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减肥新理论真的有效吗?

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The New New Thing in Dieting


Forget carbs. The new diet buzzword is "leptin." Several new diet books promise to show you a way of eating that increases the body's sensitivity to the appetite-suppressing hormone leptin. But many scientists and weight-loss experts say the books' claims aren't supported by fact.

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Leptin, named for the Greek word leptos, which means "thin," is a hormone made by fat cells that sends a signal to your brain to stop eating. When it was discovered in 1994, some scientists initially hoped that overweight people were deficient in leptin, and that injecting it would help them slim down. However, early human experiments were a disappointment -- and scientists found that most overweight people actually have unusually high levels of leptin. For reasons not yet understood, overweight people's brains appear to stop hearing leptin signals -- a phenomenon some scientists call "leptin resistance."

At least three books in recent years propose diet plans they say will naturally reverse leptin resistance. "Mastering Leptin," by nutritionist Byron J. Richards and Mary Guignon Richards, offers a range of advice from reducing carbohydrates to avoiding monosodium glutamate, which it says "damages the leptin message in the brain." "The Rosedale Diet," by physician Ron Rosedale, who specializes in metabolic medicine, proposes tackling leptin resistance by eating a lot of fish, skinned poultry and vegetables, and cutting back on starchy carbohydrates and sugars, including those in most types of fruits.

"The Fat Resistance Diet," by internist Leo Galland, who specializes in inflammatory disorders, says that leptin resistance is caused by "chronic inflammation" in the body. He proposes a diet based on 40 inflammation-reducing "superfoods," including blueberry juice, green tea, grapefruit, arugula, basil and cardamom. He also suggests eating fish three times a week, as well as nine servings of fruits and vegetables a day and at least 25 grams of fiber a day.

The advice in the three books varies significantly. Mastering Leptin, for example, advises swearing off between-meal snacks, while the other two books encourage eating healthy snacks when hungry.

Scientists who have researched leptin -- including Jeffrey Friedman at Rockefeller University, who led the team that discovered leptin -- say there is no scientific basis for claims that diet can change leptin resistance. Research does show that leptin levels fall with weight loss, though the particular diet followed doesn't appear to matter, says Rudy Leibel, a co-discoverer of leptin who is now co-director of the New York Obesity Research Center. "It's boring, and people don't want to hear it, but what matters is how many calories you eat," he adds.

It's unclear whether falling leptin levels make human brains more sensitive to the hormone. It does in rats, Dr. Leibel says, but if the same were true in humans, people who lose some weight would find it easier to lose more. Unfortunately, for most people the opposite is true. And it isn't known if MSG is a bad actor as far as leptin is concerned, scientists add. Studies have found that large quantities of it do damage leptin-sensing portions of the brains of baby rodents, but it hasn't been studied in humans.

Mr. Richards says plenty of science backs up the advice in his book, including the recommendation to avoid snacking. Dr. Galland says his books' assertions that his diet combats leptin resistance is his own theory based on "indirect" scientific evidence; he says he been prescribing the diet for years to patients to combat diseases such as arthritis, and noticed that it also helped them lose weight. Dr. Rosedale couldn't be reached to comment
减肥新理论真的有效吗?



忘记碳水化合物吧,减肥的最新流行语是“瘦素”。一些新推出的减肥书信誓旦旦地告诉你,只要遵循书中的饮食要求,就可以提高身体对抑制食欲的荷尔蒙──瘦素的敏感程度。不过许多科学家和减肥专家却表示,这种说法缺乏事实依据。

“瘦素”一词来自希腊语“leptos”,它的意思是“瘦”。这种由脂肪细胞生成的荷尔蒙可以向大脑传递停止进食的信号。当这种物质在1994年被首次发现时,一些科学家认为超重人群可能缺少瘦素,因此注射这种物质可以帮助肥胖者降低体重。不过,早期的人体实验令人失望──科学家发现大多数肥胖者的瘦素水平很高。由于尚不清楚的原因,肥胖者的大脑似乎接收不到瘦素的信号──有些科学家称这种现象为“瘦素抵抗”。

最近几年,至少有三本书宣称他们推荐的饮食可以扭转瘦素抵抗。营养学家拜伦?理查兹(Byron J. Richards)和玛丽?理查兹(Mary Guignon Richards)所著的《征服瘦素》(Mastering Leptin)一书提出了包括减少摄入碳水化合物和避免食用味精在内的一系列建议,他们认为这些物质“破坏了大脑中的瘦素信息”。研究代谢药的医生罗恩?罗斯达尔(Ron Rosedale)所著的《罗斯达尔饮食》(The Rosedale Diet)建议通过多食鱼类、去皮家禽和蔬菜,少吃淀粉质的碳水化合物和糖──包括水果中的糖份来对付瘦素抵抗问题。

研究炎症的内科医生莱奥?加朗(Leo Galland)所著《抗脂饮食》(The Fat Resistance Diet)认为,瘦素抵抗是由身体中的“慢性炎症”引起的。他建议通过食用40种消炎的“超级食品”来减肥,其中包括蓝莓汁、绿茶、葡萄柚、芝麻菜、罗勒和豆蔻。他还建议每周吃三次鱼,每天吃9次水果和蔬菜,纤维食品的摄入量一天至少达到25克。

这三本书提出的建议大相径庭。比如,《征服瘦素》建议不要在三餐中间吃零食,而另外两本书则鼓励在饥饿时吃健康的零食。

研究过瘦素的科学家们──包括曾领导科研小组发现瘦素的洛克菲勒大学的杰弗里?弗里德曼(Jeffrey Friedman)在内──表示,饮食可以改变瘦素抵抗的说法没有科学根据。瘦素的共同发现者、现在主管纽约肥胖研究中心(New York Obesity Research Center)的鲁迪?利贝尔(Rudy Leibel)说,确有研究表明瘦素水平随著体重减轻而降低,不过特别的饮食似乎与此并无关联。他还补充说,“这是老生常谈,人们也不愿再听这些唠叨,不过再说一次:重要的是你每天摄入多少卡路里。”

现在还并不清楚瘦素水平降低是否有助于人脑对瘦素的感应。利贝尔说,老鼠试验证明事实如此,不过如果人类也遵循这一规律,那么体重减轻的人会发现继续减肥很容易。不幸的是,大多数人的情况正好相反。科学家们还补充说,现在并不知道味精对瘦素到底是否会产生不利影响。研究表明,大量味精确实破坏了小老鼠脑子中的瘦素感知系统,不过这还没有得到人类研究的证实。

理查兹说,他书中的建议都有足够的科学根据,包括少吃零食的建议。而加朗说,他推荐的饮食可以战胜瘦素抵抗是根据“间接的”科学依据得出的自己的理论;他说,他多年来一直要求患有关节炎等病症的病人遵照他的饮食要求,并发现这帮助他们减轻了体重。记者未能联系到罗斯达尔置评。
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