Every year each person eats many times the body’s weight in food, yet his or her own weight is little changed. The explanation for this apparent miracle lies largely in metabolism, a complex of chemical life processes whose name comes from the Greek metabole, meaning “conversion.”
Catabolism and anabolism
Metabolic processes bring about two major kinds of change: catabolism (the breaking down of organic compounds derived from food) and anabolism (the synthesis or building up of complex compounds from simple ones, also derived from food).
Catabolism releases energy by breaking down digested fats and carbohydrates. The breakdown products serve as fuels combining with oxygen from air for oxidation, a gradual “burning” process, which releases energy for cell-building and the activities of cells and muscles. Anabolism uses energy to manufacture proteins, fats, and certain carbohydrates. Meanwhile, old and damaged cells are being broken down and removed. The whole metabolic process is carefully controlled by subtle feedback systems.
Metabolism maintains life by processes that largely balance the body’s food input with its output of heat, mechanical energy, and processed body waste, to which is added waste from undigested roughage.
Enzymes in action
Metabolism involves complex chains of chemical reactions that would be impossible at body temperatures without help from the giant protein molecules called enzymes—biological catalysts that speed up chemical reactions between other substances without themselves undergoing metabolic change.
Specific enzymes operate on specific types of molecule (called the substrate), inducing chemical changes in the substrate molecules, which then break free from the enzymes and so allow these to tackle further substrate molecules. This happens astonishingly fast. Even at freezing point, one molecule of the enzyme catalase can break down 40,000 molecules of hydrogen peroxide in just one second. Accordingly, most enzymes are needed, and occur, in only tiny quantities.
Metabolism involves many kinds of enzyme most acting on only one kind of substrate, in special conditions of acidity or alkalinity, in the presence of auxiliary activators called coenzymes.
Different enzymes cooperate in systems geared to the step-by-step breakdown or building-up of compounds. The major carbohydrate breakdown mechanism, glycolysis, occurs mainly in muscle, where the sugar glucose or the animal starch glycogen break down via intermediate compounds to pyruvic acid. This then enters the Krebs, or citric acid cycle a complicated series of enzyme-controlled reactions that break down pyruvic acid in the presence of oxygen to yield carbon dioxide and ATP (adenosine triphosphate) a source of energy that can be used for internal cell activity or for the external work of muscular contraction. Step-by-step release of energy involving oxidation may yield 673 kilocalories from just one gram molecule of glucose.
Oxygen required for energy release comes usually from air breathed in by the lungs. But the heart cannot always deliver enough oxygen to muscles. If energy demand exceeds oxygen supply, muscles can go on working for a time by anaerobic (oxygenless) metabolism. But this soon uses up all high-energy phosphate stores, and the resulting oxygen debt produces much lactic acid waste that prevents cells from working properly and causes muscular fatigue.
Catabolism breaks down most compounds to acetyl coenzyme A, a product that can be oxidized, or used as building blocks for making many complicated compounds. Almost all the fats and carbohydrates that the body needs can be built up like this, with help from different enzymes cooperating to form special metabolic pathways. In fact, most of the amino acid ingredients of proteins can be synthesized inside the body. Minerals and vitamins (except vitamin D) cannot be synthesized.
Metabolic facts and figures
The body releases about four kilocalories of energy per gram of carbohydrates or protein catabolized, and nine kilocalories of energy per gram of fat.
The amount of energy released by the body in a given time is called its metabolic rate. This can be measured from the amount of oxygen consumed or carbon dioxide given off. A man’s basal metabolic rate energy output at rest per hour per unit surface area is about 40 kilocalories per square yard of skin, compared with 32 for a woman. Sedentary male office workers expend about 2,520 kilocalories, about 400 more than women. A man performing heavy manual work may use as many as 3,600 kilocalories a day.
While the kilocalories content of food eaten matches the kilocalories of energy expended, body weight remains unchanged. If energy input exceeds output, the body may store the surplus energy as fat and put on weight. But if energy output exceeds input, the body first burns up its reservoir of fat and then starts to break down proteins as a source of energy, in time producing the muscle wastage and weight loss seen in malnutrition.
The liver’s role
Besides producing the digestive fluid bile, the liver helps to process the carbohydrate and amino acid products of digestion brought by the hepatic portal vein from the digestive tract. It converts surplus glucose into the animal starch glycogen, storing it for reconversion and releasing it as glucose in later time of need. If the liver’s glycogen stores are already full, liver cells begin transforming any extra glucose into fat, which travels around the body and collects just below the skin in adipose tissue cells in the abdomen and other places.
The liver also converts potentially poisonous nitrogenous wastes derived from protein into urea, which is released into the bloodstream and excreted harmlessly by the kidneys.
Other valuable liver functions include storing vitamins, notably B12 which is needed in manufacturing red blood cells, and minerals, including iron, which is required for hemoglobin. Lastly, the liver makes a range of blood proteins.