When you think of sodium, salt probably comes to mind. Although the two terms, “sodium” and “salt” are often used interchangeably, they are different substances. The chemical name for salt, sodium chloride, reveals that sodium is in fact a component of salt. By weight, salt is composed of 40 percent sodium and 60 percent chloride. One teaspoon of salt weighs 5 grams and contains about 2,300 milligrams of sodium.
Sodium is essential for life and for good health. It is a mineral that the body cannot manufacture itself so it must be supplied by food. Sodium is readily available from various sources—foods that contain sodium naturally, foods containing salt and other sodium-containing ingredients, and from salt added to foods during cooking and at the table. As a component of salt, sodium’s most recognized role is to make foods more flavorful. Less well-known, yet important roles of sodium-containing ingredients include helping to preserve foods, improving the texture of foods, and ensuring the safety of some foods.
Compared to other minerals, the human body needs sodium in relatively large amounts. Yet, much of the world’s population consumes more than the body’s minimum requirement for sodium. In some individuals, research suggests a link between high sodium and salt intake and high blood pressure, a major risk factor for heart disease, stroke, and kidney disease. However, this relationship may be affected by concurrent intake of other key minerals, including potassium, magnesium, and calcium. Regulating sodium intake is also believed to be important in preventing and treating other health conditions.
This IFIC Review provides background on the use of sodium as a food ingredient and sodium’s role in sustaining health. It also summarizes current knowledge on the effects of sodium and salt intake on the development and treatment of disease.
HISTORY OF SALT
Salt has a long history as a highly-valued commodity. Over the years, salt has served many diverse purposes and roles beyond its use in seasoning foods. One of salt’s most recognized uses, dating back to early centuries, has been in preserving foods, including meat, fish, vegetables, and even fruit. Salting foods prevented spoiling by drawing water out of the food, depriving bacteria of the moisture needed to thrive. Without salt, the world’s food supply would have been considerably less plentiful and less safe.
Until about two hundred years ago, salting was the primary method available for preserving foods. As a result, salty flavors were common and well-accepted. By the nineteenth century, other methods of food preservation emerged, including canning, freezing, and refrigeration. This led to a change in taste preferences and a desire for less salty flavors. By the twentieth century, commercially available methods of food preservation, such as pasteurization, freeze-drying, irradiation, and the use of preservatives, allowed food manufacturers to preserve many foods on a larger scale without relying on salt. Today, salt is still used for food preservation, but the majority of the world’s salt is used for industrial purposes, including highway salting, water conditioning, and the manufacture of chlorine and other chemicals.
At various times and places throughout history, salt also played an economic role. The ancient Greeks used salt as currency. Roman soldiers received a salt ration as part of their pay, known as “salarium argentum,” from which the English word for “salary” was derived. Other cultures relied heavily on salt production and trade, and salt even has been the cause of a bitter war. Salt also has played a vital part in religious rituals in certain cultures. Over many centuries, it has been used symbolically—as a symbol of wisdom when given to a newborn in ancient Rome and, in Europe, a pinch of salt was tossed over the left shoulder three times to fend off evil. Even today, “take it with a grain of salt” is a well-known phrase that conveys the thought to not take something too seriously.
In the early 1920s, it was discovered that adding iodine to salt could prevent Iodine Deficiency Disorders (IDD), which can cause goiter and other more serious complications, including mental retardation. In the United States, salt producers made both iodized salt and plain salt available at the same cost to encourage its use. Ultimately, the widespread use of iodized salt eliminated iodine deficiency in North America. In other parts of the world, however, iodine deficiency remains a health problem because many countries lack manufacturing and packaging technologies needed to iodize salt. The elimination of iodine deficiency worldwide has been identified as one of the highest health priorities by the World Health Organization and UNICEF.
Today, salt is inexpensive and universally available. It comes from either salt mines or from the sea. Most salt is mined from large deposits left by dried salt lakes throughout the world.
SCIENCE OF SODIUM IN FOOD
Salt and other sodium-containing ingredients are commonly added during food preparation and processing. Although enhancing flavor is a key role of salt, other functional roles of sodium in food are much broader.
