Sodium

Sodium is the primary extracellular cation (Na⁺) found within the body’s extracellular fluid. Sodium in the diet is found bound to anion chloride as NaCl commonly referred to as table salt. Sodium, chloride, and potassium are the main electrolytes that control water balance and action/electric potentials to conduct muscle/nerve impulses.

Absorption and Excretion:  Unlike some minerals, sodium absorption rate ranges between 90-95%. Sodium is absorbed in the small intestine and colon by three main pathways: sodium-glucose cotransport system (in the small intestine), sodium-chloride cotransport system (in the small intestine and proximal colon), and electrogenic sodium transport mechanism (in the colon).¹ A sodium-hydrogen system is also used.¹ Sodium is responsible for transporting other molecules such as amino acids, peptides, and water-soluble vitamins.¹ Any excess sodium is regulated and excreted by the kidneys through urine.¹ Sodium is regulated within the blood by sodium-potassium ATPases.¹ 

Storage and Circulation: Sodium is found within bones - specifically within the surface of mineral crystals (30-35%).¹ It may serve as a source of sodium to prevent hyponatremia (low sodium levels).¹ The majority of sodium is found within extracellular fluid with a minimal amount found in intracellular fluid.¹ There is 142 mEq/L of sodium in extracellular fluid compared to 10 mEq/L in intracellular fluid.¹

Functions: Sodium has many functions within the body which include conducting electric/action potentials and controlling water balance, blood pressure, and blood volume.¹ Electric/action potentials are required for muscle contractions and nerve transmissions.¹ They are regulated by gated channels that influx sodium into the cell and potassium outside the cell to trigger muscle contraction or nerve transmissions.¹ The concentration gradient is reestablished after the action/electric potential once sodium leaves the cell and potassium reenters the cell.¹

Fluid balance is controlled through the difference between the concentration gradient and charge of ions to maintain isotonicity - this is referred to as an electrochemical gradient. Sodium causes the body to retain water, increasing blood volume and pressure. The heart, blood vessels, and kidneys regulate how water and blood are used within the body through sensory receptors and hormones. Receptors in the heart termed baroreceptors note a decrease or increase in blood pressure and send input to the brain and kidneys.² The brain and kidneys release nerve impulses and hormones that target the heart and blood vessels to correct blood pressure.² The following provides more detail:

Blood pressure increases when: heart rate and contractility increase, blood vessels constrict, and when blood volume increases. 

• To lower blood pressure, the brain decreases sympathetic responses and increases parasympathetic responses to slow heart rate and dilate blood vessels.²

• Hormone atrial natriuretic peptide (ANP) produced by the heart causes vasodilation of vessels and excretion of sodium and water in urine to reduce blood volume and therefore blood pressure.²

Blood pressure decreases when: blood vessels dilate and blood volume decreases. 

• To increase blood pressure, the brain decreases parasympathetic responses and increases sympathetic responses to increase heart rate and constrict blood vessels.²

• Hormones from the kidney raise blood pressure by producing angiotensin II to constrict blood vessels and aldosterone to promote sodium and water reabsorption to increase blood volume and therefore blood pressure.²

• Hormones epinephrine and norepinephrine from the kidney increase the rate and force of heart contractions, promote vasoconstriction of skin and gut vessels, and cause vasodilation in cardiac and skeletal muscle to raise blood pressure.²

• Hormones from the brain (hypothalamus) increase blood pressure by producing antidiuretic hormone (ADH - also called vasopressin) to constrict blood vessels and move water from the kidneys into the bloodstream which increases blood volume and therefore blood pressure.²

Dietary Reference Intake (DRI) - The recommended dietary allowance (RDA) for adult men and women (14 years and older ) is 1500 milligrams (mg).³ It is recommended for adults to consume no more than 2300 mg of salt per day or less than one teaspoon.³

Deficiency/Toxicity - As described, the body regulates sodium reabsorption in relation to fluid and blood volume/pressure. Excess sodium in the blood is termed hypernatremia and can be caused by excess consumption of dietary salt. Too little sodium in the blood is termed hyponatremia and occurs more commonly in older adults. 

Dietary Sources of Sodium

Sodium is not found within raw or minimally processed foods compared to packaged, highly processed foods.  Dairy and meat products naturally contain sodium but can be processed to enhance flavor or shelf-life, increasing sodium content. Highly processed foods such as hot dogs, sausage, ham, and other luncheon meats are high in salt. Sodium is found in snack foods such as chips, pretzels, and crackers. It can also be found within condiments such as salad dressings, ketchup, and soy sauces and within ingredients used for cooking like bouillon cubes, cheeses, gravies, and broths. Canned vegetable foods may have salt added such as canned corn, green beans, carrots, pickles, and beans. Salt is often added to foods and can contribute significantly to sodium intake. It is recommended to stick to low-sodium or salt-free options commonly found in fresh fruit and vegetables, whole grains, low-fat dairy, and lean meats. 

Examples of foods containing sodium include

Meat/Seafood/Animal Product- ham, hot dogs, bologna, bacon, sardines

Dairy - cheddar cheese, milk, cream-based soups

Grains - crackers, bread, muffins

Other - soy sauce, french fries, fast food, gravies, broths, pickle

Source(s)

1. Jerry Tortora and Bryan Dickerson, Principles of Anatomy & Physiology, 16th ed. (New Jersey: John Wiley & Sons, 2021).

2. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

3. https://www.fda.gov/food/nutrition-education-resources-materials/sodium-your-diet#:~:text=Americans%20eat%20on%20average%20about,recommended%20limits%20are%20even%20lower.