Three in the afternoon. The bottle of water you bought in the morning is still almost full next to the keyboard, while the second cup of coffee is already empty. Your shoulders ache, your head feels a little heavy, and you have to read a work message twice to understand it. Standing up after a long meeting, both knees feel stiff and creaky for the first few steps. You haven’t felt thirsty all day, so nobody thinks about water.
Thirst often comes last
Water dissolves substances so the body’s chemical reactions can take place, carries nutrients to cells, takes waste away, keeps body temperature steady and lubricates the joints. The body keeps its water balance under very tight control. A shortfall of around 1% is usually made up within a day, thanks to very small changes in how concentrated the blood is, often before we clearly feel thirsty, as described by Jéquier and Constant (2010). Young children and older people are the most likely to run short of water.
Even mild dehydration leaves its mark. Armstrong and colleagues (2012) had 25 young women do moderate exercise until they lost about 1.4% of their body weight as water. They felt sluggish, found it hard to concentrate, had headaches, and found even familiar tasks more of an effort, including while sitting at rest, although most of the cognitive tests did not change. The sample was small and made up entirely of healthy young people, so it serves only as a reference point for the afternoon described above. As for tired muscles and joints that feel stiff when you first stand up, which people who have drunk too little water for a long time often mention, these have not been measured directly the way the two studies above measured their outcomes; the doctor treats them as a lead for further checking.
How much to drink in a day
For adults who are not very active, Jéquier and Constant give a reference level of about 1.5 liters of drinking water a day, more in hot weather, when working in the sun or when exercising. A bowl of soup, a plate of boiled vegetables and a little fruit with a meal also bring in a fair amount of water. Spreading your drinking through the day is easier to keep up than drinking most of it in the evening: a glass when you get up, a glass with each meal, a few sips each time you get up from your desk.
Sweat carries sodium away, fruit and vegetables bring potassium back
Sweat tastes salty because it contains sodium, along with a little potassium. People who work outdoors in the sun, or who run or cycle long distances while drinking only plain water, can get enough water yet run short of salt, then get cramps or feel weak and shaky after a workout. People who eat very little starch for a long time also often lose extra water and salt through their urine, and some lose potassium as well.
Potassium comes mainly from vegetables, fruit, beans and lentils, and potatoes and other tubers. Aburto and colleagues (2013) pooled 22 randomized trials for the World Health Organization and found that eating more potassium helped lower blood pressure in people with high blood pressure, while in people with normal blood pressure the effect was unclear. People who ate more potassium also had a stroke risk about a quarter lower.
People with kidney disease or heart disease, and people taking diuretics or blood pressure medicines, should not add potassium on their own, including potassium tablets, salt substitutes containing potassium, or rehydration packets taken over a long period. Some medicines hold potassium in the blood, some make you lose it, and weak kidneys let potassium build up easily. Blood potassium that is either too high or too low can make the heart beat irregularly. For this group, whether to replace potassium is decided by the treating doctor after looking at blood tests.
Ready-made oral rehydration salt packets are meant for diarrhea, vomiting or very heavy sweating. They are quite salty, are not a substitute for everyday drinking water, and do not suit people with high blood pressure or kidney disease.
Calf cramps in the middle of the night
Every time a muscle contracts and relaxes, it needs many ions. Sodium starts the signal, calcium produces the force of contraction, magnesium helps the muscle let go, and potassium brings it back to rest. Cramps, or a calf that is slow to soften when you stand up after sitting for a long time, usually come from several of these being out of balance together, sometimes with too little water on top.
There are two explanations for cramps during exercise: on one side, water loss and a shortage of electrolytes; on the other, a disturbance in the nerve–muscle reflex loop. Putting the two hypotheses side by side, Giuriato and colleagues (2018) found that recent studies lean toward the second. That is why the doctor looks at both the ions and the muscle reflexes. Magnesium and calcium each have their own article.
With diarrhea, vomiting or very little urine, don’t wait it out drinking at home
Confusion, muddled speech, weak arms and legs, dizziness when standing up and a fast heartbeat are the signs of severe dehydration that Jéquier and Constant list.
Beyond these, see a doctor right away for:
- Heavy diarrhea or vomiting, especially in young children and older people;
- Passing very little urine all day;
- Drowsiness, or being slow to answer when spoken to;
- Cramps together with severe muscle weakness;
- Palpitations or an irregular heartbeat.
A sip of water and muscle testing
Dr. Dinh Ngoc Duc (MD, MSc in Traditional Medicine) reviews the medicines you are taking and your most recent electrolyte test, if you have one, before discussing whether anything needs replacing. What you have drunk today, how many cups of coffee, whether you have sweated a lot or a little, and what kind of diet you are on are all asked at the same time.
Next, the doctor finds a muscle that is weak on muscle testing. You take a sip of water and hold it in your mouth, chew gently a few times, then swallow. If that muscle becomes clearly stronger right afterwards, that is a concrete sign the body needs water. Sometimes the doctor gently rolls a lock of your hair between the fingers and watches how the muscle responds, a familiar check in Applied Kinesiology when dehydration is suspected. These two methods only mean something as part of the doctor’s muscle testing; please don’t use them at home to reach your own conclusions.
For potassium, the doctor may place a very small amount on the tip of your tongue and watch how the indicator muscle changes; the method is described in our article on the health triangle. In people taking diuretics or with kidney disease, blood tests remain the deciding basis.
People who often break out in a sweat and feel tired, short of breath and dry in the mouth are usually prescribed an herbal decoction that boosts qi and generates body fluids, because in traditional medicine heavy sweating lets qi escape along with it (“qi escapes with the sweat”, 气随汗脱 / 氣隨汗脫, qì suí hàn tuō). In people who already get enough water and ions but still feel tired for a long time, the clinic sometimes also looks at the body’s energy, meaning the chakras, the aura field (Aurafield) and the meridians, but only when muscle testing points in that direction.
Worth keeping
- Keep a water bottle where you can see it and sip throughout the day; don’t wait until you feel thirsty;
- If you sweat a lot in the sun or during long workouts, replace both water and salt, not just plain water;
- Have vegetables, fruit or beans at every meal to get potassium from food;
- If you have kidney disease or heart disease, or take diuretics or blood pressure medicines, ask your doctor before adding potassium, salt or electrolyte packets;
- With heavy diarrhea or vomiting, very little urine, or drowsiness, see a doctor right away.
If you have heart failure or kidney disease, or your doctor has told you to limit fluids, ask how much water is right for you before following the amounts in this article.
References
- Jéquier E, Constant F (2010). Water as an essential nutrient: the physiological basis of hydration. Eur J Clin Nutr 64(2):115-23. https://doi.org/10.1038/ejcn.2009.111
- Armstrong LE, Ganio MS, Casa DJ, Lee EC, McDermott BP, Klau JF et al. (2012). Mild dehydration affects mood in healthy young women. J Nutr 142(2):382-8. https://doi.org/10.3945/jn.111.142000
- Aburto NJ, Hanson S, Gutierrez H, Hooper L, Elliott P, Cappuccio FP (2013). Effect of increased potassium intake on cardiovascular risk factors and disease: systematic review and meta-analyses. BMJ 346:f1378. https://doi.org/10.1136/bmj.f1378
- Giuriato G, Pedrinolla A, Schena F, Venturelli M (2018). Muscle cramps: A comparison of the two-leading hypothesis. J Electromyogr Kinesiol 41:89-95. https://doi.org/10.1016/j.jelekin.2018.05.006
