Sodium

Sodium is the electrolyte your body runs on. It maintains blood volume, blood pressure, nerve signalling, muscle contraction, and the gradient that pulls water into and out of every cell. Without enough of it, you get tired, headachy, lightheaded, foggy, crampy, and your workouts fall apart. With far too much of it sustained over decades, your blood pressure drifts up and your cardiovascular risk climbs. The interesting thing is that the band where most people actually live, somewhere between 3 and 5 grams of sodium per day, is exactly where mortality is lowest in the largest prospective studies. The conventional advice to drive sodium as low as possible is, for most people, the wrong target.

Most people don't need to supplement sodium, they get plenty from food. The people who actually do are a specific list: anyone sweating heavily (training, sauna, hot climate), anyone on a low-carb or ketogenic diet (insulin drop drives sodium loss through the kidneys), anyone fasting, anyone on diuretics, and anyone with chronically low blood pressure or symptoms like dizziness on standing, brain fog, or persistent low energy that nothing else explains. For these groups, adding 1-3g of sodium per day, ideally with potassium, is one of the cheapest and most effective interventions you can make.

Deep-dive

Sodium sits in the extracellular fluid (the blood plasma and the fluid bathing your cells) at roughly 135-145 mmol/L. That concentration is one of the most tightly defended numbers in human physiology, because every cell's volume depends on it. If extracellular sodium drops, water moves into cells and they swell, which is why severe hyponatremia causes brain edema, seizures, and death. If extracellular sodium rises, cells shrink. The kidneys, the renin-angiotensin-aldosterone system (RAAS), antidiuretic hormone, and the thirst mechanism all coordinate to keep sodium concentration stable, mostly by adjusting how much water you drink and how much you pee.

Sodium intake itself is buffered, not concentration. When you eat more salt, you don't push your blood sodium concentration up much, you push your fluid volume up, because the body holds onto more water to keep the concentration the same. This is the link between salt and blood pressure: more sodium means more blood volume means more pressure on the arterial walls, in people whose vasculature responds that way. Roughly 25-50% of the population is salt-sensitive, meaning blood pressure rises measurably with higher sodium intake. The rest are salt-resistant and barely move.

The U-shaped mortality curve. This is the most important thing to understand about sodium and it contradicts a lot of public health messaging. The largest prospective cohort study, PURE, tracked over 100,000 adults across 18 countries and found cardiovascular events and mortality lowest at sodium intakes of roughly 3-5g/day, with risk rising at both higher (above 5g) and lower (below 3g) intakes. A 2025 Chinese cohort of 270,991 adults found the lowest risk at 4.6-5.4g/day, with a clear U-shape. The same pattern shows up in pooled analyses across multiple international cohorts. Below 3g, mortality starts climbing again, likely because sodium restriction at that level activates the renin-angiotensin-aldosterone system, raises plasma noradrenaline, worsens insulin sensitivity, and shifts lipid metabolism unfavourably. The PURE result has been criticised on methodology, mostly that single spot urines underestimate intake, but the U-shape replicates across studies using different measurement methods and different populations.

The practical read: if your intake is below about 3g/day, you almost certainly benefit from more. If you're consistently above 5g/day and you have hypertension or are salt-sensitive, you probably benefit from less. If you're somewhere in between and your blood pressure is fine, the evidence doesn't support pushing it lower.

Why low-carb and fasting amplify the need. Insulin tells the kidneys to retain sodium. When you cut carbs hard or fast, insulin drops, the kidneys dump sodium and water, and you can lose 2-3g of sodium and a few kilos of water in the first week. This is what most of "keto flu" actually is. Headache, fatigue, lightheadedness on standing, exercise intolerance, brain fog, all of it is mostly sodium and volume depletion, and almost all of it resolves with 2-3g extra sodium per day plus adequate water. The same applies during prolonged fasting, especially water-only fasts longer than 24 hours.

Sweating, heat, and exercise. A typical sweat rate during moderate exercise in moderate heat is 0.5-1.5L/h, with sodium concentrations ranging widely from about 200 to 2000 mg of sodium per litre depending on the person, training status, and heat acclimatisation. Heavy sweaters and "salty sweaters" can lose 1.5-3g of sodium per hour during prolonged hard exercise in heat. For sessions under 60-90 minutes in moderate conditions, you don't need to replace sodium during the session, your body handles it. For sessions over 2 hours, in heat, or for repeated days of training, replacing sodium during and after becomes meaningful. The classic argument that sodium prevents cramping is weaker than the marketing suggests, systematic reviews have struggled to consistently link sodium intake to cramp prevention, and exercise-associated cramps appear to have more to do with neuromuscular fatigue. But sodium does meaningfully reduce the risk of exercise-associated hyponatremia, which is genuinely dangerous and is caused mostly by overdrinking plain water during long events.

