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由于生酮饮食中只摄取少量的糖类，肝脏便会将脂肪转换成脂肪酸和酮体。酮体运经脑部，取代葡萄糖作为能量来源
测试中可能遇到的词汇和知识：
starvation饿死；挨饿；绝食[stɑː'veɪʃn]
glycogen糖原['glaɪkədʒ(ə)n]
carbohydrate[有化] 碳水化合物[kɑːbə'haɪdreɪt]
ketone酮['kiːtəʊn]
rationale基本原理[,ræʃə'nɑːl]
hitting the wall撞墙期（长跑运动员突然跑不动了）
lean瘦的；贫乏的[liːn]
ketogenic diets生酮饮食
saturated fat饱和脂肪
one-size-fits-all万全之策
hormone激素，荷尔蒙['hɔːməʊn]
阅读马上开始，建议您计算一下阅读整篇文章所用的时间，对照下方的参考值就可以评估出您的英文阅读水平。
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
请根据你所读到的文章内容，完成以下自测题目：
A.vitamin
B.inorganic salt
C.lipid
D.carbohydrate
答案 (1)
A.30 minutes
B.40 minutes
C.50 minutes
D.60 minutes
答案 (2)
A.1
B.2
C.3
D.5
答案 (3)
A.it may cause constipation
B.it is best for long -term periods of training
C.the effect varies with the individual
D.vegetable fat is better than animal fat
答案 (4)
(1) 答案：D.carbohydrate解释：饥饿模式下身体会燃烧糖原，这些糖原来自于食物中的碳水化合物。
(2) 答案：B.40 minutes解释：糖原大约可以在腿部肌肉中存储40分钟，这就是为什么在马拉松途中运动员需要补充大量甜食和在三分之二处会觉得突然跑不动了。
(3) 答案：C.3解释：Dr. Attia坚持了三年生酮饮食，并且感觉“新陈代谢提高，并且更加高效了”。
(4) 答案：B.it is best for long -term periods of training解释：生酮饮食最好短期时间的训练，而不应被视为一种生活方式的选择，长期影响还没有评估。