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StillnessHydration fact

Mild dehydration, a fluid loss of about 1 to 2 percent of body weight, can bring tiredness, headache and low mood, and in some tests it makes people less accurate at attention tasks. The effects show most clearly in the heat or during exercise.

Hydration science

Health

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Two controlled studies at the University of Connecticut, published in 2011 and 2012, made healthy young adults slightly dehydrated by having them walk on a treadmill, in some sessions after taking a diuretic, a pill that makes the kidneys pass more water. In 25 women, a loss of about 1.4 percent of body weight lowered mood, made tasks feel harder and brought on headaches and poorer concentration, although most thinking tests were unaffected. In 26 men, a loss of about 1.6 percent increased fatigue and tension and led to more errors on tests of attention and working memory, the short-term mental notepad we use to hold information while using it. For a 70 kg person, 1.5 percent is about one litre, an amount that can be lost as sweat in an hour of hard exercise in the heat.

Türkçe oku
Vücudun hafifçe susuz kalması, yani ağırlığının yüzde 1-2'si kadar sıvı kaybetmesi, yorgunluğa, baş ağrısına ve moral bozukluğuna yol açabilir. Bazı testlerde dikkat gerektiren işlerde daha çok hata yapıldığı da görülmüştür. Bu etkiler en çok sıcakta ya da egzersiz sırasında belirginleşir.

ABD'deki Connecticut Üniversitesi'nde yapılan ve 2011 ile 2012'de yayımlanan iki kontrollü çalışmada, sağlıklı genç yetişkinler koşu bandında yürütülerek hafifçe susuz bırakıldı. Bazı denemelerde katılımcılara, böbreklerin daha fazla su atmasını sağlayan idrar söktürücü bir ilaç da verildi. 25 kadında vücut ağırlığının yüzde 1,4'ü kadar sıvı kaybı morali bozdu, işleri daha zor hissettirdi, baş ağrısına ve dikkat dağınıklığına yol açtı; ancak düşünme testlerinin çoğunda fark çıkmadı. 26 erkekte ise yüzde 1,6 civarındaki kayıp yorgunluğu ve gerginliği artırdı; dikkat testlerinde ve bilgiyi kullanırken aklımızda tuttuğumuz çalışma belleği testlerinde daha çok hata yapıldı. 70 kiloluk biri için yüzde 1,5 yaklaşık bir litredir; sıcak havada bir saatlik zorlu egzersizde bu kadar su terle kaybedilebilir.

Sources and further reading

Background reading: Dehydration
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English quotation books have called this an “Arabian proverb” since at least the 1860s; Tryon Edwards's A Dictionary of Thoughts (1891) is one, but these books give no Arabic original, so the label is best taken loosely. The idea is widely shared: good health gives people the strength to plan and to recover from setbacks. Turkish says “Her şeyin başı sağlık”, health comes first, and when a glass breaks people say “Sağlık olsun”: never mind, as long as everyone is well.

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Protein helps with satiety, the feeling of fullness that lasts after a meal. Meals that include beans and lentils, eggs, fish, dairy or other protein sources can help people feel full for longer, although appetite and digestion vary from person to person.

In studies that compare meals with the same number of calories, the higher-protein version usually keeps people satisfied for longer, although the effect is modest. One reason is that protein prompts the gut to release more of the hormone PYY, which tells the brain that food has arrived; a 2006 study led by Rachel Batterham at University College London found the strongest PYY response after high-protein meals. Another is that the body works harder to process protein: a 2004 review by the Maastricht University physiologist Klaas Westerterp reports that digesting and using it burns 20 to 30% of its calories, against 5 to 10% for carbohydrate and up to 3% for fat. UK guidance puts adults' needs at about 0.75 g of protein per kilogram of body weight a day, around 45 g for women and 56 g for men, and average intakes are well above that.

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The fibre we eat comes from plant foods: fruit, vegetables, wholegrains, beans and lentils, nuts and seeds. Meat, fish, eggs and dairy naturally contain none.

Fibre is the part of plant food that our digestive enzymes cannot break down. Much of it comes from the walls of plant cells, which give stems, peel and seeds their structure. Animals do not make these substances, so meat, fish, eggs and milk contain none. Mushrooms, which are fungi rather than plants, also supply a little fibre from their own cell walls. Nutritionists divide fibre into two kinds. Soluble fibre, found in oats, barley, apples and beans, dissolves in water in the gut, can form a gel and helps lower cholesterol. Insoluble fibre, in wholegrains, brown rice, nuts and the skins of fruit and vegetables, adds bulk and helps food pass through. Once fibre reaches the large intestine, gut bacteria ferment part of it into short-chain fatty acids, compounds that feed the cells lining the bowel.

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Whole fruit keeps its fibre and cell structure, so it takes longer to eat and fills you up more than juice. Juicing breaks the cells open and frees their sugars, which are harder on teeth. The NHS therefore counts juice as one of your five daily portions at most, and advises no more than 150ml a day.

In a 2009 study at Pennsylvania State University, Julie Flood-Obbagy and Barbara Rolls gave 58 adults an apple, apple sauce or apple juice 15 minutes before lunch, each with the same weight and calories. After the whole apple, people felt fuller and ate about 15% fewer calories across the starter and lunch together than on days with no starter. Juice did not have that effect, even with the apple's fibre added back. A larger Harvard analysis of about 187,000 American health professionals, published in the BMJ in 2013, found that people who ate more whole fruit, especially blueberries, grapes and apples, were less likely to develop type 2 diabetes, while those who drank more fruit juice were slightly more likely to. Long-term observational studies like this show a link rather than proof of cause, but the two kinds of evidence point the same way.

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Water makes up about 60% of an adult's body weight. The body needs it to control its temperature, keep blood flowing, digest food and keep organs working. When lost fluid is not replaced, thirst can come with tiredness, headache, dizziness and poorer concentration.

Every day we lose water in urine, in sweat and even in our breath, and more in hot weather or during exercise. About a fifth of what we take in comes from food, especially fruit and vegetables, and the rest from drinks. The NHS suggests 6 to 8 glasses of fluid a day, and water, lower-fat milk, tea and coffee all count. Thirst is the body's alarm: when the blood becomes slightly too concentrated, sensors in the hypothalamus, a small region at the base of the brain, trigger the urge to drink. This alarm weakens with age, which is one reason older people become dehydrated more easily. A simple check is the colour of urine: the NHS advises drinking enough to keep it a clear pale yellow, and darker urine is a sign to drink more.

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