Hydration and electrolytes: what the reference values actually say

The most quoted hydration rule, eight glasses of water a day, does not come from the reference values. The US numbers come from the National Academies' 2004 report on water and electrolytes, and they are both larger and more forgiving than most people assume.

How much water

These are Adequate Intakes, set from the median intake of healthy Americans rather than from a proven requirement. The report's own conclusion is that most healthy people meet their needs by letting thirst guide them. It also found no convincing evidence that caffeine causes a body water deficit: the diuretic effect is short-lived, so coffee and tea count towards the total.

The 2025 to 2030 Dietary Guidelines put it simply: hydration is a key factor in health; choose water, still or sparkling, and unsweetened drinks. Sugar-sweetened drinks, energy drinks and fruit drinks are on the avoid list.

Signs you are behind

Dark yellow urine, needing to pass urine much less often than usual, a dry mouth, headache, dizziness on standing and unusual tiredness. Pale straw-coloured urine through the day is the simplest check. Older adults feel thirst less, so they may need to drink by habit rather than by thirst.

Electrolytes

Sodium, potassium, chloride and magnesium carry the electrical signals that run nerves and muscle and keep water where it belongs. The numbers worth knowing:

• Sodium: the US guidelines set less than 2,300 mg a day for everyone 14 and over, about a teaspoon of salt. They add that highly active people may benefit from more sodium to offset sweat losses.
• Potassium: the Daily Value is 4,700 mg. Fruit, vegetables, beans, potatoes and dairy are the main sources.
• Magnesium: the Daily Value is 420 mg, from nuts, seeds, legumes and whole grains.

Sodium on a label

The Nutrition Facts panel shows sodium as a percentage of the 2,300 mg Daily Value. The FDA's rule of thumb is that 5 percent of the Daily Value or less per serving is low and 20 percent or more is high. Bread, cured meats, cheese, soups, sauces and ready meals are common sources, which is one reason the new guidelines tie sodium to cutting highly processed food.

Exercise and heat

The 2004 reference values cover healthy, mostly sedentary people in temperate climates. The report notes that people who are very active, or who live and work in hot conditions, need more, often six litres or more a day in those studies. A practical approach for long sessions is to drink to thirst, weigh yourself before and after on hot days to see how much you lose, and add salt or an electrolyte drink when sweating heavily for more than about an hour. The US guidelines make the same point about sodium for highly active people.

When an electrolyte drink helps

For most people on most days, water and food cover electrolytes. Electrolyte drinks earn their place in three situations: long or hot exercise with heavy sweating, illness with vomiting or diarrhoea, and very low food intake. For vomiting and diarrhoea, an oral rehydration solution from a pharmacy is designed for the job; sports drinks are designed for exercise and carry more sugar and less salt. If you have heart failure, kidney disease or high blood pressure, ask before adding salt or potassium.

Hydration on a GLP-1

Many people on a GLP-1 receptor agonist drink less along with eating less, without noticing. The bigger risk is fluid loss. The 2025 joint nutrition advisory points out that dehydration from severe nausea, vomiting or diarrhoea can cause acute kidney injury, with or without existing kidney disease, and that post-marketing reports of kidney problems cluster around starting and stepping up the dose. Its advice: prevent dehydration, and monitor kidney function in people at risk.

• Sip through the day rather than drinking a lot with meals, which can worsen fullness and nausea.
• On the days after a dose, when appetite and thirst are lowest, set a target and a reminder.
• If you are vomiting and cannot keep fluids down, that is a same-day call to your prescriber, not a wait-and-see. The side effects page has the full list.

Constipation, the other common GLP-1 complaint, also gets worse with low fluid, especially as fibre goes up. See the fibre guide.

Medicines that change fluid needs

Some medicines increase urine output, including diuretics (water tablets) and the SGLT2 inhibitors used for diabetes, heart failure and kidney disease. Others, for heart failure or kidney disease, come with a fluid limit. If you take any of these, ask your prescriber how much to drink rather than following a general target.

Alcohol and heat

Alcohol increases urine output, and hot weather or saunas increase sweat. Both raise needs. The alcohol guide covers the rest.

Tracking it

Water rarely needs tracking for long, but on a GLP-1 a week or two of it can explain headaches, dizziness or constipation. In Peptide AI, fluids go in nutrition tracking alongside meals, next to the dose log, so a low-fluid day after the shot is easy to spot. If your clinician checks kidney function, the creatinine and eGFR results can sit on the same timeline in bloodwork tracking.