Different tissues have specialized metabolic profiles that reflect their unique functions. Understanding tissue-specific metabolism is essential for appreciating how metabolic diseases affect particular organs and how metabolic pathways are coordinated across the body.
The Liver
The liver is the central metabolic hub, processing, storing, and distributing nutrients. Hepatocytes contain the full complement of metabolic enzymes and can perform both glucose utilization and glucose production. The liver expresses glucokinase rather than hexokinase, which has a higher Km for glucose and is not inhibited by glucose-6-phosphate. This allows the liver to phosphorylate glucose only when blood glucose is elevated, preventing futile cycling between glucose uptake and glucose production.
The liver is the primary site of gluconeogenesis, urea synthesis, ketogenesis, and lipoprotein production. It has a high capacity for fatty acid oxidation and can use amino acids as gluconeogenic substrates. The zonation of the liver creates metabolic specialization across the lobule, with periportal hepatocytes being more active in gluconeogenesis and urea synthesis, while perivenous hepatocytes are more active in glycolysis and lipogenesis.
Skeletal Muscle
Skeletal muscle is the largest tissue by mass and accounts for a major portion of energy expenditure. Muscle fibers are classified as type I, slow-oxidative fibers that rely primarily on fatty acid oxidation and have high mitochondrial density, and type II, fast-glycolytic fibers that rely more on glycolysis and have lower mitochondrial density. Most human muscle contains a mixture of fiber types.
During rest, muscle primarily uses fatty acids. During moderate exercise, glucose uptake increases through AMPK-mediated GLUT4 translocation, and both glucose and fatty acids are used. During intense exercise, glycolysis predominates, producing lactate. Muscle has large glycogen stores, about 300 to 400 grams total, which provide glucose for glycolysis without requiring blood glucose. During recovery, muscle glycogen is replenished through the insulin-dependent uptake of glucose.
Adipose Tissue
Adipose tissue exists as white and brown types. White adipose tissue stores triacylglycerols and releases fatty acids during fasting. It expresses hormone-sensitive lipase and adipose triglyceride lipase, which are activated by catecholamines and inhibited by insulin. Adipose tissue also functions as an endocrine organ, secreting adipokines including leptin, adiponectin, and resistin that regulate appetite, insulin sensitivity, and inflammation.
Brown adipose tissue is specialized for thermogenesis through uncoupling protein 1, which dissipates the mitochondrial proton gradient as heat rather than generating ATP. Brown adipose tissue is abundant in infants and is present in adults at metabolically significant levels, particularly in the supraclavicular and paravertebral regions. Cold exposure activates brown adipose tissue through sympathetic stimulation, increasing energy expenditure.
The Heart
The heart is an obligate aerobic organ with high energy demands, consuming approximately 6 kilograms of ATP daily. Unlike skeletal muscle, the heart maintains continuous contractile activity and cannot rely on anaerobic metabolism. The heart primarily uses fatty acids, which supply 60 to 90% of its energy requirements. The remaining energy comes from glucose, lactate, and ketone bodies in varying proportions depending on substrate availability.
The heart expresses a unique isoform of lactate dehydrogenase, LDH1, which is inhibited by high pyruvate levels. This favors the conversion of lactate to pyruvate, allowing the heart to use circulating lactate as a fuel. The heart’s metabolic flexibility allows it to switch between substrates, but in conditions such as heart failure, metabolic inflexibility develops with reduced fatty acid oxidation and increased glucose utilization.
The Brain
The brain has the highest metabolic rate of any organ, consuming about 20% of the body’s oxygen despite representing only 2% of body weight. Under normal fed conditions, the brain uses glucose almost exclusively. Glucose is transported across the blood-brain barrier by GLUT1 and taken up by neurons through GLUT3, which has a low Km ensuring glucose uptake even when blood glucose is low.
During prolonged fasting, the brain adapts to use ketone bodies, primarily beta-hydroxybutyrate and acetoacetate. Ketone bodies enter the brain through monocarboxylate transporters and are converted to acetyl-CoA for energy production. After about 3 days of fasting, ketone bodies supply about 30% of the brain’s energy, increasing to 70% after several weeks of starvation. This adaptation reduces the need for gluconeogenesis from muscle protein.
Red Blood Cells
Erythrocytes lack mitochondria and rely entirely on anaerobic glycolysis for ATP production. They metabolize glucose to lactate at a rate of about 40 grams of glucose daily. Red blood cells also use the pentose phosphate pathway to generate NADPH for maintaining reduced glutathione and protecting against oxidative damage.
