Experts are warning that staying hydrated should remain a daily priority, particularly during heatwaves when dehydration becomes far more common. Dehydration can range from mild to severe, explains Dr Naveed Asif, GP at The London General Practice.
"Most of us are probably mildly dehydrated the majority of the time and experience things like a dry mouth, dry lips and dry eyes," Dr Asif warns. "Then once it goes into moderate that means it's beginning to impact you more, and then we have the more severe side where you can become significantly unwell with it." Dr Asif adds that even mild dehydration can quickly exacerbate pre-existing medical conditions, causing chronic symptoms to spiral out of control.
While thirst is the most obvious indicator, health experts have highlighted six subtle and surprising symptoms that signal your body is desperately running low on fluids.
1. Mood Changes
"When people get quite dehydrated they might get a bit restless or irritable," says Dr Hugh Coyne, from Coyne Medical, who explains that the brain is highly sensitive to hydration levels. "People might notice that they feel a little bit more tense and agitated when they wake up on a hot morning and really want to have a drink of water straight away."
2. Muscle Cramps
The body relies on a delicate balance of water and electrolytes - the vital salts that keep our muscles and nerves functioning. "If they're not diluted by water and are too concentrated because you've not drunk enough, that can cause flare-ups of cramping and tingling," says Dr Asif. "It means that the nerves that control the muscles aren't working as efficiently and often the muscles begin to seize up because they just can't function."
3. Headaches
Experiencing a throbbing headache is a frequent side effect of low fluid intake. Dr Coyne explains that when total body water drops, the brain tissue undergoes a temporary shrinkage, becoming less 'plump' than usual. This places physical tension and traction on the meninges - the protective lining of the brain - while simultaneously decreasing blood flow and increasing pain sensitivity.
4. Skin Laxity
Skin behaviour can also offer clues about hydration levels. Doctors frequently check a patient's fluid levels using a metric called skin turgor. To test this, experts take a gentle pinch of the skin on the back of the hand and release it. In a well-hydrated person, the skin retains its elasticity and snaps back instantly. If you are dehydrated, the skin will take a noticeable moment to settle back down. However, Dr Coyne notes this test is less reliable for older adults, who naturally lose standard skin elasticity over time.
5. Persistent Fatigue
If you suddenly feel a wave of lethargy or an overwhelming urge to collapse onto the sofa, it might not just be the ambient heat. Tiredness and physical exhaustion are common early warning signs that your body lacks the fluids required to run efficiently.
6. Increased Heart Rate
When body fluid is low, your heart has to work significantly harder to maintain stable blood pressure and pump blood to your vital organs. As a result, you may notice your pulse starting to race. "The body also releases additional stress hormones, such as adrenaline, endorphins, and cortisol, to try and speed up the heart and speed up the process to increase delivery of vital blood and the associated salts and fluids to our vital muscles and organs," adds Dr Asif.
When to Act
Ignoring these symptoms can have devastating consequences. Dr Coyne warns that unaddressed dehydration can quickly escalate into heatstroke or exertional heatstroke, both of which carry high mortality rates. Any sign of an altered mental state - including sudden confusion, slurred speech, agitation, or a loss of consciousness - is a medical emergency and requires an immediate call for an ambulance. Experts advise that high-risk groups, such as infants, young children and the elderly, have a much lower tolerance threshold and medical attention should be sought early, as their symptoms can be harder to spot.
How to Stay Hydrated in Hot Weather
Experts recommend taking simple, proactive steps to avoid dehydration:
- Don't wait until you're thirsty: Thirst is an unreliable indicator, especially for older adults. Take a proactive approach and drink 10 sips of water at once regularly throughout the day.
- Check your urine colour: This is the easiest way to monitor your body. Aim for a light yellow or almost clear colour - if it is turning dark amber, you need to drink water immediately.
- Fuel up during exercise: Plain water is fine for activities under an hour. For longer runs or cycle rides in the heat, use a carbohydrate-electrolyte sports drink to replace lost salts.
- Keep your environment cool: Lower your internal body temperature by keeping curtains or blinds closed during the day, using fans, or turning on air conditioning where possible.
- Moisturise your skin: Applying regular moisturiser helps maintain your natural skin barrier and locks in vital topical hydration during dry, hot spells.
