In nutrition, adaptation has specific meanings, and understanding what these meanings are provides the health professional with the insight necessary to meet a client's needs. When we say, "He isn't adapted to such a rich diet yet, let's build up to it gradually," or "Before we attempt surgery on this client we had better build her up nutritionally; she isn't equipped to withstand it at this point", we are speaking of concrete physical realities. Also, adaptation explains otherwise incomprehensible everyday happenings. Why is it, for example, that a person can drink large amounts of alcohol on one occasion without getting drunk, while on the other he can't? Nutrition books often make statements about the body's response to stress, illness, medication, drugs, or nutrients based on its "prior nutrition status", or its "health", or "physiological state". This discussion is intended to help students understand specifically what these statements mean.

 

            For any adaptive change to come about, the body's proteins have to be altered, and this can occur in two ways. First, the rate at which the proteins work can be either sped up or slowed down. Second, the number of working protein units can be increased or decreased. The first kind of change, the change in rate of action, makes possible quick responses to short-term needs, while the change in number of working protein units brings about adjustments to longer-term, more gradual changes in demand. This type of change will be discussed here. Whenever a demand is placed on the body such that its ordinary responses (rate changes) are not enough to rise to the occasion, there is a need to alter the body's physical equipment (number of protein molecules available to do the job). The ability to respond to this need has been referred to as a kind of "second order homeostasis".

 

            A living cell is like a factory in many ways, but the equipment inside a cell is different in a major way than in a factory, and this difference makes the cell much more adaptable to changing circumstances. A factory is built of heavy, permanent equipment, which can turn our a specific number of products—say, cars—per day for as long as it is operated. Should demand increase, a major renovation would have to be undertaken to enlarge the factory or build a new one; should demand decrease, some of the equipment could be idled; but in either case a certain amount of inefficiency would be inevitable. The factory is inelastic; it is not prepared to shrink or grow as a cell can do. But in a cell, the protein machinery itself is being replaced all the time anyway—every hour—and so its nature can be quickly altered.

 

            To explain how this replacement takes place: most of the body's machinery is composed of proteins—enzymes, transport proteins, "pumps", membrane receptors, and the like. Each type of machinery is represented by a population of identical molecules, each with a certain life expectancy. At any point in time, a study of this population of protein molecules would reveal that some of its members are newly made and just beginning to work. Others are established and hard at work, still others are at the end of their useful lives and about to be dismantled and their parts recycled. The making and the taking apart balance so that the number of protein molecules present and working remains constant. These rates may be very fast (9 out of every 10 protein molecules in the population may be replaced every hour) or very slow (1 out of every 10 may be replaced in a month). The rate at which a cell replaces any particular set of proteins determines the half-life of the proteins—a term that characterizes the stability of the population size.

 

            Some proteins have a short half-life, some a long one. Under constant conditions, the difference between the two kinds is not obvious, but if a disturbance arises, the differing half-lives have a marked effect. For example, suppose protein synthesis suddenly shuts down (perhaps and essential amino acid is missing). The proteins with short half-lives will rapidly disappear from the cell because they are still being rapidly degraded, while those with long half-lives will linger and continue to work because their degradation is proceeding very slowly.

 

            The explanation just given has presented several concepts that biochemists usually express in technical terms. They speak not only of the half-life of proteins, but of the rate of synthesis and the rate of degradation. Using these terms they devise precise mathematical formulas to describe quantitatively what is going on within a cell or organ system.  The description given here in everyday terms, however, should suffice to permit understanding of the kinds of changes that take place in the body as it continuously adapts to its changing environmental circumstances. The examples that follow should deepen that understanding.

 

 

            In the last stages of kwashiorkor, when the disease has become very severe, the digestive system shuts down and the pancreas stops producing digestive enzymes. In this case adaptation involves making less of something. This confers a survival advantage on the child, because energy and building blocks are thereby conserved to serve higher purposes such as maintaining the brain, heart, and lung tissue and their minimum vital functions. The making of less equipment for the GI tract and pancreas is reflected in the atrophy of those organs. They lose so much protein (and associated supporting lipid and other material) that they become physically smaller. The intestinal villi shrink, and the areas of the pancreas that produce digestive enzymes disappear.

 

            When a kwashiorkor child has been brought into the hospital very near death and the attempt is being made to save a child, the person responsible for feeding the child has to take care to go slowly, because the child is not equipped—literally doe not have the equipment—to digest food. The first choice is simple carbohydrate in small quantities, with only a very little protein, sufficient to engage available enzymatic machinery. If this little can be digested, the body will begin to adapt to eating protein again (make more digestive enzymes, using the newly supplied amino acids and energy). Then, more foods containing protein, and finally fat, can be supplied. The body will not produce fat-digesting enzymes and the proteins necessary for carrying fat until many other proteins (that are higher priority for recovery) have been made.

 

            Not all children with kwashiorkor can recover. In some, the process of degeneration has gone too far. The healthy body has the resources to adapt, but they can be exhausted. This example shows that, as far as possible, the body is directly responsive to the tasks it is given to do. The precise enzymes the pancreas produces at each stage of malnutrition have not been studied in the human being, but it is likely that they are exactly those which it must have to take care of its needs most efficiently. In kwashiorkor, carbohydrate has been present in the diet, so the pancreas has probably kept on producing the enzyme amylase to digest it. Fat has not been present, so production of pancreatic lipase has probably been ceased.

 

            Providing kcalories in the usable form—as carbohydrate—lifts the constraints on producing digestive enzymes, and providing just a little fat at first gives the signal for the pancreas to start making the lipases again. Step by step, nutritional therapy can then build up to a normal diet, but if too much were given too soon, the system would be overwhelmed and the child might die. A diet manual might say only "Avoid whole milk in the first week, because fat is poorly tolerated at the start", but the person with an understanding of enzyme adaptation might see deeply into the simple statement to the underlying reality: the enzymes aren't there are first. (They come back slowly, in response to being presented with the material they work on, a phenomenon the biochemists call enzyme induction.)

 

 

            Animals fed high-protein diets experience hypertrophy of their livers and kidneys. The reason may be obvious after the examples given previously. What should jump mind is:   "Proteins: the liver and the kidneys must be making additional enzymes to manage all of the additional amino acids that are flooding the system." In fact, amino acids are metabolized in the liver. A large and active group of enzymes is necessary to perform this metabolism. Their presence is reflected in an enlarged liver. The kidneys have grown because they have built additional nitrogen-excreting machinery.

 

            There is still another way in which adaptation takes place, and this should clinch the point of the discussion. Any protein the body can make is a candidate for regulation. A protein can be made or degraded rapidly, slowly, or not at all. Normally the steady state prevails—a comfortable rate of synthesis accompanied by a matching rate of degradation—but when circumstances change, the responses can be as finely tuned as instruments of a symphony orchestra. Depending on what that person did yesterday or ate last week, or is accustomed to eating or drinking or smoking or breathing, his body may be equipped with considerably different enzymatic and other protein machinery than that of the client in the next bed in the same hospital room. The very number of molecules devoted to serving each of the body's multitudinous purposes differs from one individual to the next.

 

            It is hoped that this discussion has revealed these differences and will provide a deeper insight into what is meant by such vague phrases as: "prior nutrition status", or its "health", or "physiological state". Understanding the impact of a person's nutrition history on his present health will help equip students to practice diet therapy wisely, and also, hopefully, will lead them to realize that what a person eats and drinks today will affect his health tomorrow and in the months and years to come.