“Drink… Drink… Hydration is important”. You have been told to drink more water. You are drinking more water. You are drinking what feels like a personally irresponsible amount of water. And approximately twenty minutes later you are sitting on the bathroom floor wondering why you still feel like someone replaced your blood with warm air.
This is not a personal failing. This is not an electrolyte brand problem. This is your circulatory system failing to do a specific thing with the fluid you are giving it, for reasons that are biological, measurable, and frankly a little insulting given how hard you are trying.
The hydration problem in POTS and Long COVID is not simply about drinking enough. It is about what your body does, or fails to do, with what you drink. Understanding that distinction changes everything about how you approach it.
The blood volume problem
Up to 70 percent of people with POTS have measurably less circulating blood volume than a healthy person of the same size and sex should have. The average deficit is roughly 13 percent, or about 700 millilitres, nearly a pint and a half missing from the circulatory system. This is not a feeling. It is a measurement. There are direct blood volume tests that show it. And that shortfall is what drives the racing heart on standing, the dizziness, the brain fog and the profound unsteadiness of POTS, as documented in this comprehensive overview of hypovolemic POTS pathophysiology.
When blood volume is low and you stand up, there is not enough volume to maintain adequate pressure and flow to the brain. The heart compensates by racing, the brain receives less blood than it needs, and you experience dizziness, cognitive dimming, nausea and the specific quality of feeling like you might simply cease to function upright.
A 2025 computational haemodynamics study confirmed that hypovolemia reduces cerebral blood flow by approximately 100 millilitres per minute due to a 30 percent decrease in blood volume, meaning the brain is receiving significantly less oxygen supply than it should, as detailed in this pathophysiological mechanisms study of POTS haemodynamics.
Hydration is really a circulation problem. The question is not how much water you drink, but whether that water stays inside your bloodstream long enough to increase blood volume. So yes, hydration is central. What is less obvious is why the standard approach does not adequately fix it.
Why your kidneys are not solving this on their own
In a body with normal regulatory systems, low blood volume triggers a predictable response. The kidneys detect the shortfall. The renin angiotensin aldosterone system, or RAAS, activates. Aldosterone rises, telling the kidneys to retain sodium and therefore water, expanding blood volume back toward normal. The system self corrects.
In POTS, this response is paradoxically broken. Research has found inappropriately low aldosterone and renin levels in POTS patients despite hypovolemia, meaning the kidneys are not receiving the signals that should prompt sodium retention. The body is low on blood volume and yet the hormonal cascade that should correct this is not firing properly, as described in this ScienceDirect pathophysiology review of POTS. A 2025 study added a further complication: elevated angiotensin II type 1 receptor autoantibodies, which could directly disrupt the volume retaining arm of the RAAS, suggesting an autoimmune mechanism in some patients.
The practical consequence is that even though your blood volume is low, your kidneys are not retaining sodium and water the way they should. They are quietly excreting what you drink rather than holding onto it. This is why plain water passes through so quickly. There is no adequate hormonal signal telling the kidneys to keep it.
Why plain water makes this worse rather than better
When you drink plain water without sodium, your blood is momentarily diluted. The kidneys register this dilution and excrete the excess relatively quickly to restore balance. Some absorption happens, but the retention is poor because the hormonal system that should signal the kidneys to hold fluid is not working correctly.
Water without sodium in POTS is, at least in part, hydration that goes straight back out.
A plasma volume deficit of nearly 13 percent has been documented in POTS patients via radiolabeled serum albumin investigation, with intravascular hypovolemia exaggerated in the upright posture by orthostatic blood pooling in the abdomen, pelvis and legs, as shown in this ScienceDirect review of oral and IV hydration in orthostatic intolerance. Sodium matters because it changes what the kidneys do with the water alongside it. Sodium is the signal that tells the body to retain fluid in the bloodstream rather than excrete it. Without sufficient sodium, the fluid you drink does not effectively expand blood volume. It just passes through.
The Heart Rhythm Society expert consensus statement on POTS recommends 2 to 3 litres of fluid daily alongside 10 to 12 grams of sodium, making salt loading a first-line non pharmacological therapy, as referenced in this POTS hydration and fluid management guide. That is substantially more sodium than a typical Western diet provides, and it is a clinical recommendation, not a wellness suggestion.
The MCAS layer: when hydration itself becomes complicated
If you have MCAS alongside POTS and Long COVID, everything described above is true, and there is an additional problem sitting on top of it.
Mast cells are immune cells distributed throughout the body, heavily concentrated in the gut lining and around blood vessels. In MCAS, they are prone to releasing histamine and other mediators in response to triggers that should not be significant: specific foods, fragrances, temperature changes, stress, or products that contain certain additives.
