Electrolytes: When Water Alone Is Not Enough
Your sweat carries water and electrolytes, sodium among them, and how much of each depends on you, the weather and what you are doing. Sweat rates vary enormously between people. For an ordinary day or a short session, plain water almost always covers it. Illness changes the picture entirely, and generic formulas have nothing useful to say about that.
When a sodium plan is worth the trouble
Long sessions, heavy sweating, heat and back-to-back training days are what make fluid and sodium planning worth doing. What you normally eat counts too, and so does whether you can reach a drink mid-session, because a plan you cannot execute at mile nine is a plan on paper. The NATA position statement on fluid replacement covers replacement in physically active people.
Salt crust on your kit tells you your sweat is salty. It tells you nothing about how salty, and a clean shirt is just as quiet on the question. Pick your sodium from your conditions and from a professional, never from your laundry.
For a demanding event, a sports dietitian or another qualified professional can build you a plan. The sweat-rate guide explains a rough way to work out your fluid loss, and where that method breaks down. The daily water calculator handles fluid only, so your sodium is yours to work out.
Sodium cannot make overdrinking safe
Exercise-associated hyponatremia is low blood sodium during or after exercise, and drinking too much is a common route into it. Sports drinks can be part of that route, because sodium in a bottle does not reliably protect somebody who keeps drinking. See the international consensus statement and the 2017 research update.
Let thirst set your pace, and treat finishing a session heavier than you started as a warning sign. Thirst serves many healthy adults well. Prolonged work or sport is where you want a plan built around your own conditions, your medical needs and the heat you will be standing in. Coffee and tea count toward your fluid for the day, which the caffeine and hydration evidence sets out.
When to call for help
Confusion, collapse, seizures or severe illness during exercise needs urgent medical assessment. Heat illness and fluid imbalance share symptoms, so puffy hands, thirst and urine colour will never tell you which one you are looking at. Do not pour water or salt into someone who is confused. Call your local emergency number.
The CDC guide to heat-related illness describes warning signs and first aid. Severe dehydration and heat illness are both capable of killing someone. Treat each of them as the emergency it is.
Illness and prescribed restrictions
Vomiting or diarrhoea may call for proper oral rehydration treatment, which a homemade sports-drink recipe does not replace. Kidney or heart disease, blood-pressure treatment and any sodium or fluid restriction all need medical guidance. Ask your doctor first. The daily water guide explains why population intake figures are averages and not prescriptions.
Scope and correction note, 13 September 2026: This guide provides general adult education. It has been revised to distinguish estimates from measurements and to remove unsupported guarantees. It has not received independent clinical review. See the editorial policy and medical disclaimer.