What's actually happening when you feel thirsty
The human body is made up of roughly 55–60% water. That water isn't just sitting there — it's involved in almost every physiological process going on inside you, from regulating body temperature and cushioning your joints to transporting nutrients and oxygen around the body.
Dehydration occurs when your fluid intake falls below your fluid losses. And here's the part most people don't realise: you don't need to feel parched for it to be happening. Even a mild fluid deficit — around 1–2% of body weight — has been shown in research to negatively affect concentration and focus, short-term memory, mood, energy levels and exercise capacity.
For a 70kg person, 1% is just 700ml. Less than a standard water bottle.
By the time thirst kicks in, you've often already crossed that threshold. Thirst is a lagging indicator, not an early alarm.
Why the 8 glasses a day rule doesn't apply to you
It's probably the most famous piece of health advice on the planet. It's also not how hydration works. Your fluid needs depend on your body size, activity levels, the temperature around you, what you're eating and your individual sweat rate.
Food counts too — fruits, vegetables, yoghurt and milk all contain significant amounts of water, quietly topping up your intake at every meal.
One practical way to keep tabs: check your pee. Pale straw = generally well hydrated. Yellow = probably fine. Dark, like apple juice = your body may need more. (Note: vitamin supplements can turn urine bright yellow — don't read too much into a single trip.)
The best hydration target is the one that matches your body and your day — not an arbitrary number.
Why water alone sometimes isn't enough
Here's something that surprises a lot of people: drinking plain water doesn't automatically mean your body holds onto it. Sodium is what helps make that happen.
Sodium is the key electrolyte in hydration — it supports fluid absorption in the gut and helps the body retain the fluid you drink. When sodium is present, it encourages you to keep drinking and reduces how much fluid the body loses through urine, especially useful when rehydrating after exercise, a long commute or a hot day.
This is why, on particularly sweaty days, water can feel like it goes straight through you. It often does.
Thirst is your body asking for fluids. Sodium is what helps your body actually use them.