Serum sodium

Serum sodium is the concentration of sodium in your blood, reported on almost every routine blood test. People whose level sits at the high end of the normal range tend to age faster and get more heart failure, high blood pressure and dementia over the following decades, and habitual low fluid intake is the most likely reason — but the evidence is observational, comes mostly from one research group, and no trial has shown that drinking more changes it.

Treat a reading in the low 140s or above as a nudge to drink more steadily through the day, not as a diagnosis. Don't chase the bottom of the range either: lower does not look better.

What it measures

A standard laboratory reports anything from 135 to 145 mmol/L as normal. In healthy people the body defends that number tightly: when you drink less, you carry less water, sodium drifts up, and vasopressin — the hormone that tells the kidney to hold water — rises to pull it back down. So serum sodium is a proxy for how much you habitually drink, not a direct readout of it. Almost all of the risk described below sits inside the normal range.

The main study: 25 years of middle-aged Americans

The signal comes from the Atherosclerosis Risk in Communities (ARIC) study, which enrolled 15,752 middle-aged US adults and followed them for 25 years. The researchers scored a "biological age" from a panel of 15 blood and organ-function markers, nine of which carried the calculation.^[1]

Biological age. In a healthier subset of 6,956 people taking neither blood-pressure nor cholesterol drugs, those above 142 mmol/L had about 10–15% higher odds of being biologically older than their age, rising to roughly 50% only above 144 mmol/L.
Chronic disease. Among the 5,168 who survived to the 25-year visit, those above 142 mmol/L had roughly 39% higher risk of a combined measure of heart failure, dementia, chronic lung disease and stroke than people at 138–140.
Early death. Above 144 mmol/L, risk of dying early was about 21% higher than in the 137–142 reference band: 34.5% died during follow-up against 26.2%. That estimate only just clears no effect.

A separate snapshot (cross-sectional) analysis of US adults found the curve is U-shaped rather than flat: biological age bottomed out at about 139 mmol/L and rose below that as well as above it, though the differences were small.^[2]

Blood pressure and dementia: the same group, a second population

An Israeli cohort of 407,187 adults who were healthy at the start saw the same pattern. Sodium of 140–142 mmol/L carried about 13% higher risk of later high blood pressure, rising to about 29% above 143, with heart-failure risk about 20% higher in that top band. Roughly six in ten of the health service's current healthy members sit at 140 mmol/L or above.^[3]

A 2026 analysis of ARIC plus 395,802 members of the Israeli health service put a number on dementia: risk starts climbing around 141 mmol/L and runs 1.5- to 3-fold higher at the top of the normal range and above it.^[4]

The two populations put the turning point in different places — ARIC's lowest-risk band runs to 142 mmol/L, while the Israeli data show risk climbing from 140 — so the low 140s are best read as a grey zone rather than a safe plateau.

Why this isn't proof

It is one research group. The Israeli analyses share senior authors with the ARIC one, so they corroborate rather than independently replicate it. No genuinely independent cohort has reproduced the serum-sodium result. A Dutch population study of 8,028 adults followed for about 11 years found no link between sodium and new dementia — though it was designed around low sodium and modelled the rest of the range as a straight line, so it could miss a threshold at the high end.^[5]

The best independent support uses a different measure. Among 63,488 Japanese adults followed for a median 19.4 years, the lowest fifth of estimated daily water turnover carried about 26% higher mortality in women and 18% in men. The curve was L-shaped: it argues against being under-hydrated, not for drinking ever more.^[6]

Kidneys muddy the measure. Blood in ARIC was drawn after an overnight fast, so a high morning sodium partly reflects how well the kidneys concentrate urine overnight, and six of the fifteen markers behind the biological-age score measure kidney function. Weaker kidneys could push both sides of the association at once.^[7]

The mechanism: plausible, not proven

The proposed link is vasopressin. Short term it saves water; kept switched on for years, it may be costly. Its stable blood marker, copeptin, tracked with about 63% higher heart-failure risk (top quarter against bottom) in 5,297 older Swedish adults followed for a median 11 years. Per step up in copeptin, the association shrank only modestly after adjusting for kidney function, but adjusting for a heart hormone that tracks fluid volume removed about half of it, which suggests much of the copeptin signal is ordinary fluid-volume physiology.^[8] Copeptin also travels with arterial stiffness and coronary calcium, but only in snapshot (cross-sectional) imaging data.^[9]

The researchers' own review puts it no more strongly than that persistent activation of these water-saving responses may be harmful, with causation still being tested.^[10] And when inflammatory and oxidative-stress markers were measured directly, they explained about 1% of the link between serum sodium and mortality — and in that analysis the risk sat with low-normal sodium, not the high-normal readings this page is about.^[11]

What hasn't been tested

The step from "high-normal sodium predicts disease" to "drinking more prevents it" is an inference, not a finding. Three randomised trials have tested drinking more against an organ outcome:

Two in adults with kidney disease were null: coaching people with moderate chronic kidney disease to drink about 1 L a day more did not slow their decline over a year, and three years of high water intake did not slow kidney growth in polycystic kidney disease.^[12]^[13]
The third encouraged 175 healthy children, median age 8.6, to drink more for a year and found office blood pressure about 2 mmHg lower than in controls. It was small and unblinded, and its own authors attribute the effect to more efficient sodium excretion by the kidney rather than to vasopressin.^[14]

No trial has shown that drinking more slows aging or extends life. The full picture on how much to drink is in Water.

What to do

Read a high-normal value as a hint, not a verdict. One morning reading partly reflects your kidneys, not just your drinking.
If you sit in the low 140s or above, drink steadily through the day until your urine stays pale and you rarely feel thirsty (under 65; past 65, drink on a schedule).
Don't aim for the bottom of the range. Biological age rose below about 139 mmol/L too, and in the Japanese data the risk sat with drinking too little, not with failing to drink ever more.
Don't overdrink during endurance exercise. Drinking too much plain fluid dilutes blood sodium dangerously (exercise-associated hyponatremia), which has killed athletes; sports drinks don't prevent it. Drink to thirst. Details are in Water.