北京协和医院刘燕萍:补叶酸莫过量

2016-11-18 张思玮 科学网

  “目前,叶酸缺乏的问题已经引起了大家足够的重视,但也不要盲目补充,否则叶酸过量了也会对身体有害。”北京协和医院营养科副主任医师刘燕萍在接受《中国科学报》记者采访时说。 叶酸是一种水溶性的B族维生素。几十年来,孕妇和准备受孕的妇女都被建议提前服用叶酸,以降低新生儿的某些出生缺陷风险,尤其可以在某种程度预防婴儿的大脑和脊柱等重大出生缺陷。 业内


 

“目前,叶酸缺乏的问题已经引起了大家足够的重视,但也不要盲目补充,否则叶酸过量了也会对身体有害。”北京协和医院营养科副主任医师刘燕萍在接受《中国科学报》记者采访时说。

叶酸是一种水溶性的B族维生素。几十年来,孕妇和准备受孕的妇女都被建议提前服用叶酸,以降低新生儿的某些出生缺陷风险,尤其可以在某种程度预防婴儿的大脑和脊柱等重大出生缺陷。

业内通常认为,叶酸作为水溶性维生素在体内无法储存,每天都需要补充,即使天天过量食用也无大碍。

不过,这一观点在刘燕萍看来有些站不住脚。“血清叶酸水平受进食的影响显著,而红细胞叶酸则能反映一段时间内的叶酸营养状况,这说明叶酸是可以在细胞内蓄积的,至少在红细胞内是可以的。如果一段时间内过量补充叶酸会造成红细胞叶酸浓度过高。”

那么,到底叶酸补充多少量合适呢?根据最新的《中国妇幼人群膳食指南》(2016)建议,备孕期即应开始补充叶酸,整个孕期应特别注意避免叶酸的缺乏。除了每天选择摄入叶酸含量丰富的食物以外,还推荐了含叶酸400ugDFE/d的制剂来保底。

“一个国家的膳食指南,是综合全民的营养现况给出的意见,具体到个体,则未必合适。”刘燕萍说,过多的叶酸摄入会干扰锌的代谢,引起锌的缺乏;还会增加某些肿瘤发生风险;还可能掩盖血液中维生素B12缺乏的症状,影响贫血的诊断和治疗。

以《中国妇幼人群膳食指南》中推荐的含叶酸400ugDFE/d制剂来说,制剂合成叶酸不同于自然食物来源的还原型叶酸,它属于氧化型叶酸,具有稳定和吸收度高的优势,它的吸收率是天然叶酸的1.7倍。

“所以,我们在计算叶酸当量(DFE)时,制剂叶酸的剂量应乘以1.7。假如我们成年人每天应该摄入400ug叶酸,选择服用维生素药片的话,则只需要含有235ug叶酸制剂就足够了。”刘燕萍说。

此外,《中国妇幼人群膳食指南》还指出,夫妻双方怀孕三个月前应该补充叶酸避免缺乏,推荐量为400ugDFE/d。

但问题是,谁能准确预测三个月后怀孕?“这就会导致夫妻双方只要萌生要孩子的念头,就开始补充叶酸制剂,如果再加上膳食来源的叶酸,实际获得的叶酸量则超得更多。”刘燕萍告诉记者,《中国居民膳食营养素推荐量标准》中明确注明,叶酸的可耐受最高量是每天1000ug/d。

其实,一个合理饮食的人,从膳食中是能够获得充足的叶酸。但也有研究发现,我国南方人比北方人体内叶酸水平差不多高两倍,其原因主要在于南方四季出产新鲜的绿叶菜,而在北方漫长的冬季则多以白菜、土豆和萝卜为主。

不过,除了膳食的差别,人与人在补充叶酸上的个体差异还可能与基因有关。

研究发现,亚甲基四氢叶酸还原酶(MTFHR)的基因型不同,会造成叶酸代谢的巨大差异。MTFHR为CC型的人体内叶酸活化的能力正常,而CT或TT型的人则有不同程度的缺陷,后两者往往需要更多的叶酸摄入才能满足机体的需要。

采访中,刘燕萍给记者打了个比方:如果一个南方女孩儿,叶酸代谢能力正常,平时爱吃蔬菜和肉类,偶尔还会吃点炒肝或卤煮,她可能根本就不缺叶酸。但为了备孕,她仍然选择“海淘”维生素来加强营养,这就会导致叶酸过量。

那么,人们如何才能知道需不需要补充叶酸呢?刘燕萍建议,应该去营养科门诊咨询专业的医生。“通常医生会先做个营养摄入的评估,同时进行一次空腹抽血,查一查是否贫血,还有血清叶酸和红细胞叶酸的水平,必要时还可以做MTFHR的检测,综合上述结果医生才会制定一个合理的膳食方案。”

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