Taste and sensory factors
“Salty” is known as one of the five basic tastes, along with sweet, sour, bitter, and umami. Unlike the other four tastes, which have many triggers, only sodium chloride is associated with the unique taste of saltiness. Scientists have attempted to mimic this taste with salt substitutes with limited success. While lower sodium food products generally taste less salty, they often require the use of other ingredients to add or enhance flavor.
Although scientists believe that the preference for salty flavor is innate, there is evidence that the level of salt preference is learned. In fact, early experiences with low or high salt diets may have a long-term impact on an individual’s preferred salt level. [Beauchamp, 1990] Dietary exposure has been shown to influence adult salt preferences. Studies have demonstrated that a gradual reduction in sodium intake over 8 to 12 weeks can decrease preference for salty foods and increase acceptance of foods with less sodium. [Bertino et al., 1982; Mattes, 1997]
Salt serves other taste-related functions. Adding just a few grains of salt can bring out a food’s natural flavor without contributing a salty taste. Interestingly, foods with higher levels of sodium do not necessarily taste salty. For example, some baked goods may contain more sodium than some frozen entrees. Similarly, foods with surface sodium, such as French fries, typically have a greater salty taste than foods with sodium already incorporated, such as baked goods, even though they may contain less total sodium. Foods with less moisture, such as potato chips, generally require more salt for a salty taste as compared to foods with more moisture, such as French fries.
In crackers, pretzels, and other dry foods, salt is believed to decrease the perception of dryness. Salt is also used to help balance any metallic or chemical aftertaste in products such as soft drinks.
Salt is available in various crystal sizes and shapes, each with different purposes.
Food preservation and safety
Salt and sodium-containing ingredients preserve the quality and safety of foods by inhibiting the growth of bacteria, yeasts, and molds, which in turn help to prevent food spoilage and foodborne illness. Foods most commonly preserved with salt and sodium-containing ingredients to prevent the growth of bacteria include cured, ready-to-eat meats and processed cheese products. Microorganisms of particular concern include Clostridium botulinum, which produces a toxin responsible for foodborne botulism, and Listeria monocytogenes, which can cause listeriosis, a serious infection with high mortality.
In ready-to-eat meats, salt, in combination with either sodium nitrate or sodium nitrite, plays an important role in preventing the growth of spoilage organisms and C. botulinum. (Jay, 2000; Doyle 1989/Hauschild) Sodium or potassium lactate and sodium diacetate in cured, ready-to-eat meat products interacts with salt to reduce or prevent Listeria growth. [Seman, 2002] In shelf-stable processed cheese products, moisture, pH, salt, and phosphate emulsifying salts all contribute to preventing the growth and toxin production by C. botulinum. (Tanaka, 1986)
Other foods also use salt as a means of preventing spoilage. For example, natural cheeses contain salt, which helps prevent growth of yeasts and molds. In salad dressings, salt along with acidic ingredients prevent the growth of spoilage bacteria, yeasts, and molds. In fermented foods, such as pickles, salt suppresses the growth of spoilage organisms while allowing the lactic acid bacteria to produce acid. The increased acidity contributes to flavor and helps limit further microbial growth. Salt is added to butter and cheese to enhance flavor and to prolong their refrigerated life.
Other functional uses of salt
Other uses of salt include as a texture aid, fermentation control, binder, and stabilizer. Adding salt to bread dough controls the fermentation action of the yeast, strengthening the gluten in bread dough to provide a uniform grain and texture. Gluten holds more water and carbon dioxide when salt is added to bread dough, allowing the dough to expand evenly without tearing. Bread made without salt will have a coarse texture and a bland flavor.
During pickling, the concentration of salt brine is gradually increased, reducing the fermentation rate. This helps to draw moisture out of foods, producing a pickled food with a crisp, firm texture.
In cured meats such as ham, salt improves tenderness by promoting the binding of water by protein. It also enables large pieces of meat to bind together so the product can be sliced. Hotdogs rely on salt for the interaction of meat proteins with fat and water. Cheese depends on salt to produce the desirable, even consistency, and the formation of a rind in certain types of cheeses.
Salt promotes the development of color in ham, bacon, and hotdogs. Used with sugar and nitrate or nitrite, salt produces an appealing color in processed meats. In baked goods, salt enhances the golden color produced when sugars are caramelized.
