Measurement of energy balance during voluntary overeating in rats unequivocally establishes the quantitative importance of diet-induced thermogenesis in energy balance. Like cold-induced thermogenesis, this form of heat production involves changes in the activity of the sympathetic nervous system and brown adipose tissue which suggest that this tissue may determine metabolic efficiency and resistance to obesity.

A role for brown adipose tissue in diet-induced thermogenesis

https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.1550-8528.1997.tb00591.x

Obesity is among the most common and costly chronic disorders worldwide. Estimates suggest that in the United States obesity affects one-third of adults, accounts for up to one-third of total mortality, is concentrated among lower income groups, and increasingly affects children as well as adults. A lack of effective options for long-term weight reduction magnifies the enormity of this problem; individuals who successfully complete behavioral and dietary weight-loss programs eventually regain most of the lost weight. We included evidence from basic science, clinical, and epidemiological literature to assess current knowledge regarding mechanisms underlying excess body-fat accumulation, the biological defense of excess fat mass, and the tendency for lost weight to be regained. A major area of emphasis is the science of energy homeostasis, the biological process that maintains weight stability by actively matching energy intake to energy expenditure over time. Growing evidence suggests that obesity is a disorder of the energy homeostasis system, rather than simply arising from the passive accumulation of excess weight. We need to elucidate the mechanisms underlying this "upward setting" or "resetting" of the defended level of body-fat mass, whether inherited or acquired. The ongoing study of how genetic, developmental, and environmental forces affect the energy homeostasis system will help us better understand these mechanisms and are therefore a major focus of this statement. The scientific goal is to elucidate obesity pathogenesis so as to better inform treatment, public policy, advocacy, and awareness of obesity in ways that ultimately diminish its public health and economic consequences.

/pdf/obesity-pathogenesis-an-endocrine-society-scientific-gjpkxgtl5r.pdf

Obesity Pathogenesis: An Endocrine Society Scientific Statement

In rodents, brown adipose tissue (BAT) regulates cold- and diet-induced thermogenesis (CIT; DIT). Whether BAT recruitment is reversible and how it impacts on energy metabolism have not been investigated in humans. We examined the effects of temperature acclimation on BAT, energy balance, and substrate metabolism in a prospective crossover study of 4-month duration, consisting of four consecutive blocks of 1-month overnight temperature acclimation (24 °C [month 1] → 19 °C [month 2] → 24 °C [month 3] → 27 °C [month 4]) of five healthy men in a temperature-controlled research facility. Sequential monthly acclimation modulated BAT reversibly, boosting and suppressing its abundance and activity in mild cold and warm conditions (P < 0.05), respectively, independent of seasonal fluctuations (P < 0.01). BAT acclimation did not alter CIT but was accompanied by DIT (P < 0.05) and postprandial insulin sensitivity enhancement (P < 0.05), evident only after cold acclimation. Circulating and adipose tissue, but not skeletal muscle, expression levels of leptin and adiponectin displayed reciprocal changes concordant with cold-acclimated insulin sensitization. These results suggest regulatory links between BAT thermal plasticity and glucose metabolism in humans, opening avenues to harnessing BAT for metabolic benefits.

https://diabetes.diabetesjournals.org/content/diabetes/63/11/3686.full.pdf

Temperature-Acclimated Brown Adipose Tissue Modulates Insulin Sensitivity in Humans

Evidence to support the hypothesis that the ratio of fat to body weight in the human female is a determinant of female reproductive ability is presented and the mechanism through which the degree of fatness may operate to increase or decrease reproductive capacity is described. During maturation the proportion of bone muscle and fat changes in the human female. The high proportion of fat at menarche (24%) and the even higher proportion of fat at physical maturity (26%-28%) may influence reproductive ability. Previous investigations indicate that a minimal ratio of fatness to body weight is required before menarche can be achieved and menstrual cycle regularity maintained. The mechanism by which these fat deposits influence reproduction was suggested by a recent study which revealed that androgens are converted to estrogens in the fatty tissue of the breast. Furthermore the degree of fatness apparently determines whether the most potent or least potent form of estrogen will be metabolized. Periodic assessment of the ratio of total body water to body weight (an index of fatness) among 118 females who were followed from menarche to physical maturation at age 16-18 facilitated the identification of minimal fatness thresholds which must be reached in order to establish menarche in cases of primary amenorrhea and to re-establish menstrual cycles in cases of secondary amenorrhea. Dancers and athletes frequently experience amenorrhea and delayed menarche. Although their body weights may be high the ratio of fat to lean in the body may be reduced below the critical level. In populations undergoing food shortages the mechanism whereby body fat regulates reproduction may have an adaptive value. Malnourished women will experience amenorrhea when their body weight is reduced to the critical level. Energy which might otherwise be expended in reproduction will be conserved. Furthermore births are prevented in a population already experiencing food shortages. This hypothesis may also explain why some populations in developing countries have small completed family size. Chronic malnutrition rather than the use of traditional birth control methods may account for their low fecundity.

Body fat, puberty and fertility.

1. In 125 cases of perirenal fat samples derived from human necropsies (from 1 month to 86 years), the occurrence of the brown adipose tissue was examined. Brown fat cells were contained in 72% (90: 125) of the cases. 2. In the infants, all samples contained maximal amounts of brown fat cells: the perirenal fat was composed almost exclusively of brown fat cells: in children and teenagers they began to diminish, and in younger adults further reductions were observed: after the fifth decade conspicuous diminutions occurred. In later decades the brown fat in the perirenal fat was small in amount. 3. This change in the amount of the brown fat tissue undergoes individual variations. In some cases, the brown fat tissue disappeared from perirenal fats in the early stages of life, while in others it persisted to very late stages of life. A man as old as 86 years possessed brown adipose cells in the perirenal fat. 4. Multilocular brown fat cells were classified into the following types: Type 1, fat-depleted cells: Type 2, small-locular cells: Type 3, middle-locular cells: Type 4, large-locular cells: Type 5, monolocular brown fat cells with a thick cytoplasmic rim and pseudomonolocular brown fat cells and Type 6, multilocular brown fat cells rich in cytoplasm. In the infants, all cell types were identified: the small-locular cells were in general scanty in all decades following infancy: in later decades of life, the most common cell types were middle-locular and large-locular cells. The fat-depleted cell is a particular cell type and may occur usually accompanied by multilocular brown fat cells rich in cytoplasm. 5. The fat lobules are composed of a centrally located brown fat cell area and a peripheral layer of monolocular white fat cells, which in the infants was very thin but in the following ages was gradually widened to invade the interior of the lobules. 6. The present findings suggest a continuous replacement of brown fat cells by white fat cells during advancing age. The monolocular brown fat cell with a thick cytoplasmic rim and the pseudomonolocular brown fat cells probably represent transitional forms between multilocular brown fat and monolocular white fat cells. These cell types were found throughout life, suggesting continuous transformation of the brown fat cell into the white fat cell.

C Yokochi

Papers

The occurrence of brown adipose tissue in perirenal fat in Japanese.