SALT. Because salt is indispensable to life, acts as a food preservative, and uniquely flavors foods, humans have been preoccupied with it since the beginning of recorded history. The desire to obtain salt politically or militarily has influenced the histories of countries in Asia, Africa, Europe, South America, and the Middle East. Indeed, salt was used as a form of currency and had greater value than gold in some ancient societies. Even religious and magical significance has been attributed to this mineral.

In chemistry, the term "salt" generally refers to any compound that results from the interaction of an acid and a base. In the fields of geology and agriculture, the term "salt" is used as a synonym for the word "mineral." Although numerous salts are essential to human health (for example, potassium chloride, sodium hydroxide), in the following paragraphs the term "salt" will refer specifically to the inorganic, white crystalline substance that is known as sodium chloride (abbreviated NaCl), unless otherwise noted. It is also known as table salt, rock salt, sea salt, and saline. The reader should be aware that some paragraphs below refer to sodium chloride, whereas others refer to sodium, the mineral/ion/electrolyte.

When sodium chloride enters the body, it dissociates almost completely into its constituent particles, the ions sodium and chloride. Sodium chloride is soluble in water and glycerin. Sodium is the most plentiful ion in blood. As electrically charged particles, positively charged sodium (Na⁺) and negatively charged chloride (Cl^-) are classified as electrolytes because they conduct electricity when dissolved in water.

Dietary Salt

Sodium exists in many foods that are commonly consumed in Western diets including processed sandwich meats, cheese, canned vegetables, pickled foods, salty snacks, and soft drinks. Other sources of sodium are not as well recognized: condiments, sauces, baking soda, baking powder, and bread. In restaurant foods, fast-food meals, and Chinese cuisine the sodium levels can be very high. Only about 10 percent of the sodium in Western diets is due to discretionary salt added at the table.

The sodium content of plants and vegetables depends on numerous factors. These include plant maturity, genetics, agricultural practices, soil salinity, soil fertility, soil pH, the rate at which water percolates through soil, as well as meteorological factors such as rainfall, cloud cover, and sunlight.

For most Americans today, eating preserved and processed foods has become a way of life. Sodium chloride is the most common food additive. Approximately 75 percent of sodium in Western diets originates from processed foods. Because salts of all kinds, including sodium chloride, are very stable, it is virtually impossible to remove sodium from foods that have been canned in glass or metal containers. In fact, the addition of sodium may occur during home meal preparation as well as commercial processes. For example, it is possible that a vegetable contains only 2 mg of sodium per 100 g on the vine but may contain 2 to 310 times that amount after canning. Processes such as adding a salt solution to prevent discoloration of vegetables (that is, brining), or the use of sodium salts as processing aids, also result in the addition of sodium to the final product.

Salt in Food Processing

In the late nineteenth and early twentieth centuries, before modern processing techniques existed, food preservation consisted primarily of heat sterilization used in combination with the addition of salts and spices. Salt was used to suppress the growth of unwanted bacteria. Today, sodium is added to processed foods in several forms. Sodium nitrate and sodium nitrite are added to meats as preservatives. Sodium citrate monobasic is added as a pH buffering agent. Both sodium fumarate and malic acid sodium salt are added to foods as buffering agents and flavor enhancers. These salts are used in concert with numerous other food additives in the United States (for example, antioxidants, stabilizers, colors, sweeteners, enzymes, and emulsifiers), under the direction of the U.S. Food and Drug Administration.

Fergus Clydesdale, a professor at the University of Massachusetts at Amherst, explained in 1988 that the loss of sodium during processing is solely due to leaching (that is, extraction, rinsing, or filtration). Canning, boiling, steaming, blanching, and cooking are the processes most likely to cause leaching of sodium and other salts. However, the extent to which these electrolytes are lost varies with the food product, type of processing, and properties of each ion. The amount of water used in a given commercial process also affects mineral losses. Steaming, for example, uses less water than boiling. Further, the total processing time may affect sodium losses from foods. Brief procedures will likely extract less salt than lengthy ones.

