Lipid Composition of Adipose Tissue. Adipocyte Distribution. Brown Adipose Tissue

Although triglyceride is the major storage form, adipose tissue has been shown to also be a major site for cholesterol storage. Low-density lipoproteins add to cholesterol storage, whereas high-density lipoproteins both add and remove cholesterol. Apparently, human adipocytes synthesize little cholesterol but derive it from interstitial lipoproteins.

The fatty-acid composition of lipid stored in human adipose tissue differs somewhat between nonobese and obese individuals. In samples taken from subcutaneous adipose tissue by needle biopsy, the dominant fatty acid, oleic, was similar in both the obese and nonobese subjects, but the obese had a significantly lower percentage of the saturated myristic, palmitic, and stearic fatty acids. In contrast, the obese had a higher percentage of palmitoleic, a partially unsaturated fatty acid.

There were no significant differences between the two groups for any of the polyunsaturated fatty acids. Because of the significant group differences in the saturated fatty acids, the polyunsaturated to saturated (P/S) ratio was 0.49 in the obese and 0.41 in the nonobese (P < 0.05) (Table I). Lipid composition of adipose tissue will vary somewhat with a person’s habitual diet.

TABLE I. Adipose Tissue Fatty-Acid Composition in Nonobese Subjects and Obese Patients^a

Adipocyte Distribution. Gender effects are quite obvious in adipose tissue distribution in humans, with women having a larger fat depot generally but particularly in the femoralgluteal region. Such gender differences are associated with differences in sex hormone concentrations. In contrast, men, particularly as they grow older, tend to have relatively enlarged abdominal depots. However, such gender differences are not always clear-cut. Excess abdominal obesity, the so-called android type, appears to present an increased risk of cardiovascular disease, whereas the femoral- gluteal accumulation, or gynoid type, appears to pose no known risk.

With gross obesity, both the internal and the subcutaneous depots become quite large, with the latter relatively larger than the former. Although some differences in the regional mobilization of stored fat are possible, it is generally accepted that all depots participate with dietary restriction associated with considerable body weight loss.

Brown Adipose Tissue. What has been discussed thus far is white adipose tissue, which is by far the dominant form in mature humans. Brown fat exists as well in the newborn and particularly in rodents. Brown fat has distinctly different features than white fat and is regarded as the “flash heater,” important as part of the central heating system that functions in an animal’s arousal from hibernation and provides nonshivering thermogenesis in cold environments, the latter undoubtedly a protective mechanism in the newborn. Brown fat has considerable innervation and is quite responsive to adrenergic stimuli. It is noteworthy that denervated brown adipose tissue assumes the morphology of white adipose tissue.

Brown adipose tissue (BAT) has been identified by the color brought about by its extensive vascularity, mitochondrial density, and associated cytochrome content. It is characterized by round nuclei and multivesicular fat droplets. The most extensive sites of BAT are the perirenal and axillary-deep (i.e., at relatively central and internal sites) cervical areas.

Studies of mitochondrial energetics have revealed that BAT is specialized for sympathetically regulated heat production associated with uncoupled oxidative phosphorylation in which energy provided by metabolic substrates such as free fatty acids, is used to produce heat, instead of being used to generate ATP, the energy currency of the cell. In the infant, 30 g of BAT (1% of body weight) may well account for the thermogenesis response to cold exposure or norepinephrine challenge. By measuring blood flow along with temperature gradients, it has been calculated that the perirenal adipose tissue of young men which contains brown fat, produced 25% of the thermogenesis brought about by oral ephedrine administration. Thus, this small amount of BAT-containing tissue had a measurable thermic effect.

TABLE II. Some Known and Unknown Alterations in Brown Adipose Tissue (BAT) Function

TABLE III. Features Distinguishing Brown from White Adipose Tissues

A summary of the modifications in BAT function produced by different conditions in animals and man appears in Table II. Sites that contained BAT in the child presumably retain a biochemical distinction from white adipose tissue by virtue of the fact that the protein responsible for uncoupling oxidative phosphorylation can be identified at these sites in the adult. Also, evidence indicates that intra-abdominal fat is more biochemically active in terms of lipolysis and lipogenesis (i.e., in response to insulin and catecholamines). A comparison of features that distinguish brown from white adipose tissue appears in Table III.