Hydration 101: how much fluid do I need?

One week we’re told to drink eight glasses of water a day; the next we’re told we need electrolyte supplements, alkaline water or the latest hydration product. It’s hardly surprising that people get confused and experience information overload about how much water we should drink.

Hardly a day goes by without a new nutrition headline telling us what we should, or shouldn’t, be eating, or an article refuting previous nutrition claims. This often leaves people wondering what to believe. The fact is, science evolves; knowledge and understanding change. Making sense of the science can help us make informed choices, without getting caught up in every new trend.

Hydration is one of the foundations of good health and a healthy diet, but it is not exempt from the influence of marketers trying to sell the latest innovations in water bottles, sports drinks or electrolyte supplements. Their job is to sell; their effect is to confuse and overwhelm.

The good news is that it doesn’t have to be complicated. Understanding how your body regulates water can empower you to make informed choices.

Why it matters

The human body is composed of 50-70% water, and this water is involved in a wide range of physiological processes; it helps regulate body temperature, transport nutrients, lubricates joints, supports digestion, and normal muscle and brain function.

Even mild dehydration can affect how you feel. You may notice:

• fatigue
• poor concentration
• headaches
• dry mouth
• reduced daily function
• irritability

Sometimes thirst can be mistaken for hunger, resulting in grazing when what we are lacking is fluid, not food. While this doesn’t happen to everyone, it’s a good reason to pay attention to your own body’s thirst signals.

For most healthy people, thirst is an effective reminder to hydrate. However, thirst is a delayed mechanism, meaning that by the time we are aware of it, we are already in a mild state of dehydration. Sipping fluids throughout the day can avoid this – maintaining visual reminders such as a glass of water or a water bottle near you where you can see it can help.

It's also important to remember that the thirst response can be impaired in older adults, those with cognitive disorders, and in some people taking certain medications. If this applies to you, it’s especially important to drink fluids regularly throughout the day.

If hydration is so important, the next logical question is how much do we really need?

How much water should I drink?

This is probably one of the most common questions I get asked. The honest answer is that there isn’t a magic number, or a one-size-fits-all recommendation.

Fluid requirements change from day to day depending on a variety of factors, including activity level, weather, body size, diet, and overall health. Hydration isn’t about achieving a magic intake every day; it’s about replacing fluid losses. The losses change day to day, and your fluid requirements will change accordingly.  

Although we may be aware of general guidelines of fluid intake, for instance the UK Eatwell Guide recommends 6-8 cups/glasses of fluid per day, personal circumstances will affect our requirement.

For instance:

• pregnancy/lactation
• climate
• physical activity
• illness or recovery from illness
• age

A simple way to monitor your hydration is by looking at the colour of your urine. Pale yellow usually indicates adequate hydration, while darker-coloured urine may be a sign that you need more fluids (remember, though: certain medications and high doses of B-vitamins may also cause the colour of urine to appear different).

Contrary to popular belief, tea and coffee, lower-fat milk, and sugar-free drinks all count towards your fluid intake. Water isn’t the only source of hydration. Did you know that 20-30% of the water we consume each day comes from food, rather than beverages? 

Some fruits and vegetables are composed of approximately 90% water; these include watermelon, cantaloupe, cucumber, strawberries, tomatoes, lettuce, celery, and spinach.

What are electrolyte drinks and when do we need them?  

Hydration is big business. The shops are packed with electrolyte supplements, enhanced waters and sports drinks, but do most of us need them? The simple answer is that they are really only needed after prolonged strenuous exercise, during periods of illness, for example gastrointestinal illness, or when working in extreme heat.

An electrolyte drink is simply water supplemented with essential minerals, namely sodium, potassium, magnesium, and calcium. These carry a minute electrical charge necessary for controlling functions such as fluid balance, nerve function and muscle contractions.

We lose some of those minerals when we sweat and, after heavy sweating, for example after strenuous exercise over a prolonged period (one hour plus) if you drink only plain water, you dilute the remaining minerals in your body possibly, leaving you feeling fatigued, crampy or headachy.

Electrolyte drinks replace both the water and the lost minerals at the same time and, if they contain sugar, they may help your body absorb the water and sodium faster.

The good news is that you can easily make an effective isotonic electrolyte drink at home that matches your body’s own concentration of sugar and salt using a 4:1 ratio of fruit juice and water, plus a pinch of salt.

5 practical tips for staying hydrated

Staying hydrated doesn't have to be complicated. These simple habits can help you maintain your fluid intake throughout the day.

1. Keep water visible by keeping a glass or a bottle nearby to act as a visual prompt to drink.
2. Carry a water bottle with you when you are out and about.
3. Eat hydrating foods including water-rich fruit and vegetables in every meal and in snacks.
4. Adjust fluid intake according to the environment, weather and levels of activity.
5. Monitor your urine, ensuring it stays a pale yellow colour.

Always remember to seek nutritional support when appropriate.