The bib pick-up area of a 70 km trail race (running on mountain and hill paths), on a Thursday afternoon. The corridor is crowded with stalls, and nearly every one has jars of salt tablets on display. The seller counts out twelve tablets into a small plastic box and tells you, “one an hour, so you don’t cramp and your sodium doesn’t drop.” Buy the box and you get a free soft flask with pre-printed marks. Each mark is one drink, one every fifteen minutes, thirsty or not.
The box of salt and the marked flask rest on the same belief: take in enough salt and enough water on a schedule, and the body will be fine. Sweat really does carry salt away, so the first half of that belief has a basis. The second half is not supported by data from ultra races (races longer than the 42 km marathon).
Two-thirds of ultrarunners believe in salt tablets
There is a reason the salt stalls are busy. In 2018, Hoffman’s group surveyed 1,152 ultrarunners (published in 2019). Two in every three wanted race organizers to have sodium tablets ready for them (66.4%), and almost exactly as many (65.5%) believed the tablets keep blood sodium from dropping during exercise.
It sounds reasonable. Low sodium, so replace sodium. The catch is that in long-distance runners, sodium usually drops because the blood becomes diluted when the water you drink exceeds what you lose in sweat. To fix it, you have to fix the amount of water.
The international consensus statement on exercise-associated hyponatremia (low blood sodium during exercise) (Hew-Butler and colleagues, 2015) and the guidelines of the Wilderness Medical Society (WMS; Bennett and colleagues, 2020) both conclude that salt tablets or sodium supplements do not prevent hyponatremia if the runner still drinks too much. The consensus statement rates this advice as strong, even though the evidence is still low. WMS rates it more softly: a weak recommendation, based on moderate evidence. The evidence is not yet high, but the two documents point in the same direction.
What worries us about the box of salt is the reassurance it gives. With salt on hand, a runner may easily think a few more cups of water won’t hurt. No study has measured this specifically, but thinking that way runs exactly against the advice above.
A runner who has nausea, vomiting, a headache or confusion after drinking a lot should not be given more to drink, and should be taken to medical care. Heatstroke also makes runners confused, and its management is almost the opposite. That emergency topic has its own article in this series.
Cramps in the last few kilometers
Salt tablets are also promised to stop cramps. Cramps tend to come in the last few kilometers, when your shirt has been soaked and dried several times over, so it is easy to blame the salt lost in sweat.
Hoffman and colleagues followed runners in a 161 km ultramarathon in two studies, in 2015 and 2016. Taking sodium tablets was not linked to cramps, nor to nausea or vomiting. These two studies were only observational, not randomized trials. On the other hand, they were done in exactly the right people: long-distance runners.
Miller and colleagues (2022) reviewed exercise-associated muscle cramps and also found that the data do not support the idea that losing water and electrolytes is the main cause. A meta-analysis of observational studies by Costa and colleagues (2026) found that the strongest associated factor was a history of cramps, with an odds ratio of about 4.4, meaning people who have had cramps before are about four times more likely to get them again than people who never have. Blood potassium after the race in people with cramps was actually higher, not lower, with moderate-level evidence.
If your calf seizes up mid-race, stop and gently stretch that muscle until it lets go. Anyone who cramps on every long run should see a doctor to rule out low potassium, low sodium, or the effects of medicines they are taking, such as diuretics or statins (cholesterol-lowering drugs).
Harrison’s internal medicine textbook describes a specific type of heat cramp, seen in people who sweat very heavily but drink only plain water, with no salt. We read that as one more reason to use a sodium-containing drink in long races in hot weather. It does not justify swallowing salt tablets by the hour.
White salt lines on your cap
After a long summer run, many people notice white streaks spreading over the brim of their cap or the back of a dark shirt. That is salt from sweat. According to Performance Nutrition by Austin and Seebohar, sweat contains on average about 1 g of sodium per liter, and this figure varies a great deal from person to person. People who are used to the heat lose even less. Brown and colleagues (2022) recorded sweat sodium falling by about 22–59% after heat acclimatization.
In hot, humid weather like ours here in Vietnam, the body needs more water and electrolytes (Austin and Seebohar, 2011; International Society of Sports Nutrition, ISSN: Tiller and colleagues, 2019). Each person loses a different amount of salt, and the weather differs from race to race, so there is no single salt dose for every runner. Someone running five or six hours in the sun, whose sweat leaves thick white streaks, usually needs more salt than someone running 10 km in the early morning. “One tablet an hour” ignores exactly that difference.
Salt belongs in your flask and your bowl of rice porridge
The best place for salt is usually in drinks and food.
The US Institute of Medicine advises that sports drinks contain 460–1,150 mg of sodium per liter (Austin and Seebohar, 2011). Ready-made sports drinks often contain only about 460–575 mg per liter, while in ultras, needs may exceed 575 mg per liter (Tiller and colleagues, 2019). Bottle labels often list sodium per serving or per bottle, so you need to convert it to mg per liter before you can compare it with these benchmarks. The clinic places sodium in drinks in the group “can be used, depending on the person”, best suited to long races in hot weather.
