How much water should you drink on a hot day, and do cold drinks really cool you down?

Summer is here, and things are getting hot.

It's shaping up to be a scorcher, with dangerous heatwaves predicted, and we humans can only take so much heat.

Our ideal body temperature sits at around 37 degrees Celsius, but basically none of us will survive if our core temperature reaches 43C.

So as the mercury rises, you may find yourself asking: how much water should I drink to stay safe in the heat?

It's important to know that while being dehydrated can increase your risk of heat-related illness, it is only part of the story. You can still be seriously ill, with your vital organs under stress, even if you're not thirsty.

That said, most healthy adults are generally pretty good at regulating body temperature as we start to warm up. Our skin has lots of glands from which salty sweat oozes, and it's the evaporation of that sweat that helps keeps us cool.

But this also means we can lose water easily, and must replenish it regularly.

Most adults lose two to three litres of water a day. As we tend to sweat more in hot weather, your water intake may need to increase accordingly.

Given we're all different, how much each person needs to drink varies. The amount that's right for you will depend on your age, body size and any health conditions you have.

But when we're dehydrated, we feel thirsty. It's our body's signal that it needs fluids.

Mild to moderate dehydration symptoms include:

• feeling thirsty
• a dry mouth, lips and tongue
• feeling dizzy or light-headed, particularly when standing up
• headache
• dark urine (wee) and not as much wee as normal.

More severe dehydration symptoms may also include a fast heart or breathing rate, fever, drowsiness and confusion.

How to tell if you're drinking enough

University of Queensland community health and wellbeing researcher Lauren Ball says a strong indicator of dehydration is in the toilet bowl — the colour of our urine. The darker it is, the more dehydrated we are.

"Dark yellow urine [is] a very good sign that you are dehydrated."

She says pale yellow urine, closer to the colour of water, is an indication you're drinking enough.

Professor Ball adds you shouldn't try and guess how much water you need based on how much you're sweating, as it's sometimes hard to gauge.

For example, "when you're swimming, you're definitely sweating, you just can't tell because it's going into the water.

"Any kind of wind might make you feel like you're not sweating [too], because the water on your skin is being … blown away."

So if you are hitting the pool or the beach on a hot day, make sure you stay hydrated.

But is also important to note that you can still suffer from heat-related illnesses such as heat stroke even if you are hydrated, warns Ollie Jay, director of the Heat and Health Research Centre at the University of Sydney.

"Dehydration is an aggravating risk factor for sure, but not the root cause of all heat illnesses that it is sometimes suggested to be.

This means people who are susceptible to heat-related diseases, such as those with chronic disease, young children, the elderly or are pregnant should be particularly cautious during hot weather.

"Anyone with any heightened heat stress risk ... should avoid exertion during the hottest time of the day irrespective of their hydration practices," Professor Jay says.

Do cold drinks cool you from within?

When it's hot outside, you probably want to grab the coldest beverage you can find to cool yourself down.

But it's not so simple.

"The reality is that the temperature of ingested water makes little to no difference to body temperature," Professor Jay says.

"The volume of cooling that can be delivered through a cold drink is comparable to cooling a bathtub full of hot water by pouring a small glass of cold water into it."

Research from Professor Jay's team found drinking a cold beverage slightly suppresses sweating.

While this sounds great on hot days, less sweat means less evaporative cooling on your skin, so your core body temperature doesn't end up changing much at all.

Coffee, tea and alcohol

When it comes to staying hydrated, water is usually all that anyone really needs, Professor Jay says.

But many people also reach for other drinks to quench their thirst or celebrate over the summer season.

Some public health guidelines suggest avoiding coffee and tea in hot weather if they contain caffeine, which is a very mild diuretic.

Diuretic substances make you pee more, meaning they remove some of the fluid you need to stay hydrated.

But Professor Jay says there is "no real evidence" that tea and coffee are risk factors, with Professor Ball adding that these drinks have water in them as well.

"You could make an iced tea or iced coffee. There's all sorts of variations you could make healthy," Professor Ball says.

"If [coffee or tea] is going to help someone from a flavour perspective to get enough fluid in, then I think that's a good thing."

Alcohol, however, is a stronger diuretic, and shouldn't be your hydration go-to on a hot day.

If you are drinking in the heat, it's especially important to have a glass of water between drinks, for instance.

Tips to keep up your water intake

If you struggle to drink enough water in a day, creating habits for yourself like keeping a bottle of water in your car or by your desk can help, or you could even consider a phone app which sends you reminders to take a sip.

In addition to tea and coffee, Professor Ball there are plenty of ways to spice up your fluid intake.

She suggests sugar-free fruit infusions or cordials, fruit-flavoured tea bags, wedges of lime or lemon or even adding bubbles with a soda machine.

But Professor Jay and Professor Ball don't recommend sipping sugary drinks like juice or soft drink to replenish your liquids.

"Given the [high] volume of fluids that need to be replaced from sweating … it would be a generally unhealthy approach to [rehydrate] with sweet drinks considering the calories that would be consumed," Professor Jay says.

Can electrolyte sports drinks help?

Sports drinks are really only necessary if you lose a substantial amount of salt, Professor Jay says — for example, if you do more than one hour of continuous moderate to intense physical activity on a hot day.

So unless you're training like an athlete, most people will adequately replace lost salt through their diet.

In saying that, Professor Ball says GPs usually recommend drinking electrolyte-containing oral rehydration solutions or diluted apple juice if you or a child have mild gastro or food poisoning.

Gastro is more common in the summer months, so it can be a double whammy on hot days.

Should I worry about over-hydrating?

You might have come across the term hyponatraemia, which is a serious condition found in extreme cases of drinking too much water in a short period.

Hyponatraemia cases tend to be found in people with certain diseases or mental illnesses (such as in some cases of schizophrenia), endurance athletes or in infants who are fed infant formula that is too diluted.

While there are cases of people who have died of hyponatraemia, Professor Ball says the likelihood of it happening doing daily tasks is low.

"You would need to drink litres and litres and litres.

"The risk is much higher of us being chronically dehydrated, whether we realise it or not, from having insufficient fluid that I don't think we should really cap our intake, as long as we're going about just generally daily habits."

If you are going to take part in endurance fitness this summer — such as a marathon, long bike ride or an extended bushwalk — you should first find out the right level of water intake for you.