Various other salts (for example, potassium chloride, magnesium chloride, sodium nitrate, sodium benzoate, and sodium acetate) are added to foods during commercial processing. They serve to cure meats, provide or intensify the flavor of numerous products, decrease caking of dry products, stabilize pH (that is, when used with jams, gelatins, baked goods, pasteurized cheese), fortify nutrients, and enhance texture. Sodium nitrite, for example, reacts with meat pigments to develop a characteristic pink color. In bread and baked goods, salt serves a variety of functions including the control of the rate of fermentation in yeast-leavened products. Fermented vegetables such as sauerkraut require salt for flavor and to extract water and other nutrients from the plant tissue to form brine, in which desirable organisms flourish and undesirable ones are subdued. The firmness and color of fruits and vegetables are preserved by the calcium salt of lactic acid. In cheese products, salt is added to the curd or applied to the cheese surface to remove whey and to slow the production of acid. Sorbic acid and its salts are antimicrobial agents that work to suppress the growth of bacteria; molds in cheese, sausages, fruits, jellies, bread and cakes; and yeasts in salad dressings, tomato products, syrups, candies, and chocolate syrup.

Biological and Physiological Considerations

The various minerals in the human body serve to maintain acid-base balance, blood volume, and cell membrane permeability, and provide the constituents of bones and teeth. Sodium chloride is important in maintaining the proper concentration of body fluids (that is, osmolality), expediting fluid movement between cells, enhancing glucose absorption, and allowing proper conduction of impulses along nerve and muscle tissues.

Body fluids are distinguished as either intracellular (that is, existing inside muscle and organs) or extracellular (that is, circulating blood plus the interstitial fluid that lies between cells). To accomplish their functions, body tissues maintain intracellular and extracellular ions in different concentrations. This requires considerable energy, approximately one-third of all resting metabolism, and is accomplished by molecules that are embedded in cell membranes throughout the body; these large protein molecules are known as pumps because their action causes an unequal distribution of an ion on the inside and outside of a membrane. In blood, the concentrations for some ions (for example, potassium and calcium) are maintained within narrow limits. Table 1 illustrates these concepts for sodium, chloride, potassium (K⁺), and magnesium (Mg²⁺). Chloride is the most common negative ion that combines with sodium in the extracellular fluid. Sodium and chloride account for more than 80 percent of all particles in the extracellular fluid. Potassium, magnesium, and phosphate are the most abundant intracellular ions. Potassium speeds energy metabolism and is involved in the synthesis of proteins and a storage form of carbohydrate (that is, glycogen). Magnesium allows the body's chemical reactions and biochemical pathways to function efficiently. Approximately 60 percent of the body's magnesium exists in the skeleton, in combination with calcium and phosphorus; in fact, 99 percent of all calcium exists in bones and teeth. The remaining magnesium is present in red blood cells and muscle, supporting the transport and storage of oxygen.

The concentrations of ions in sweat and urine, which constitute the major avenues of loss, may vary markedly between individuals. This large range exists in sweat and urine because diet, acute exercise, chronic physical training, and heat acclimatization alter the loss of these ions—especially sodium and chloride—at the sweat glands and kidneys.

Sodium Metabolism

Sodium is so intimately related to other intracellular ions, extracellular ions, and water that it is difficult to consider the factors that regulate its metabolism independently. Nevertheless, the following text is limited to the regulation of sodium retention and excretion.

Sodium, chloride, potassium, and magnesium ion concentrations (mmol/L) in intracellular fluid and in four extracellular fluids
Source	Sodium	Chloride	Potassium	Magnesium
Intracellular fluid	8	150	31	10
Extracellular fluids
Sweat	15–53	4–8	2–5	15–70
Urine	32–224	43–60	8–10	39–218
Blood plasma	96–110	3–6	1–2	135–145
Saliva	11–45	11–23	0.1–0.4	10–75

At rest, the kidneys filter circulating blood at the rate of 1.0 to 1.5 L/min, causing the kidneys to generate approximately 180 L of fluid during a 24-hour period. Because the average urine volume of normal adults totals 1.3 L/day, almost all of the renal filtrate is reabsorbed and returned to the bloodstream. The amount of sodium excreted into the urine depends upon the body's need for sodium. If excess sodium is consumed without water, the kidney excretes urine with a high concentration of sodium. If dietary sodium is restricted, the kidneys are capable of producing a dilute urine that maintains the concentration of sodium in body fluids at a normal level.

