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长时间以来，人们一直对咖啡对人体的作用争论不休。一些人说，常饮咖啡对心脏有益，而另外一些人则持相反的观点。加拿大研究人员日前表示，咖啡对人的心脏究竟是有利还是有害要取决于喝咖啡者的基因情况而定。
据《新科学家》杂志网站3月7日报道，一项新研究结果显示，在那些因基因构成导致咖啡因代谢较慢的人中，每天喝2杯至3杯咖啡的人较之拥有相同基因构成，但每天只喝1杯或更少量咖啡的人，其心脏病发作的危险增加36％。如果他们每天饮用咖啡的量达4杯以上，那么其心脏病发作的危险会相应增加64％。
对此，加拿大多伦多大学的艾哈迈德· 埃尔-索海米（Ahmed El-Sohemy）是此项研究的负责人，他说：“我们的数据资料显示，咖啡因在人体内逗留的时间越长，那么它所带来的危害也就越大。”
另一方面，那些咖啡因代谢较快的人，如果他们每天喝2杯至3杯咖啡，那么他们就会比那些拥有同种基因构成，但每天只喝1杯或更少量咖啡的人，患心脏病的几率少22％。
自然刺激物
近期的其它一些研究突出说明了喝咖啡带来的积极作用和消极作用。此前，一项针对荷兰男性与女性的研究显示，大量饮用咖啡的人，其患心血管疾病和II型糖尿病的几率较低。
2005年公布的另外一项研究结果显示，那些常常饮用无咖啡因咖啡的人和饮用普通咖啡的人相比，前者血液中含有的某些胆固醇水平较高。
但是，人们关注更多的则是咖啡中含有的咖啡因这种物质对人体健康潜在的负面影响。这一新的研究成果对下面这一理论表示支持，即咖啡因这种天然的刺激物会防碍一种被称为腺苷（adenosine）的激素发挥作用，从而使血管变得狭窄。腺苷的作用则是让血管保持宽敞。
一种基因的变异
在上个世纪90年代末期，研究人员发现，人类携带的一种酶的突变基因能够分解人体内的咖啡因。埃尔-索海米说，那些体内携带有两份CYP1A2*1A基因的人，分解咖啡因的速度比那些体内携带CYP1A2*1F基因的人快4倍。
埃尔-索海米和他的同事对2000余名心脏病患者和年龄、性别及居住区域相对应的相同人数的健康人进行了DNA分析。他们这一研究显示，携带有CYP1A2*1F基因的人，每天喝2杯至3杯咖啡会使心脏病发作的危险增加36％；每天喝4杯或更多的人，其心脏病发作的危险将增加64％。血液中咖啡因水平的增加会阻碍腺苷的作用，导致血管收缩，最终则可能导致心脏病发作。埃尔-索海米说，在这项研究中，大约有55％的人携带有减缓咖啡因代谢的基因。
而那些携带有同型结合的CYP1A2*1A基因（即携带有两份快速代谢咖啡因基因）的人，喝咖啡却能减少他们心脏病发作的危险。研究显示，在那些参与研究的志愿者中，每天喝2杯至3杯咖啡会使他们心脏病发作的危险降低22％。
报道说，这是科学家们首次对基因如何影响咖啡对心脏病发作危险的作用进行系统的分析研究。埃尔-索海米强调说，尽管很少有人知道自己是否携带有CYP1A2*1F基因，但每天只喝一杯咖啡对身体是没有什么负面作用的。
Coffee can raise or reduce your chances of suffering a heart attack – it all depends on your genes, researchers suggest.
People with a genetic makeup that causes them to metabolise caffeine more slowly have a 36% greater risk of heart attack if they drink two to three cups of coffee a day than people with the same gene who drink one cup or less a day, according to a new study. And if they drink more than four cups, this risk rises to 64%.
"Our data suggest that the longer caffeine is lingering in the system, the more harm it can do," says Ahmed El-Sohemy at the University of Toronto, Canada, who led the study.
On the other hand, individuals who metabolised caffeine quickly and consumed two to three cups of coffee a day had a 22% reduction in the risk of heart attack compared with those with the same genetic makeup who consumed just one cup or less each day.
Natural stimulant
Other recent studies have highlighted both positive and negative effects of caffeine consumption. A previous survey of Dutch men and women suggested that heavy coffee drinkers had a lowered risk of developing the cardiovascular disease, type 2 diabetes.
Another investigation published in 2005 found that people who consumed decaffeinated coffee had higher levels of certain cholesterols in their blood than those who consumed regular coffee.
But much of the concern surrounding coffee's potentially negative health effects has centred on the caffeine it contains. Research has supported the idea that caffeine, a natural stimulant, narrows blood vessels by blocking the effects of a hormone called adenosine, which normally keeps them widened.
Variations on a gene
In the late 1990s researchers discovered that humans carry variants of the gene for an enzyme that breaks down caffeine in the body. People who carry two copies of the CYP1A2*1A gene may break down caffeine up to four times faster than those carrying the CYP1A2*1F gene, according to El-Sohemy.
He and his colleagues analysed the DNA of more than 2000 patients along with an equal number of healthy people matched for age, sex and area of residence.
Their retrospective study showed that two to three cups of coffee per day caused a 36% rise in risk of heart attack among people who carried the CYP1A2*1F gene - and drinking four or more cups a day caused a 64% increase in the same group. Increased levels of circulating caffeine may block adenosine's action, causing blood vessels to constrict, subsequently triggering a heart attack, says El-Sohemy.
About 55% of those involved in the study carried the gene for slow caffeine metabolism. Cup half full
People who were homozygous for CYP1A2*1A -meaning they carried two copies of gene for fast caffeine-metabolism - actually reduced their risk of heart attack by drinking coffee. Among these volunteers, two to three cups of coffee caused a 22% decrease in heart attack risk. Drinking more than four cups of coffee only nominally reduced their chances of a heart attack.
This is the first time that scientists have done a systematic analysis of how genes can influence the effect of coffee on heart attack risk. The study is part of a larger trend in which scientists have untangled how genes make some diets and habits more beneficial or addictive for some people than for others.
Few people know whether they carry the CYP1A2*1F gene or not. But El-Sohemy stresses that having just one cup a day does not appear to have any adverse health effects.
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