Performance. A pre-exercise sodium load (about 164 mEq, roughly 3.8g sodium, taken 1 hour pre-exercise) expanded plasma volume by ~3% and improved time-trial performance and thermoregulation in trained cyclists. In hot conditions, higher-sodium beverages during exercise help maintain plasma volume and reduce body mass loss. In cool conditions and shorter events, sodium supplementation during the session doesn't improve performance. The pattern is similar to what you'd predict, sodium matters most when you're losing a lot of it.

Salt sensitivity, hypertension, and salt substitutes. About a quarter to half of people with hypertension are salt-sensitive, meaning their blood pressure responds meaningfully to salt restriction. The DASH-Sodium trial is the cleanest evidence: combining the DASH diet (high in potassium, calcium, and magnesium from fruit, vegetables, and dairy) with low sodium (~1.5g/day) lowered systolic blood pressure by ~7 mmHg in normotensive adults and ~11 mmHg in hypertensive adults, more than DASH alone or sodium reduction alone. The strongest evidence for a hard cardiovascular benefit comes from the Salt Substitute and Stroke Study (SSaSS), where replacing regular salt with a 75% sodium chloride / 25% potassium chloride blend in over 20,000 older Chinese adults (most with hypertension or prior stroke) reduced stroke by 14%, major cardiovascular events by 13%, and all-cause death by 12% over five years. Notably, the win came from the swap, more potassium and less sodium together, not from cutting sodium alone.

Why the sodium-to-potassium ratio matters more than either nutrient alone. Sodium and potassium aren't independent variables, they're the two halves of the same physiological system. Every cell in the body maintains a steep gradient: high potassium inside, high sodium outside, with the Na+/K+-ATPase pump burning roughly 20-30% of your resting energy budget to keep it that way. That gradient runs nerve signalling, muscle contraction, fluid balance, and vascular tone. The cardiovascular effects of each ion mostly oppose each other. Sodium drives water retention, raises blood volume, and in the presence of aldosterone stiffens the endothelium and reduces nitric oxide release, which raises blood pressure and impairs vasodilation. Potassium does the opposite, it hyperpolarises endothelial cells, increases nitric oxide release, softens the vascular wall, promotes natriuresis (the kidneys dump sodium when potassium is high), and reduces sympathetic vasoconstriction. The same potassium intake produces a much bigger blood pressure drop in someone eating high sodium than in someone already eating low sodium, because the lever is the imbalance, not the absolute number of either ion.

This is why the ratio outperforms either nutrient as a predictor. A 10-year cohort of 143,050 adults and a longitudinal Iranian study both found the sodium-to-potassium ratio more strongly associated with cardiovascular events and mortality than sodium or potassium alone. The TOHP follow-up showed the same pattern for coronary events. The WHO target is a molar ratio of 1:1 or lower, which translates to roughly 1g of sodium per 1.7g of potassium by mass. Most people globally sit around 3.7:1 by molar ratio, heavily sodium-skewed, almost always because potassium intake is too low rather than because sodium is unusually high. The practical implication is that for most people, eating more potassium-rich food (vegetables, fruit, beans, potatoes, dairy) is a more effective lever than aggressively cutting salt, and salt substitutes that bake potassium into the salt itself are an underrated intervention.

Women. Women are more salt-sensitive than men at every age, premenopausal and postmenopausal, and the gap widens after menopause as oestrogen-mediated nitric oxide signalling drops and the kidneys become less efficient at excreting sodium. The practical implication is that postmenopausal women with rising blood pressure are one of the groups most likely to benefit from a sodium-lowering, potassium-raising approach, and the SSaSS data backs this up. Across the menstrual cycle, the high-hormone luteal phase shifts fluid balance: progesterone increases aldosterone and modestly increases sodium losses, oestrogen lowers the osmotic threshold for thirst and vasopressin release, and plasma volume drops by around 8%. Women doing long endurance work in the luteal phase carry a slightly higher risk of exercise-associated hyponatremia, mostly through overdrinking plain water rather than greater sodium loss per se. Female athletes also have lower absolute sweat rates and sweat sodium concentrations than men on average, which means lower total sodium losses in most training, but the variability between individuals is huge and personal sweat testing matters more than sex-based generalisations.

