作者：Susan Jeffrey
出處：WebMD醫學新聞
November 24, 2009 — 一篇新的後設分析證實，攝取高鹽量與中風和整體心血管疾病(cardiovascular disease，CVD)風險增加有關。
研究者報告指出，彙整的相關風險顯示，平均每天鹽量攝取增加5g，中風風險增加23%、CVD風險增加17%。多數西方國家的每天習慣用鹽量為10g，是目前世界衛生組織建議量的2倍。
第一作者、義大利拿波里大學醫學院、"Federico II" ESH高血壓精進中心的Pasquale Strazzullo醫師等研究者結論表示，據估計，中風的案例致命率約為三分之一、心血管疾病為五分之一，減少鹽攝取量可降低23%的中風比率、17%整體心血管疾病比率，每年可減少125萬人死於中風，和減少約300萬人死於心血管疾病。
他們指出，此外，因為在這些世代研究中有些鹽量攝取測量不精確，實際影響有可能低估。
這項研究線上發表於11月25日的英國醫學期刊(British Medical Journal)。
【地球的鹽】
作者們寫道，過去一世紀中，與攝取高鹽量有關之風險的證據令人注目。例如，已經由多種研究，包括隨機試驗，證實攝取鹽和血壓的關聯。他們寫道，之前的研究指出，每天減少攝取約6g鹽量，代表高血壓患者的血壓可降低7/4-mm Hg、血壓正常者可降低4/2-mm Hg，這種變化預期可減少24%中風風險，以及減少18%的冠心病。
研究者指出，缺乏長期隨機試驗，因實務上的困難以及需要長期間也不太可能進行，前瞻性世代研究可以提供間接證據。在本研究中，Strazzullo醫師等人對1966至2008年間發表的前瞻性研究進行了一個系統性回顧和後設分析，檢視各種資料庫的差異。
他們的分析檢視13篇適合之研究的19個世代樣本，包括了177,025研究對象，這些人在3.5到19年的追蹤期間內，有超過11,000件血管事件。至於納入的每個研究，摘錄其相對風險和95%信心區間資料，使用隨機效果模式彙整，也進行異質性、發表偏見、次組分析、總迴歸分析等。
作者們報告指出，鹽攝取量較高與中風風險顯著增加及CVD風險較高(略為顯著)有關；不過，排除一篇離群值的研究之後，與CVD的風險也是顯著。
表、較高鹽攝取量和中風與CVD風險之關聯
事件 | 彙整相對風險 (95% CI) | P 值 |
中風 | 1.23 (1.06 – 1.43) | .007 |
整體 CVD | 1.17 (1.02 – 1.34) | .02 |
CI = 信心區間；CVD =心血管疾病
人類減少鹽攝取量的努力有一些進步，但是多數國家的攝取量依舊高於建議值。作者們寫道，更有效推廣此項公衛政策的障礙之一是，食品業界過去的抵制，其論點是現有之證據未顯示該政策對臨床試驗有意義的療效指標有顯著助益。
在英國，有些食品業者主動和政府及公衛部門合作，減少鹽攝取量到約10%，但是整體數值依舊篇高。他們結論表示，雖然許多政府偏好採用自主方式管理，強制規定有其優勢，是達到公衛目標最有效率、符合成本效益的方法。
【受歡迎的見解】
約翰霍普金斯大學、Welch預防、流行病學與臨床研究中心的Lawrence J. Appel醫師在編輯評論中，形容Strazzullo醫師等人的報告是對醫學文獻有用且受歡迎的見解。
Appel醫師指出，少鹽以降低血壓、降低血壓以減少心血管事件。不過，此一推論的理由仍是間接性的，因為進行此類研究有方法學上的挑戰。不過，他指出，這些間接證據支持少鹽的好處。
觀察型研究之結果的主要問題在於，攝取鹽量的測量、每天鹽攝取量的變化等。Appel醫師寫道，這些問題在目前的研究中也有，鹽量攝取測量的差異和不佳品質，可能是Strazzullo醫師等人分析這些研究之結果異質性的主要原因。
他指出，政策制定者以前放棄前瞻觀察研究的結果，支持考慮更明確的鹽量攝取和血壓關聯的證據。至少，Strazzullo醫師等人的分析可消除減少鹽量可能有害(分析錯誤而錯誤詮釋研究造成之謬傳)的想法。
Appel醫師結論表示，所有人減少鹽量攝取的立場現在更強烈了，減少鹽量攝取不只可降低血壓，還可預防中風和其他心血管疾病等嚴重後遺症。
EC Grant部份資助本研究。作者們與Appel醫師皆宣告沒有相關財務係。
BMJ. 線上發表於2009年11月25日。
High Salt Intake Boosts Stroke, CVD Risk
By Susan Jeffrey
Medscape Medical News
November 24, 2009 — A new meta-analysis confirms that high salt intake is associated with increased risks of stroke and total cardiovascular disease (CVD).
