隨著年齡增長,人體逐漸虛弱、認知能力下降、慢性病發病率增加。身體老化會令長者生活方式改變,如飲食質量下降、體力活動減少、藥物治療和手術等,都對長者的免疫系統、腸道微生物平衡以至整體健康有相當大的影響。
前文曾提及細胞老化過程中,NAD+減少的影響。而長者常見的免疫老化(immunosenescence)和慢性炎症(chronic inflammation),則對整體健康,如對抗病原,以至代謝調控及肌肉量等,都有影響。日本有研究分析四十多名平均過百歲人瑞的免疫細胞,並與中年人士作比較。結果發現人瑞們的免疫細胞,如NK自然殺手細胞(Natural Killer Cells)及其伴隨的自然殺手T細胞(NKT cells),數目都較多,免疫細胞的吞噬能力都較中年人士佳。團隊指出這或能幫助身體清除老舊細胞,從而防止老舊細胞不斷刺激免疫系統、導致慢性炎症並促進老化。
人體超過七成的免疫細胞都源自腸道,因此腸道健康就是長壽之道。意大利和四川都有研究人瑞的腸道菌組成,發現他們腸道菌多樣性比常人高,部分益菌如Akkermansia和Bifidobacterium 都比較多。亦有不少研究指出慢性炎症能增加炎症細胞因子如IL-6及IL-8,並與能製造有益短鏈脂肪酸的毛螺菌(Lachnospiracese)數目減少、與腸炎有關的丹毒絲菌(Erysipelotrichaceae) 增加等都有關聯。
益生菌護腸利免疫 助腸道菌平衡
益生菌除了能平衡腸道菌、維持長者腸道健康外,亦對免疫系統有所助益。例如益纖菌中的雙歧桿菌HN019和嗜酸乳桿菌NCFM,都有助長者提升免疫系統、延緩免疫老化等功效。
食用益生菌可增加腸道益菌數量。新西蘭有研究把80位健康長者分為四組,進食不同分量HN019和一組對照組。四星期後,服用益生菌長者糞便樣本中的益菌如雙歧桿菌(Bifidobacterium)和乳桿菌(Lactobacillus)數量在第二和第四星期都有顯著增加。就算停止食用兩星期,與對照組相比,曾服用HN019的長者雙歧桿菌和乳桿菌數量仍然比較多。
加拿大有研究邀請25名長者,每天分別飲用加有HN019的奶粉及純奶粉。六週後發現飲用HN019奶粉的長者,血液中負責清除老舊細胞、病菌等有害物質的巨噬白血球(PMN)細胞數目有明顯增長。而這些白血球消滅外來細菌的能力亦有所提升。台灣亦有相似研究發現,HN019能提升巨噬白血球及自然殺手淋巴細胞(Natural Killer Cell)的活性。
除了巨噬白血球和淋巴細胞外,其他免疫細胞亦可因食用 HN019 而得到促進。新西蘭有研究發現30位年齡介乎63到84歲的健康長者,經過3星期服用雙歧桿菌HN019後,免疫細胞CD3+和CD4+ T細胞和自然殺手細胞CD56+都有明顯上升。雖然停止服用益生菌3星期後免疫細胞數量有所下降,但仍比未服用前高。再有後續硏究發現,HN019除能提升免疫細胞外,更能有效提升七十歲以上受試者的免疫系統抗癌能力(Tumoricidal activity) 。
年紀越來越大,少不免會有病痛。芬蘭有七成75歲以上長者需服用最少一種非類固醇抗發炎藥或止痛藥。然而,這些非類固醇抗發炎藥對腸道細菌帶來負面影響,特別是年長人士的腸道微生態。有研究邀請了47位70歲以上服用非類固醇抗發炎藥的長者參與研究,其中24位長者被安排在兩星期內,每天服用含嗜酸乳桿菌 NCFM和乳糖醇(lactitol)的共生質。從腸道微生物數量見到,服用了共生質的長者雙歧桿菌有所增加,而且與健康成年人的數量相約。前列腺素E2(PGE2)在正常的腸道功能上擔當著重要的角色,如保護細胞、對抗發炎藥引起腸道傷害等。可是,一般長者的PGE2水平會較年輕成年人低。在這研究中,長者服用兩星期益生菌後,PGE2反而有所增加,從而保護長者腸道細胞。因此服用抗發炎藥物的長者,尤其需要補充益生菌。
益生菌輔助免疫 擊退病源防感染
益生菌對免疫的好處不止於改善長者的先天免疫(Innate immunity)如NK細胞及T細胞,它亦能直接影響造成感染的病原體,協助免疫系統抵禦入侵。
艱難梭菌(Clostridium difficile)是在人類腸道中能引致系統性感染的細菌,能導致腹瀉、發燒、甚至能併發結腸炎以至敗血症而影響生命。有數據指出65歲以上長者受艱難梭菌感染機會比年青人高十倍。芬蘭有研究給長者食用加入了嗜酸乳桿菌 NCFM和HN001的芝士四週,發現在測試前腸道已有艱難梭菌的長者服用益生菌芝士後,艱難梭菌的數目有明顯減少,而且效果仍然持續至測試後。除益生菌外,益生元如益纖菌中的聚葡萄糖(Polydextrose)對預防艱難梭菌感染亦有幫助。有模擬大腸實驗將艱難梭菌與人類腸道菌一同培植,而加入聚葡萄糖後,艱難梭菌的數目有顯著下降,製造短鏈脂肪酸的毛螺菌則有所增加。
除了直接預防感染外,益生菌如HN019能亦協助免疫系統產生更多的抗體去抵禦病源入侵。有研究餵食小鼠含HN019益生菌的奶粉10天,再以霍亂毒素(Cholera toxin)刺激其免疫系統以產生抗體。結果發現食用HN019小鼠血液中的抗體量明顯比沒服食益生菌的高兩倍以上。
我們雖然不能阻止歲月流逝,但亦能以益生菌及益生元保持腸道及免疫健康、維持身體基能、繼續享受人生。
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