■跑一次全馬的消耗量其實是RMR的15.6倍。 資料圖片【原文】人類耐力（endurance）有沒有極限？美國杜克大學（Duke University）一項研究發現，無論是普通人抑或精英運動員，人類運動時的能量消耗量（energy expenditure）長遠而言都不會超過休息代謝率（RMR，resting metabolic rate，即休息時維持身體功能所需的能量）的2.5倍，以一般成年人計，即每日最多4,000卡路里。研究又指出，人類耐力極限其實是受制於消化系統（digestive system），而非心肺或肌肉功能。
研究刊登於美國科學期刊《Science Advances》，研究人員分析了「橫越美國」（Race Across the USA）馬拉松參加者（marathoner）的數據。選手需要於140日內由加州（California）跑到華盛頓（Washington, D.C.），全程4,957公里，平均每周要跑6個全馬。研究人員記錄選手們在極限下的卡路里消耗量，發現選手初期卡路里消耗量雖然很高，但隨日數增加，每日耗量會持續下跌，最終降至RMR的2.5倍，因此認為這就是人類耐力的極限。
研究人員進一步分析其他運動的消耗量，發現很多所謂挑戰耐力的運動（endurance sport），其實都未達到人體極限。例如跑一次全馬的消耗量其實是RMR的15.6倍，環法單車賽（Tour de France）選手連踩23日的消耗量也只是4.9倍，徒步南極（Antarctic）95日的消耗則為3.5倍，顯示當運動時間愈長，卡路里消耗量會愈接近2.5倍的極限水平。
研究人員相信，耐力極限主要和人體的消化系統有關，與心肺或肌肉功能無關。報告指出，持續消耗大量能量時，人體不能夠一直消化（digest）、吸收（absorb）及產生足夠的卡路里和營養（nutrient），而只能透過燃燒脂肪或肌肉進行補充，但在到達極限後，人體就會重新平衡（rebalance）能量的消耗。杜克大學教授蓬策爾（Herman Pontzer）認為，研究有助提升運動員表現。
研究亦發現，在懷孕（pregnancy）期間，婦女每日能量消耗量最高為RMR的2.2倍，相當接近極限，意味耐力極限或許不單適用於運動，亦可能適用於人體其他功能，例如胎兒大小可能便受制於母體的RMR。
Limit of human endurance: burning 4,000 calories per day
【譯文】Is there a limit to human endurance? A study by Duke University of the United States suggested that no matter for ordinary people or elite athletes, the upper limit of energy expenditure is 2.5 times the body's resting metabolic rate （RMR）, or for an average person, burning 4,000 calories a day. The research also found that a major limiting factor for human endurance in fact lies in the digestive system, but not the cardiopulmonary or muscular function.
The results appeared in the American scientific journal Science Advances. The research team analyzed the data of the marathoners participating in the Race Across the USA, who would need to finish a 4,957-kilometer race from California to Washington, D.C. within 140 days, equivalent to six marathons a week on average. The researchers recorded the calorie consumption of the players and discovered that their energy expenditure started out relatively high, but eventually dropped and flattened out at 2.5 times their RMR as time passed by, which is considered the maximum possible limit of human endurance.
The researchers further studied the energy consumption of a wide variety of sports and found that many so-called endurance sports had actually yet to make athletes reach their maximum level of exertion; for example, people may expend energy up to 15.6 times their RMR for a single marathon, 4.9 times for the 23 days of the Tour de France, and 3.5 times in a 95-day trek across the Antarctic. It is proved statistically that the longer the event, the closer it is to the limit of energy expenditure.
The research team believed that the endurance limit is mainly attributable to the human digestive system, rather than anything to do with the heart, lungs or muscles. The report indicated that when a large amount of energy is consumed, the human body cannot digest, absorb and produce enough calories and nutrients but to use up its own resources by burning through fat or muscle mass to sustain a higher level of energy use; but after reaching the limit of human exhaustion, the body will have to rebalance its energy expenditure. Professor Herman Pontzer of Duke University said the findings might eventually help boost the performance of athletes.
It was also discovered that the energy consumption of a pregnant woman peaks at 2.2 times their RMR, which is very close to the limit, showing that physiological limit is a factor constraining not only the breakthrough of athletes but also other functions of the human body, for instance, how big babies can grow in the womb.■龐嘉儀
Q&A
1. 研究顯示人類的潛能已被激發了多少個百分比？
2. 研究認為人類競技水平已逼近生理極限，哪一年後再創造世界紀錄的可能性甚微？
3. 哪個競技項目最能顯示人類破紀錄的速度正在放緩？
4. 極限運動在全球哪個國家最受歡迎？
5. 承上題，該國佔全球參與極限運動人口的多少個百分比？
Answer
1. 99%
2. 2060年
3. 男子100米項目（Men's 100 metres）
4. 美國
5. 6%