This matters for hydration in several ways.
Dehydration itself can be a mast cell trigger. The physiological stress of low blood volume and the autonomic instability that comes with it can provoke mast cell activation, which then causes additional vascular symptoms, flushing, heart rate changes and gut disturbance, that compound the POTS picture. Staying adequately hydrated is therefore not just about blood volume. It is also about reducing the mast cell activation that dehydration can provoke.
The other problem is that many electrolyte products, flavoured drinks and oral rehydration solutions contain ingredients that trigger mast cells in some people: citric acid, artificial colours, preservatives, natural flavours, fruit extracts, and certain sweeteners. Finding hydration products you can tolerate when MCAS is present is genuinely difficult and often requires trial and error, because what triggers one person’s mast cells is tolerated without problem by another.
The practical working approach for MCAS and hydration is to start with the simplest possible ingredients. Water with sodium chloride (plain salt or sodium chloride tablets) is usually well tolerated because there are very few additional ingredients to react to. Potassium through food sources like fresh cooked vegetables, if tolerated, is gentler than potassium supplements which can be more reactive for some people. Adding ingredients one at a time rather than starting with a complex commercial formula allows you to identify what works before scaling up.
Why Electrolytes Work Better Than Water
One of the most common questions people ask is, “Why do electrolytes help when plain water doesn’t?”
The answer is that hydration is not just about replacing fluid. It is about keeping that fluid inside the bloodstream long enough to increase blood volume.
Water is absorbed quickly from the gut, but without enough sodium much of it is rapidly filtered by the kidneys and excreted in urine. For people with POTS, this problem is often made worse because the hormonal system that normally tells the kidneys to retain sodium and water—the renin-angiotensin-aldosterone system (RAAS)—does not appear to respond appropriately.
Electrolytes, particularly sodium, help solve this problem. Sodium increases the osmotic pressure within the bloodstream, encouraging water to remain inside the circulation rather than moving rapidly into the urine. The result is a greater expansion of plasma volume, improved blood flow back to the heart and better delivery of oxygen to the brain and muscles.
This is why many people with POTS feel noticeably better after drinking an oral rehydration solution or a high-sodium electrolyte drink than after drinking the same amount of plain water. It is not because electrolytes are absorbed faster. It is because the body is able to retain the fluid more effectively.
For most people with POTS, the goal is not simply to drink more. The goal is to increase and maintain circulating blood volume throughout the day.
The morning problem and timing
The hardest time of day for most people with POTS is the first thirty to sixty minutes after waking. You have been horizontal overnight, blood has redistributed, and standing up is a cardiovascular demand you are meeting with whatever blood volume you have after seven or eight hours without fluid. That is often the worst possible starting conditions.
The most evidence supported and practically useful intervention is a fluid bolus before standing. Drinking 500 millilitres of water with sodium before getting out of bed, or sitting upright in bed to drink before attempting to stand, pre loads blood volume before you impose the orthostatic challenge of being upright. Some people drink their morning fluid while still lying down. The mechanics are slightly awkward. The functional difference in how they feel standing up twenty minutes later is often significant enough to make it worth the awkwardness.
The other timing principle is to front load hydration toward the morning and early afternoon rather than trying to drink two litres across the whole day, because afternoon and evening fluid has less time to expand blood volume before your cardiovascular system is working hardest during peak activity. Drinking most of your daily fluid in the morning and tapering toward the evening also reduces the disruption to sleep from overnight bathroom visits, which matters when Long COVID sleep is already fragile.
Large amounts of fluid consumed quickly without sodium can also cause a temporary drop in blood sodium concentration, which in POTS can worsen dizziness and brain fog. Smaller, more frequent amounts with consistent sodium intake avoids this.
The goal is not simply hydration. It is sustained blood volume expansion across the waking hours, with sodium doing the work of keeping the fluid where the circulatory system needs it rather than flushing it through.
Frequently asked questions
Why does drinking water not seem to help my POTS symptoms?
Because up to 70 percent of POTS patients have a genuine blood volume deficit that the body is not correctly compensating for through its normal hormonal systems. Plain water without sodium is not reliably retained in the bloodstream. The kidneys excrete it too quickly because the renin angiotensin aldosterone system that should signal fluid retention is not working properly in POTS.
How much fluid and sodium do I actually need with POTS?
The Heart Rhythm Society expert consensus recommends 2 to 3 litres of fluid daily alongside 10 to 12 grams of sodium for POTS patients with suspected hypovolemia. This is significantly more sodium than a standard Western diet provides and is a clinical recommendation from specialist consensus, not a wellness suggestion.