Whole-body sodium balance is maintained over a wide range of dietary and environmental conditions, primarily due to the action of the hormone aldosterone on the kidneys. When dietary sodium is high, urinary sodium increases to excrete the excess. When dietary sodium is low, aldosterone reduces the loss of sodium in urine appropriately. Thus, a sodium deficiency is rare, even among individuals who consume very low-sodium diets (see below). The body may experience a sodium deficiency when sweat losses are large and persistent, or when illness (for example, chronic diarrhea, renal disease) results in inadequate sodium retention by the kidneys. Following major changes in dietary sodium levels, concentrations of the following hormones also adapt, suggesting that they minimize perturbations of extracellular fluid-ion balance: renin, angiotensin II, atrial natriuretic peptide, and nitric oxide. The latter compound plays a pivotal role in blood pressure maintenance by regulating sodium and water excretion at the kidneys. Despite our knowledge of these facts, scientists cannot explain the exact mechanism by which the brain assesses whole-body sodium status.

A predictable sequence of events occurs when a normal individual limits the intake of sodium (for example, 230 mg daily). During the initial days of salt restriction, urinary sodium levels progressively decrease until about the fifth day, when the 24-hour losses become small (for example, 115 mg or less). This individual ordinarily loses 1 or 2 kg of body weight, which is attributable to the loss of sodium and an appropriate volume of water. Initially, the reduced body water comes almost exclusively from the extracellular fluid; as time passes, the intracellular fluid compartment also shrinks. For the next few days, urinary sodium concentration remains low, and the body continues to maximize salt conservation until a reduced whole-body sodium equilibrium is established. Sweat sodium levels decrease in a manner similar to urine during dietary restriction; both are due to the action of the hormone aldosterone.

Toxicity

As is true for virtually all nutrients and compounds, salt can be detrimental or lethal in large quantities. Direct contact with sodium chloride can cause skin irritation, and heating it to high temperatures emits a vapor that irritates the eyes. When heated to the point of decomposition, it emits toxic chloride and disodium oxide (Na₂O) fumes. When consumed in large amounts, sodium chloride can cause stomach irritation. In addition, laboratory experiments have shown the following dose-response effects: 50 mg/24 hr, skin irritation in rabbits; 100 mg/24 hr, moderate eye irritation in rabbits; 125 ml/L, inhibition of DNA synthesis in isolated human cells; 27 mg/kg body weight, abortion of a human fetus; and 3,000 mg/kg body weight, lethal oral dose for 50 percent of the animals tested. Potassium chloride causes physiological responses at the following doses: 500 mg/24 hr, mild eye irritation in rabbits; 125 g/L, lung cell death in hamsters; 2,600 mg/kg body weight, lethal oral dose for 50 percent of the animals tested. Calcium chloride is lethal for 50 percent of the animals tested at a dose of 1,000 mg/kg body weight, when administered orally, and at an intraperitoneal dose of 264 mg/kg body weight. Studies have shown that magnesium chloride is lethal for 50 percent of rats tested at an oral dose of 2,800 mg/kg body weight.

The preservatives known as sulfites (see Sodium and Hypertension, below) can produce deleterious side effects, when consumed in large quantities. Investigations involving laboratory animals have shown that sulfites may inhibit some of the body's biochemical reactions and retard whole-body growth in infants; cause gastrointestinal distress; and induce reversible anemia, nutrient deficiency (for example, thiamine), and gene mutations. A lethal oral dose of sodium bisulfite (50 percent of the animals tested) was 498 mg/kg body weight in rats and 300 mg/kg body weight in mice.

