Jethalal S. Zaveri
We, human beings, like the other living things on planet Earth, are chemical beings, whose bodies are made up mainly of carbon compounds. We are active organisms and we get the energy to power our activities from chemical compounds contained in the foods and drinks consumed by us. Building materials for growth and repair are also obtained from foods, but they rarely come in ready-to-use forms. Carbohydrates, proteins and fats in the foods must be first broken down into simpler components (by digestive processes) in order to pass through cell-membranes and enter the body-cells; within the cells the products of digestion are subjected to further chemical reactions. In catabolic reactions, food materials are torn down into smaller units or oxidized to release the energy stored in their chemical bonds. In anabolic reactions, substances are built up into more complicated compounds—the characteristic chemicals of the body. Both breakdown and building up—or synthetic reactions—go on constantly in the body cells. The term metabolism refers to the sum total of all the anabolic and catabolic reactions of the body.
The catabolic reactions supply energy for activities such as muscle-contractions, transmission of nerve-impulses etc. The chemical energy stored in foodstuffs is also converted to other energy-forms such as heat and electricity. The energy is temporarily stored in convenient units in the ATP molecules. This "energy currency" permits a controlled release of energy as it is needed; if the energy were released all at once, as in a fire, it might destroy the cells.
Metabolic reactions must go on continually as long as life continues. If the energy-substrates are not replenished by food intake, the body-tissues themselves will be metabolized. The first preference for energy supply is given to the carbohydrates and fats. The amino acids from proteins are used mainly as building materials. Catabolism takes first priority. Anabolic processes are deferred if there are not enough materials available for both.
Carbohydrate Metabolism
A lump of sugar or a piece of fruit can give "instant energy" to a tired person. Food-sugar in the form of glucose is quickly absorbed through the stomach-lining and is carried by the blood-stream to the cells. Simple food substances such as water, simple salts, simple sugars, alcohol, some drugs do not need further digestion and can be absorbed directly through the stomach-lining. Complex sugars and starches make their effect felt less rapidly, but they too contribute to the body's energy reserves. Glucose is the main product of starch digestion and virtually all the sugar in the blood is in the form of glucose. Some of the major alternatives in the body for the sugars absorbed from the digestive tract are as follows: they may—
- pass into the circulation as blood-sugar.
- be carried to the liver and converted to glycogen and stored there,
- be converted to glycogen in skeletal muscles,
- be converted to lipids and stored in fat deposits,
- be converted to amino-acids,
- be oxidized in the tissues as energy sources,
- be excreted in the urine (as in the disorder diabetes).
Blood-sugar
When food is eaten, the blood-sugar level rises to a peak and then falls rapidly. A high blood-glucose concentration triggers an automatic regulatory mechanism in which the endocrine portion of pancreas pours out the hormone insulin which promotes the transport of glucose into the tissue cells. With the drop in sugar-level, secretion of insulin also decreases. On the other hand, where blood-sugar level falls below normal, other regulatory mechanisms come into play. These promote breakdown of glycogen in the liver and the release of glucose into the blood decrease glucose utilization, mobilize lipids from the cells and enhance glucose-absorption from the intestine.
The liver plays a pivotal role in the carbohydrate metabolism. Here, much, of the glucose is removed and synthesized into glycogen. If in abundance, some of the glucose is converted into fat and stored. When the glucose-supply is low, liver breaks down glycogen to glucose. Under some conditions, liver can synthesize glucose from non-carbohydrate sources. During starvation, first the glycogen-reserves are exhausted. Then the fat-deposits are called upon. Finally the tissue-proteins are broken down and their amino-acids used for producing glucose and energy.
Protein Metabolism
Apparently the body goes to a great deal of wasted effort. Proteins are first broken down into amino-acids by the digestive processes, absorbed into the blood-stream, delivered to the cells and then build back into proteins again. However this round-about procedure is necessary for two reasons: first, the food-protein molecules are too large to pass through cell-membranes, and second, the food proteins are not the (chemically) right proteins for the human body. All the proteins of all living organisms on earth, though differing from species to species, are built up from the same basic set of building materials viz. about twenty odd different amino-acids. Thus after the food-proteins are broken into their components, no matter from where they originally come from—a plant or an animal - they can be synthesized into human proteins.
Anabolism usually predominates over catabolism in protein metabolism. Besides syntheses of tissue-protein, a large portion of the food-proteins is used for a variety of non-protein synthesization.