Adding a pinch of salt to cream or egg whites before they’re whipped helps increase the volume and serves as a stabilizer. Adding salt to water will raise the boiling point slightly. When cooking vegetables or pasta in boiling water, this enables the food to cook more quickly. A shorter cooking time allows vegetables to better retain color and nutritive value.
Functions of Common Sodium-Containing Ingredients
Sodium-containing food ingredients serve a variety of important functions.
ESSENTIAL ROLE OF SODIUM IN MAINTAINING HEALTH
Sodium is an essential mineral that the body requires to maintain health. To survive, sodium must be consumed regularly in adequate amounts.
Sodium balance in the body
Sodium is a vital component of all fluids in the human body, including blood and sweat. Often working with other minerals, such as potassium and chloride, sodium’s primary role is to maintain the proper balance of fluids in the body and the acid-base balance of body fluids. Sodium and chloride mostly work outside body cells, and potassium works mainly inside cells. Together these minerals help regulate the movement of fluids in and out of body cells. This movement of fluids carries nutrients into body cells and wastes are carried out.
The kidneys regulate the body’s sodium level and the volume of water circulating in the body. In healthy individuals, excess sodium passes out through urine, and to a lesser extent, through perspiration. To counteract the effects of higher intakes of sodium, the body produces and excretes more urine. If excess sodium is not removed, such as when kidney function is impaired, swelling of body tissues may occur because sodium causes the body to retain fluid. When extra fluid is retained, blood volume increases, which in turn increases blood pressure, making the heart work harder to move blood through the blood vessels. This explains why sodium’s role in maintaining the delicate balance of fluid in the body also makes sodium a key component in regulating blood volume and pressure.
Sodium, along with chloride and potassium are referred to as electrolytes because they possess a mild electrical charge when dissolved in body fluids. With their electrical charge, sodium, chloride, and potassium help to transmit nerve impulses throughout the body. For example, the neural control of muscle contraction depends on the signals generated by these electrolytes. All three electrolytes must be in proper balance to assure normal nerve function.
Sodium deficiency is relatively uncommon. Even when sodium intake is low, the body conserves this mineral by reducing the amounted excreted through urine and sweat to maintain the balance between sodium and other fluids. However, normal healthy kidneys are not effective at conserving potassium and thus are not able to prevent a deficiency when potassium intake is low.
Chloride is also essential for good health. It helps preserve the acid-base balance in the body, aids potassium absorption, is a component of digestive stomach acid, and enhances the ability of blood to carry carbon dioxide from body tissues back to the lungs where it is exhaled from the body.
Dietary Sodium Requirements
In the Institute of Medicine’s (IOM) 2004 release of Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, the report recommends an Adequate Intake (AI) level based on the minimum amount of sodium needed for a diet adequate in other essential nutrients and to replace sodium lost daily through perspiration for individuals engaged in recommended levels of physical activity. [DRI report]
The AI, or daily amount of sodium sufficient to meet the needs of most healthy people, is 1,500 milligrams per day (3,800 milligrams of salt) for 19- to 50-year-olds with normal blood pressure. This amount is less than half of the estimated 3,200 milligrams per day consumed on average by individuals in the United States. [NHANES III] Older adults and the elderly require somewhat less sodium based on lower energy intakes. See Table 1 for Dietary Reference Intakes for sodium for various life stage groups. Individuals engaged in higher levels of physical activity or in humid climates resulting in excessive sweating may require more than the recommended AI levels for sodium. [DRI report]
The IOM also suggests a maximum level for daily sodium consumption, known as the Tolerable Upper Intake Level (UL). For healthy individuals through the age of 50 years, the UL is 2,300 milligrams per day (5,800 milligrams of salt). The scientific report of the 2005 Dietary Guidelines Advisory Committee advises a daily sodium intake level consistent with the UL. According to this report, the general goal for adults is to reduce sodium intake to less than 2,300 milligrams of sodium per day. Older adults, African Americans, and those with chronic diseases such as hypertension, diabetes, and kidney disease are advised to reduce their intake even further. When making their recommendations, the Committee not only took into consideration sodium intake as it relates to chronic disease risk, but also current daily consumption of sodium in the United States.