The second place is the food table at aid stations. For ultras, the ISSN (Tiller and colleagues, 2019) favors real food, including salty dishes. When an aid station has rice porridge, noodle soup, or boiled sweet potato dipped in salt, the salt goes in along with starch and water, at the pace you eat and drink rather than by the clock.
If you still carry salt tablets, remember two things. Salt tablets cannot replace keeping an eye on how much water you drink. And Harrison’s textbook clearly warns against swallowing salt tablets that have not dissolved.
The marks on the side of the flask
On water, the advice of the medical associations is very simple: drink to thirst. The hyponatremia consensus statement (Hew-Butler and colleagues, 2015) and WMS (Bennett and colleagues, 2020) rate this as a strong recommendation with low evidence. The WMS heat illness guidelines (Eifling and colleagues, 2024) rate it as strong with moderate evidence, and set the goal of not losing more than 2% of body weight. For someone weighing 60 kg, 2% is 1.2 kg.
The marks on the flask from the stall point the opposite way. Drinking on a schedule before you are thirsty, or trying to drink back all the weight you lose while still running, are two habits worth dropping.
The scale is still useful. The American College of Sports Medicine (ACSM; Sawka and colleagues, 2007) uses weighing before and after a run to work out each person’s fluid needs. The two approaches can fit together if you still drink to thirst and use the scale only as a reference. Weigh yourself before and after a few long training runs, and you will know how much you usually lose in that kind of weather. Then you can work out how many flasks to carry, and which long stretches between aid stations need extra. That number is not for setting a forced drinking schedule on race day.
The 1.5–2 liters a day recommended by the Vietnamese Ministry of Health is for the general population and does not account for a 6-hour run.
Our grandparents mocked people who “dig the well only once they are thirsty.” For runners, the saying is right about preparation: know at which kilometer the aid stations are, and carry enough flasks for long stretches without one. When it comes to drinking itself, let thirst tell you. The traditional healer Nguyen Huu Duc also advised drinking enough water when your throat is dry or when you feel hot (Stories of Eastern Medicine, vol. 10). That everyday advice points the same way as the medical associations.
The bathroom scale
Back home, you step on the scale. If you really have lost weight compared with before the run, Austin and Seebohar suggest drinking about 1.5 liters for every kg lost, that is, about 150%, containing sodium from about 500 mg per liter upward, sipped gradually over the next few hours. A loss of 0.8 kg means about 1.2 liters. This 150% figure comes from the book Performance Nutrition, even though it is often quoted as ACSM advice. It is only a reference. Once you are no longer thirsty, there is no need to force down the rest.
Oresol is the oral rehydration and electrolyte sachet that every Vietnamese household knows. For mild heat exhaustion, the Ministry of Health chooses Oresol mixed at the correct strength. Our clinic’s medical records include the instruction “take Oresol after running,” and we keep that instruction for people who sweat heavily or feel slightly heat-exhausted after a run. Along with it: mix it with exactly the amount of water written on the sachet, drink it when thirsty, and don’t force yourself to finish it. Mixing it stronger to make it “saltier” or weaker to make it easier to drink both move away from that correct strength.
Sweat is the fluid of the Heart
The textbook on the theory of Chinese medicine says “exertion consumes qi” (劳则气耗 / 勞則氣耗, láo zé qì hào): with hard exertion you breathe fast, sweat comes out, and qi is used up. Sweat is regarded as the fluid of the Heart (the heart organ as understood in Eastern medicine); heavy sweating depletes blood and damages the body fluids, that is, the water and fluids that nourish the body (Basic Theory of Chinese Medicine, 中医基础理论 / 中醫基礎理論, p. 386, 498). People who tend to lose a lot of salt in their sweat, and who after a whole hot season of running feel thirsty, short of breath and worn out, are classed in Eastern medicine as having dual deficiency of qi and yin (气阴两虚 / 氣陰兩虛, qì yīn liǎng xū), meaning both qi (strength) and yin (the fluids that nourish the body) are lacking.
For dual deficiency of qi and yin, after an examination, the doctor may prescribe a decoction in ready-to-drink 50 ml pouches for the recovery weeks between race seasons. The pouches do not replace water and salt while running. The formula often contains herbs that reduce sweating, so on days with a hot run or on race day, take it well away from your run, and not within 2 hours before you run.
Gan cao (Glycyrrhiza uralensis root, licorice) is also connected with salt. If you often drink teas or suck lozenges containing Gan cao, you should know that in high doses or with long-term use, this herb causes salt retention, swelling, high blood pressure and low potassium (Chinese Materia Medica, 中药学 / 中藥學, p. 647; Takao and colleagues, 2015). The clinic adds up Gan cao from all sources and keeps the total to no more than 2 g a day.
