Basal metabolism, also known as Basal Metabolic Rate (BMR), is the minimum amount of energy required by the human body to maintain essential physiological functions while the body is at complete physical and mental rest. These essential functions include breathing, blood circulation, maintenance of body temperature, functioning of the heart, brain activity, kidney function, liver function, cellular repair, hormone secretion, and other vital metabolic activities necessary for sustaining life. Basal metabolism represents the largest component of the body's daily energy expenditure and usually accounts for approximately 60–75 percent of the total energy requirement in a healthy adult, depending on age, body composition, and level of physical activity. The concept of basal metabolism is fundamental in nutrition because it helps determine the energy needs of individuals and communities. Proper estimation of BMR is essential for planning balanced diets, preventing malnutrition, managing obesity, and promoting overall health. Community nutrition programs rely on knowledge of basal metabolism to estimate calorie requirements for different population groups such as infants, children, adolescents, adults, pregnant women, lactating mothers, and elderly individuals. Basal metabolism is measured under highly standardized conditions. The individual should be in a post-absorptive state, usually after fasting for 10 to 12 hours, should have had adequate sleep, should be physically and mentally relaxed, and should be in a comfortable environmental temperature. The person should not have engaged in recent physical activity, consumed stimulants, or experienced emotional stress before the measurement. Under these controlled conditions, the oxygen consumed and carbon dioxide produced by the body are measured using indirect calorimetry, which provides an estimate of energy expenditure. Basal Metabolic Rate is generally expressed in kilocalories per day or kilocalories per square meter of body surface area per hour. Since body size influences metabolism, BMR may also be expressed relative to body weight or lean body mass. Basal metabolism occurs because every cell in the body requires a continuous supply of energy. The heart beats continuously to pump blood throughout the body. The lungs perform respiration by exchanging oxygen and carbon dioxide. The kidneys filter waste products from the blood. The liver carries out numerous metabolic reactions, including detoxification and nutrient metabolism. The brain remains active even during sleep, controlling body functions and maintaining consciousness. Cells constantly synthesize proteins, repair tissues, and maintain ionic gradients across cell membranes. All these activities require ATP, the energy currency of the body, which is generated through metabolic reactions involving carbohydrates, fats, and proteins. The physiological basis of basal metabolism involves complex biochemical processes. Nutrients consumed in food are digested and absorbed into the bloodstream. Glucose enters cells and undergoes glycolysis followed by the citric acid cycle and oxidative phosphorylation to produce ATP. Fatty acids undergo beta-oxidation before entering the citric acid cycle. Amino acids contribute to protein synthesis and may also be used for energy when necessary. These metabolic pathways continue even during complete rest because the body's organs require uninterrupted energy to maintain life. Several factors influence basal metabolism, making it different among individuals. Age is one of the most important factors. Infants and young children have a very high BMR because they experience rapid growth and development. Their cells divide quickly, tissues grow actively, and energy demand is high. During adolescence, BMR remains elevated due to growth spurts and hormonal changes. As people age, muscle mass gradually decreases while fat mass often increases. Since muscle tissue is more metabolically active than fat tissue, the BMR declines with advancing age. Elderly individuals therefore require fewer calories than younger adults, although they still need adequate nutrients to maintain health. Sex also affects basal metabolism. Males generally have a higher BMR than females because they possess greater lean body mass and larger organ size. Testosterone promotes muscle development, increasing metabolic activity. Females naturally have a higher percentage of body fat and relatively lower muscle mass, leading to a lower basal metabolic rate under similar conditions. Body size and body composition significantly influence BMR. Larger individuals require more energy because they possess more tissues that require maintenance. However, body composition is even more important than total body weight. Muscle tissue consumes more energy than adipose tissue even at rest. Therefore, individuals with greater muscle mass have higher basal metabolism compared to those with higher body fat percentage. Body surface area also influences heat loss from the body. Individuals with larger body surface areas lose more heat to the environment and therefore require more energy to maintain normal body temperature. Taller individuals generally have slightly higher BMR than shorter individuals of similar weight. Genetic factors contribute to variations in