Table 1: Dietary Reference Intakes for Sodium
|Life Stage/Age||Adequate Intake (AI)1|
|Tolerable Upper Intake Level (UL)2|
|1 AIs may be used as a goal for individual intake. For healthy, breastfed infants, the AI is the mean intake. The AI|
for other life stage and gender groups is believed to cover the needs of all individuals in the group, but lack of data
prevent being able to specify with confidence the percentage of individuals covered by this intake.
2 UL is the maximum level of daily intake that is likely to pose no risk of adverse effects. Unless otherwise specified,
the UL represents the total intake from food, water, and supplements.
3 ND = Not determinable due to lack of data of adverse effects in this age group and concern with regard to lack of
ability to handle excess amounts. Source of intake should be from food only to prevent high levels of intake.
|Source: Adapted from Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate.|
Institute of Medicine, 2004. www.nap.edu
Effects of physical activity and temperature. Loss of sodium through increased sweating is relative to the length and level of physical activity as well as the air temperature. Individuals who exercise strenuously in the heat on a daily basis can lose significant amounts of sodium through sweat. Thus, the AI does not apply to highly active individuals, such as endurance athletes. However, the higher calorie intake required with increased physical activity is likely to provide enough sodium to meet the increased requirement. The loss of sodium through sweat is dependent on overall diet, sodium intake, sweating rate, hydration status, and degree of acclimatization to the heat. [Allsopp et al, 1998] See Sodium and Athletic Performance for additional discussion.
Effects of potassium and calcium. A high potassium intake from supplementation has been shown to increase urinary sodium excretion. [van Buren et al, 1992] Potassium is believed to inhibit sodium reabsorption in the kidneys until equilibrium is reached. Increased urinary sodium results in fluid loss and a reduction in blood volume, which is generally considered to be an important component of the blood pressure lowering effect of potassium, particularly in individuals with hypertension. Most clinical studies have used potassium supplementation to achieve the varying higher intakes of potassium. However, at least one study concluded that a sufficient increase in potassium intake (approximately 2,300 mg potassium per day) could be achieved with a potassium-based salt substitute and an increased intake of fruits and vegetables. (MacGregor et al, 1982] In view of a potential blood pressure lowering effect and reduction in the risk of kidney stones and bone loss, the Dietary Guidelines Advisory Committee Report recommends a daily potassium intake of at least 4,700 mg. [DG Report, 2004]
Higher intakes of sodium are known to result in increased urinary loss of calcium, with possible adverse effects on bone health. See the discussion of the effects of sodium on bone health. Yet effects of calcium intake on sodium excretion are not clear. Limited evidence suggests that there is little or no effect of high or low calcium diets or calcium supplementation on urinary sodium excretion. [Cappuccio et al, 1986, Weinberger et al 1993]
Effects of diuretics. Diuretics work by promoting diuresis—an increase in urine production. When the kidneys are stimulated to produce more urine, excess fluids and minerals, including sodium and chloride, are flushed from the body. The reduction in total body water helps to reduce blood volume and pressure. Diuretics may be used to treat a variety of conditions, including high blood pressure, congestive heart failure, or kidney disease. Each type of diuretic affects the body differently and can affect the balance of sodium, chloride, and potassium. When a diuretic is used to treat hypertension, a diet moderately restricted in sodium (less than 2,400 mg sodium per day) offers a balance between efficacy of the medication and safety for most individuals. [JNC, NIH, 2003] With some diuretics, potassium losses are replaced with supplements or increased intake of potassium-rich foods.
Effects of disease and body weight. In cystic fibrosis (CF), the transport of sodium and chloride between body fluids and cells is impaired, causing the production of abnormally thick and viscous mucus. As a result, the sodium and chloride content of sweat is very high. This leads to increased requirements for sodium and chloride for individuals with CF, although the exact amount is not known.
Sodium requirements also may be affected by diabetes. In situations of high blood glucose (hyperglycemia) and high levels of glucose in the urine (glycosuria), the kidneys compensate by increased excretion of water and sodium. In acute cases of hyperglycemia, blood volume and sodium levels can become severely depleted, requiring intravenous administration of water and sodium chloride, along with insulin to correct the imbalance. High blood pressure and other cardiovascular diseases are common in individuals with diabetes. These conditions often are treated with reduced sodium intake. However, some oral hypoglycemic medications, such as chlorpropramide, are associated with low blood levels of sodium caused by increased water re-absorption. [Gardenswartz and Berl, 1981]
Body weight does not appear to influence daily sodium requirements. However, a well-established relationship between body weight and hypertension indicates that weight loss reduces blood pressure. [Neter et al, 2003] One explanation for this relationship may be that overweight and obesity amplifies blood pressure response to a higher sodium intake. Thus, weight loss may help decrease blood pressure by enhancing blood pressure response to a lower sodium intake.
