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Losing just of the water in your body (mild dehydration), can impair your cognitive performance, attentiveness, short-term memory and . These symptoms, which affect your ability to think, are often collectively described as “brain fog”, a non-medical, colloquial term. There are many potential causes of “brain fog”, but one you can control is hydration.

Up to of the adult human body is water. The brain, however, is around 73 percent.

"The water in our body needs to constantly be changed because we use it to flush out toxins and support in-cell processes,” said dietitian Sophie Medlin. “None of the cells in our body can work optimally when we’re dehydrated.” Your body regularly loses water through breathing, sweating and urination but if the water isn’t replaced at the same rate as it is lost, you become dehydrated.

Are you dehydrated?

Urine colour can indicate if you’re dehydrated. Your wee is yellow because it contains urobilin, a waste product from your kidneys. The amount of water you drink dilutes the urobilin, making the colour of your wee a good indicator of hydration. It can change daily or even hourly. You should drink enough during the day so your wee is a pale colour.

Dehydration can happen more easily if you have “diabetes, vomiting or diarrhoea, been in the sun too long (heatstroke), drunk too much alcohol, sweated after exercising, have a high temperature of 38C or more [and] and been taking medicines that make you pee more," according to the .

Older adults are to dehydration due to a lower thirst response, meaning you may not feel as thirsty. Because of this, and the increased risk associated with health conditions and medications, “It may be prudent for the elderly to learn to drink regularly when not thirsty,” suggests .

How much water should you drink?

The UK suggests we have 6 to 8 glasses of fluid a day, including sugar-free drinks. For people under 60 years of age, “35 millilitres of water per kilogram of body weight per day is the calculation dietitians use clinically,” according to Medlin. For most healthy people this is between two to three litres per day, she added.

The total amount of water you need also includes the contribution from water in food, so you may not need to drink as much liquid, depending on your diet. Your total fluid intake can also include tea and coffee, in moderation.

It's true, caffeine can cause your body to produce more urine by increasing the blood flow to your kidneys, encouraging them to flush out more water. However, when consumed in moderate amounts, some research suggests caffeinated drinks, such as and , can be as hydrating as water. Medlin suggests having a glass of water alongside caffeinated drinks to combat any potential diuretic effects.

Michael Mosley, ���˿��� health journalist and broadcaster suggests in his Radio 4 series, to drink a glass of water with every meal as a simple way to stay hydrated.

Sipping water than gulping it. “Ideally, we will keep hydrated through the day by having a drink on the go at all times. When we consume a large volume all at once, our body has to adjust for this," said Medlin. "The electrolyte shifts that happen can mean we lose more of that water in our urine than we would if we had drunk it slowly.” A reusable flask makes it easier to sip on water when you’re out and about. If it is insulated, water can stay cool or hot, as you prefer.

In general, drinking water is beneficial to those who are dehydrated, according to one , but there is “little research” to support the notion that additional water in hydrated people has any benefit. So going above and beyond the recommendations isn’t necessary.

Get at least five a day

Many foods, especially fruits and vegetables , and contribute to the overall quantity of water you need each day. Melons, lettuce, and cooked squash are between 90 to 99 percent water. Yoghurt, apples, oranges, broccoli and carrots are 80 to 89 percent water. Whereas high-fat foods such as pizzas, cakes and biscuits contribute very little water to the diet and high-salt foods like sausages, cheese and crisps may dehydrate you as more water is required to remove the salt from the body.

A healthier, Mediterranean diet contributes more water to your daily intake, as well as conferring many other and your physical health.

Some drinks may worsen "brain fog"

“Drinks containing sugar may give a quick energy high, which mentally alerts you, but this is followed by sugar dip which leads to an energy low and mental tiredness,” said nutritionist Sonal Shah. Drinking sweetened drinks or juices in consuming more calories than you need, so check ingredient and nutritional labels before you buy. Or just stick to water.

“Alcohol is a toxin and in order for our bodies to eliminate it, we first dilute it in our blood by diverting water into our bloodstream from other tissues,” said Medlin. “Our liver then kicks in to process, neutralise and excrete the alcohol through our kidneys.” It's “an expensive process in terms of water use.” Fatigue, decreased alertness, physical discomfort and emotional disturbance are also an alcohol hangover. Anyone concerned about "brain fog" does not need an additional hangover.

To combat dehydration during the festive season, drink water or non-fizzy soft drinks in between each alcoholic drink. Fizzy drinks speed up the absorption of alcohol into your body. Drink water before you go to sleep and keep a glass of water by your bed to sip if you wake up during the night. Festive drinks like mulled wine or cider can be made less alcoholic, while tasting delicious, by diluting with orange, apple or grape juice.

Make water tasty

If you're not used to drinking plain water, you can jazz it up with some natural flavourings. Chef Donal Skehen adds mint, ginger, cucumber, citrus fruit, frozen fruits and a dash of fruit juice to his water to introduce some flavour.