Monosodium glutamate (MSG) is added to foods by chefs to potentiate various flavors. This effect is greatest in meat-and vegetable-based soups, sauces, gravies, and spice blends. The levels of MSG in foods range from approximately 0.3 percent in spinach and tomatoes to about 10 percent in parmesan cheese and 20 percent in dehydrated soup mixes. Some consumers also mix additional MSG into foods in the form of sauces. This may be strongly influenced by cultural food preferences. In Korea and Taiwan, for example, the average adult consumes six to ten times more MSG each day than the average person in the United States. Because sodium is a part of the molecular structure of MSG, it becomes available as free, metabolically active sodium. Therefore, individuals who consume restricted-sodium diets (see Sodium and Hypertension, below) should monitor both the natural levels of MSG in foods as well as the amount that is intentionally added. Monosodium glutamate also produces unwanted side effects in some individuals, including warmth, tingling, tightness, headache, swelling of the liver, and a feeling of pressure in the upper body or face. This phenomenon is often associated with consumption of Chinese food because of its high MSG content. The toxicity of MSG has been studied extensively and it is relatively low, compared to other salts. It has been estimated, for example, that an average adult, weighing 70 kilograms, would have to consume more than 3 pounds of MSG at one time to experience a toxic effect. This does not mean, however, that detrimental effects are nonexistent. A large quantity of MSG has been associated with convulsions, vomiting, and nerve cell damage in research animals, although there are great differences between species. Studies have shown that MSG is lethal, for 50 percent of rats tested, when consumed as an oral dose of 17,300 mg/kg body weight. Thus, when consumed in typical amounts, MSG does not appear to induce illness or toxicity. Because the scope of this article does not allow detailed considerations of the toxicities of other salts, the reader may refer to the book Food Additive Toxicology for further information.

Sodium and Hypertension

Because the kidneys regulate the volume of circulating blood, they are intimately involved in the genesis of high blood pressure (that is, hypertension). This disease often involves excessive retention of extracellular fluid, especially in the bloodstream. For unknown reasons, resistance to blood flow through the kidneys is increased two-to fourfold. And, unfortunately, even though blood pressure may be reduced by prescription medications, the kidneys do not excrete normal amounts of salt and water in urine. This scant urine output causes water and sodium retention until blood pressure rises again to an elevated level. Treatment for this fluid and electrolyte retention often involves diuretics, which increase hourly water and salt losses in urine markedly. Considering these facts, a multiple-stage scientific hypothesis has evolved. This concept proposes that a high dietary sodium intake (1) overloads the kidneys' capacity to excrete sodium and results in fluid retention, (2) increases endocrine gland secretion (that is, natriuretic hormone), (3) inhibits cell membrane function, (4) increases the sodium concentration inside cells and calcium levels in the smooth muscles that encircle blood vessels, which (5) subsequently increases the resistance to blood flow and blood pressure. Interestingly, some research indicates that hypertension may be dependent on the coexistence of sodium and chloride in the diet. Consumption of chloride salts (for example, potassium chloride and calcium chloride) is associated with hypertension, in a way similar to that of sodium.

Forty-three million Americans live with persistently high blood pressure, defined as readings of 140/90 mm Hg or above; this represents 24 percent of the adult population of the United States. This makes it one of today's most prevalent disease conditions. High blood pressure increases the risk of stroke, heart disease, and kidney failure. Individuals with a family history of hypertension, the elderly, middle-aged men, and middle-aged black women are at greatest risk. Yet, everyone is vulnerable because blood pressure typically rises with age.

It is important to acknowledge that heredity plays a critical role in hypertension and that this complex disease is affected by many different genes. Present wisdom states that, without these genes, a person will not develop high blood pressure. Such individuals, whose blood pressure increases with increasing sodium consumption, are salt-sensitive. This explains why there are great differences in human responses to sodium chloride.

Several factors play a role in reducing high blood pressure. In hypertensive adults, for example, a single aerobic exercise session (45 minutes) reduces blood pressure for 12 to 24 hours. A healthy diet (high in fruits, vegetables, low-fat dairy products; low in saturated and total fat) also reduces blood pressure. But salt has received the most attention. There is a large body of evidence, and consensus within the scientific community, that dietary sodium chloride is a risk factor for high blood pressure, independent of other risk factors such as alcohol and obesity. During the last 25 years, numerous professional organizations and advocacy groups have supported reductions of sodium in commercially processed foods, including the American Academy of Pediatrics, American College of Cardiology, Food Research Action Center, American College of Preventive Medicine, American Health Foundation, National Alliance of Senior Citizens, and National Urban Coalition.