Effects of life stage. Limited research is available on the amount of sodium required for normal growth and development. However, growth failure has been observed in young children with salt-wasting disorders, suggesting the need for adequate sodium early in life. No studies have evaluated the effects of varying levels of sodium intake on growth in normal full-term infants. In pre-term infants, sodium appears to be necessary for normal growth. [Al-Dahhan et al, 1984] Dietary sodium is required in sufficient quantities to permit the normal expansion of blood and fluid volume that accompanies tissue growth.
Sodium requirements for infants are estimated based on the average amount of sodium in human milk and complementary foods consumed by infants at various ages. For children and adolescents, sodium recommendations, like those for adults, are based on meeting nutrient needs for other essential nutrients. The AI is thus based on the adult requirement at lower energy intake levels for children and adolescents. During pregnancy and lactation, there is a lack of evidence to suggest that sodium requirements differ from that of non-pregnant or non-lactating women. [DRI report, p6-41]
Sodium Recommendations from other Experts and Organizations
There is an ongoing dialogue on an appropriate level of daily sodium intake and the necessity of reducing sodium intake for the general population. As presented previously, the IOM and the 2005 Dietary Guidelines Advisory Committee recommendations call for a significant reduction in sodium intake for the general U.S. population. These recommendations are based on the amount of daily sodium needed to maintain health and to reduce the risk of developing hypertension. In 1994, the U.S. Food and Drug Administration established a Daily Value (DV) for sodium of 2,400 mg for use as a reference value in food labeling.
The U.S. National High Blood Pressure Education Program set a sodium intake recommendation of no more than 2,300 mg per day as a means to prevent hypertension in nonhypertensive individuals [Whelton et al, 2002] and as first line adjuvant therapy in hypertensive individuals. [NHLBI, 2003] The American Heart Association recommends no more than 2,400 mg of sodium per day for healthy Americans. In 2003, Great Britain’s Scientific Advisory Committee on Nutrition also set an upper limit of 2,400 mg of sodium per day. [British Heart Foundation, 2003]
The Canadian Hypertension Education Program urges lifestyle modifications such as exercise and weight reduction as important and actionable changes to prevent and control hypertension, but does not call for a reduction in sodium intake for individuals with normal blood pressure. [Touyz, 2004] According to a 2004 report, Canadian recommendations for the management of hypertension are to keep sodium intake at 1,500 mg to 2,300 mg per day in hypertensive individuals and less than 2,300 mg per day in normotensive individuals at high risk for developing hypertension. [Touyz, 2004] In the World Health Organization’s 2003 report, Diet, Nutrition and the Prevention of Chronic Disease, a recommendation to restrict daily sodium intake to less than 2,000 mg per day is cited in addition to an upper limit of 1,600 mg of sodium per day as a means to lower blood pressure. [WHO, 2003]
Although sodium positions vary somewhat between health and expert organizations, intake recommendations are fairly consistent. However, some guidelines also emphasize other dietary and lifestyle recommendations, in addition to reducing sodium intake, for prevention and treatment of hypertension and other health conditions.
Estimates of Actual Sodium Intake
The major form of dietary sodium is salt, which accounts for approximately 90 percent of the total sodium consumed in the United States. [Mattes and Donnelly,1991]. Other forms of sodium which contribute to the total sodium content of food include seasonings with sodium, such as monosodium glutamate, and sodium-containing ingredients, such as sodium benzoate, sodium nitrite, and sodium citrate. The primary food sources of sodium are processed and canned foods, which often contain sodium from salt or sodium-containing ingredients added during processing for flavor and a variety of other functions, such as food safety and preservation. Many condiments, including Worcestershire sauce, soy sauce, ketchup, mustard, onion salt, and bouillon cubes, also contain sodium from salt and sodium-containing ingredients.