MOST of the 14 minerals which appear to be essential in human nutrition are widely distributed in foods and for many of them there is little danger that dietary deficiency will occur. In a number of pathologic states, however, abnormalities of mineral nutrition develop which require diagnosis and therapy. The essential minerals are sodium, potassium, chlorine, calcium, phosphorus, sulfur, magnesium, iron, copper, iodine, maganese, cobalt, zinc and molybdenum. Until recently, mineral metabolism has been a somewhat neglected field of study, yet minerals, like vitamins, have many vital physiologic functions including important roles in enzyme systems.
WATER AND ELECTROLYTES
Disturbances of water and electrolyte metabolism are extremely common and important in clinical medicine. Our understanding in this field has been increased greatly through the use of isotopic techniques and by investigations of the role of the endocrine glands in water and electrolyte metabolism. The method of flame photometry for estimation of sodium and potassium likewise has made a distinct contribution to clinical investigation. Only a brief resume of knowledge in this field, with particular emphasis on nutritional aspects, can be undertaken in this discussion. A recent symposium gives many interesting details of some of the newer concepts and their application to medical problems.
For centuries water has been recognized as indispensable and man can live for only short periods without it. The daily requirement is about 1 ml. for each calorie of food or approximately 2.5 liters for the normal adult living in a temperate climate. This water is supplied approximately as follows: 1200 ml. as liquids, 1000 ml. from the water in solid foods and 300 ml. from the water produced by oxidation. When water balance is being maintained, the loss is likewise about 2.5 liters of which approximately 1500 ml. is excreted by the kidneys, 500 ml. by the skin as insensible perspiration, 500 ml. in the expired air and 100 ml. in the stool. The minimal intake of water which will meet requirements during fasting,, if activity is limited, is 0.8 liter daily which represents about 50 per cent of the need. The remainder is furnished by oxidation and release of cellular water. Water requirement may be in-creased markedly in conditions associated with profuse sweating, vomiting, diarrhea or polyuria.
Fluid is continually being exchanged between the various compartments of the body and balance is maintained by a number of regulatory mechanisms. Water which is excreted into the intestinal tract each day may amount to as much as 8 liters of which all but 100 ml. is reabsorbed under normal conditions. Normally, no electrolyte is lost in the expired air or in insensible perspiration and little in the intestinal discharge. Accordingly, the kidney is primarily responsible for water and electrolyte excretion and for maintaining the constancy of the body content of these materials. Adrenal cortical hormones and the antidiuretic hormone of the pituitary play important roles in the regulatory mechanism (61b). The primary action of the anti-diuretic hormone (A.D.H) is to decrease renal excretion of water by increasing reabsorption from the kidney tubules. The stimulus for A.D.H. secretion is an increase in the osmotic pressure of the fluid bathing a receptor organ in the hypothalamus. Perhaps a deficit in volume in some key portion of the body fluids may promote secretion also. Painful and disagreeable stimuli increase secretion.
The normal adult kidney can excrete the catabolites formed in 24 hours in as little as 500 ml. of urine. In the infant, the ability to concentrate urine is limited, the metabolic rate is higher, more metabolites per unit of body size must be excreted and insensible loss of water from the skin is greater than in the adult. As a result, depletion of body water can occur more rapidly and be of more serious consequence.
The total amount of water in the body comprises about 50 to 55 per cent of total body weight; the volume of extracellular fluid amounts to approximately 23 per cent, the plasma volume to about 5 per cent of body weight. Methods for measuring the fluid content of the body and its compartments consist chiefly of dilution techniques many of which employ radioactive isotopes. Details of a number of these procedures, and findings in certain disease states are given in a recent report by Moore and associates (61a).
The syndrome of water depletion was studied in normal subjects by Black, McCance and Young . The initial and most prominent symptom was intense thirst; the mouth became dry and there was difficulty in swallowing. Weakness, oliguria and loss of weight in proportion to water deficit were observed. Other findings included a change in temperament, confusion and hallucinations. Plasma volume decreased slightly, serum sodium increased and serum potassium decreased. Severe dehydration resulted in prostration, peripheral circulatory collapse, anemia, azotemia and serious electrolyte disturbances.
Dehydration accompanying disease shows many of these findings in conjunction with the primary abnormalities of the pathologic state. Acute dehydration may result from excessive sweating, an increase in pulmonary or renal water loss, from vomiting, fistulae or diarrhea. The daily loss may amount to as much as 14 liters in sweat or urine and 8 liters from the gastrointestinal tract. In such situations, total body water is reduced but the percentile charge is not great; an acute loss of 7000 ml. may result in only a 5 to 7 per cent decrease in total body water. However, severe clinical effects ensue. Careful measurement of changes in body weight is more useful in detecting the extent of acute water depletion and in following rehydration than the more complicated estimations of body water. Determination of the volume of packed erythrocytes, of plasma protein concentration, or specific gravity of plasma are of assistance in following the course of dehydrated patients.