Brown adipose tissue in the postnatal period

Brown adipose tissue (BAT) is crucial for a newborn’s survival in the postnatal period because it plays a decisive role in thermoregulation and energy adaptation. In the first days of life, newborns are particularly exposed to the risk of hypothermia due to a high surface-area-to-body-weight ratio, reduced muscle mass, and the immaturity of heat production mechanisms based on shivering. In this context, BAT provides non-shivering thermogenesis—an autonomic process that generates heat without the activation of skeletal muscles—regulated by stimulation of the sympathetic nervous system and by mobilization of intracellular energy substrates.

This thermogenic activity helps maintain body temperature within optimal physiological ranges, protecting vital organs such as the brain, heart, and kidneys, and ensuring an adequate energy supply even during normal physiological fasting or under conditions of environmental metabolic stress. BAT function is therefore indispensable not only for immediate survival, but also for the newborn’s metabolic adaptation and overall physiological stability.

Morphology and differences from white adipose tissue

BAT differs markedly from white adipose tissue (WAT) in structural, metabolic, and functional characteristics. Brown adipocytes contain numerous mitochondria—highly metabolic organelles capable of generating heat—and multiple lipid droplets that provide readily available energy substrates. Vascularization is abundant, enabling rapid delivery of oxygen and nutrients, which is essential for a metabolism with high energy demand. In contrast, WAT consists of cells with a single large lipid vacuole, few mitochondria, and more limited vascularization. The main function of white adipose tissue is long-term energy storage and endocrine regulation through the secretion of hormones and adipokines such as leptin and adiponectin, which modulate systemic energy balance and insulin sensitivity. The functional interplay between BAT and WAT reflects evolutionary adaptive strategies: while brown fat responds rapidly to thermal and metabolic stimuli, white fat ensures energy accumulation and long-term metabolic regulation.

Thermogenic function and metabolic role

BAT-mediated non-shivering thermogenesis is activated by sympathetic stimulation, which triggers norepinephrine release and the mobilization of fatty acids stored in intracellular lipid droplets. The energy derived from lipid catabolism is then converted into heat, allowing a rapid rise in body temperature without engaging skeletal muscles. Beyond its immediate thermogenic role, clinical and experimental evidence suggests that BAT may influence long-term metabolism. The amount and activity of brown fat in the first days of life appear to modulate energy balance, insulin sensitivity, and obesity risk, indicating a role in neonatal metabolic programming. In addition, BAT contributes to acute lipid metabolism by rapidly mobilizing fatty acids for energy production, thereby protecting vital organs during periods of physiological fasting or metabolic stress, with potentially lasting effects on systemic metabolism.

Location in the newborn

In full-term newborns, BAT is distributed in strategic areas that maximize thermogenic capacity and protect vital organs, contributing to autonomous regulation of body temperature. The main BAT depot lies in the interscapular region, between the shoulder blades, and can often be visualized through radiologic imaging or MRI. This area represents a “central thermogenic hub,” because the heat produced can quickly spread toward the trunk and neck, helping stabilize the temperature of surrounding tissues.

Other significant concentrations are found in the neck and shoulders, regions that protect the upper trunk and support thermoregulation of the head, which is particularly sensitive to heat loss. The perirenal and paravertebral regions contain BAT that helps preserve the temperature of the kidneys, spinal cord, and vertebral column—organs essential for neonatal survival. The mediastinum also hosts brown adipose tissue that may contribute to thoracic thermoregulation and to maintaining optimal cardiopulmonary activity.

This uneven distribution of BAT explains why some areas of a newborn’s body may feel warmer to the touch—such as the interscapular region and neck—while others, such as the peripheral limbs, tend to be relatively cooler. This reflects both a lower presence of brown fat and limited peripheral vascularization. The sensation of differential warmth is therefore an indirect indicator of thermogenic activity and BAT distribution. Reduced local thermogenesis may often translate into cold limbs and a less thermally stable chest, making careful environmental and clinical management necessary—such as thermoregulated incubators after birth, skin-to-skin contact, and monitoring of central and peripheral body temperature. BAT localization determines not only the efficiency of thermogenesis, but also the perception of warmth and cold in different regions of the newborn, with direct implications for clinical care and prevention of hypothermia-related complications.

BAT can be considered a refined natural protective mechanism for the newborn: a highly specialized biological system that ensures thermal continuity and metabolic stability at a stage of life when autonomous adaptation to the environment is still limited. Through non-shivering thermogenesis, this tissue enables the newborn to defend effectively against environmental variations, preserving homeostasis and ensuring proper function of vital organs. Its activation—coordinated by complex neuroendocrine circuits—testifies to the high level of sophistication with which the human body protects life in its earliest and most vulnerable stages.