Can Sugar Substitutes Make You Fat?

When it comes to dieting, most of us are willing to resort to a trick or two to help us curb our appetite and eat less — drinking water to fill up when we're hungry, for example, or opting for artificial sweeteners instead of sugar to get the same satisfying sweetness without the offending calories. But new research suggests that the body is not so easily fooled, and that sugar substitutes are no key to weight loss — perhaps helping to explain why, despite a plethora of low-calorie food and drink, Americans are heavier than ever.

In a series of experiments, scientists at Purdue University compared weight gain and eating habits in rats whose diets were supplemented with sweetened food containing either zero-calorie saccharin or sugar. The report, published in Behavioral Neuroscience, presents some counterintuitive findings: Animals fed with artificially sweetened yogurt over a two-week period consumed more calories and gained more weight — mostly in the form of fat — than animals eating yogurt flavored with glucose, a natural, high-calorie sweetener. It's a continuation of work the Purdue group began in 2004, when they reported that animals consuming saccharin-sweetened liquids and snacks tended to eat more than animals fed high-calorie, sweetened foods. The new study, say the scientists, offers stronger evidence that how we eat may depend on automatic, conditioned responses to food that are beyond our control.

What they mean is that like Pavlov's dog, trained to salivate at the sound of a bell, animals are similarly trained to anticipate lots of calories when they taste something sweet — in nature, sweet foods are usually loaded with calories. When an animal eats a saccharin-flavored food with no calories, however — disrupting the sweetness and calorie link — the animal tends to eat more and gain more weight, the new study shows. The study was even able to document at the physiological level that animals given artificial sweeteners responded differently to their food than those eating high-calorie sweetened foods. The sugar-fed rats, for example, showed the expected uptick in core body temperature at mealtime, corresponding to their anticipation of a bolus of calories that they would need to start burning off — a sort of metabolic revving of the energy engines. The saccharin-fed animals, on the other hand, showed no such rise in temperature. "The animals that had the artificial sweetener appear to have a different anticipatory response," says Susan Swithers, a professor of psychological sciences at Purdue University and a co-author of the study. "They don't anticipate as many calories arriving." The net result is a more sluggish metabolism that stores, rather than burns, incoming excess calories.

Swithers stops short of saying that the animals in her study were compelled to overeat to compensate for phantom calories. But she says that the study does suggest artificial sweeteners somehow disrupt the body's ability to regulate incoming calories. "It's still a bit of a mystery why they are overeating, but we definitely have evidence that the animals getting artificially sweetened yogurt end up eating more calories than the ones getting calorically sweetened yogurt."

Though it's premature to generalize based on animal results that the same phenomena would hold true in people, Swithers says, she notes that other human studies have already shown a similar effect. A University of Texas Health Science Center survey in 2005 found that people who drink diet soft drinks may actually gain weight; in that study, for every can of diet soda people consumed each day, there was a 41% increased risk of being overweight. So even though her findings were in animals, says Swithers, they could lead to a better understanding of how the human body responds to food, and explain why eating low-calorie foods doesn't always lead to weight loss. "There is lots of evidence that we learn about the consequences about eating food," she says. "And we have physiological responses to food that are conditioned."

So does that mean you should ditch the artificial sweeteners and welcome sugar back into your life? Not exactly. Excess sugar in the diet can lead to diabetes and heart disease, even independent of its effect on weight. But it's worth remembering that when it comes to counting calories, it's not just the ones you eat that you have to worry about. The calories you give up matter too, and they may very well reappear in that extra helping of pasta or dessert that your body demands. Your body may actually be keeping better count than you are.

