Why Do Men Accumulate Abdominal Visceral Fat?

Abstract

Men have a higher tendency to accumulate abdominal visceral fat compared to pre-menopausal women. The accumulation of abdominal visceral fat in men, which is a strong independent predictor of mortality, is mainly due to the higher dietary fat uptake by their abdominal visceral fat. Since dietary fat is absorbed by the enterocytes and transported to the circulation in the forms of chylomicrons and very low density lipoproteins (VLDLs), it is crucial to understand how these lipoproteins are different between men and women. The chylomicrons in men are generally bigger in size and more in quantity than those in women. During the postprandial state, these chylomicrons congest the lamina propria and the low-pressure lymphatics. In this paper, we propose that this congestion predisposes the chylomicron triglycerides to hydrolysis by lipoprotein lipase (LPL). The liberated fatty acids are then stored by the nearby abdominal visceral adipocytes, leading to the accumulation of abdominal visceral fat. These mechanisms perhaps explain why men, through their bigger and higher production of chylomicrons, are more likely to accumulate abdominal visceral fat than pre-menopausal women. This accumulation eventually leads to belly enlargement, which confers men their apple-shaped body.

Keywords: absorption; adipocyte; belly; chylomicron; gender; intestine; lipoprotein; visceral.

FIGURE 1

The storage of dietary triglycerides by abdominal visceral fat. Dietary triglycerides are digested and taken up by the enterocytes that line the intestinal lumen. The enterocytes secrete the dietary triglycerides in the form of VLDLs and chylomicrons to the lamina propria. Within the lamina propria, there are numerous blood capillaries (red/blue) and lymphatic capillaries (green). Due to their smaller size, some of the VLDLs can readily enter the lumen of the blood capillaries. In contrast, chylomicrons that are produced more by males are more likely to be retained in the lamina propria as they attempt to gain access to the lumen of the lymphatics. The higher retention of chylomicrons in the lamina propria predisposes their triglycerides to LPL hydrolysis. The liberated fatty acids, which are the products of LPL hydrolysis, can then be delivered to the abdominal visceral adipocytes that are located within the retroperitoneum and mesenteries. The fat that lies within the retroperitoneum is called retroperitoneal fat, and the fat in the mesenteries is known as intraperitoneal fat. The intraperitoneal fat depots shown here are mesocolonic, lesser omental, greater omental, and mesenteric fat. Note that the liberated fatty acids supply the abdominal visceral adipocytes prior to the subcutaneous adipocytes. VLDL = very low density lipoprotein; LPL = lipoprotein lipase.

FIGURE 2

Types of body fat in relation to abdominal visceral fat. Body fat can be categorized into brown, beige, and white fat. Based on its location, white fat can be further categorized into ectopic, subcutaneous, and visceral fat. Some of the examples of ectopic fat are intrahepatocellular, intrapancreatic, intramyocellular, and intracardiomyocellular fat. Subcutaneous fat includes abdominal, femoral, and gluteal subcutaneous fat. Visceral fat includes epicardial, retroperitoneal, and intraperitoneal fat. The intraperitoneal fat can be further subcategorized into mesocolonic, lesser omental, greater omental, and mesenteric fat. Abdominal visceral fat includes both retroperitoneal and intraperitoneal fat (bold/blue).

FIGURE 3

The proposed mechanisms for the sex differences in the development of the abdominal visceral fat. The size of lipoproteins is proposed to serve as an important factor in regulating the abdominal visceral adiposity. Due to the higher intake of dietary triglycerides and the potential hormonal regulation, men produce bigger and more chylomicrons. These chylomicrons trigger congestion within the lamina propria and lymphatics, subjecting their triglycerides to LPL hydrolysis. The subsequent uptake of the hydrolysis products by the surrounding adipocytes results in the accumulation of the abdominal visceral fat. LPL = lipoprotein lipase.