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.

一些极限长跑运动员与环法自行车赛(Tour de France)的运动员有什么共同点呢？突然之间，似乎所有人都在谈论高脂饮食。

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 carbohydrates², to burning stored body fat which is released into the bloodstream as ketones. These extremely low carbohydrate¹ diets are called ketogenic.

选择高脂饮食背后的想法是，当人进入饥饿状态时，身体可以从燃烧糖元（食用包含碳水化合物的食物后产生的糖类）转变为燃烧体内储存的脂肪，分解的酮体进入血液中。这些碳水化合物含量极低的饮食就被称为生酮饮食。

The rationale is that there is only about 40 minutes of glycogen stored in leg muscles, which is why runners have to keep gulping³ 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储存的糖元仅够维持40分钟左右，这就是为什么运动员会在跑马拉松期间大量摄入糖分，或者在跑完三分之二路程时就体力不支了。

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，即便体形瘦削的运动员也有几磅的脂肪储备，如果能够利用这些储备，你就可以连续几个小时骑车，而不用担心耗尽能量。因此一些环法自行车赛的骑手们已开始采用这种饮食方法了。

What’s tempting⁴ more athletes to try ketogenic diets is recent research that casts doubt on the link between saturated⁵ 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 enthusiast⁶, his view on the persistent⁷ medical concerns about fat.

最近的一项研究吸引了更多运动员尝试生酮饮食，该研究对饮食中的饱和脂肪与心脏病之间的关系提出质疑。我向在纽约和南加州执业的医学博士彼得•阿提亚(Peter Attia)问到他对于脂肪这个长期医学问题的看法。他还是一名骑行爱好者。

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.

阿提亚坚持过3年的生酮饮食，他在博客中写下了自己的经验，从科学家角度给出了有关这种饮食如何发挥作用的解释。

He says it made him “far more metabolically⁸ 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，作为一名骑行者，这种饮食让他“的新陈代谢更灵活和更高效”。那么他会建议他的患者采用生酮饮食吗？这通常意味着摄入非常少的碳水化合物。他会建议大约三分之一的患者这么做。

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 determined⁹ by genetic¹⁰ make-up.

他的观点是没有一种适合所有人的饮食。对某一个人有益的饮食可能不适用于其他人。一些人在摄入更多脂肪后表现会更佳，还有一些人在摄入大量未经加工的碳水化合物（例如蔬菜）后会拥有更强的耐力。个人对于营养的回应很多取决于基因组成。

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.

阿提亚建议，在采用生酮饮食之前，运动员应与运动医生合作，确定这种饮食是否有益甚至安全。你需要在餐前和用餐期间查血，以确保你的小型低密度脂蛋白(LDL)颗粒或甘油三酯没有突然升高。人们认为，甘油三酯会引发心脏病。

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 hormone¹¹ and, at high levels, has been associated with cancer. Healthy fats can be obtained through foods such as coconut¹² oil, avocados and nuts but many prefer to pile on the bacon and butter.

他还指出，生酮饮食的碳水化合物含量很少，但实际上并非高蛋白饮食，太多的动物蛋白会导致类胰岛素一号增长因子(IGF-1)升高，它控制着生长激素，如果它处于高水平，会与癌症产生关联。健康脂肪可以通过椰子油、牛油果和坚果等食物获得，但很多更喜欢堆积在熏肉和黄油上。

Other concerns about low-carb diets include the effect these regimes have on gut¹³ bacteria, which mainly ferment¹⁴ undigested starches¹⁵ 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.

其他对于低碳水化合物饮食的担忧包括这类饮食对于肠道细菌的影响，肠道细菌主要是把植物纤维等未消化的淀粉发酵，并产生有利于保护肠道的短链脂肪酸。这类饮食纤维含量很低，因此运动员报告过有便秘问题。

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.

生酮饮食或许最适用于短期训练，而不应被视为一种生活方式选择。其长期效果尚未得到评估。