Older adults. Salt sensitivity rises with age, partly through declining kidney function and partly through reduced vascular compliance. At the same time, older adults are at higher risk of hyponatremia from overdrinking, certain medications (SSRIs, thiazide diuretics), and a blunted thirst response. Chronic mild hyponatremia in older adults is associated with falls, gait imbalance, attention deficits, and impaired cognition, all of which were previously thought to be benign. If you're over 65 and you feel chronically low-energy, foggy, or lightheaded, getting a serum sodium check is a reasonable first step before reaching for any compound.

The downsides at the high end. Above 5g/day chronically, sodium is associated with rising blood pressure, increased left ventricular mass, vascular stiffness, and some evidence of independent stroke risk that's not fully explained by blood pressure alone. There's also a fairly consistent link between very high sodium intake and gastric cancer risk in epidemiological studies. None of this changes the U-shape, but it's the reason cutting back makes sense if you're consistently in the 6-10g/day range that comes from a heavy ultra-processed food diet.

Dosage:

Baseline daily target for most adults: 3-5g of sodium per day (roughly 7.5-12.5g of salt). This is where prospective cohort data show the lowest mortality. Most people on a normal mixed diet land in this range without trying. If you cook from whole foods and salt to taste, you're probably fine.

Generally salt contains about 400mg of sodium per 1g of table salt. Salt is roughly 40% sodium, 60% chloride. So if you use 5g of salt in a day of cooking, that's about 2,000mg of sodium.

If you're on a low-carb, ketogenic, or carnivore diet: add 2-3g of supplemental sodium per day, especially in the first 2-4 weeks. A teaspoon of salt is about 2.3g of sodium. Spread it across the day in water, broth, or food. This single change resolves most "keto flu" complaints.

If you're fasting longer than 24 hours: 1-3g of sodium per day in water or broth, plus potassium and magnesium. Without it, expect headache, dizziness, and severe fatigue.

For training in moderate conditions, sessions under 90 minutes: no sodium needed during the session. Eat a normal meal afterwards.

For training over 2 hours, especially in heat: aim for roughly 500-1500 mg of sodium per hour during the session, depending on sweat rate and how salty you sweat. Heavy or salty sweaters are at the top of that range, light sweaters in cool conditions at the bottom. A simple way to gauge: if you have visible white salt crusts on your kit after training, you're a heavy salt loser and you need more.

Pre-exercise loading for hot or long sessions: roughly 3-4g of sodium with 500-1000 mL of fluid 1-2 hours before the session expands plasma volume and improves heat tolerance. Useful for races in heat or hot saunas, less relevant for everyday gym work.

For chronic low blood pressure, lightheadedness on standing, or POTS-like symptoms: 3-6g of additional sodium per day plus increased fluid is a standard non-pharmacologic intervention. Discuss with a doctor if symptoms are severe.

Pair with potassium, and think in ratios, not absolutes. The sodium-to-potassium ratio predicts cardiovascular outcomes better than either intake alone. The WHO target is a molar ratio of 1:1 or lower, which works out to a mass ratio of roughly 1:1.7 (sodium:potassium), or about 1g of sodium for every 1.7g of potassium. Most people globally sit closer to 3.7:1, heavily sodium-skewed. Practically: don't obsess over the exact number, just push potassium up rather than just pulling sodium down. Most people miss the target on the potassium side, not the sodium side. Fruit, vegetables, beans, potatoes, and dairy are the highest-yield sources. If you swap regular table salt for a 75/25 sodium/potassium chloride blend (lite salt, low-sodium salt), the trial evidence supports a real cardiovascular benefit, especially if you're older or hypertensive.

Forms. Plain table salt (sodium chloride), sea salt, and Himalayan salt are nutritionally near-identical for sodium purposes. The mineral content differences are real but trivial at typical intakes. Iodised salt is worth using if you don't eat much seafood or dairy, since iodine deficiency is making a comeback in low-salt populations. Electrolyte powders (LMNT, Liquid IV, etc.) are convenient but expensive, you can replicate them with table salt, lite salt, and a citrus drink for pennies.

Timing. Sodium with water is faster-absorbing if you're cramping, lightheaded, or pre-exercise. Sodium with food is fine for daily intake.

Here's what you can expect:

If you've been under-eating sodium (low-carb, fasting, heavy training, or just chronically restricting salt), the difference from adding 2-3g/day is often striking within 24-72 hours. Energy comes back, headaches resolve, you stop feeling lightheaded when you stand up, your training tolerance improves, your appetite normalises. The common reaction is "I didn't realise how bad I felt until I fixed it."