Catabolism occurs in the liver. Amino-acids in excess of the body's synthetic needs are de-aminised to, ultimately, form urea which is excreted. A gram of protein releases about 4 calories of heat, the same as a gram of carbohydrate and less than one-half that of fat.
Protein-digestion is a long drawn-out process. Small amounts of amino-acids absorbed from the digestive tract are transported through the body and quickly resynthesized into body-proteins. Cells and tissues can store protein, but each type of tissue has an upper limit to the amount of protein it can store, and excess amino-acids are catabolized or excreted in the urine.
Protein-synthesis is so important a factor in growth and tissue-formation that its metabolism is influenced by many endocrine hormones. Growth hormone, androgens (especially testosterone), thyroxin as well as adrenal cortical hormones all directly or indirectly promote cell and tissue-formation.
Fat (Lipid) Metabolism
The sedentary way of life, combined with a diet with more calories than the body needs each day, turns one of the strong point of human metabolism into a negative factor. Excess fats, above what the body needs, are deposited in characteristic deposits. Excess carbohydrates and even protein can be converted into fats and stored. Fat yields 9 calories per gram.
Immediately after a meal, the fat-concentration in the blood may rise. As the fat-laden blood passes through certain tissues—adipose[1] tissue, heart-muscle and skeletal muscles - fat is converted to glycerol or fatty acids. Glycerol is catabolised like glucose; the fatty acids can be oxidized for energy (except in nerve-tissue which uses only glucose for energy), or can be stored after resynthesization by the fat-cells of the adipose tissue. Fat is also a vital constituent of tissue and cells.
Though the fat deposits scattered through the body often seem all too permanent, actually they are in a dynamic state, with a breakdown, re-synthesis and exchange with the plasma fat. It has been found that as much as half of the total fat-reserve changes position each day, and even in the most obese person, the fat is not the same fat that was stored two or three weeks before.
The liver plays a key role in fat metabolism. It synthesizes fatty acids from carbohydrates, and saturates, unsaturates and oxidizes them by a specialized process. When carbohydrate metabolism is abnormally low, as in starvation or diabetes, increased amounts of fatty acids are utilised for energy.
The role of cholesterol is still cloaked in controversy. It is absorbed from the intestine; but even when kept on a cholesterol-free diet, the body synthesizes its own cholesterol. It is a constituent of bile-salts and is used in the formation of several hormones including cortisone and progesterone.
Insulin is the main regulator of fatty acid mobilization facilitating their transport into the cells. Several other hormones—epinephrine, adrenal cortical hormones, growth hormone, and the thyroid hormone—increase fat-mobilization. Sympathetic innervation of the adipose tissue has a similar effect. The levels of blood-glucose and fatty acids are inversely related. When the glucose is high, the fatty acid level is low and vice versa.
Water Metabolism
We produce a great deal of metabolic water. Water is formed in the oxidation of glucose, proteins and lipids. A total of about 375 millilitres of water is produced in the human body every day, but in order to maintain homeostasis[2], we need about 10 time that amount. Water not only participates in reactions of digestion and other reactions but is also lost in urine, feces, sweat etc. Food and drinks make up the difference of our daily fluid needs. Most solid foods also contain substantial amounts of water.
Energy Metabolism
Virtually all the processes and activities that go on in the human body require a continual supply of energy. The energy that powers the human body is chemical energy. The energy metabolism of the body consists of a sequence of reactions that has much in common with the burning of a sigree or a stove. Fuel is oxidized by oxygen in both cases, releasing the energy and forming carbon-dioxide and water. But there are a number of important distinctions. A fire burns rapidly and the energy released is given off all at once in the form of heat and light. The burning of the fuel in the body proceeds in a series of small controlled steps; much of the energy released in each step is recaptured and stored. Chemical energy is used in the body to do work. It may be converted to mechanical energy (to lift a load) or electrical energy (for transmission of nerve-impulses) or it may be used as chemical energy to build up complex body chemicals.
Part of the body energy is given off as heat at the time of release. Further energy is lost as heat during the performance of the work. Ultimately all the chemical energy is directly or indirectly converted to heat, but heat-production is not entirely useless. It helps to maintain body-temperature at the optimum level for enzyme-catalyzed reactions. But often the heat production is in excess of the body's needs and special mechanisms are provided to get rid of the excess. Although the energy of food is not used directly as heat, it is customarily measured in heat units; the unit commonly used in physiology is called Calories.[3]