Two inner-thigh muscles that keep the salt ledger
The body’s salt–potassium balance depends partly on aldosterone, a hormone of the adrenal glands. Applied Kinesiology (AK), a way of testing muscle strength by hand to read the state of the body, reads this gland through the sartorius and gracilis muscles on the inner thigh (Walther, Synopsis, p. 322–323). Training hard and sleeping little through a hot race season is a type of stress Walther names for this gland (p. 515); when the gland is overloaded, these two muscles often work below threshold, meaning they weaken when tested (p. 512).
Picture a runner who comes in during the rest week after a race, describing exhaustion and a cap brim caked with thick salt lines. The first thing is a glass of water. In someone who is dehydrated, every muscle may test weak, and drinking or holding a sip of water in the mouth brings the muscles back (p. 162); skip this step and the whole session is easily misread. Then the doctor tests the sartorius and has the person taste a little salt, then a little potassium. Walther uses exactly this pair to assess aldosterone activity: if salt makes the muscle weak, potassium makes it strong, and vice versa (p. 514). Potassium sits mainly inside the cells, so blood tests rarely show it out of range (p. 512), and in the clinic’s experience, some people have normal blood sodium and potassium yet their body still does not have enough to work with; in that case the muscle test results are still read together with the lab figures. People who sweat heavily also have the teres major, behind the shoulder, tested. For this muscle, Walther notes a link with minerals in people who sweat heavily, and with zinc in people who find it hard to taste their food (p. 352).
For a substance that weakens a muscle, Walther reports that Schmitt treated it right while the patient still had that substance in the mouth (p. 514). The doctor re-stimulates the nerve signals, retests the sartorius, and only then discusses whether the salt should go in the drinking flask or in the bowl of rice porridge next season.
All of this belongs to the weeks of preparation and recovery. Muscle testing does not tell you how much to drink each hour. A runner who is vomiting or confused after drinking a lot at a race needs a blood sodium measurement and medical care, not a session on the muscle-testing table.
The salt box stays at home
The box of twelve tablets from the stall can stay at home. The soft flask clipped to your running vest with water pockets (hydration vest) should hold a sodium-containing drink, especially when the race runs through midday, but don’t rely on the marks on its side. Stop at any aid station you like, but drink when thirsty, and have an extra bowl of rice porridge or a sweet potato dipped in salt at the long stops. Back home, step on the scale, and whatever you have lost, drink it back gradually, guided by thirst.
If you get cramps again and again across several races, feel worn out and sweat heavily after every hot running season, or want to know how much salt your body really needs, bring the salt tablets, electrolyte sachets and sports drinks you are using to the appointment. Book a consultation.
And at the first long aid station of the race, fill your flask with a sodium-containing drink, eat a few spoonfuls of rice porridge, take a few more sips if your mouth is still dry, and keep going.
Worth keeping
- Drink when you are thirsty, not on a schedule set by the clock;
- Don’t rely on salt tablets to prevent hyponatremia or cramps;
- In long, hot and humid races, use a sodium-containing drink, read the label and convert it to mg per liter;
- Eat some salty food at the aid stations in long races;
- If you use salt tablets, don’t swallow tablets that have not dissolved;
- Weigh yourself before and after running as a reference; if you have lost weight, drink back gradually about 1.5 liters per kg, with sodium;
- Mix Oresol with exactly the amount of water written on the sachet, drink to thirst, and don’t force it.
References
Sources in Vietnamese
- Bộ Y tế (Cục Quản lý Môi trường y tế). 2023. Hướng dẫn chăm sóc sức khỏe mùa nắng nóng cho cộng đồng và người lao động (ban hành kèm văn bản số 356/MT ngày 19/5/2023). Cổng thông tin Bộ Y tế, đăng ngày 22/5/2023. https://moh.gov.vn/tin-lien-quan/-/asset_publisher/vjYyM7O9aWnX/content/bo-y-te-huong-dan-cham-soc-suc-khoe-mua-nang-nong-cho-cong-ong-va-nguoi-lao-ong (trang gốc nay không mở được; bản lưu ngày 7/12/2025: http://web.archive.org/web/20251207001617/https://moh.gov.vn/tin-lien-quan/-/asset_publisher/vjYyM7O9aWnX/content/bo-y-te-huong-dan-cham-soc-suc-khoe-mua-nang-nong-cho-cong-ong-va-nguoi-lao-ong).
- Nguyễn Hữu Đức. Chuyện kể Đông y. Quyển 10, T106.
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- Bennett BL, Hew-Butler T, Rosner MH, Myers T, Lipman GS. 2020. Wilderness Medical Society Clinical Practice Guidelines for the Management of Exercise-Associated Hyponatremia: 2019 Update. Wilderness Environ Med. 31(1):50-62. doi:10.1016/j.wem.2019.11.003. PMID: 32044213.
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Sources in Chinese
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