死亡率高近1倍

〔編譯管淑平／綜合報導〕喝牛奶向來被認為能強化骨骼，不過一份29日在英國醫學期刊（BMJ）發表的瑞典研究指出，牛奶喝太多可能無助減少骨折，反而「或許與較高的死亡率有關係」。研究顯示，婦女1天喝3杯以上、680毫升牛奶，死亡率是1天喝不到1杯者的將近2倍。

這份研究分析6萬1000名女性、4萬5000名男性的牛奶、發酵乳和乳酪食用頻率，分析牛奶飲用量與死亡率、骨折率之間的關係。女性平均追蹤約20年，男性追蹤11年。結果發現，牛奶飲用量與整體死亡率和骨折率、尤其是髖部骨折呈現正相關。

1天喝3杯以上、平均680毫升牛奶的女性，在追蹤期結束時死亡比率是每天喝不到1杯牛奶、約60毫升女性的將近2倍；發生骨折機率也隨著牛奶飲用量增加而穩定上升，前者髖部骨折風險是後者的1.6倍。男性也有類似現象，但比較不明顯；男性1天喝3杯以上（平均830毫升）死亡比率是喝不到1杯（平均50毫升）的1.1倍。

報告主要作者、烏普薩拉大學教授麥可森（Karl Michaelsson）說：「全脂、低脂和脫脂，各種形式牛奶都呈現這種較高風險死亡率現象，從每天喝2杯者開始出現。」

研究人員揣測，這種現象可能與D-半乳糖（D-galactose）有關，過去動物研究發現這種醣類會加速老化、縮短壽命。麥可森說，半乳糖或許導致氧化壓力和輕度發炎反應，影響死亡率和骨折機率。牛奶中含高量D-半乳糖，但乳酪不含。這份研究發現，飲用優格等發酵乳多的人骨折率較低。另外，每天喝半杯到1杯低飲用量族群，髖部骨折風險比完全不喝者「稍微降低」；但死亡率未呈現這種關係。

部分專家不認同

但報告提醒，不應就此解讀攝取高量牛奶會造成骨折，還需要更進一步研究才能下定論。一些專家在評論時強調，該研究有可能顯現出逆因果關係，也就是病人因骨質疏鬆而大量攝取牛奶，結果死亡後卻把原因歸咎於牛奶。

http://news.ltn.com.tw/news/world/paper/825851

Milk intake and risk of mortality and fractures in women and men: cohort studies

Abstract

Objective To examine whether high milk consumption is associated with mortality and fractures in women and men.

Design Cohort studies.

Setting Three counties in central Sweden.

Participants Two large Swedish cohorts, one with 61 433 women (39-74 years at baseline 1987-90) and one with 45 339 men (45-79 years at baseline 1997), were administered food frequency questionnaires. The women responded to a second food frequency questionnaire in 1997.

Main outcome measure Multivariable survival models were applied to determine the association between milk consumption and time to mortality or fracture.

Results During a mean follow-up of 20.1 years, 15 541 women died and 17 252 had a fracture, of whom 4259 had a hip fracture. In the male cohort with a mean follow-up of 11.2 years, 10 112 men died and 5066 had a fracture, with 1166 hip fracture cases. In women the adjusted mortality hazard ratio for three or more glasses of milk a day compared with less than one glass a day was 1.93 (95% confidence interval 1.80 to 2.06). For every glass of milk, the adjusted hazard ratio of all cause mortality was 1.15 (1.13 to 1.17) in women and 1.03 (1.01 to 1.04) in men. For every glass of milk in women no reduction was observed in fracture risk with higher milk consumption for any fracture (1.02, 1.00 to 1.04) or for hip fracture (1.09, 1.05 to 1.13). The corresponding adjusted hazard ratios in men were 1.01 (0.99 to 1.03) and 1.03 (0.99 to 1.07). In subsamples of two additional cohorts, one in males and one in females, a positive association was seen between milk intake and both urine 8-iso-PGF2α (a biomarker of oxidative stress) and serum interleukin 6 (a main inflammatory biomarker).

Conclusions High milk intake was associated with higher mortality in one cohort of women and in another cohort of men, and with higher fracture incidence in women. Given the observational study designs with the inherent possibility of residual confounding and reverse causation phenomena, a cautious interpretation of the results is recommended.

http://www.bmj.com/content/349/bmj.g6015