This blog post explores the physiological reasons behind how sex hormones and the characteristics of fat cells interact to create differences in fat accumulation and breakdown speeds across body regions in men and women.
The processes by which fat is stored and broken down within the body have been clearly elucidated through extensive research. Fat accumulates within fat cells in the form of triglycerides. Examining this process, dietary fat is converted into triglycerides through the action of various enzymes during digestion. It is then absorbed in the small intestine, transported via the bloodstream, and subsequently stored in adipose tissue. Since triglycerides cannot be directly absorbed into the cells of the small intestine, they are first broken down into fatty acids and glycerol by lipase, a fat-digesting enzyme secreted by the pancreas, before being absorbed. The fatty acids and glycerol absorbed into the small intestine cells are then recombined through a chemical reaction called esterification to form triglycerides again. This triglyceride is released from the small intestinal cells into the bloodstream and transported to various parts of the body.
When triglyceride reaches the capillaries near fat cells, it is broken down again into fatty acids and glycerol by lipase attached to the cell membrane of the capillary cells, and then absorbed into the fat cells. The lipase acting here is secreted by the fat cells and transported for fat absorption. Adipocytes re-esterify the absorbed fatty acids and glycerol, storing them as triglycerides. If the amount of triglycerides in the blood becomes excessively high, and existing adipocytes cannot store any more through expansion alone, the number of adipocytes increases to store the excess amount.
The triglycerides stored in fat cells are later broken down back into fatty acids and glycerol, secreted into the bloodstream, and serve as a crucial energy source for generating the energy needed by bodily organs. This triglyceride breakdown is categorized into catecholamine-stimulated lipolysis, where the neurotransmitter catecholamine activates hormone-sensitive lipase within fat cells, and basal lipolysis, which occurs without catecholamine action. Basal lipolysis occurs during normal, non-stressful periods without special energy demands, while catecholamine-stimulated lipolysis activates during periods requiring significant energy, such as intense exercise. Generally, the rate of triglyceride breakdown during basal lipolysis tends to increase with larger adipocyte size.
Therefore, regulating the process of triglyceride storage within fat cells or controlling the breakdown of triglycerides stored in fat cells becomes a key method for controlling body fat accumulation. Endocrine substances such as growth hormone and sex hormones are involved in this regulation of fat accumulation. Among these, growth hormone is known to promote fat breakdown by increasing sensitivity to catecholamine stimulation while simultaneously reducing the activity of lipase secreted by fat cells, thereby decreasing triglyceride storage within them. For this reason, triglyceride accumulation within fat cells increases during adulthood, when growth hormone secretion decreases, compared to adolescence, when secretion is high.
Meanwhile, blood concentrations of sex hormones increase during puberty, remain above a certain level in adulthood, and then decrease in old age. Although the specific mechanisms by which sex hormones influence fat accumulation and breakdown are not yet fully understood, recent studies have focused on the fact that in women, fat accumulates more in the subcutaneous fat cells beneath the skin of the buttocks and thighs, while in men, fat accumulates more in the visceral fat cells around the abdominal organs. These findings are used to explain the function of sex hormones.
These attempts to elucidate gender differences in fat accumulation can be discussed from two perspectives. The first is the difference in the patterns of fat accumulation and breakdown between genders. In adult visceral fat cells, the rate of catecholamine-stimulated lipolysis is faster in women than in men, and the activity of lipase secreted from fat cells is higher in men than in women. Conversely, in adult subcutaneous fat cells of the buttocks and thighs, the rate of catecholamine-stimulated lipolysis is faster in men than in women, and the amount of triglycerides esterified is greater in women than in men. Second is the difference in fat breakdown patterns by body region. In women, catecholamine-stimulated lipolysis occurs faster in visceral fat cells than in subcutaneous fat cells of the buttocks and thighs, whereas in men, there is almost no difference in rate between these two sites.
These differences in triglyceride storage and breakdown capacity across fat cells, based on sex and location, demonstrate that sex hormones are involved in a highly complex manner in regulating the processes of triglyceride storage and breakdown within fat cells.
