The word "metabolism" is used frequently in both scientific and popular discourse, often with a looseness that can obscure rather than clarify the underlying biology. In its most precise sense, metabolism refers to the totality of chemical reactions occurring within a living organism that are necessary to sustain life. These reactions encompass the conversion of nutrients into usable energy, the synthesis of biological molecules, the elimination of waste products, and the ongoing maintenance of cellular structures. This article presents the foundational vocabulary and conceptual framework of metabolism, without framing any component as a target for intervention or optimisation.
Catabolism and Anabolism: The Two Directions
Metabolic reactions can be broadly divided into two categories that represent opposite directions of chemical flow. Catabolism refers to the breakdown of complex molecules into simpler ones, releasing energy in the process. The digestion of carbohydrates into glucose, and the subsequent breakdown of glucose through glycolysis and the citric acid cycle, are examples of catabolic pathways. Anabolism, by contrast, refers to the synthesis of complex molecules from simpler precursors, a process that requires an input of energy. The construction of proteins from amino acids, or the assembly of glycogen from glucose units, are examples of anabolic processes.
These two directions are not sequentially distinct phases of metabolism; they operate simultaneously throughout the body, with different tissues and organs tending toward different metabolic orientations depending on their function and the broader physiological state of the organism at any given moment.
Energy Currency: The Role of ATP
The universal energy currency of cellular metabolism is adenosine triphosphate, universally abbreviated as ATP. Energy released from catabolic reactions is not used directly to power cellular work; instead, it is captured in the form of ATP through the phosphorylation of adenosine diphosphate (ADP). When energy is required for cellular processes — muscle contraction, active transport of molecules across membranes, biosynthesis — the terminal phosphate group of ATP is cleaved, releasing energy and regenerating ADP.
This system functions as a rechargeable currency rather than a storage reservoir; the body's total pool of ATP is relatively small and is continuously regenerated from the catabolic processing of macronutrients. The three main classes of macronutrients — carbohydrates, fats, and proteins — each feed into distinct pathways that ultimately converge on the regeneration of ATP, though via different routes and with different efficiencies.
Basal Metabolic Rate
Even in a state of complete rest, the body requires a continuous supply of energy to maintain basic physiological functions — cardiac activity, respiration, neural function, thermoregulation, and the ongoing maintenance of cellular structures. The rate at which energy is expended under these minimal conditions is referred to as the basal metabolic rate (BMR). Measuring BMR requires strict conditions: the individual must be in a post-absorptive state (typically meaning no food intake for a defined period), at thermal neutrality, and at complete physical rest.
In practice, a related and more practically accessible concept is the resting metabolic rate (RMR), which is measured under less stringent conditions and provides an estimate of baseline energy expenditure applicable to everyday contexts. BMR and RMR account for the largest single component of total daily energy expenditure in most adults — generally estimated at somewhere between 60 and 75 percent — with physical activity and the energy cost of digestion (referred to as the thermic effect of food) accounting for the remainder.
BMR is influenced by a range of factors including body composition, age, sex, hormonal status, and ambient temperature. It is not a fixed or static value and changes over time in response to alterations in body composition and physiological state. The common simplification that metabolic rate is a single, fixed number that determines body weight is a considerable oversimplification of a dynamic and multi-factorial system.
Metabolic Rate During Sleep
Sleep is not metabolically inert. While total energy expenditure declines modestly during sleep relative to waking resting states, the body continues to perform extensive physiological work across the night. The pattern of metabolic activity during sleep is not uniform; it varies across the sleep cycle and between sleep stages. During slow-wave sleep, metabolic rate reaches its lowest levels, while REM sleep is associated with a marked increase in brain glucose utilisation and, in some measurements, an elevation of overall metabolic activity toward levels approaching quiet wakefulness.
Various hypotheses link the metabolic activity of sleep — particularly slow-wave sleep — to processes of cellular maintenance and the clearance of metabolic waste products from the brain via the glymphatic system, a pathway that appears to be substantially more active during sleep than during wakefulness. This area of research is relatively recent and continues to develop; the materials here reflect the current conceptual frameworks rather than settled conclusions.
Hormones and Metabolic Regulation
Metabolic processes are coordinated in large part through hormonal signalling. Insulin and glucagon, produced by the pancreas, regulate the balance between glucose uptake and storage (insulin) and the mobilisation of glucose from stored reserves (glucagon). Cortisol, a steroid hormone produced by the adrenal glands, influences metabolic rate, glucose availability, and the balance between catabolism and anabolism. Thyroid hormones set the baseline pace of cellular metabolism across most tissues.
These hormones do not act in isolation; they function within a complex network of feedback relationships and interact with signals from the nervous system, the circadian clock, and external inputs such as food intake and physical activity. The circadian regulation of hormonal secretion — in which the timing of hormonal peaks and troughs is programmed by the biological clock — is one of the mechanisms through which sleep and metabolic function are interrelated, as the hormonal profile of the sleeping body differs substantially from that of the waking body across the full 24-hour cycle.