inherited traits and the body’s hormonal environment. Genetic factors play an influential role in dictating body fat distribution, with heritability estimates for distribution between the trunk and extremities being high.
The metabolic activity of the fat cells contributes to the area’s reputation for being “stubborn.” Fat cells contain two types of receptors that respond to adrenaline: alpha-2 and beta-adrenergic receptors. Beta-receptors promote lipolysis (the breakdown of fat for energy), while alpha-2 receptors inhibit this process.
Fat deposits resistant to mobilization, such as those in the flanks, typically have a higher concentration of alpha-2 receptors compared to beta-receptors. This imbalance means that when the body signals a need for energy, fat cells in the lumbosacral region are slower to release stored energy than fat cells in other areas. This biological resistance explains why fat loss through general exercise can be challenging in this area.
The distribution is also modulated by sex hormones. Estrogen promotes fat storage in the lower body (hips, thighs, and gluteal-femoral region), a pattern known as gynoid fat distribution. Conversely, lower levels of estrogen, such as after menopause, often lead to a shift in fat storage toward the central, abdominal, and flank areas.
Stress hormones, specifically cortisol, also influence fat deposition in the central and upper-body regions, including the flanks. Chronic stress leading to elevated cortisol levels can promote fat accumulation because cortisol receptors are denser in central adipose tissue. The combined effect of these hormonal signals and the receptor ratio contributes to localized fat storage.
Health Context of Subcutaneous Lower Back Fat
Understanding the health implications of lower back fat requires distinguishing it from other types of fat. The fat above the bum is overwhelmingly subcutaneous fat, the layer situated directly under the skin. This is biologically different from visceral fat, which is stored deeper within the abdominal cavity, surrounding the internal organs.
Subcutaneous fat, including fat in the flank area, is metabolically less detrimental than visceral fat. Visceral fat is highly active, releasing inflammatory compounds and fatty acids directly into the liver, which is linked to an increased risk of heart disease and type 2 diabetes. Therefore, subcutaneous fat in the lower back does not pose the same immediate health risk as excessive visceral fat.
The presence of fat in the flanks is a component of the “pear” or gynoid body shape, characterized by fat storage around the hips and lower body. Studies show that individuals with a pear-shaped distribution have a lower risk of cardiometabolic disorders compared to those with an “apple” or android shape, where fat is concentrated in the abdomen.
When measuring health risk, the waist-to-hip ratio is often used, and flank fat is factored into the hip circumference measurement. While the fat is primarily subcutaneous, a large volume in the flank area can indicate an overall positive energy balance and may coincide with the accumulation of visceral fat in the abdomen.