If you're already eating a normal mixed diet with adequate salt, adding more sodium does basically nothing. You'll pee out the excess, your weight might bump up half a kilo from water retention, and that's it.

If you have salt-sensitive hypertension and you reduce sodium from 6g/day to 3g/day while raising potassium, expect 5-11 mmHg drops in systolic blood pressure over a few weeks. The DASH-style approach (more vegetables, fruit, dairy, less processed food) tends to work better than just cutting salt in isolation.

During exercise, the difference sodium makes during a hard 2-hour session in heat versus none is meaningful: better thermoregulation, more stable plasma volume, less fatigue toward the end, and a much lower risk of cramps and hyponatremia. In cool conditions for shorter sessions, you won't feel a difference.

Side effects & risks:

Blood pressure rise in salt-sensitive individuals. Roughly 1.6 mmHg systolic per gram of sodium added, on average, much more in the salt-sensitive subset. If you have hypertension, monitor BP when changing sodium intake significantly.

Water retention and bloating in the first few days of higher intake. Self-resolves as the kidneys adjust.

Hyponatremia from overdrinking water without sodium, particularly during long endurance events, hot yoga, or with certain medications (SSRIs, thiazides, MDMA). Symptoms: headache, nausea, confusion, lethargy. Severe cases (sodium below 120 mmol/L) can cause seizures and death. The fix during long events is to drink to thirst rather than on a fixed schedule, and to include sodium with fluids.

Hypernatremia is rare in healthy people with intact thirst, since the kidneys can excrete enormous sodium loads. Risk is real in infants, the very elderly, and anyone with impaired access to water or impaired thirst response.

Heart failure, advanced kidney disease, cirrhosis with ascites: sodium restriction is genuinely indicated. Don't follow general population guidelines if you have these conditions, follow your specialist's plan.

Lithium interaction. Sodium intake significantly affects lithium levels. If you're on lithium, large changes in sodium intake can cause toxicity (if you cut sodium) or loss of efficacy (if you increase it). Don't change intake without your psychiatrist's input.

Kidney stones. High sodium intake increases urinary calcium excretion and can raise stone risk in people prone to calcium stones. If you've had kidney stones, keeping sodium below 4g/day and increasing fluid is reasonable.

Gastric cancer. Epidemiological evidence consistently links very high salt intake (especially from preserved/pickled foods) to elevated gastric cancer risk. Another reason the high end of the range is worth avoiding chronically.

"Salt addiction" is not a thing in any meaningful pharmacological sense, but ultra-processed food is engineered with salt-fat-sugar combinations that override satiety. The problem isn't sodium per se, it's the food matrix. Cooking from whole ingredients and salting to taste rarely gets anyone into trouble.

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Blood markers

Serum sodium, baseline if you have any of: chronic fatigue, brain fog, lightheadedness, frequent headaches, low blood pressure, are over 65, or take SSRIs or thiazide diuretics. Reference range is 135-145 mmol/L. Anything below 135 is hyponatremia and warrants attention. Many people with chronic low-grade symptoms sit at 133-135 and feel meaningfully better when corrected.

Blood pressure, baseline and routinely. Sodium's main downside operates through BP. If you're salt-sensitive, you'll see it here. Home cuffs are cheap and worth owning.

Potassium, baseline if you're going to use a salt substitute (sodium/potassium chloride blend), are on ACE inhibitors, ARBs, or potassium-sparing diuretics, or have any kidney issue. The risk with a high-potassium salt substitute in someone with reduced kidney function is hyperkalemia, which can be dangerous.

Aldosterone and renin (paired, ideally as a ratio), if you have unexplained hypertension that doesn't respond to lifestyle changes, especially if it's resistant or you have low potassium. Primary aldosteronism is one of the most under-diagnosed causes of hypertension and changes the entire treatment approach.

eGFR and urine albumin/creatinine ratio, baseline if you're using sodium aggressively for performance or have any cardiovascular risk factors. Tells you whether your kidneys can handle the load.

For most healthy adults eating a normal mixed diet without symptoms, none of this is needed. The people who actually benefit from baseline labs are: anyone over 65, anyone with hypertension or a family history of it, anyone with chronic unexplained symptoms (fatigue, fog, lightheadedness), and anyone on medications that interact with sodium handling.

Sold and consumed as ordinary table salt. No restrictions in any country.