深度:每天究竟该吃多少盐?BMJ细数科学家的共识和分歧
钠与健康
量化钠摄入量
相反地,长期观察性试验往往依赖于食物频率问卷、膳食记录或 24 小时回忆法来评估营养素的摄入量。这些方法很容易出现偏差,很难估计摄入食物的钠含量,特别是在烹饪中或上菜前添加的那些钠。
钠与血压
钠与心血管疾病
BOX:队列研究中关于钠与心血管疾病关系的不同结果的可能解释
随机性变异
偶然变化可能造成不同人群样本之间的不同结果,即便样本来自于同一背景人群也可能出现这一状况。
测量误差
流行病学研究通常使用便宜、实用的方法(例如,现场尿液测量),而非昂贵费劲但更准确的方法(例如,24 小时尿液测量)。这种简单的测量可能会导致个体误差,并降低发现关联性的可能。
最近的一项研究发现25,误差甚至可以改变剂量-反应曲线的形状。如果按系统分布,这种误差可能会导致病人被分到低钠摄入量组,从而错误地将高死亡率归因于低钠摄入量。不过,这项研究存在一定的局限性:虽然采用的是空腹的晨尿,但所用的公式是针对 24 小时的尿液样本。
混杂性
钠总体摄入量的混杂异质性可以解释不同研究之间的一些差异。
在研究中,许多因素都可能影响结果,如性别、年龄、能量摄入、吸烟、血压、社会地位和合并症。调整这些因素可能会减弱35或放大11,36,37钠摄入量与预后效果之间的关系。尽管这些研究进行了校正,但不同研究之间仍可能存在无法解释的差异(混杂性)。
反向因果
当暴露的概率受到所研究结果的因果影响时,就会出现反向因果关系38。
例如,高血压或心血管疾病患者可能被建议降低钠摄入量。这将明显减少他们的摄入量,同时只改变他们心血管风险的某一个方面。而其他疾病患者可能会由于食欲下降造成钠摄入量较低。因此,低钠组的死亡率较高。同样地,高钠摄入组中也有食物摄入量高、患有糖尿病和高血压的肥胖人群。
对脂质和肾素-血管紧张素-醛固酮系统(RAAS)的影响
将钠摄入适度降低到约 2000mg,不会激活交感神经系统或增加血清中的脂质,对肾素-血管紧张素-醛固酮(RAAS)系统只有很小的影响39。然而,将钠摄入量大幅降低到 2000mg 以下的水平,会导致交感神经系统和 RAAS 的显著激活,以及血清脂质的显著增加22。这些结果是否会影响心血管状况和死亡率等健康结果,尚存分歧。
此外,最近的研究表明,钠摄入量可能由神经激素系统来调节以达到生理上的最佳量,而不是生理上的最低值40。这可以解释为什么世界人口的钠摄入量在一个相对狭窄的范围内。
已存在的疾病
患有心脏病和高血压的人通常使用利尿剂或限制 RAAS 的药物来治疗。所有这些治疗方法都可能引发低钠血症,尤其是对于心力衰竭的患者,低钠摄入量可能会导致低钠血症。由于低钠血症会增加死亡率41,所以这种效应可能会造成低钠摄入组观察到的死亡率增加。
结果为何不同?
说得通的解释包括研究研究人群的异质性、测量误差、混杂因素、颠倒的因果关系或是在低水平条件下相反的生物功能(BOX)。
如何解决争议
另外还有一项在约 1000 名心力衰竭患者中进行的 SODIUM_HF 试验(临床试验,政府标识:NCT02012179)。
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