Sodium is essential for nearly every major system in your body. It carries electrical signals through your nerves, controls how water moves between cells, helps your muscles contract, and even powers the absorption of nutrients from food. Your body needs between 200 and 500 milligrams of sodium daily just to keep these basic functions running, though most people consume far more than that. Here’s what sodium actually does once it enters your body.
Carrying Signals Through Your Nerves
Your nerve cells communicate using tiny bursts of electricity called action potentials, and sodium is the ion that makes them possible. Nerve cells can’t fire without voltage-dependent sodium channels embedded in their membranes. These channels are normally closed, but when a small electrical change reaches them, they snap open and let sodium rush into the cell. That influx creates a chain reaction: as more sodium enters, more channels open, which lets even more sodium in. This rapid feedback cycle is what generates the electrical spike that travels down a nerve fiber.
The size of these signals depends directly on how much sodium is available. When researchers reduce the concentration of sodium outside nerve cells, action potentials get smaller. Block sodium channels entirely, as the toxin from puffer fish does, and nerve signals stop altogether. Every sensation you feel, every thought you form, and every command your brain sends to the rest of your body depends on sodium flooding through these channels millions of times per second.
Making Your Muscles Contract
Muscle movement starts at the junction where a nerve meets a muscle fiber. When an electrical signal reaches the end of a nerve, it triggers the release of a chemical messenger called acetylcholine. That messenger binds to receptors on the muscle cell and opens channels that let sodium (and potassium) flow through. The resulting wave of sodium into the muscle cell creates a depolarization, essentially a voltage shift, that’s large enough to trigger the muscle’s own action potential. That action potential then kicks off the mechanical process of the muscle fiber shortening and generating force.
This sequence happens in skeletal muscles you control voluntarily, like your biceps, and also in your heart. Without adequate sodium, the electrical trigger that tells a muscle to contract weakens or fails, which is why muscle cramps and weakness are among the first symptoms of low sodium levels.
Controlling Water Balance Between Cells
Sodium is the most abundant positively charged ion outside your cells, while potassium fills the same role inside them. This separation isn’t random. It creates an osmotic force that determines where water goes. Water moves freely across cell membranes, always flowing toward whichever side has a higher concentration of dissolved particles. Because sodium dominates the fluid outside your cells, it is the major determinant of how much water stays in your blood plasma and in the spaces between cells.
When sodium levels in your blood rise, water follows, increasing blood volume. When sodium drops, water shifts into cells, potentially causing them to swell. This is why drinking large amounts of water without replacing sodium (during a marathon, for example) can be dangerous. The brain is particularly sensitive to swelling, which is why confusion, headaches, and in severe cases seizures and coma are hallmarks of dangerously low sodium.
Regulating Blood Pressure
Your body has a dedicated hormonal system for managing blood pressure, and sodium sits at the center of it. When blood pressure drops, your kidneys release an enzyme called renin, which sets off a cascade that ultimately produces the hormone aldosterone. Aldosterone tells your kidneys to reabsorb more sodium back into the bloodstream rather than letting it leave through urine. As sodium levels rise in your blood, your body retains more water alongside it, increasing total blood volume and pushing blood pressure back up.
This system works in both directions. When you consume a lot of sodium, the extra water your body holds can raise blood pressure beyond healthy levels over time. That’s the reason public health guidelines recommend limiting intake. The World Health Organization advises adults to consume less than 2,000 milligrams of sodium per day, roughly the amount in just under a teaspoon of table salt.
Powering Nutrient Absorption
Your small intestine relies on sodium gradients to pull glucose and other nutrients out of food and into your bloodstream. Cells lining the intestine use a pump that constantly moves sodium out of the cell, creating a steep difference in sodium concentration between the inside and outside of the cell. A transporter protein called SGLT1 then exploits that difference: it grabs two sodium ions trying to flow back into the cell and uses their momentum to drag one glucose molecule along with them.
This is why oral rehydration solutions, used worldwide to treat dehydration from diarrhea, contain both salt and sugar. The sodium helps the intestine absorb the glucose, and both together pull water into the body far more effectively than water alone.
Fueling Cellular Energy Demands
Maintaining the sodium gradient across every cell membrane is one of the most energy-intensive jobs your body performs. The sodium-potassium pump, which pushes sodium out of cells and pulls potassium in, consumes a staggering share of your energy budget. In the brain alone, this pump is estimated to use roughly 50% of all energy the organ consumes, both during active thinking and at rest. Across the entire body, keeping sodium where it belongs accounts for a significant fraction of your baseline calorie burn.
That energy expenditure isn’t wasted. The gradient the pump maintains is what makes nerve signaling, muscle contraction, and nutrient absorption possible. It’s the biological equivalent of charging a battery that the rest of your cells constantly draw from.
How Much You Need and How Much You Lose
A healthy, active adult needs between 200 and 500 milligrams of sodium per day to cover basic physiological needs. That’s a remarkably small amount, less than a quarter teaspoon of salt. Most people in developed countries consume several times that. The WHO’s recommended ceiling of 2,000 milligrams per day is meant to balance the body’s needs against the cardiovascular risks of chronic overconsumption.
Exercise changes the equation. Sodium is the primary electrolyte lost in sweat, with concentrations typically around 50 to 60 millimoles per liter. For someone sweating heavily during prolonged exercise, losses can add up quickly. There’s no universal consensus on exactly how much sodium athletes should replace after a workout, but replenishing both water and sodium during and after exercise is broadly recommended to maintain fluid balance and prevent hyponatremia.
What Happens When Sodium Drops Too Low
Hyponatremia, the clinical term for low blood sodium, occurs when levels fall below 135 millimoles per liter (the normal range is 135 to 145). Early symptoms include nausea, headache, fatigue, and irritability. As levels drop further, you may experience confusion, muscle spasms, and weakness. Severe hyponatremia can cause seizures and loss of consciousness.
The condition is most common in endurance athletes who overhydrate, older adults on certain medications that increase sodium loss through urine, and people with conditions affecting kidney or hormone function. The underlying problem is almost always too much water diluting sodium rather than too little sodium in the diet, since modern diets tend to provide sodium in excess.