主編評論：運動能有效減肥但並非靠燃燒脂肪

Editor-in-Chief's Paper: Fat Reducing Effect of Exercise Training is not Mediated by Fat Burning

10.5297/ser.1702.editorial

郭家驊(Chia-Hua Kuo)

大專體育學刊

17卷2期（2015 / 06 / 30）

i - iv

繁體中文;英文

除了水以外，構成人體最主要的成分是以碳元素為骨幹的各類分子（醣類、脂肪、蛋白質、DNA、RNA……）。從癌症的臨床與動物研究可以經常觀察到當腫瘤高速成長時，身體的肌肉與脂肪組織快速萎縮。這個結果給予我們兩個重要啟示：一、全身碳資源可快速重新分配；二、成長中的組織可攫取其他組織的碳資源，重塑體型。脂肪為人體最主要 碳資源的供應庫，除了作為能源也作為建構細胞的材料。因此這說明了為何成長中的青少年比起食慾與飲食量已下降的中年人其腹腰囤積脂肪的情況較少。雖然肌肉收縮過程消耗了許多含碳資源與結構破壞，運動反而促進肌肉更新與成長，挑戰愈強效果愈好，脂肪組織相對萎縮愈快，運動改變身體組成的現象成為全身碳資源可重新分配的另一個明證。張世沛教授於將大專體育學刊發表的研究證據顯示，肌肉損傷／再生指標增加愈多的運動選手，體脂肪減少的效果愈好。在他的研究中柔道選手經過數星期的訓練，血液中磷酸激酶 (creatine kinase) 濃度愈高者，體脂肪減少量愈多。無氧運動與阻力訓練，涉及快速的肌肉收縮與較高的物理性挑戰，所造成的肌肉損傷效果通常較有氧運動或走路多。這類運動反而可誘發肌肉加速成長，肌肉的成長使飯後的含碳資源回補到肌肉組織較多、返回脂肪組織儲存減少，進而造成比較快的塑身效果。運動使全身碳資源重新分配的觀念，相較於運動靠燃燒脂肪來減肥的說法更能解釋運動改善身材的實際效果。運動主要依賴醣類做為能源，燃燒脂肪來減肥的說法，無法完美解釋運動減肥的結果，在應用上也產生一些矛盾。事實上，脂肪酸為安靜狀態支撐身體運 作的主要能源，由於運動強度愈低，能源使用愈依賴脂肪 (Romijn et al., 1993)，因此不少專家推薦強度較低的有氧運動與走路來減肥。但相對於這兩類慢速活動，安靜休息反而依賴脂肪燃燒的比例更高，使得這種追求低強度運動訓練的說法難以自圓其說。幾年前的研究證實，每星期 3 天的高強度間歇型運動 (high intensity intermittent exercise, HIIE) 可有效減少體脂肪，消耗相同熱量的連續性有氧運動 (steady state exercise) 反而效果比較差 (Trapp, Chisholm, Freund, & Boutcher, 2008)。如果我們的觀念停留在運動靠燃燒脂肪來減肥，將造 成體育老師或健身教練傾向給予較為低強度的無效運動處方。運動靠燃燒脂肪減肥的觀念最大的致命傷在於科學證據大多無法支持。實際上運動訓練者當天 24 小時內的總熱量燃燒較高主要歸咎於醣類燃燒的增加，脂肪燃燒並不會增加 (Melanson, Gozansky, Barry, MacLean, Grunwald, & Hill, 2009; Melanson, Sharp, Seagle, Donahoo et al., 2002)，無論阻力訓練與有氧訓練均無法促進運動後 24 小時內的脂肪燃燒 (Melanson, Sharp, Seagle, Horton et al., 2002)。因此，運動減肥的實際效果，無法從運動時與運動後燃燒脂肪的角度來解釋。運動靠燃燒脂肪來減肥的說法，面臨另外一個挑戰是低氧反而更能有效的幫助減肥。氧氣稀薄顯然不利燃燒脂肪，但實際上卻可產生極快的減脂與增肌的效果。我們與新加坡共同進行的研究發現 10 位青少年游泳選手在每天 12 公里的固定訓練目標下，移地到高地訓練中心三星期雖體重沒有改變，選手以DEXA 測量體脂肪均一致減少，無一例外，肌肉質量也小幅增加。這個現象似乎與低氧促進飯後養分與胰島分配到肌肉有關(Chia et al., 2013)。由於身體脂肪組織平時即不斷釋放出脂肪酸到血液中讓肌肉使用(Coppack et al., 1990)，飯後小腸吸收的養分與胰島素透過血液回補到脂肪組織的量是否平衡，才是決定腹腰部脂肪細胞是否肥大的最終關鍵。針對快速增肌減脂，專業建議如下：排除燃燒脂肪的舊觀念，提高運動強度，儘量動員較多的肌纖維與肌群，使飯後含碳資源與胰島素分配進入肌肉組織增加，藉以儘量減少飯後能源回補脂肪組織的情形。研究證實用餐時間遠離運動訓練時間將使運動減脂效果幾乎歸零(Trapp et al., 2008)，一天的主餐時間離運動愈近愈好，在身體能自然承受的範圍內，運動前或運動後立即用餐均可。其他以外時間必須嚴格控制熱量攝取，甚至減少，成年人如無飢餓感時可不必強迫用餐。

Abdominal fat-reducing effect of exercise training has been commonly reported. Since exercise appears as an energy consuming behavior, it is intuitively thought that the outcome of abdominal fat decrease by exercise training is mediated by what has been commonly called "fat-burning effect." Thus aerobic training at moderate intensity is widely recommended for obesity management. Such interpretation raises several controversies. First, if fat burning contributes size reduction of fat-storage tissues, skeletal muscle should be the first tissue showing size reduction due to increased breakdown of carbon-based fuel. However, exercise training increases muscle mass with enhanced intramuscular triglyceride storage. In a contrary, exercise training decreases abdominal fat storage despite fat cell contains very few mitochondria that are required for fat burning during exercise. Second, increasing exercise intensity results in energy reliance on carbohydrate than fat. Yet, anaerobic-based high-intensity intermittent exercise has better fat-reducing effect than aerobic training at moderate intensity with the same energy consumed. Furthermore, the 24-h fat utilization is not altered by exercise at either low or high intensity compared to resting condition. It is therefore unjustified of saying that fat burning contributes to the actual fat reducing effect of exercise training. Third, oxygen is required for fat burning. However, lowering oxygen availability by chronic hypoxia exposure has been consistently shown to decrease body fat in humans and animals. A significant fat reduction without weight changes occurs when swimmers switched their training site from sea-level to altitude at the same daily training volume. Oxygen deficit due to systemic hypoxia (16%) increases postprandial blood distribution to skeletal muscle, suggesting that increased deliveries of insulin and ingested carbon source towards skeletal muscle against adipose tissue might be the major determinant of body composition in humans. Conclusion: Exercise training increases energy demand of skeletal muscle against adipose tissue resulting favorable fuel distribution towards muscle than adipose tissue, which may be a better explanation for the abdominal fat reducing effect of exercise training. This concept can better explain the fact that anaerobic training produces better fat reducing benefit than aerobic training with similar energy expenditure.

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