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Caffeine lingers in the body for hours: healthy adults usually take three to seven hours, often about five, to clear half of what they consume. This is its half-life. Smoking shortens it, while pregnancy, the contraceptive pill and some medicines lengthen it, so the same coffee lasts longer in some people.

Pharmacology

Health

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Caffeine keeps you alert by blocking adenosine, a chemical that builds up in the brain while you are awake and makes you feel sleepy. From a drink it is absorbed within about 45 minutes and peaks in the blood after one to two hours. The liver then breaks it down, mainly with an enzyme called CYP1A2. In smokers the half-life is 30 to 50 percent shorter; the contraceptive pill can make it considerably longer, and late in pregnancy it can reach 15 hours. In a 2013 study at Henry Ford Hospital in Detroit, 400 milligrams of caffeine, about three mugs of filter coffee, cut total sleep by more than an hour even when it was taken six hours before bed. The researchers advised avoiding large amounts of caffeine for at least six hours before going to sleep.

Türkçe oku
Kafein vücutta saatlerce kalır: Sağlıklı bir yetişkin, aldığı kafeinin yarısını genellikle üç ile yedi saat arasında, çoğunlukla da beş saat civarında vücudundan atar. Bu süreye kafeinin yarı ömrü denir. Sigara bu süreyi kısaltır; gebelik, doğum kontrol hapı ve bazı ilaçlar ise uzatır. Bu yüzden aynı kahvenin etkisi bazı kişilerde çok daha uzun sürer.

Kafein, uyanık kaldıkça beyinde biriken ve uyku hissi veren adenozin adlı maddenin etkisini engelleyerek insanı dinç tutar. İçecekle alınan kafein yaklaşık 45 dakikada emilir; kandaki düzeyi bir iki saat içinde en yükseğe çıkar. Ardından karaciğer onu, çoğunlukla CYP1A2 adlı bir enzimle parçalar. Sigara içenlerde yarı ömür yüzde 30-50 kısalır; doğum kontrol hapı ise onu belirgin biçimde uzatabilir. Gebeliğin son aylarında yarı ömür 15 saate kadar çıkabilir. ABD'nin Detroit kentindeki Henry Ford Hastanesi'nde 2013'te yapılan bir çalışmada, üç kupa filtre kahveye denk gelen 400 miligram kafein, yatmadan altı saat önce alındığında bile toplam uykuyu bir saatten fazla kısalttı. Araştırmacılar, yatmadan önceki en az altı saat boyunca yüksek miktarda kafeinden kaçınmayı öneriyor.

Sources and further reading

Background reading: Caffeine
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He who has health has hope, and he who has hope has everything.

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.

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