糖替代品会让你变胖吗？

图：一名女子向咖啡杯里倒低糖（摄影：Michael A. Keller / Corbis）

不得不节食的时候，我们大多会诉诸这一两种方法来遏制我们的胃口，少吃点东西——例如，靠喝水来与饥饿作斗争，又或者食用一些人工甜味剂来代替糖，这样就能得到同样的满足感而不至于吸取过多的热量。但是最新的研究告诉我们，身体可不是那么容易上当的，糖替代品可不是减肥的特效药——这就解释了为什么尽管食用了低热量的食物和饮料，美国人的体重还是决绝的保持上升趋势。

在一系列的实验中，普渡大学的科学家们用小白鼠来研究体重增加与饮食习惯的关系——分别用零卡路里的糖精和含糖食物来喂养小白鼠。研究结果已经刊登在《行为神经科学》上，与我们所以为的一些观点正好相反：与那些食用葡萄糖口味（一种天然的高卡路里的甜味剂）的酸奶的同类们相比，坚持食用人工甜味酸奶的小白鼠们，在经过两周以后，他们摄取了更多的卡路里，体重增加也更加明显——绝大多数卡路里在体内以脂肪的形式保存。普渡研究小组的这项长期研究始于2004年，当时他们报告说动物食用糖精（包括含糖液体和小吃）时往往比食用高卡路里和含糖食物的时候吃得多。科学家们认为这个最新研究对以下结论提供了一个强有力的证据：获取食物这一动作更多的表现为一种自发的行为，也就是说对食物的条件反射超乎我们的控制。

研究的揭示了正如Pavlov的狗被训练成对钟产生条件反射那样，动物们也会同样的认为当他们吃到甜食的时候他们就可以摄取更多的卡路里——显然，甜食包含了足够多的卡路里。当动物们吃到那些含零卡路里的人工甜品时——打乱了原来的甜品与卡路里“理所当然”的联系——他们进食的数量会增加（以保证获取足够的卡路里——译者注），体重也随之增长。这就是研究所揭示的内容。这项研究证明，在生理层面上，动物们对人工甜味剂的反应不同于那些高卡路里的含糖食物的反应。让小白鼠食用糖类，在用餐时体温出现了预期中的上升，与他们要燃烧一定卡路里的需要相辅——（燃烧卡里路）来进行相应的新陈代谢。另一方面，食用糖精的小白鼠并没有出现体温升高的现象。“动物们对这些人工甜味剂有着不同于预期的反应，”普渡大学心理学教授以及本次研究的主要参与者之一，Susan Swithers如是说，“他们对卡路里的摄入没有反应。”最终的结果就是导致了新陈代谢的缓慢进行，摄入的过量的卡路里被用于储蓄而不是消耗。

Swithers没有回应那些质疑——研究中的动物被强制摄取过量食物，以此补充足够的卡路里。但是她说，这项研究的确表明了，人工甜味剂确实打乱了身体对摄入的卡路里的调控能力。“为什么他们会过度进食的谜题还没有完全解开，但是我们有明确的证据证明，食用人工甜味酸奶的动物吃的比那些食用天然含糖酸奶的动物要多得多。”

尽管把这个基于动物的实验结果同样套用于人类还嫌过早，但是Swithers说她所关注的其它的一些基于人类的调查也得出了类似的结果。德州大学健康科学中心2005年的一项调查发现：经常饮用软饮料的人们更容易发胖；在调查对象中，每天饮用一听苏打碳酸饮料的人们，发胖风险提高41%。所以尽管研究的对象是动物，Swithers说，这一结果依然能帮助我们更好的理解人类身体对事物的反应，也解释了为什么食用低卡路里的食物并不一定能减肥。“我们有大量的证据支持对于食用食物的研究结论，”她说，“我们会对食物有反应是有条件的。”

这是否表示你应该抛弃人工甜味剂，回到糖的世界？不完全是。在饮食中摄入过量的糖会导致糖尿病和心脏病，以及提升你的体重。但是你应该记住的是，当你计算卡路里的时候，不应该只考虑这些你已经花费了这么多心思的饮食。为了卡路里所放弃的事情实在太多了，并且他们很可能蜂拥而至，仅仅是因为你食用了每日必需的意大利面或者饭后甜点。要知道你的身体的计算能力可比你强得多。（看来不知不觉就会变胖亚！似乎没有什么法子，如果不和我们的胃口作斗争的话——译者的小意见）