The advice that "everyone should drink at least 3 litres of water a day" is not a scientific rule that applies to every person. How much water you need depends on age, sex, weight, physical activity, diet, climate and general health, and drinking too much water in a short time can lead to hyponatraemia. It is better to drink according to your body's signals, such as the colour of your urine and thirst, than to aim for a fixed amount.
One thing I often hear in consultations is: "I try to drink 3 litres a day but I can't manage it." Yet this figure does not apply to everyone. An amount that is enough for some people may be more than others need.
What Determines How Much Water You Need?
People who exercise, are on the move all day or live in a hot climate need more fluid; people who are inactive need less. The main factors that affect your needs are:
- Age and sex
- Body weight
- Physical activity
- Air temperature and climate
- The water content of your food
- General health
Someone who eats plenty of vegetables and fruit gets part of their fluid needs from food.
Can Drinking Too Much Water Be Harmful?
It can. Drinking a great deal of water in a short time upsets the balance of electrolytes in the blood and lowers the sodium level. This condition, known medically as hyponatraemia, can cause headache, weakness, nausea, muscle cramps and confusion. So it is just as important to keep the balance right as it is to avoid drinking too little.
How Can You Tell If You Are Drinking Enough?
The most practical indicator is the colour of your urine. Dark yellow urine may suggest you are not drinking enough, while pale yellow or almost colourless urine may indicate that you are drinking enough.
A few simple rules also make things easier:
- Spread your water through the day rather than drinking a lot in one go.
- Drink more on active and hot days.
- Pay attention to thirst.
- Do not count caffeinated drinks as water.
If you have a condition such as kidney stones, heart failure or kidney disease, decide how much fluid you should drink together with your doctor.
