極限長跑與環法自行車賽的共同點:生酮飲食
What do some extreme distance runners have in common with the athletes at the Tour de France cycle race? Suddenly, everyone seems to be talking about adopting a high-fat diet.
一些極限長跑運動員與環法自行車賽(Tour de France)的運動員有什麼共同點呢？突然之間，似乎所有人都在談論高脂飲食。
The idea behind these diets is that when humans enter starvation mode, their bodies are able to switch from burning glycogen, the sugars produced by eating a meal including carbohydrates, to burning stored body fat which is released into the bloodstream as ketones. These extremely low carbohydrate diets are called ketogenic.
選擇高脂飲食背後的想法是，當人進入飢餓狀態時，身體可以從燃燒糖元（食用包含碳水化合物的食物後產生的糖類）轉變爲燃燒體內儲存的脂肪，分解的酮體進入血液中。這些碳水化合物含量極低的飲食就被稱爲生酮飲食。
The rationale is that there is only about 40 minutes of glycogen stored in leg muscles, which is why runners have to keep gulping down sweets during marathons or they end up “hitting the wall” two-thirds of the way through the race.
其原理是大腿肌肉中儲存的糖元僅夠維持40分鐘左右，這就是爲什麼運動員會在跑馬拉松期間大量攝入糖分，或者在跑完三分之二路程時就體力不支了。
But even lean athletes have several pounds of stored fat reserves, so if those could be tapped, you could cycle for hours without running out of fuel. So some Tour de France cyclists have started using this approach.
然而，即便體形瘦削的運動員也有幾磅的脂肪儲備，如果能夠利用這些儲備，你就可以連續幾個小時騎車，而不用擔心耗盡能量。因此一些環法自行車賽的騎手們已開始採用這種飲食方法了。
What’s tempting more athletes to try ketogenic diets is recent research that casts doubt on the link between saturated fat in the diet and heart disease. But I asked Peter Attia, a medical doctor who practises in New York and southern California, and who also happens to be a cycling enthusiast, his view on the persistent medical concerns about fat.
最近的一項研究吸引了更多運動員嘗試生酮飲食，該研究對飲食中的飽和脂肪與心臟病之間的關係提出質疑。我向在紐約和南加州執業的醫學博士彼得•阿提亞(Peter Attia)問到他對於脂肪這個長期醫學問題的看法。他還是一名騎行愛好者。
Dr Attia, who spent three years on a ketogenic diet, has written about his experiences on his blog, where he gives a scientist’s explanation of how the diet works.
阿提亞堅持過3年的生酮飲食，他在博客中寫下了自己的經驗，從科學家角度給出了有關這種飲食如何發揮作用的解釋。
He says it made him “far more metabolically flexible and efficient” as a cyclist. So would he recommend that his patients go on a ketogenic diet, which usually means eating very few carbohydrates? He would in about only a third of cases.
他表示，作爲一名騎行者，這種飲食讓他“的新陳代謝更靈活和更高效”。那麼他會建議他的患者採用生酮飲食嗎？這通常意味着攝入非常少的碳水化合物。他會建議大約三分之一的患者這麼做。
His point is that there are no one-size-fits-all diets out there. What works for one person may not work for others. Some people perform better by eating more fats, while others actually do better in endurance by eating a lot of nonprocessed carbs such as vegetables. And a lot of an individual’s response to nutrition is determined by genetic make-up.
他的觀點是沒有一種適合所有人的飲食。對某一個人有益的飲食可能不適用於其他人。一些人在攝入更多脂肪後表現會更佳，還有一些人在攝入大量未經加工的碳水化合物（例如蔬菜）後會擁有更強的耐力。個人對於營養的迴應很多取決於基因組成。
Dr Attia suggests that before going ketogenic, athletes work with a sport-orientated physician to determine if such a diet would be beneficial or even safe. You need to get blood tests before and during the diet to make sure you are not experiencing a sudden increase in small-sized low-density lipoprotein (LDL) particles, or triglycerides, which are known contributors to heart disease.
阿提亞建議，在採用生酮飲食之前，運動員應與運動醫生合作，確定這種飲食是否有益甚至安全。你需要在餐前和用餐期間查血，以確保你的小型低密度脂蛋白(LDL)顆粒或甘油三酯沒有突然升高。人們認爲，甘油三酯會引發心臟病。
He also points out that, while it might be low in carbohydrates, the ketogenic diet is not actually a high protein diet — too much animal protein can cause a rise in IGF-1, which controls growth hormone and, at high levels, has been associated with cancer. Healthy fats can be obtained through foods such as coconut oil, avocados and nuts but many prefer to pile on the bacon and butter.
他還指出，生酮飲食的碳水化合物含量很少，但實際上並非高蛋白飲食，太多的動物蛋白會導致類胰島素一號增長因子(IGF-1)升高，它控制着生長激素，如果它處於高水平，會與癌症產生關聯。健康脂肪可以通過椰子油、牛油果和堅果等食物獲得，但很多更喜歡堆積在燻肉和黃油上。
Other concerns about low-carb diets include the effect these regimes have on gut bacteria, which mainly ferment undigested starches such as vegetable fibre and produce short-chain fatty acids that help protect your gut. Because these diets are low in fibre, athletes have reported issues with constipation.
其他對於低碳水化合物飲食的擔憂包括這類飲食對於腸道細菌的影響，腸道細菌主要是把植物纖維等未消化的澱粉發酵，併產生有利於保護腸道的短鏈脂肪酸。這類飲食纖維含量很低，因此運動員報告過有便祕問題。
Ketogenic diets are probably best for short-term periods of training only and should not be considered a lifestyle choice. The long-term effects have not been assessed.
生酮飲食或許最適用於短期訓練，而不應被視爲一種生活方式選擇。其長期效果尚未得到評估。