In countries where dietary sodium is low, high blood pressure is rare. According to clinical investigations, when hypertensive adults reduce salt consumption their blood pressure usually decreases, although not always to a normal level. Additional evidence suggests that a high-salt diet aggravates other illnesses including asthma, gastric cancer, kidney stones, and osteoporosis. Therefore, consuming a low-salt diet will, for many people, reduce their risk of developing or aggravating a chronic illness such as cardiovascular stroke.

Individuals who are placed on sodium-restricted diets often consume other salts in place of sodium chloride. This increases the daily potassium intake because salt substitutes usually contain a high percentage of potassium chloride. This dietary strategy offers potential health benefits in the form of lowered blood pressure and reduced risk of stroke. For some individuals, however, the use of a potassium-containing salt substitute can cause illness or death. Individuals with a disease, those taking medications, and the elderly should be advised that these salt substitutes ought to be used only to enhance taste, and not for cooking purposes. Sulfites also should be considered. These compounds preserve food by retarding deterioration, rancidity, or discoloration and thus are categorized as antioxidants. At least three sulfites are commonly used as food additives: sodium sulfite (Na₂SO₃), sodium metabisulfite (Na₂S₂O₅), and sodium bisulfite (NaHSO₃). Because these preservatives contain sodium that becomes free and metabolically active in cells, each contributes to the diet's total sodium load.

Unfortunately, reducing the salt content of foods, to restrict sodium consumption, affects the quality and properties of foods. In the meatpacking industry, for example, reducing sodium chloride extremely results in inferior meat cohesion and water retention, and reduces shelf life. These and other unwanted effects explain why commercial food processors usually do not reduce the sodium chloride levels in their products voluntarily.

Managing Dietary Sodium

Compared to the average daily intake in the United States, ranging from 2,300 to 6,900 mg/day, the minimum physiological need for sodium (40 to 300 mg/day) and the intake necessary for good health (500 mg/day) are very small. In fact, the amount of sodium in fresh vegetables alone may be enough to meet an adult's basal requirement. Eight simple procedures make reducing salt intake effective. First, cook with only small amounts of added salt. Second, add little or no salt to food at the dinner table. Third, limit your intake of salty foods such as potato chips, salted nuts, pretzels, popcorn, soy sauce, steak sauce, garlic sauce, pickled foods, and cured meats. Fourth, request that the chef omit salt from your restaurant meal. Fifth, educate yourself about foods that contain large qualities of sodium and seek low-sodium brands when shopping for crackers, pasta sauce, canned vegetables, bread, and other commercial products. Sixth, develop a taste for the unsalted flavor of foods. The taste preference for salty foods can be altered with patience. Seventh, evaluate your diet by reading food labels carefully to determine the sodium content. This can be especially helpful in the aisles of a supermarket because you cannot eat what you do not purchase. Eighth, make a mental list of foods that you will avoid because they contain too much sodium. Here are a few examples, presented in units of milligrams per 100 g of food: fried crisp bacon, 2,400; baking soda, 9,000; beef bouillon cube, 24,000; bologna, 1,300; celery salt, 28,000; cured ham, 1,100; dill pickle, 1,400; frankfurters, 1,100; salt pork, 1,800; green pickled olives, 2,400; and processed cheese, 1,500.

Careful selection of low-sodium food items also will prove to be useful. Table 2 provides a comparison of the sodium content of several vegetables, in fresh and canned forms. Obviously, individuals who desire to reduce their total dietary sodium levels should substitute fresh vegetables for canned varieties, whenever possible. The exception to this recommendation lies in vegetables that lose sodium during processing, due to leaching. This provides the added benefit of ensuring that other dietary nutrients are not lost during commercial packaging (that is, leaching, boiling, blanching).

Another excellent way to lower sodium intake is to alter food preparation practices in the home. Many spices, herbs, and other flavorings do not contribute significant amounts of sodium but may be used to improve the flavor of low-sodium meals. These include allspice, basil, bay leaf, chives, cinnamon, cloves, curry, dill, garlic, ginger, leeks, lemon juice, mint, mustard, nutmeg, orange extract, oregano, paprika, parsley, pepper, peppermint, pimento, poppy seed, saccharin, saffron, sage, sesame, brown and white sugar, tarragon, vanilla extract, and wine.

