补钙须谨慎 过量有风险
Dutifully Taking Your Calcium Pill? It May Be Too Much
来源：华尔街日报    2012-07-19

        While many people aren't getting enough calcium, new research cautions that some people may have the opposite problem: They could be getting too much.          
        Americans spend more than $1 billion a year on calcium supplements in hopes of staving off osteoporosis, the brittle bone disease that cripples many elderly women and some men.           
        Yet recent studies link calcium supplements to a higher risk of heart attacks and kidney stones. Last month, the U.S. Preventive Services Task Force issued a draft recommendation against taking calcium and vitamin D, saying there wasn't enough evidence of benefit to justify the risk.          
        For generations of Americans who grew up exhorted to drink their milk to maintain strong bones, the reports raised troubling questions: Is calcium not so important after all? Are the supplements unsafe? And how much is too much?         
        'It's gotten very confusing but it doesn't need to be,' says Ethel Siris, director of the Toni Stabile Osteoporosis Center at Columbia University Medical Center in New York.         
        The links to heart attack and kidney stones involved calcium supplements, not calcium from food sources, Dr. Siris and other experts note. Many other studies have not found such health risks, so more research is needed to understand what levels, if any, might be hazardous.        
        Meanwhile, experts say they are still more concerned that too many Americans aren't getting enough calcium, since the body can't make it on its own.        
        'Unless you take in enough calcium, by mouth, every day, you have to keep borrowing it from your skeleton, so over your lifetime, you need to get enough,' says Dr. Siris. 'If you have low bone mass, or are at risk for fractures, you want to minimize any need to take calcium from the skeleton.'        
        How much calcium people need varies by age and gender. Adults generally need 1,000 milligrams daily, rising to 1,200 mg for women over 50 and men over 70, according to guidelines issued in 2010 by the Institute of Medicine, an independent advisory group. Children need 1,300 mg daily during the peak growing years of 9 to 18.        
        People also need sufficient levels of vitamin D to absorb the calcium. The IOM recommends 600 international units a day for most adults, and 800 daily after age 70, although many physicians recommend more. It is difficult to take in that much vitamin D from food sources, and prolonged sun exposure, the best source of vitamin D, can lead to skin cancer, so experts say many people should take vitamin D in supplement form. 
        Getting adequate calcium from food is easier. For example, 8 ounces of milk or 6 ounces of yogurt has 300 mg of calcium, and one cup of spinach has 270 mg. But studies show on average, Americans get only about 750 mg of calcium from their diets.         
        Many of the more than 30 million Americans who are lactose intolerant don't get enough calcium or rely on calcium supplements.       
        'For people who can't or won't get more in their diet, it's better to fill that gap with supplements than to stay on the low side,' says Bess Dawson-Hughes, director of the Bone Metabolism Laboratory at the USDA Nutrition Center at Tufts University.        
        Consuming too little calcium can contribute to osteoporosis, a thinning of the bones that causes more than 2 million fractures a year. Once osteoporosis sets in, taking extra calcium and vitamin D won't prevent fractures. But not getting enough of the nutrients can make bone loss worse.       
        On the other hand, it is easy to consume more than the 2,000 mg a day that the IOM considers the safe upper limit for adults. 'A number of health-care providers say, 'Oh, the requirement is 1,200 mg daily, so take 1,200 mg in supplements,' regardless of what the patient is consuming in food,' says Dr. Dawson-Hughes. 'If you have a bowl of Total in the morning and a yogurt and a glass of milk for lunch, you can get to 2,000 mg easily.' Multivitamins also contain calcium in widely varying amounts. Chewable chocolate, caramel and gummy-bear varieties have made calcium supplements as appealing as candy.         
        In general, any vitamins or minerals the body can't absorb are simply excreted. But studies linking calcium supplements to heart attack and kidney stones have made experts more wary of excess calcium than before.        
        In a 2006 report from the Women's Health Initiative, a large government study, women who took 1,000 mg of calcium daily had 17% more kidney stones than those who got a placebo. But subjects were allowed to eat their usual diet, and take calcium supplements on their own, no matter what group they were in, so their total calcium intake was unknown.         
