Do you need electrolytes to stay hydrated — or is water enough?

As summer temperatures climb, hydration becomes a shared concern — from protecting vulnerable people such as the elderly and infants to fuelling bodies working hard in the heat.

But for those sweating heavily, questions about electrolyte balance often come into play too.

Experts say hydration isn't a one-size-fits-all approach. The common advice to drink eight cups of water a day is often treated as a standard, but it's only a guideline, they say.

Associate professor Fiona Lithander, a nutrition scientist and dietitian at the Liggins Institute at the University of Auckland, says the eight-cups a day advice is "a good amount to aim for", but fluid needs vary depending on age, gender, activity levels, weather and lifestyle.

Rush says the advice itself isn't backed by the literature. So, understanding your hydration needs comes down to basic physiology.

"What goes in equals what comes out," Rush explains. Fluid losses through sweat, breathing, urine and faeces determine how much you need to replace.

But there's no harm in having water on hand when outdoors in hot weather, she adds.

Hydration is only one part of coping with heat. Keeping the body cool is just as important

"In a heatwave, one way of keeping cool is to dampen your clothes, to have a breeze blowing through, to be in the shade, not to overexert yourself, certainly not to go to sleep in the sun on the beach," Rush says.

"If your head is cool, it helps cool the rest of the body. I sometimes wet my sun hat with water and put it on my head or wash my hair, dampen my hair."

Dehydration happens when the body's water content drops too low, Lithander says. Symptoms can include headaches, tiredness, thirst and dry or cracked lips.

Urine colour and how many trips you take to the toilet are other useful indicators, according to Rush.

"Healthy, well-hydrated urine - unless you're eating some things that can make your urine change colour, like beetroot - is usually a sort of pale straw colour."

On the other hand, if you're passing too dark- or light-coloured urine, you're not going enough and you're thirsty - those could be signs you need to drink more, she says.

Rush says her research shows children tend to self-regulate fluid intake better than adults, who often end up over-consuming fluids, including water, tea, coffee, milk and foods with high water percentages.

"And I think we tend to over-consume these days, just walking around with a water bottle and drinking and sipping all the time can actually do harm if you wash your body out, as it were, because when you pass urine, you also lose electrolytes and things that the body also uses. So it's a balance."

Although Lithander says it's possible to experience overhydration, which can lead to hyponatraemia (low sodium blood levels), it is rare among the general population.

"You would need to be drinking many, many litres in a short period of time for hyponatraemia to occur.

"It can occur, for instance, in endurance athletes who are competing or training in a sport in particular conditions, but it's pretty rare in the general population."

Electrolytes are minerals, such as sodium, potassium, magnesium and calcium, which help regulate hydration, muscle contraction and nerve signals.

"That's how cells communicate and how they maintain their integrity, they're charged... and they all perform functions in the body that are really important for homeostasis, the state of being at the right hydration."

For most people, taking extra electrolytes through things like drinks or powders isn't necessary, Lithander and Rush say. Most people in New Zealand would get enough of these electrolytes from the food they already eat, Lithander says. But they can be recommended by health professionals for people who have lost a lot of blood, or are vomiting or have diarrhoea.

"Unless you're a super athlete and really running marathons and working out very hard, you don't need to worry so much about it, as long as you have a good diet," Rush says.

Lithander says one concern about too many electrolytes is that they may negatively affect health. For instance, sodium is an electrolyte and whilst we get enough sodium from our food here in New Zealand, too much can contribute towards raised blood pressure.

"We generally have plenty sodium through the food that we eat day in, day out in New Zealand. So we don't need to take electrolytes in addition to water to hydrate. Water alone in the general population is absolutely fine."

Foods and drinks high in sugar or salt can make you thirstier. Rush draws the analogy of water drawn out of raw potatoes when sprinkled with salt.

"Because what you put in your mouth shouldn't be more concentrated than what's in your body… It's called osmosis," Rush explains.

She recommends a varied diet with fewer ultra-processed foods.

"Ultra-processed foods like potato crisps and sugary drinks don't help so much. Although, I guess if you're getting to feel tired, a bit of sugar and water would help, but do you really have to buy it at great expense in packaging?"

For those who struggle with plain water and are resorting to fizzy, sugary or energy drinks, Lithander suggests simple alternatives.

“Some people say, ‘oh, gosh, I don't really like water’. So you could put a slice of lemon, lime or orange in your water and eat some extra fresh fruit which is full of water."