Your intestines absorb nearly all the salt you eat, with about 90% taken up in the lower small intestine and colon. From there, your kidneys fine-tune how much sodium stays in your body and how much leaves through urine. These two organs work together to keep sodium levels in a tight range that supports blood pressure, fluid balance, and nerve function.
Where Salt Gets Absorbed in the Gut
Salt absorption happens along the entire length of your digestive tract, but different segments handle it in different ways. The small intestine does the bulk of the work, pulling sodium in alongside nutrients like glucose and amino acids. This is efficient because sodium hitches a ride with these nutrients through shared transport channels on the surface of intestinal cells.
The colon picks up what the small intestine leaves behind. In the ascending (right-side) colon, sodium and chloride are absorbed together through a paired exchange process. Further down, in the descending colon, sigmoid, and rectum, sodium is pulled in through dedicated sodium-selective channels that are tightly controlled by hormones. This final stretch acts as a safety net, reclaiming sodium before waste leaves the body.
The system is remarkably thorough. Under normal conditions, the intestine absorbs nearly all ingested sodium, leaving very little to be lost in stool.
How Intestinal Cells Actually Move Sodium
Three main mechanisms pull sodium from the inside of your gut into the cells lining it. The most important is a protein called NHE3, which swaps sodium from the gut lumen for hydrogen ions going the other direction. NHE3 is responsible for absorbing the majority of dietary sodium in both the small and large intestines. When researchers knocked out this protein in mice, the animals developed severe diarrhea and massive sodium loss, and their intestinal segments became visibly swollen with accumulated fluid.
The second mechanism links sodium absorption to nutrient absorption. A transporter called SGLT1 carries one sodium ion into the cell alongside each molecule of glucose. This is why oral rehydration solutions contain both sugar and salt: the glucose actively pulls sodium (and therefore water) across the intestinal wall. Without SGLT1, glucose absorption fails entirely.
The third mechanism uses epithelial sodium channels, or ENaC, which are most active in the lower colon. These channels let sodium flow directly into cells down its concentration gradient and are regulated by the hormone aldosterone.
None of these entry points would work without a pump on the opposite side of the cell. The sodium-potassium pump sits on the base of each intestinal cell and continuously pushes sodium out into the bloodstream while pulling potassium in. This keeps sodium levels low inside the cell, which is what creates the driving force for sodium to enter from the gut side in the first place. It’s an energy-intensive process: the pump burns ATP with every cycle.
How the Kidneys Regulate What Stays
Once sodium enters the bloodstream, the kidneys decide how much to keep. Your kidneys filter all the sodium in your blood many times per day, then reabsorb most of it before it reaches your bladder. The numbers are striking: the proximal tubule (the first segment of each kidney filtering unit) reclaims 60 to 70% of filtered sodium. The loop of Henle grabs another 20 to 30%. The distal tubule picks up 5 to 10% more. By the end, less than 3% of filtered sodium actually leaves in urine, and in sodium-conserving states, that number drops below 0.1%.
The kidneys use many of the same transport proteins found in the gut, including NHE3 in the proximal tubule and ENaC in the collecting duct. This overlap means hormones like aldosterone can coordinate sodium handling in both the intestines and kidneys simultaneously.
How Aldosterone Controls Sodium Absorption
Aldosterone is the primary hormone that turns up sodium absorption when your body needs to retain salt. Produced by the adrenal glands, it acts on both the intestines and the kidneys in a two-phase process.
In the first phase, aldosterone triggers cells to insert more sodium-potassium pumps into their outer membranes. This ramps up the engine that drives sodium out of cells and into the blood, which in turn pulls more sodium in from the gut or kidney tubule. In the second phase, the hormone increases the total production of both the sodium-potassium pump and the NHE3 transporter, building more long-term absorption capacity. Aldosterone also activates ENaC channels in the colon and kidney collecting duct, opening another route for sodium to enter cells.
When aldosterone levels rise (from dehydration, low blood pressure, or blood loss), your body becomes highly efficient at holding onto sodium. When levels drop, more sodium passes through to urine.
What Can Disrupt Salt Absorption
Several conditions interfere with normal sodium absorption in the gut, and they share a common mechanism: they disable NHE3.
Bacterial infections are a major cause. Toxins from Salmonella, Campylobacter, and Shigella all trigger a spike in calcium levels inside intestinal cells, which shuts down sodium and chloride absorption. This is a key reason why infectious diarrhea causes such rapid dehydration. The gut isn’t just leaking fluid; it’s actively failing to absorb sodium, and water follows sodium. Serotonin, which the gut produces in large quantities during inflammation, has a similar inhibitory effect on sodium transport.
Chronic inflammatory bowel diseases like ulcerative colitis also reduce sodium absorption. In biopsies from patients with ulcerative colitis, researchers found significantly decreased levels of both NHE3 and ENaC proteins, meaning the colon loses its ability to reclaim sodium effectively. This contributes to the chronic diarrhea and electrolyte imbalances common in these conditions.
Why Glucose Helps You Absorb Salt
The coupling of sodium and glucose transport is one of the most practical facts about salt absorption. Because SGLT1 requires both sodium and glucose to function, consuming them together significantly boosts sodium (and water) uptake. This principle is the entire basis of oral rehydration therapy, which has saved millions of lives from dehydration caused by cholera and other diarrheal diseases.
The effect works because even when NHE3 is knocked out by infection or inflammation, the SGLT1 pathway can still operate as long as glucose is present. A simple mixture of water, salt, and sugar exploits this backup route, bypassing the damaged absorption pathway and pulling sodium and water into the bloodstream through a channel that bacterial toxins don’t typically shut down.