        In the 12-year Nurses' Health Study of 90,000 women, those who consumed a high level of calcium in food had fewer kidney stones than those who consumed less.         
        Studies linking calcium intake and heart-attack risk are similarly confusing. A study of 24,000 Germans published in the journal Heart last month, found that those who consumed about 820 mg of calcium a day had a 31% lower risk of heart attack than those who consumed much less. But those who got their calcium exclusively from supplements were more than twice as likely to have a heart attack as those who took no supplements.         
        In 2010, researchers in New Zealand analyzed 11 clinical trials involving 12,000 people, and found that those taking calcium supplements had a 30% higher risk of heart attack than those who didn't. However, the studies didn't include people taking vitamin D, which some researchers believe is protective for the heart.           
        Exactly how calcium supplements might contribute to heart attacks baffles cardiologists. 'Nobody has associated the calcium in your bloodstream with calcification in your arteries,' says Nieca Goldberg, medical director of the Joan H. Tisch Center for Women's Health at New York University Langone Medical Center. Still, she says she now urges patients get their recommended calcium from food than from supplements to avoid possible problems.          
        Osteoporosis experts also urge patients not to take more than the recommended amount of calcium. 'Women should definitely stop taking two big calcium supplements a day,' says Dr. Dawson-Hughes. Even if the risks remain unclear, taking more than the body can absorb doesn't benefit bones, 'so it's not worth any risk -- real, imaginary, uncertain, possible or probably,' she adds.         

　　尽管有很多人钙摄入量不足，但新的研究提醒我们，也有一些人面临的问题可能恰好相反：他们的摄入量太多了。
　　美国人一年花费逾10亿美元购买各类钙补充剂，期望能推迟骨质疏松症的发生。骨质疏松症是一种让骨头变脆的疾病，许多老年女性和部分男性都会因骨质疏松致残。
　　但近期的研究揭示，钙补充剂有可能导致心脏病和肾结石风险升高。美国预防医学工作组(U.S. Preventive Services Task Force)上月发布了一份建议书草案，不主张服用钙和维生素D补充剂，称并没有充足的证据显示这样做利大于弊。
　　对从小就被劝说要多喝牛奶以维持强健骨骼的几代美国人来说，这些报告引发了一些麻烦的问题：钙是不是其实并不太重要？钙补充剂是不是不安全？摄入钙量多少算超量？ 
