If the human body is working as designed, when we need food, we’ll feel hungry and when we need water, we’ll feel thirsty.
In practice, most people also eat because the clock tells us it’s the time of day or night when we usually eat. Or we eat because we happen to see, smell, or are given some food that pleases our tastebuds. Some people also eat because they’re following a diet that prescribes what they can and can’t consume. The best time to eat, however, is when you get pangs of gut-rumbling hunger – the true indication we need some food.
Our bodies also need water but social customs and marketing have distorted our fluid consumption as well. From a young age, many now drink something from a bottle or can, often a sweet or alcoholic drink, whether they actually feel thirsty or not.
When we feel hungry, it would be odd to assume we are therefore malnourished. However, we often hear people (who don’t understand the way the body works) claim that if we feel thirsty, we are already ‘dehydrated’.
Actual dehydration is a serious problem that can have adverse effects, especially on the kidneys, but also on the heart and brain. Most of the time, thirst is an excellent way to ensure dehydration doesn’t occur.
Occasionally, the thirst response doesn’t work as well as it should. This mostly only occurs in the very young and the very old.
For the first six months, a breast-fed baby needs no fluid except its mother’s milk. However, once past this phase, some young children may become so involved in an activity that they don’t stop for a drink, although a healthy child will eventually feel thirsty.
If a child has vomiting or diarrhoea, thirst may take time to recover. If so, the child will need frequent sips of water to prevent their small bodies becoming severely dehydrated.
Some frail aged people deliberately ignore their thirst and reduce their fluid intake to avoid any struggle they may fear in accessing a toilet. This can have adverse effects on their kidneys and their health, so it’s important to ensure such people develop good fluid habits. Elderly people with dementia may also fail to respond to thirst.
During strenuous exercise or in very hot weather – especially if you haven’t ‘acclimatised’ to the heat – the thirst response can be a bit slow. If you’re in this boat and you’re feeling weak or faint, make sure you drink some water. However, athletes should avoid over-hydration, as this can cause hyponatraemia with nausea, vomiting, severe headaches and cramps.
For most people and in most circumstances, thirst is an excellent inbuilt indicator that you should have a drink.
Which water?
For millions of years, humans responded to thirst by drinking water. That’s still usually the first choice of those whose thirst has been stimulated by physical activity or sudden heat.
However, successful marketing campaigns have convinced many people that they need to buy something to drink. Indeed, it’s hard to think of any marketing exercise that has been as successful as that of the drinks industry. Just over half of all sugar intake comes from sweet drinks. The drink industry’s ‘solution’ to evidence of the harmful effects of added sugars is to sell artificially-sweetened versions. This may save some kilojoules but the acidity of these products makes them just as damaging to teeth as the sugared varieties.
Recent research in animals indicates artificial sweeteners can have several adverse effects, including on the all-important ‘good’ gut bacteria. Experiments in mice show that some artificial sweeteners cause changes in gut bacteria can alter the animals’ ability to control blood glucose levels. Research in humans already shows that some other additives in many ultra-processed foods and drinks (see article in KV Voice November 2023) have adverse effects on gut bacteria. Studies are currently underway to see if this also applies to artificial sweeteners in humans. There may be many reasons to avoid these products.
Any attempts to curtail marketing of human foods and drinks is difficult. Perhaps the prime example of the greatest marketing coup of all time has been convincing people with perfectly safe tap water to pay up to 2,000 times as much for water in a plastic bottle. As well as the massive global problem of dealing with billions of single-use plastic water bottles, many bottled waters in Australia are just tap water. It may have been filtered – indeed, if it comes direct from a spring, it must be filtered. If you’re tempted to swallow the marketing hype, it’s worth noting that tap water in Australia must meet much more stringent quality criteria and is monitored far more carefully than bottled water.
To add to the environmental problems caused by the 373 million plastic drink bottles ending up in landfill in Australia each year, research has found microplastics from this type of bottle can have harmful effects on body cells. Tests show water from single use bottles has higher levels than tap water.
Water is an ideal drink. If you want it fizzy, the good news is that researchers can find no adverse effects from straight carbonated or straight soda water. Studies show no harmful effects on teeth or bones (unlike any cola drinks) and some people find slightly fizzy water it easier to swallow than plain water. If it makes you burp, that’s just the body’s way to get rid of the carbon dioxide.
However, use a soda maker and avoid single use plastic bottles.
How much?
There’s a popular belief we need eight glasses of water a day. That figure came from a recommendation in 1945 from the Food and Nutrition Board of the USA’s National Research Council. It was intended for the average person, but it included the water already present in foods.
If you are small or inactive, you probably won’t need eight glasses a day, whereas that quantity would be woefully insufficient for an athlete, a farmer working in the sun, or during a heatwave. If you’re sweating heavily, it makes sense to drink more and you may need at least a litre an hour.
But under most circumstances, you don’t need eight glasses a day and you don’t need to take in all your water as a liquid. Most fruits and vegetables are about 85-90% water, lean meat averages 65% water, eggs clock in at 75% and most breads are almost 40% water.
It’s also a myth that tea and coffee are ‘dehydrating’ and therefore don’t count as part of the body’s fluid needs. One of the most important functions of fluid is to assist the kidneys to filter the blood and send many toxic substances out through the bladder in urine. Tea and coffee have a mild diuretic action and can certainly be counted as part of our fluid intake. Problems with caffeine usually only occur in those taking caffeine supplements.
Beer also has a diuretic action, but the adverse effects of exceeding safe drinking limits extend far beyond dehydration. My advice is always quench your thirst with water before drinking any alcohol.
If you are concerned about whether you are drinking enough, take a look at the colour of your urine. It should be pale yellow or almost clear.
Take home message
Thirst is almost always a good guide as to fluid needs.
