Nutrition
Hydration
Where the eight-glasses rule came from, what dehydration does to training performance, how to judge your own status, and when electrolytes matter.
Hydration advice is dominated by one number that nobody can source and a general sense that more is always better. Both are worth correcting, because the actual guidance is simpler and the failure modes run in both directions.
Where “eight glasses” came from
A 1945 US recommendation suggested roughly 2.5 litres of water a day for adults. The next sentence noted that most of that quantity is contained in prepared foods.
That sentence did not survive. What propagated was eight glasses of plain water, on top of everything else you consume, as a daily minimum with no reference to your size, your climate or your activity.
The current Institute of Medicine adequate intake is about 2.7 litres a day for women and 3.7 for men — again, from all sources, with food supplying roughly 20 percent. Coffee and tea count toward it, despite the persistent belief that caffeine dehydrates you; at normal intakes the fluid in the drink more than offsets the mild diuretic effect.
What dehydration does to training
The threshold that matters is around 2 percent of body mass lost as fluid. For an 80 kg person that is 1.6 kg.
Past that, endurance performance declines measurably, perceived effort rises, and thermoregulation gets harder. Effects on maximal strength are smaller and appear at larger deficits — which is why a runner notices dehydration long before a lifter does.
Most people doing an hour in an air-conditioned gym never approach that threshold. Someone doing a long ride in summer heat can pass it comfortably.
Judging your own status
Three practical checks, in order of usefulness.
Urine colour. Pale straw is well hydrated. Dark yellow means drink more. It is crude, free, and adequate for daily purposes — with the caveat that B vitamins turn urine bright yellow regardless.
Thirst. For healthy adults going about a normal day, thirst is a reliable signal. It lags during hard exercise, and it blunts with age, which is why older adults and athletes should not rely on it alone.
Weight change around a session. Weigh yourself before and after a long workout. The difference is almost entirely fluid, and it tells you your personal sweat rate rather than an average one.
When electrolytes actually matter
Sodium and other electrolytes are lost in sweat, and replacing them matters when losses are large.
That means beyond roughly 60 to 90 minutes of continuous hard exercise, in significant heat, or if you are a heavy sweater — the person whose kit is visibly salt-crusted afterwards. Endurance athletes and people training outdoors in summer have a genuine case.
For an hour of lifting indoors, water is sufficient and an electrolyte drink is mostly a flavoured way to add calories.
More is not better
This is the part that gets left out.
Drinking large volumes of plain water faster than your kidneys can excrete it dilutes blood sodium. Exercise-associated hyponatraemia is rare but genuinely dangerous, and it has killed marathon runners who followed advice to drink at every station regardless of thirst.
The protection is straightforward: drink to thirst rather than to a schedule, and when sessions run long and sweaty, include some sodium rather than only water.
One note for lifters
If you supplement creatine, you will hold more water inside your muscle cells. That is the mechanism working, not a hydration problem, and it does not mean you need to drink extra beyond normal thirst.
Common questions
How much water should I drink a day?
Roughly 2.7 litres a day for women and 3.7 for men from all sources combined, which is the US Institute of Medicine's adequate intake. That total includes food, which supplies around 20 percent. Training, heat and body size all raise it, which is why thirst and urine colour beat any fixed number.
Where did eight glasses a day come from?
A 1945 US recommendation of about 2.5 litres a day that also stated most of it comes from food — a sentence that got dropped in the retelling. The figure is not wrong as a rough total; the version that survived, eight glasses of plain water on top of everything else, was never the advice.
Does dehydration affect workout performance?
Yes, measurably. Losing about 2 percent of body mass in fluid is where endurance performance starts to decline and perceived effort rises. Effects on maximal strength are smaller and appear at greater losses, so a lifter feels it less than a runner does.
How do I know if I am dehydrated?
Urine colour is the practical check: pale straw is well hydrated, dark yellow means drink more. Thirst is a reasonable signal for most healthy adults, though it lags during hard exercise and blunts with age. Weighing yourself before and after a long session tells you what you actually lost.
Do I need electrolyte drinks?
Not for a normal gym session. They earn their place beyond about 60 to 90 minutes of continuous hard exercise, in significant heat, or if you are a heavy or salty sweater. For an hour of lifting in an air-conditioned gym, water is enough.
Can you drink too much water?
Yes, and it is dangerous. Drinking large volumes of plain water faster than the kidneys can excrete it dilutes blood sodium — hyponatraemia — which has killed endurance athletes. Drinking to thirst rather than to a schedule is the protection.
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