Why does adding salt to water help more than drinking water alone?
Sodium is the signal that tells the kidneys to retain fluid in the bloodstream rather than excrete it. Without adequate sodium alongside fluid, water passes through the body too quickly to effectively expand blood volume. The combination of fluid and sodium is what produces meaningful plasma volume expansion.
Why is my blood volume low in the first place?
In hypovolemic POTS, the hormonal system that normally corrects low blood volume, specifically aldosterone and renin, is paradoxically underactive despite the deficit. The kidneys are not retaining sodium and water as they should. Some research has also found autoantibodies that directly disrupt the volume regulating hormonal cascade, suggesting an autoimmune contribution
Why is morning the worst time for POTS symptoms?
Because you have been horizontal for hours without fluid, blood has redistributed overnight, and standing up is a cardiovascular demand you are meeting with diminished blood volume. A fluid bolus with sodium before standing is one of the most practically effective single interventions for the morning period specifically.
How does MCAS complicate hydration?
Dehydration itself can trigger mast cell activation, compounding POTS symptoms. Many commercial electrolyte products contain additives including citric acid, artificial flavours and sweeteners that can trigger MCAS reactions. Starting with the simplest ingredients, water and plain sodium chloride, and adding things one at a time allows you to find what works without provoking reactions.
Can I drink too much water with POTS?
Yes. Large amounts of plain water without sodium can dilute blood sodium concentration and worsen dizziness and brain fog in POTS. Consistently including sodium with fluid intake and avoiding very large amounts of plain water in a short window reduces this risk.
What is a fluid bolus and does it actually work?
A fluid bolus means drinking 400 to 500 millilitres of fluid with sodium over a short period, typically before standing in the morning or before activity. Studies in orthostatic intolerance have found it produces a measurable increase in blood pressure and reduction in heart rate on standing. It is one of the most evidence supported non pharmacological interventions for POTS.
If I have MCAS, what is the safest way to start adding sodium to my hydration?
Start with plain salt or sodium chloride tablets in still water, which have the fewest potential triggers. Add other minerals or ingredients one at a time with a gap of several days between each, so you can identify reactions. Keep a log of what you tried, the dose, and your response. Commercial products with multiple ingredients are useful if tolerated but make identification of triggers harder.
Should I see a specialist about my blood volume?
If POTS is significantly affecting your daily life and standard fluid and salt measures are not adequately managing symptoms, it is worth discussing blood volume assessment and possibly IV fluid support with a cardiologist or autonomic specialist. Direct blood volume testing exists, though it is not widely available, and 24 hour urine sodium testing can indicate whether adequate sodium retention is occurring.
This article is for general information and education. It does not replace personalised medical advice. If you are considering significant changes to your fluid or sodium intake, particularly if you have kidney disease, heart disease or hypertension, please discuss this with your GP or a clinician familiar with POTS and Long COVID first.
Sources and further reading
Blood volume and RAAS in POTS Hypovolemic POTS mechanisms, blood volume deficit, and clinical assessment: https://www.cognitivefxusa.com/blog/hypovolemic-pots-symptoms-causes-and-treatment-for-low-blood-volume-potsRAAS dysfunction and inappropriately low aldosterone in POTS, ScienceDirect 2024: https://www.sciencedirect.com/science/article/abs/pii/S0146280624006121 Pathophysiological mechanisms of POTS including cerebral blood flow and haemodynamics, PMC 2025: https://pmc.ncbi.nlm.nih.gov/articles/PMC12220990/
Fluid and sodium recommendations Oral and intravenous hydration in orthostatic intolerance and POTS, ScienceDirect: https://www.sciencedirect.com/science/article/pii/S1566070222000108 Heart Rhythm Society consensus on fluid and salt loading in POTS, summary: https://pwaterapp.com/hydration-pots-syndrome/ ENDOFFILE
About This Hydration Guide for Long COVID
This guide combines current research on POTS, Long COVID, hypovolemia, autonomic dysfunction and hydration with practical strategies commonly recommended by autonomic specialists. It explains why fluid, sodium and blood volume are closely linked, while recognising that hydration needs vary from person to person, particularly when conditions such as MCAS, kidney disease or heart disease are present. As research continues to evolve, this article will be reviewed and updated to reflect new evidence. It is intended for educational purposes and should not replace personalised medical advice from your GP or healthcare professional.
Disclaimer: This article is based on personal experience and available research at the time of writing. It is not a substitute for medical advice. Always speak to a healthcare professional before making changes to your treatment plan or trying new supplements or medications.