纽约哥伦比亚大学医学中心(Columbia University Medical Center)托尼•斯塔比尔骨质疏松研究中心(Toni Stabile Osteoporosis Center)主任西里斯(Ethel Siris)说，“问题变得令人非常费解，但其实不至于如此。” 
　　西里斯博士和其他专家指出，可能引起心脏病和肾结石的是钙补充剂，而不是从食物中获取的钙。许多其他研究并未发现这类健康风险，因此，若要了解钙摄入量过多是否会有危险以及达到多少可能会有危险，还需要做更多研究。
　　专家们同时也说，他们更担心的仍然是有太多美国人钙摄入量不足，因为人体无法自己制造钙。 

　　西里斯博士说，“除非每天都能经口服摄入充足的钙，否则你不得不一直向骨骼借用钙，所以人一生中必须摄取足够的钙。如果你骨密度低，也就是说有骨折风险的话，你就得尽量避免从骨骼中获取钙。” 

　　人体需要的钙量依年龄和性别而有不同。根据独立顾问组织药学研究所(Institute of Medicine)2010年发布的指导原则，成年人一般需要每天摄入1,000毫克钙，50岁以上的女性和70岁以上的男性每天需要摄入的钙量会增加到1,200毫克。孩子在九到18岁的生长高峰期每天需要1,300毫克钙。 
　　人体要吸收钙，还需要摄取充足的维生素D。药学研究所建议多数成年人每天摄入600国际单位的维生素D，并建议70岁以上的人每天摄入800国际单位，不过许多医生推荐的摄取量要大于这个数。从食物中摄入这么多的维生素D很难，而长时间阳光照射（维生素D的最佳来源）可能导致皮肤癌，因此专家称，许多人都应该服用维生素D补充剂。 
　　从食物中获取充足的钙则比较简单。比方说，八盎司牛奶或六盎司酸奶中含有300毫克钙，半品脱菠菜中含有270毫克钙。但研究显示，美国人平均只能从食物中获得约750毫克钙。 
　　逾3,000万乳糖不耐受的美国人无法获得充足的钙，或者要依赖钙补充剂。 
　　塔夫茨大学(Tufts University)美国农业部营养中心(USDA Nutrition Center)骨骼新陈代谢实验室(Bone Metabolism Laboratory)主任道森休斯(Bess Dawson-Hughes)说，“对无法或没能通过饮食摄取更多钙的人来说，用营养片剂来补钙比缺钙要好。” 
　　摄入钙量过少会引起骨质疏松，骨质疏松症导致每年逾200万例骨折事件发生。一旦得了骨质疏松症，即使摄入额外的钙和维生素D也无助于防范骨折。但如果这些营养物质摄入不足的话，骨耗损会加重。 
从另一方面来说，人一天的钙摄入量很容易超出2,000毫克（被药学研究所视为成年人的安全上限）。道森休斯博士说，“许多医疗服务机构会说，‘哦，一天必须摄入1,200毫克钙，所以要服用1,200毫克补充剂，’他们不管患者吃的是什么样的饮食。如果你早晨喝一碗Total牌谷物片，中餐喝一盒酸奶和一杯牛奶的话，很容易就能摄入2,000毫克钙。”复合维生素片中也或多或少地含有钙。钙补充剂能制成可以嚼着吃的巧克力、太妃糖和小熊糖等品种，像糖果一样吸引人。 
　　总体而言，任何人体无法吸收的维生素或者矿物质都会被排泄出去。但由于研究发现钙补充剂与心脏病和肾结石存在关联，专家们对过量摄入钙这一问题比从前更加警惕。 
　　大型政府研究项目“妇女健康倡议”(Women's Health Initiative)2006年的一份报告显示，每日服用1,000毫克钙补充剂的女性患肾结石的风险比服用安慰剂的女性高17%。不过在此项研究中，两个组别的实验对象都被允许按照她们的日常饮食习惯来吃东西，钙补充剂也是自行服用，因此她们摄入的钙的总量是未知的。 
　　为期12年的“护士健康研究”(Nurses' Health Study)对九万名女性进行了研究，研究发现，从饮食中摄取大量钙的女性患肾结石的风险要低于钙摄入量较少的女性。 
揭示钙摄入量与心脏病风险之间关联的研究也令人感到有些困惑。《心脏医学期刊》(Heart)上月刊登了一项针对24,000名德国人的研究，研究发现，每日摄入约820毫克钙的人患心脏病的风险比摄入量少得多的人低31%。但仅从补充剂中摄取钙的人患心脏病的风险则比不服用任何补充剂的人高出一倍以上。 
2010年，新西兰的研究人员对涉及12,000名试验对象的11项临床试验进行了分析，发现服用钙补充剂的人患心脏病的风险比不服用钙补充剂的人高30%。然而，相关研究未将服用维生素D的人包括在内，一些研究人员认为，维生素D可保护心脏。
　　钙补充剂究竟是怎么导致心脏病呢？这个问题令心脏病学家疑惑不解。纽约大学朗格尼医学中心(New York University Langone Medical Center)琼•H•蒂施女性健康中心(Joan H. Tisch Center for Women's Health)的医学主任戈德堡(Nieca Goldberg)说，“没有人发现血液中的钙与动脉血管钙化有什么联系。”不过，她说她现在会敦促患者通过膳食而非补充剂来摄取推荐量的钙，以避免潜在风险。
　　骨质疏松症专家也敦促患者不要摄取超过推荐量的钙。道森休斯博士说，“女性肯定不应该再一天服用两大片钙补充剂了。”即使相关风险我们还不清楚，但摄入超过身体吸收能力的钙对骨骼没有好处，她还说，“所以不值得去冒险──不管是真正的、想象的、不确定的、有一定可能的还是可能性很大的风险。”