Sodium
What is it?
Sodium is the major positively charged ion in blood and body fluids. The average adult body contains about 90 g of sodium. More than half of this is in the fluids that bathe the cells, about one-third is in the bones, enmeshed in the crystals of insoluble bone minerals, and some is retained within the cells.
What it does in the body
In addition to its functions as an electrolyte, sodium is also a component of ATPase, an enzyme involved in the production of energy. It is necessary for the transport of amino acids and glucose into body cells.
Absorption and metabolism
Absorption of any soluble form of sodium is passive with excess intakes easily absorbed. Excretion of sodium is mainly via the kidneys with increased sodium intake leading to increased excretion. There is a limit on the extent to which the kidneys can concentrate the urine, so large sodium intakes must be balanced by large intakes of water. The highest excretion of sodium occurs at midday and the lowest at night. Some sodium is excreted in sweat.
Deficiencies
A deficiency of sodium is unlikely in any diet, except possibly those that are very low in salt and protein. Deficiency may occur with dehydration in heat exhaustion brought on by high temperatures, hard exercise, manual work and in babies, by diarrhea. Symptoms include mental apathy, loss of appetite and sometimes vomiting and muscle cramps. In severe cases, the blood thickens, veins collapse, blood pressure is reduced and the pulse becomes rapid. Deficiency of sodium can also occur after water intoxication which happens when large amounts of water but no sodium are drunk after heavy sweating.
Other causes of low blood sodium levels include kidney disorders, hormonal imbalance, lung cancers, lung infection, liver cirrhosis, toxemia of pregnancy and high blood glucose levels.
Sources
Foods high in sodium include yeast extract, bacon, smoked fish, salami, sauces, cornflakes, canned or boiled ham, biscuits, cheese, margarine and butter. Foods of animal and fish origin usually contain more sodium than whole grains, fruits and vegetables. Large quantities of sodium chloride, sodium bicarbonate or monosodium glutamate (MSG) are often added to foods during cooking, refining, processing and preservation. Many drugs contain sodium.
In babies, the ideal sodium content of food is that found in human breast milk. Untreated cow's milk is much higher in sodium and should not be fed to babies in the first three months of life as excess salt can cause high blood pressure and even death. Babies fed bottled water may be at risk of sodium deficiency as the water does not contain the levels of sodium comparable to those found in breast milk. It should not be used instead of infant formula.
| Food | Amount | Sodium (mg) |
|---|---|---|
| Salami, pork | 100g | 2260 |
| Blue vein cheese | 100g | 1395 |
| Sausages, cooked | 100g | 1294 |
| Salami, beef | 100g | 1176 |
| Corned beef | 100g | 1134 |
| Feta cheese | 100g | 1116 |
| Baked beans, canned | 1 cup | 1008 |
| Soy sauce | 1 tbsp | 914 |
| Olives | 100g | 872 |
| Chicken soup, can, prepared | 1 cup | 849 |
| Anchovies, canned, drained | 5 fillets | 733 |
| Cheddar cheese | 100g | 620 |
| Potato chips | 100g | 594 |
| Tuna, canned | 1 can | 557 |
| Fish, battered, deep fried | 1 fillet | 484 |
| Pizza, with cheese | 1 slice | 336 |
| Gravy powder | 1 tbsp | 332 |
| Lima beans, canned | ½ cup, drained | 312 |
| Bacon, cooked | 3 slices | 303 |
| Corn flakes | 1 cup | 297 |
| Plain cake | 1 piece | 233 |
| Cod, cooked | 1 fillet | 140 |
| French dressing | 1 tbsp | 128 |
| Butter | 1 tbsp | 117 |
Recommended dietary allowances (RDA)
An amount of about 500 mg a day is considered adequate to maintain the body's salt concentration. Intake should be no more than 2.4 g of sodium per day which is the amount found in around one teaspoon of salt. Those who eat few processed foods are probably getting around 1.2 g of sodium in food, leaving around half a teaspoon to be added to stay within recommended limits. Athletes and those who perspire a great deal may need more. The average daily intake of sodium in the USA is about 3 to 6 g with one-third to one-half of this being made up from table salt.
Pregnant women may need to consume 2 to 3 g of sodium per day. This amount should be available from a varied diet of wholesome, minimally processed foods with no salt added during cooking. In Australia, the recommended intake is 920 to 2300 mg per day.
Supplements
A healthy diet without salt supplementation is sufficient to provide for the body's needs. Salt supplementation is an acquired taste. Salt tablets may be useful in cases where sodium is lost through excessive sweating.
Toxic effects of excess intake
Hypertension
Epidemiological studies show that high sodium intakes are linked with high blood pressure. As a person ages, changes in the hormonal systems which regulate the control of water and sodium balance lead to changes in blood pressure. Many studies, including the Intersalt study, have found that high salt diets accelerate the increase in blood pressure that occurs with age.