The pooled relative risk showed that an average difference in intake of approximately 5 g/day of salt was associated with a 23% increased risk of stroke, the researchers report, and a 17% increase in CVD risk. The average habitual salt intake in most Western countries is 10 g/day, double the level currently recommended by the World Health Organization.
"Given that the case-fatality rate for stroke is estimated at 1 in 3, and [the rate] for total cardiovascular disease at 1 in 5, a 23% reduction in the rate of stroke and a 17% overall reduction in the rate of cardiovascular disease attributable to a reduction in population salt intake could avert some one and a quarter million deaths from stroke and almost 3 million deaths from cardiovascular disease each year," the researchers, with lead author Pasquale Strazzullo, MD, from "Federico II" ESH Excellence Centre of Hypertension at the University of Naples Medical School, Naples, Italy, conclude.
Moreover, because of some imprecision in salt intake measurements in these cohort studies, the actual effects are likely to be underestimated, they add.
The study is published online November 25 in the British Medical Journal.
Salt of the Earth
During the past century, evidence of the risks associated with high salt intake has become "compelling," the authors write. The link between salt intake and blood pressure, for example, has been shown through a variety of study types, including randomized trials. Previous studies have indicated that a reduction of about 6 g/day of salt intake translates to a 7/4-mm Hg reduction in blood pressure in those with hypertension and a 4/2-mm Hg reduction in normotensive individuals, changes that at a population level would be expected to reduce stroke by about 24% and coronary heart disease by 18%, they write.
In the absence of a long-term randomized trial, which is not likely to ever be performed because of the practical difficulties and long duration that would be required, prospective cohort studies can provide indirect evidence, the researchers note. In this study, Dr. Strazzullo and colleagues performed a systematic review and meta-analysis of prospective studies published between 1966 and 2008, identified from a variety of databases.
Their analysis examined 19 cohort samples in 13 eligible studies, including 177,025 participants who had more than 11,000 vascular events during 3.5 to 19 years of follow-up. For each study included, relative risks and 95% confidence intervals were extracted and pooled using a random-effects model, and heterogeneity, publication bias, subgroup, and meta-regression analyses were carried out.
Higher salt intake was associated with a significantly higher risk of stroke and a higher risk of CVD that was of borderline significance, the authors report. However, when 1 study was excluded as an outlier, the relationship between higher salt intake and CVD was also significant.
Table. Risk of Stroke and CVD Associated With Higher Salt Intake
| Event | Pooled Relative Risk (95% CI) | P Value |
| Stroke | 1.23 (1.06 – 1.43) | .007 |
| Total CVD | 1.17 (1.02 – 1.34) | .02 |
CI = confidence interval; CVD = cardiovascular disease
Efforts to reduce salt intake on a populationwide basis have resulted in some progress, but most countries still have consumption high above recommended levels. "One barrier to a more effective implementation of public health policies has been the historical opposition of the food industry, based on the arguments that the available evidence does not show significant benefits on hard endpoints at a population level," the authors write. "Our study now clearly addresses those doubts."
Voluntary changes by the food industry in cooperation with governments and public health sectors have reduced salt intake by about 10% in the United Kingdom during approximately 4 years, for example, but overall levels remain high. "While the voluntary approach is the preferred choice for many governments, the 'regulatory' approach has advantages, sometimes being the most efficient, effective, and cost-effective way of achieving public health targets," they conclude.
A Welcome Addition
In an editorial accompanying the publication, Lawrence J. Appel, MD, from the Welch Center for Prevention, Epidemiology and Clinical Research at Johns Hopkins University, Baltimore, Maryland, calls the report by Dr. Strazzullo and colleagues, "a useful and welcome addition to the medical literature."
Salt reduction lowers blood pressure, and lowering blood pressure reduces cardiovascular events, Dr. Appel points out. However, this line of reasoning is still largely indirect because of the methodological challenges of conducting such studies. What evidence there is, however, supports the indirect evidence of benefit from salt reduction, he notes.
Results from observational studies have been the most problematic, largely because of problems measuring salt intake and large variations day to day in salt consumption. This problem was also seen in the current study as well, where "the disparate and often poor quality of measurements of dietary salt probably contributed to the significant heterogeneity in the study results seen by Strazzullo and colleagues," Dr. Appel writes.
"Policy makers have previously dismissed the results from prospective observational studies in favour of the considerably more robust body of evidence that links salt intake with blood pressure," he points out. "At a minimum, Strazzullo and colleagues' analyses should dispel any residual belief that salt reduction might be harmful (a canard resulting from misinterpretation of studies, often with flawed analyses).
"The case for population-wide salt reduction is now stronger," Dr. Appel concludes. "A reduced intake of salt not only lowers blood pressure but also prevents its major sequelae — stroke and other cardiovascular diseases."
The study was funded in part by an EC Grant. The authors and Dr. Appel have disclosed no relevant financial relationships.
BMJ. Published online November 25, 2009.
