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Sodium is a mineral that exists in the body as the ion Na+. Sodium is acquired through diet, mainly in the form of salt (sodium chloride, NaCl). Regulating the amount of Na+ in the body is absolutely critical to life and health.
Sodium is possibly the most important mineral in the body. It plays a major role in controlling the distribution of fluids, maintaining blood pressure and blood volume, creating an electrical gradient that allows nerve transmission and muscle contraction to occur, maintaining the mechanisms that allow wastes to leave cells, and regulating the acidity (pH) of the blood. Many different organ working together, including the kidneys, endocrine glands, and brain, tightly control the level of Na+ in the body. Researchers estimate that between 20% and 40% of an adult’s resting energy use goes toward regulating sodium. Sodium affects every cell in the body, and a major failure of sodium regulatory mechanisms means death.
In the body, sodium exists as electrolyte. Electrolytes are ions that form when salts dissolve in water or fluids. These ions have an electric charge. Positively charged ions are called cations. Negatively charged ions are called anions. Electrolytes are not evenly distributed within the body, and their uneven distribution allows many important metabolic reactions to occur. Sodium (Na+), potassium (K+), calcium(Ca 2+), magnesium (Mg 2+), chloride (Cl-), phosphate
(Illustration by GGS Information Services/Thomson Gale.)
(HPO4 2-), bicarbonate (HCO3-), and sulfate (SO4 2-) are important electrolytes in humans.
Na+ is ten times more concentrated in fluid outside cells (i.e. extracellular fluid and blood) than it is in fluid inside cells. This difference in concentration is maintained through the expenditure of cellular energy, and it is critical to many metabolic functions, including maintaining the proportion of water that exists inside and outside of cells. (See the entry on electrolytes for a more detailed explanation of how this occurs). When Na+ is too high or too low, it is almost never because an individual has eaten too much or too little salt. Instead, it is because organs such as the kidneys or endocrine glands that regulate the conservation or removal of sodium from the body have broken down.
Researchers estimate that humans can remain healthy taking in only 500 mg of sodium daily. Salt is 40% sodium by weight, and 500 mg is slightly less than the amount of sodium found in 1/4 teaspoon of salt. Humans almost never take in too little salt; their health problems result from too much salt in the diet.
The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for many vitamins and minerals including sodium. The DRIs
consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97-98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.
The IOM has not set RDAs for sodium, but instead it has set AI levels for all age groups based on observed and experimental information about the amount of sodium needed to replace what is lost by a moderately active individual each day. Sodium is lost in both urine and sweat. IAs for sodium are measured in milligrams (mg). UL levels have not been set. However, the IOM recommends that adults limit their sodium intake to less than 2,400 mg per day, and the American Heart Association recommends an adult daily intake of 1,500-2,300 mg.
The following list gives the recommended daily AL levels of sodium for each age group.
- children birth-6 months: AI 120 mg
- children 7-12 months: AI 370 mg
- children 1-3 years: AI 1,000 mg
- children 4-8 years: AI 1,200 mg
- children 9-13 years: AI 1,500 mg
- adolescents 14-18 years: IA 1,500 mg
- adults age 19-50: AI 1,500 mg
- adults ages50-70 1,300 mg
- adults 71 years or older: AI 1,200 mg
- pregnant women: IA 1,500 mg
- >breastfeeding women: AI 1,500 mg
Sources of sodium
Many people think that the main source of salt in their diet is what they add to food when they are cooking or at the table while eating. In reality, more than three-quarters of the sodium in the average American’s diet is added to food during processing. Another 12% is already naturally in the food. For example, 1 cup of low-fat milk contains 110 mg of sodium. About 6% of sodium in the diet is added as salt during cooking and another 5% from salting food while eating.
Although most sodium in diet comes from salt, other sources of sodium include preservatives and flavor enhancers added during processing. Sodium content is required to be listed on food labels of processed foods. Some common ‘‘hidden’’ sources of sodium include:
- baking soda
- baking powder
- disodium phosphate
- monosodium glutamate (MSG)
- sodium nitrate or sodium nitrite
Below are some common foods and their sodium content.
- table salt, 1 teaspoon:2,300 mg
- dill pickle, large: 1731 mg
- canned chicken noodle soup, 1 cup: 850-1,100 mg
- ham, 3 ounces: 1,000 mg
- sauerkraut, 1/2 cup: 780 mg
- pretzels, 1 ounce: 500 mg
- potato chips, 1 ounce: 165-185 mg
- soy sauce, 1 teaspoon: 304
- deli turkey breast, 1 ounce: 335 mg
Fresh fruits, vegetables, unsalted nuts, and rice, dried beans and peas are examples of foods that are low in sodium.
Sodium and health
Too high a concentration of sodium in the blood causes a condition called hypernatremia. Too much sodium in the diet almost never causes Hypernatremia. Causes include excessive water loss (e.g. severe diarrhea), restricted water intake, untreated diabetes (causes water loss), kidney disease, and hormonal imbalances. Symptoms include signs of dehydrationsuch as extreme thirst, dark urine, sunken eyes, fatigue, irregular heart beat, muscle twitching, seizures, and coma.
Hypernatremia and hyponatremia are at the extreme ends of sodium imbalance. However, high dietary intake of salt can cause less visible health damage in the form of high blood pressure (hypertension). Hypertension silently damages the heart, blood vessels, and kidney and increases the risk of stroke, heart attack, and kidney damage. A low-salt diet significantly lowers blood pressure in 30-60% of people with high blood pressure and a quarter to half of people with normal blood pressure. Some individuals are more sensitive to sodium than others. Those people who are most likely to see a rise in blood pressure with increased sodium intake include people who are obese, have type 2 diabetes, are elderly, female, and African American.
The American Heart Association recommends reducing sodium in the diet to between 1,500 mg and 2,300 mg daily. Below are some suggestions for cutting down on salt.
- Eat more fresh fruits and vegetables.
- Look for processed foods that say ‘‘no salt added’’
- Limit or eliminate salty snacks such as chips and pretzels.
- Restrict the amount processed meats such as hot dogs, pepperoni, and deli meats.
- Avoid high salt canned soups; choose heart-healthy lower salt soups instead.
- Use spices instead of salt to give foods flavor.
People who are salt-sensitive may need to keep their salt intake at levels below the suggested daily amounts to control their blood pressure.
Certain drugs cause large amounts of sodium to be excreted by the kidneys and removed from the body in urine. Diuretics (‘‘water pills’’) are among the best known of these drugs. Other types of drugs that may cause low sodium levels, especially in ill individuals, include non-steroidal anti-inflammatory drugs (NSAIDs) such as Advil, Motrin, and Aleve, opiates such as codeine and morphine, selective serotonin-reuptake inhibitors (SSRIs) such as Prozac or Paxil, and tricyclic antidepres-sants such as Elavil and Tofranil.
Health concerns about sodium have been discussed above. Most problems related to high blood pressure are chronic, slow to develop disorders that do not cause serious complications until the second half of an individual’s lifetime. Kidney failure, heart attack, and stroke are all complications of high blood pressure and potentially of high sodium intake.
Salt is an acquired taste. Parents can help their children control their salt intake and discourage the development of a craving for salt by substituting low-salt foods for high-salt foods.
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Tish Davidson, A.M.