咖啡因是敌是友?

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咖啡因能让你开始新的一天，让你的脚步充满活力. 它可以帮助你集中注意力，改善你的情绪，甚至可能帮助你长寿.

但多少才算太多?

咖啡因, 天然兴奋剂, 能在各种食物中找到吗, 比如咖啡豆, 茶叶, 可可豆, 瓜拉那浆果和巴拉圭叶. It also can be synthetically created and added to beverages such as soda and energy drinks. 研究表明，大约90%的美国人.S. 成年人每天都会摄入某种形式的咖啡因.

最受欢迎的一种方式是喝咖啡. 正因为如此，大多数咖啡因研究都以这种饮料为中心. 格雷格·马库斯, associate chief of cardiology for research and a professor of medicine at the University of California, 旧金山.

"的 literature on the whole shows that coffee consumption is generally not a detriment to health,他说. "But I am very reluctant to recommend anyone begin drinking coffee if they aren't otherwise doing so, 或者为了健康而增加消费."

研究发现咖啡因有好处也有坏处. People who regularly drink coffee may be less likely to develop chronic illnesses, 比如心血管疾病, 糖尿病。, 帕金森氏症和一些癌症. A few studies suggest they are less likely to die from heart disease and other illnesses.

根据 美国食品药品监督管理局, as much as 400 milligrams of caffeine a day – equal to four or five cups of coffee – is considered safe for healthy adults. 一杯8盎司的绿茶或红茶含有30-50毫克的咖啡因. 每8盎司能量饮料可能含有40-250毫克, 一罐12盎司的含咖啡因汽水含有30-40毫克咖啡因.

In moderate doses – up to two 8-ounce cups of coffee – caffeine can make people less tired and more alert. 一些研究表明，它可以减少食欲，降低患抑郁症的风险. 但高剂量——12杯或更多——会让人感到焦虑, 使血压升高，导致心悸和睡眠困难. 对于经常摄入咖啡因的人来说, 突然停止消费会导致戒断症状, 比如头痛, 疲劳和情绪低落.

确定多少才算太多可能很难. A moderate amount of caffeine for one person may feel like a high dose for someone else. That's because some people metabolize caffeine faster than others, Marcus说. Factors such as how much someone weighs and what medications they take also can play a role. 最重要的是，咖啡因对每个人的影响是不同的.

“这种化合物很复杂, 我们需要认识到，这不仅可能有好处也可能有坏处, 但这可能因人而异,马库斯说.

He and his colleagues recently completed one of the few randomized studies on caffeine consumption, which he presented at the 美国心脏协会's Scientific Sessions last year. 的 researchers asked participants to drink – or refrain from drinking – coffee for no more than two consecutive days each for two weeks.

的 发现, which are considered preliminary until the full results are published in a peer-reviewed journal, showed that people were more physically active and slept less on days they drank coffee than on days they went without. 的y also had more irregular heartbeats from the lower chambers of the heart but fewer episodes of abnormally rapid heartbeats from the upper chambers.

Marcus说 one limitation of the study was that people were starting and stopping caffeine consumption, which could be causing an exaggerated reaction in people who were used to drinking it every day. 他说:“如果你经常喝咖啡，咖啡因的作用就会减弱。. “身体会适应这种咖啡因水平. 经常摄入咖啡因可以加速新陈代谢."

People who metabolized caffeine faster had fewer problems sleeping than those whose bodies broke it down more slowly, 他说.

在他的心脏病学实践中, Marcus tells patients who are having trouble sleeping or experiencing abnormal heart rhythms to see what role caffeine might be playing. "I generally advise that it is reasonable for patients bothered by trouble sleeping or with palpitations to experiment with their caffeine consumption. 休息一段沙巴足球体育平台，看看咖啡因是否有效果.但他并没有给出避免咖啡因的全面建议.

Marcus doesn't distinguish between the caffeine that people get from coffee versus hot or iced tea. "的re may be health differences between the two, but they haven't been studied yet,他说.

他对能量饮料的消费没有那么灵活, 哪种咖啡的咖啡因浓度更高, as well as added sweeteners or carbohydrates and no evidence they provide any health benefits. 研究 has found energy drinks can cause abnormal electrical activity in the heart and higher blood pressure that persists for several hours.

马库斯说:“总的来说，我会提醒大家不要喝能量饮料。.

还有其他保持警惕的方法.

"的 best strategies and overall most healthy strategies to boost alertness are long-term healthy habits,比如晚上睡个好觉，经常锻炼, Marcus说. He recommends people who have trouble staying awake consult a physician to see if they have sleep apnea or another sleep disorder.

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