By Jennifer Bowden

Why does coffee keep some people awake while others drop off to sleep unaffected?

    In the 16th century, Turkish women could legitimately divorce their husbands if they failed in their duty to keep the household coffee pot full. Although that response is a little extreme, I wouldn’t debate it with a coffee lover before they’ve had their morning espresso.

    After all, coffee is a drink with a passionate following and a touch of intrigue about it. Until 2004, caffeine was on the World Anti-Doping Agency’s “banned substances list”, but the ban was lifted (fortunately for endurance athletes and cyclists) amid suggestions that caffeine had little or no effect on performance. Try telling that to anyone who has lain awake at night after one latte. For others, though, caffeine has little or no effect on their sleep. And that’s the thing: caffeine affects everybody differently.

    Coffee is a complex mix of chemicals, including caffeine, cafestol咖啡醇, kahweol咖啡白脂 and chlorogenic acid绿原酸. Caffeine stimulates the central nervous system, elevates the blood pressure and increases the metabolic rate. It is also a diuretic. The degree to which people feel caffeine’s effects depends on several factors: body mass (the smaller you are the greater the effects); your history of caffeine consumption, as regular drinkers build up a tolerance to the negative effects; and stress – the more stressed we are, the greater our sensitivity to caffeine’s effects. And then there’s our genetics.

    Caffeine is metabolised primarily in the liver by the cytochrome P450 (CYP450)细胞色素P450 enzyme system. A number of genetic variations that alter this activity have been identified. Genetic variations of N-acetyltransferase 2（N-乙酰转移酶2） – another important enzyme involved in caffeine metabolism – also alter enzymatic action. So, as a result of genetic variations such as these, the rate at which caffeine is metabolised may change. Because they metabolise caffeine more slowly, some people will have greater exposure to caffeine metabolites.

    Unfortunately, most research on coffee and health has not considered genetic variation, which may partly explain the conflicting information on coffee and “regularity”. We do know a person’s gut motility in the rectosigmoid region直肠乙状结肠区increases within minutes of consuming a caffeinated drink. This increase is more prevalent in those individuals who report that drinking coffee induces a bowel movement, which in turn suggests individuals who feel physiological effects are not imagining it.

    The results of a US Nurses’ Health Study investigation into diet and constipation shed more light on the subject. They found low-to-moderate coffee consumption (up to five cups a day) was associated with a decreased risk of constipation whereas high coffee consumption (six or more cups a day) was associated with an increased risk. Researchers suggest the inverse relationship at higher levels may be due to coffee’s diuretic effect. Genetic variation may also explain some variability in results. Further research is required.

    However, as concern remains about the long-term health effects, moderate consumption is recommended. Limit yourself to three or four cups a day (about 300-400mg of caffeine). Pregnant women should limit themselves to a maximum of three cups a day. If you consume more than four cups a day or suffer from restlessness, anxiety/irritability, sleeplessness or headaches, try to reduce your coffee consumption gradually, as this will lessen the withdrawal effects. Substituting decaffeinated beverages such as herbal teas is a good idea. If you are particularly susceptible to coffee’s effects, just limit your intake and consume coffee early in the day.

注：

latte：a drink made by mixing espresso with hot milk with lots of bubbles of air in it