一天吃多少鹽才不會有害健康?國民健康署的建議是一天不超過 6 克,也就是鈉不超過 2400 毫克。不過吃飯的當下要怎麼測量?是否有些不食人間煙火?現在美國一項研究利用一種「鹽湯匙」,可以讓你測出食物中的鹽有多少,不過一天 2400 毫克的說法最近也受到質疑,學者甚至認為,我們現在被認為是吃過量的情況,其實是剛剛好。
鹽湯匙助你避開高鹽食物
美國肯塔基大學護理學院助理教授 Misook Chung 的團隊研發一款「鹽湯匙」,只要把尖端放入熱的液體食物中,就可以快速顯示含鹽量的多寡。她表示,鹽湯匙的成本大約 40 美元,裝一顆電池就能運作。她們利用鹽湯匙測試成效,找來了包括有心臟病患在內的受試者,看看鹽湯匙能幫助他們減少多少鹽的攝取。其中一半受試者有鹽湯匙及詳細的食物含鹽量相關資訊,另外一半則是控制組,僅有普通的食物資訊。
▲鹽湯匙可以告訴你食物是否太鹹了。(圖片來源:Misook L. Chung, R.N., Ph.D.)
結果發現,三個月的實驗期間,每天的鈉攝取量可以從 3,894 毫克降到 3,604 毫克,而且受試者回報說他們都很享受吃少鹽食物,九成受試者說注意到自己更能感受到食物中少量鹽帶來的滋味。實驗中還包括照顧心臟病患者的家人,他們的成績看起來更好:從 4,123 毫克降到 3,380 毫克。
飲食中如果鈉鹽過多,會讓鈉離子在體內吸附過量水份,造成浮腫、血壓升高、增加心、腎負擔;鈉離子太多也會排擠鉀離子,容易產生四肢無力、食慾不振、嘔吐、倦怠、心律不整等心臟疾病。
Chung 表示,不僅是鹽湯匙有助人們注意鹽的攝取,這項計畫本身也就是一種衛教,讓人們瞭解高鹽食物都身體的危害。她們將繼續研究,看這項計畫的長期成果,以及受試者是否能夠一直堅持下去。這項成果發表在美國內科醫學會議。
大型研究顯示 我們現在鹽吃得剛剛好
鹽湯匙是一個好的開始,它可以讓我們知道食物的含鹽量,但目前還沒有辦法測出所有種類食物的鹽份多寡。不過在世界各國衛生單位都在宣導每日限量鹽份攝取時,稍早的一項大型研究卻給學界帶來一場極大的爭議:我們真的吃了太多鹽嗎?多吃一點鹽真的會引發心臟病嗎?
一項包括十萬兩千份樣本、歷時四年的跨國性研究認為,每日如果攝取 3000 至 6000 毫克的鈉,其實有助於降低心血管疾病的風險,例如心臟病、中風與心臟衰竭;而以研究的四年期間來觀察,攝取更多或更少鈉的人,死亡的風險還更高。研究團隊的加拿大麥克馬斯特大學 (McMaster University) 臨床流行病與生物統計學助理教授 Andrew Mente 表示,研究的目的是要發現一個合宜的鈉攝取量標準,而他們發現最佳標準就是中間值,而不是現有的建議量。
他表示,現有的標準有重新思考的必要,因為他們的研究規模夠大,觀察期也夠久,具有足夠代表性。這十萬兩千人的資料來自 17 個國家,年齡介於 35 至 70 歲之間,研究刊登在新英格蘭醫學期刊。當然,Mente 也不忘加上一段話:還需要做更進一步的研究,如果本身已經有高血壓的情形,每天鈉攝取量建議要減少到 5,000 毫克以下,同時人們也應該要注意其他方面,例如多運動,不要吸菸。
目前的統計發現,美國人一天的鈉攝取量約在 3,400 毫克,而台灣光是國、高中男生這個年齡層,一天的鈉攝取量約在 4,899 及 4,962 毫克,居各年齡層之冠。不過若是按照這項研究的結果來看,只是剛好而已。
「適量」是恆久的法則?
不過同一期雜誌另一項看起來比較符合主流看法的研究,是由美國塔夫茨大學 (Tufts University) 與哈佛大學公衛學院合作,透過重新審視數十份研究資料,尋找食鹽攝取與高血壓乃至於心血管疾病之間的關係。他們發現,每天鈉攝取量超過 2,000 毫克,一年會有 165 萬人死亡,其中有半數人每天攝取量超過 4,000 毫克。
雜誌的編輯、阿拉巴馬大學教授 Suzanne Oparil 認為,Mente 的研究缺乏一個「正常」與「較少」鈉攝取量受試者的對照研究,暗示至少就少量攝取方面,是否真會引發疾病是有疑義的。紐約時報的評論認為,Mente 的研究將不同鈉攝取量跟最後的健康結果串連起來,這是它的價值所在,但它並不能證明低攝取量與最後的疾病之間有因果關係。
羅格斯大學 (Rutgers University) 生醫與衛生學院院長 Brian Strom 則支持 Mente 的研究,認為目前並沒有其他研究說明低鈉飲食是有益的,鈉的攝取應該要適中比較好。
各方說法不一,且都有科學證據支持,對一般民眾才是真正的困擾。不過就長久的經驗來看,最喜歡吃的往往就是會造成身體產生某種疾病的原因,而醫生叫你不要吃的往往就是你最喜歡吃的食物,這樣看起來,任何食物都應該要適量,似乎才是對身體最好的原則。其實不僅是飲食,人的一生各方面都是如此。
(首圖來源:© Tim Pannell/Corbis)
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