In determining the amount of sodium that a person consumes, groundwater is often ignored. However, the sodium content of public and private aquifers in the United States varies greatly from one location to another. Although most sources of water include less than 20 mg of sodium per liter, a minor input to daily sodium, certain areas of Arizona, Texas, and Illinois report 325 to

Sodium content (milligrams per cup) of vegetables: Fresh versus canned
	Fresh, raw	Canned
Asparagus	4	285
Beets	57	36
Carrots	31	280
Green beans	8	536
Green peas	2	236
Lima beans	1	310
Sweet potatoes	24	48
Tomatoes	2	18
Tomato juice	2	230

432 mg of sodium per liter of groundwater. Considering the fact that the average adult consumes more than 2 L of fluid each day, this could mean that some Americans receive over 1 g of sodium per day from tap water alone. If a water softener is used to reduce hardness from a local water supply (for example, remove the mineral calcium carbonate), sodium content can be magnified.

Individuals on low-salt diets also should be concerned about the adequacy of other nutrients. It has been estimated that 40 percent of all low-sodium diets lack other essential nutrients, especially protein, the B vitamins, riboflavin, and calcium. These deficiencies result from the removal of food items that contain sodium.

Salt Restriction and Sodium Deficiencies

As noted above, the basal physiologic need for sodium is 40 to 300 mg/day and the amount recommended for good health is 500 mg/day. Field studies, conducted between 1931 and 1962, confirmed that adults can eat low-sodium diets and remain healthy. Interestingly, some of these populations lived in tropical climates, where sweat losses were great, including the vigorous Masai warriors of Africa who consume less than 1,955 mg of sodium per day, the inhabitants of tropical Nigeria who ingest less than 2,760 mg of sodium per 24-hour period, and Galilean naturalists who ingest only 736 mg of sodium per day.

It is difficult to deplete the body of sodium. The action of the hormone aldosterone on the kidneys, and the relatively large per capita daily intake of sodium in Western diets relative to basal physiological needs, are quite adequate to maintain whole-body sodium levels. Thus, sodium deficiencies are rare, but may be experienced in three extraordinary situations. The first involves dietary salt restriction as therapy for disease (for example, hypertension or congestive heart failure). The possibility that sodium depletion may occur in these illnesses does not contraindicate the use of a low-sodium diet when suitable, but it is important that the patient be monitored carefully. Frequent measures of serum sodium concentration are desirable during the first few weeks of a salt-restricted diet. A decline in serum sodium level should prompt a reevaluation. The second circumstance involves diseases of the kidneys or endocrine glands that alter normal sodium balance, such as Addison's disease or diabetes insipidus. The third situation, involving hot environments, is considered in the following section.

Hot Environments Exaggerate Salt Losses

Exercise or labor in cool environments increases the sweat loss and water intake, but the psychological drive to drink and fluid-electrolyte hormones regulate total body water within >0.2 percent (>150 g) of the normal body weight each day. Blood plasma volume is regulated within > 0.7 percent (> 25 g) on consecutive days.

During mild-to-moderate intensity exercise in a hot environment, voluntary water intake does not keep pace with water losses. Most humans produce 0.8 to 1.3 L of sweat per hour, but replace only one-third to three-fourths of this amount by drinking. Thus, if exercise in a hot environment is prolonged and strenuous, a 3 to 5 percent body weight loss can occur. This is significant because, at these levels, both endurance and strength decline.

Table 1 demonstrates that sweat contains sodium, chloride, and other minerals. In fact, sweat contains more than forty distinct organic compounds. Regarding the sodium chloride content, considerable interindividual differences exist among healthy adults. Physically fit athletes who are heat-acclimatized (that is, adapted to exercise in a hot environment) usually lose 400 to 800 mg of sodium chloride per liter of sweat. In contrast, the sweat of unfit, nonacclimatized adults contains from 1,000 to 3,000 mg of sodium chloride per liter. This difference occurs because physical training and heat acclimatization reduce the concentration of salt in sweat.

