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作者：Laurie Barclay, MD  
出處：WebMD醫學新聞

　　March 23, 2010 — 根據發表於3月24/31版美國醫學會期刊的一篇前瞻世代研究報告，身體質量指數(BMI)低於25 kg/m2的婦女，身體活動與體重增加較少有關，每天約60分鐘的中等強度體能活動與成功維持正常體重有關。
　　
　　麻州波士頓哈佛醫學院、布萊根婦女醫院I-Min Lee醫師等人寫道，並不清楚預防長期體重增加所需的體能活動量要多少，在2008年，聯邦指引建議每週至少進行150分鐘的中等強度活動(每週每小時為7.5等代謝量/代謝當量[MET])，以獲得實質的健康助益。
　　
　　研究目標是評估遵守一般飲食的婦女中，不同程度的體能活動和長期體重改變的關聯。研究對象包括34,079名健康的美國婦女、平均年紀54.2歲，從1992 -2007年進行前瞻觀察。這些對象在研究開始時、36、72、96、120、144、156個月時報告她們的體能活動以及體重。在這幾個時間點，體能活動情況被分類為每週每小時達到小於7.5 MET、7.5-小於 21 MET、大於等於21 MET。使用重複量數迴歸方式，前瞻檢查平均3年期間的體能活動與體重改變情況。
　　
　　整個研究期間，平均體重增加2.6公斤。相較於每週每小時達大於等於21 MET的婦女，每週每小時達7.5 -小於 21 MET的婦女中，平均3年期間內的平均體重增加為0.11公斤 ± 0.04公斤(P = .003)。同樣的，多變項分析也顯示，相較於每週每小時達大於等於21 MET的婦女，每週每小時小於7.5 MET者的體重增加了0.12公斤 ± 0.04 公斤(P = .002)。
　　
　　體能活動和BMI之間的影響有顯著的互動關係，在BMI小於25 kg/m2的婦女中，體能活動程度和體重增加之間有反向的劑量-反應關係(趨勢P值< .001)。不過，BMI 值為 25- 29.9 kg/m2的婦女並無此關係(趨勢P值 = .56)，BMI 值大於等於30.0 kg/m2的婦女也沒有(趨勢P值 = .50)。
　　
　　研究開始時，在BMI小於25 kg/m2的4,540名婦女(13.3%)中，於研究期間成功維持體重、增加不到2.3公斤者，研究期間的平均體能活動為每週每小時達21.5 MET(約相當於每天有60分鐘的中等強度活動)。
　　
　　研究作者寫道，在攝取一般飲食的婦女中，只有BMI小於25之婦女的體能活動和體重增加較少有關，在13年間成功維持正常體重且增加不到2.3公斤的婦女，在整個研究期間平均幾乎每天有60分鐘的中等強度活動。
　　
　　研究限制包括依據自我報告來確認休閒性體能活動和體重資料，缺乏其他身體組成和可能影響體重之藥物的詳細資料。此外，研究對象並不具美國人口的代表性，並未隨著時間重覆檢測飲食情況，限制熱量攝取的婦女維持體重所需的活動程度也未探討。
　　
　　研究作者結論表示，這些資料認為，2008年版聯邦建議每週運動150分鐘，雖然可以降低慢性疾病風險，但是不足以預防未限制熱量者的體重增加，只有正常體重的婦女其體能活動和體重增加存在有反向關係；較重的婦女中，沒有此一關聯，這強調了控制熱量攝取對其維持體重的重要性。
　　
　　Lee醫師擔任Virgin HealthMiles的顧問，也是其科學委員會委員。國家健康研究中心支持本研究。


Daily Physical Activity Prevents Weight Gain Only in Women With Normal BMI

By Laurie Barclay, MD
Medscape Medical News

March 23, 2010 — Daily physical activity is associated with less weight gain only among women whose body mass index (BMI) is lower than 25 kg/m2, according to the results of a prospective cohort study reported in the March 24/31 issue of the Journal of the American Medical Association. Approximately 60 minutes per day of moderate-intensity physical activity was associated with successfully maintaining normal weight.

"The amount of physical activity needed to prevent long-term weight gain is unclear," write I-Min Lee, MBBS, ScD, from Brigham and Women's Hospital, Harvard Medical School in Boston, Massachusetts, and colleagues. "In 2008, federal guidelines recommended at least 150 minutes per week (7.5 metabolic equivalent [MET] hours per week) of moderate-intensity activity for 'substantial health benefits.'''

The goal of this study was to evaluate the association of different amounts of physical activity with long-term weight changes among women following a usual diet. The study cohort consisted of 34,079 healthy US women, mean age, 54.2 years, who were observed prospectively from 1992 to 2007. Participants reported their physical activity and body weight at baseline and at months 36, 72, 96, 120, 144, and 156. At each time point, physical activity was classified as less than 7.5, 7.5 to less than 21, or 21 or more MET hours per week. Physical activity and weight change at intervals averaging 3 years were prospectively examined with repeated measures regression.

Throughout the study, mean weight gain was 2.6 kg. In women expending from 7.5 to less than 21 MET hours per week, mean weight gain during a mean interval of 3 years was 0.11 kg ± 0.04 kg (P = .003) vs women expending 21 or more MET hours per week. Similarly, multivariate analysis also showed that those expending less than 7.5 MET hours per week gained 0.12 kg ± 0.04 kg (P = .002) vs women expending 21 or more MET hours per week.

There was a significant interaction between the effect of physical activity and BMI. Among women with a BMI of less than 25 kg/m2, there was an inverse dose-response relationship between activity levels and weight gain (P for trend < .001). However, no such relationship occurred among women with a BMI from 25 to 29.9 kg/m2 (P for trend = .56) or in those with a BMI of 30.0 kg/m2 or more (P for trend = .50).

In 4540 women (13.3%) with a BMI of less than 25 kg/m2 at the beginning of the study who successfully maintained their weight by gaining less than 2.3 kg during the study, mean activity level during the study was 21.5 MET hours per week (approximately 60 minutes per day of moderate-intensity activity).

"Among women consuming a usual diet, physical activity was associated with less weight gain only among women whose BMI was lower than 25," the study authors write. "Women successful in maintaining normal weight and gaining fewer than 2.3 kg over 13 years averaged approximately 60 minutes a day of moderate-intensity activity throughout the study."

Limitations of this study include reliance on self-report for determination of recreational physical activity and weight, and lack of detailed information on other measures of body composition and on medications potentially affecting weight. In addition, participants are not representative of the US population, there were no repeated measures of diet with time, and activity levels needed for weight management among women restricting caloric intake were not examined.

"These data suggest that the 2008 federal recommendation for 150 minutes per week, while clearly sufficient to lower the risks of chronic diseases, is insufficient for weight gain prevention absent caloric restriction," the study authors conclude. "Physical activity was inversely related to weight gain only among normal-weight women; among heavier women, there was no relation, emphasizing the importance of controlling caloric intake for weight maintenance in this group."

Dr. Lee has served as a consultant to Virgin HealthMiles and sits on its scientific advisory board. The National Institutes of Health supported this study.

JAMA. 2010;303:1173-1179.