About one-third of the general population and about one-half of those suffering from high blood pressure are "salt sensitive" and show increases in blood pressure when salt intakes are high. Restriction of dietary salt usually leads to decreases in blood pressure in such cases. Family history also contributes to high sodium levels and may play a role in hypertension.
A new study of almost 1500 British people has found that those who eat the most salt tend to have the highest blood pressure. The study, which involved men and women aged 16 to 64, found that as daily salt intakes rose from 1600 mg to 9200 mg, so did blood pressures. A rise in salt consumption from 2300 mg to 4600 mg led to a 7.1 mmHg rise in systolic blood pressure for women and a 4.9 mmHg rise for men. 1
In a two-month double-blind, randomized, placebo-controlled crossover study published in 1997 in The Lancet, researchers found that modest reduction in salt in the diets of elderly people led to lower blood pressure. The study involved 29 patients with high blood pressure and 18 with normal blood pressure. The average blood pressure fall was 8.2/3.9 mm Hg in the normal subjects and 6.6/2.7 mm Hg in those with high blood pressure.2 In those with normal blood pressure, cutting salt may have little effect, according to an analysis of 83 studies published in the Journal of the American Medical Association in 1998.3
High blood pressure appears to be associated with an imbalance of minerals where sodium and possibly chloride are too high; and potassium, calcium and magnesium are too low. When sodium levels are too high, the amount of water retained in the body is increased and water is pulled from cells. The result is high blood pressure and water retention which can lead to puffy eyes and swollen feet or fingers. The ratio of sodium to potassium in the diet may also be important in the regulation of blood pressure. Diuretic drugs, which are often used to treat high blood pressure, act by forcing the kidneys to excrete water and sodium at a faster rate.
Dietary sodium restriction is used to control pregnancy-related high blood pressure. It does not seem to lead to any adverse effects on other minerals or the baby. In fact, increasing evidence suggests that the amount of salt in a baby's diet affects blood pressure later in life. In a study published in 1997, Dutch researchers compared the effects of low salt and normal salt diets in 476 children born in 1980. They measured blood pressures in the first week of life and every four weeks after that for a six month period. Fifteen years later, the study participants had their blood pressures measured again and the results showed that children who had been in the low salt group had lower blood pressures than those in the normal salt group.4
Stress and sodium
Stress may affect sodium excretion. In certain people, stress seems to contribute to high blood pressure and this may be mediated via effects on sodium excretion. In a 1995 German study, researchers tested the effects of stress on 27 people with normal blood pressure and 21 with high blood pressure. The participants in the study took part in a 30-minute video game after which their excretion of sodium was measured. Seventy per cent of the people showed increased sodium excretion and 30 per cent showed decreased excretion. Those who excreted more sodium showed less stress-associated increases in blood pressure and greater expression of anger.5
Inhibited breathing seems to decrease sodium excretion which could mediate the role of behavioral stress in some forms of hypertension. Sodium excretion patterns under stress may be altered with certain types of antihypertensive medication.
Premenstrual syndrome
Salt and water retention are often seen in women with premenstrual syndrome. High salt diets may exacerbate these symptoms, although research results are conflicting.
Osteoporosis
High salt intakes seem to increase calcium excretion, thus lowering bone mineral density and increasing the risk of osteoporosis. In a study published in 1995, Australian researchers investigated the influence of urinary sodium excretion on bone density in a two-year period in 124 postmenopausal women. The results showed that increased sodium excretion was linked to decreases in bone density.6
Asthma
Population studies have found links between consumption of table salt and asthma. Some research reports suggest that high dietary sodium intake is a cause of asthma and airway hyper-reactivity, while others show no effect. A 1993 UK study tested the effects of either a placebo or sodium supplements on asthma sufferers who had previously followed a low sodium diet. The results showed a worsening of symptoms and laboratory measurements of disease severity in those patients on the high sodium diets.7
Urinary stones
A high urinary sodium-to-potassium ratio may be linked to the formation of urinary stones. Researchers involved in the Gubbio Population Study in Italy assessed the relationship between urinary sodium-to-potassium ratio and urinary stone disease in 3625 men and women aged 25 to 74. Analysis of the results showed that higher ratios were linked to an increased risk of stone formation.8
Therapeutic uses of supplements
Sodium bicarbonate is used intravenously to treat metabolic and respiratory acidosis, in the acute treatment of excessive potassium levels, and to make urine alkaline.
Interactions
In addition to the interactions with other electrolytes described above, sodium increases urinary loss of calcium. The hypertensive effect of sodium is enhanced when calcium intake is low.