Salt Balance during Exercise and Labor

Table 3 provides estimates of the amount of fluid and salt lost in sweat, during different activities that are conducted in hot environments. Obviously, water and sodium chloride losses increase in proportion to the duration and the intensity of exercise. As a point of reference, Table 4 describes selected nutrients that are consumed by an adult in the United States. The intake of sodium chloride averages 4,600 to 12,700 mg, and water consumption averages 2.5 L/day. Comparing these two tables, it becomes obvious that 30 minutes of mild gardening produces a small fluid and sodium loss that can be replaced by a normal diet. An ultramarathon, requiring 20 to 30 hours to complete, involves extraordinary salt (14,400 to 70,000 mg NaCl) and water (18.0 to 35.0 L) losses that far exceed normal 24-hour food consumption. Clearly, constant fluid-electrolyte intake is required, during and after an ultramarathon, to replace lost nutrients.

Three fluid-electrolyte disorders involve sodium (that is, heat exhaustion, heat cramps, and exercise-related hyponatremia)

The amount of water and sodium chloride lost in sweat during labor or during exercise in hot environments
Event, Duration, Personal Characteristics	Total Water Loss (L)	Sodium Loss (mg)a
Mild gardening, 30 min, sedentary adult	0.3–0.5	240–2,000
Strenuous work, 60 min, experienced laborer	0.8–1.5	640–4,500
10-km run, 40 min, healthy adult	0.5–1.0	400–4,000
Leisure hike, 2 hr with rest, heat-acclimatized adult	2.0	1,600–6,000
Intense cycling, 2–4 hr, physically-fit cyclist	3.0–8.0	2,400–24,000
Ultramarathon, 20–30 hr, highly trained runnerb	18.0–35.0	14,400–70,000
aLoss in sweat and urine; these calculations assume a range of 800–4,000 mg sodium chloride per liter of sweat; physical training and heat acclimatization increase a person's sweat rate but decrease the sodium content of sweat and urine.
bRunning pace is slow and includes walking.
SOURCE: Average Consumption of Selected Minerals and Sodium Chloride in the United States (mg/day). National Research Council, 1989.

and have become the most common illnesses among athletes and laborers in hot environments. Heat exhaustion, an inability to continue exercise in the heat, is primarily a fluid depletion disorder in which either large sodium, water, or mixed sodium-water losses occur during exercise-heat exposure. Heat cramps occur most often in the abdominal wall and large muscles of the extremities and are due to whole-body sodium depletion. Treatment for these two heat illnesses involves replacing the sodium chloride and water that was lost in sweat and urine. Exertional hyponatremia involves a reduced serum sodium concentration (<130 mEq/L) and represents a marked dilution of the extracellular fluid. This disorder, unlike the previous two, involves overhydration. Athletes

Consumption of selected nutrients in the United States (mg per day), as published by the National Research Council in 1989.
Mineral	Amount consumed (mg/day)a
Sodium chloride	4,600–12,700
Sodium	1,800–5,000
Potassium	2,500–3,400
Magnesium	207–329
Calcium	530–1,179
aThe water intake of a 70-kg adult is approximately 2.5 L/day, in solid foods and fluid.

or laborers, who consume and retain a large volume of pure water (for example, 10 L in 5 hr), may experience a life-threatening series of physiological changes that signal water intoxication. The most serious effects are coma, fluid in the lungs (pulmonary edema), and brain swelling (cerebral edema).

Replacing Salt Losses due to Exercise

Individuals who exercise for more than two hours, and who are not hypertensive, should increase their salt intake slightly (see Table 3). Similarly, if a weight loss of 3 percent or more is due to fluid losses during work or exercise, a minor sodium deficit should be expected. The simplest means to replace these deficits after exercise involve adding salt to your meals and selecting saltier foods. Canned soup, for example, contains 1,950 to 2,450 mg of sodium chloride; canned tomato juice contains 1,525 mg. Fluid-electrolyte replacement beverages contain 150 to 300 mg, and 1 percent low-fat milk contains 300 mg sodium chloride It also is wise to eat more fruits, such as bananas and watermelon, to replace lost potassium.

See also Assessment of Nutritional Status; Body Composition; Electrolytes; Fish, Salted; Meat, Salted; Microbiology; Minerals; Nutrition; Sodium; Thirst.

Lawrence E. Armstrong