Diet Soda, Metabolic Syndrome, and Weight Loss

As I write this, I am nursing a glass of Diet Pepsi, one of the embarrassing number that I put away on a typical day (starting about five minutes after I wake up, to my boyfriend's abject horror). I've resolved to cut back on my consumption in the past, mostly because I'm cheap enough to resent paying for something with absolutely no nutritional value, but have always been defeated by the calorie factor. Besides, doesn't everyone deserve a vice? But because of my love-hate relationship with the stuff, two recent studies about artificial sweeteners caught my eye.

One, published in Circulation, came as quite a shock: Drinking diet soda, it suggested, puts me at higher risk of developing a group of risk factors like high blood pressure and unhealthy levels of "bad" cholesterol that are tied to heart disease and diabetes. Another paper, published in Behavioral Neuroscience, found that—in rats, at least—cutting the traditional link between sweet flavor and high calories seems to throw off the ability to judge the caloric content of food. That, no surprise, leads to overeating. So much for the calorie factor.

The first study, which looked at the food intake of more than 9,500 middle-aged adults, is investigating potential dietary factors behind the "metabolic syndrome," which isn't a disease in itself but a cluster of symptoms—a large waist circumference is another one. Among its many findings: Those who drank the most diet soda were 34 percent more likely to have metabolic syndrome than those who drank the least.

Before you toss your cans (or, in my case, two-liter bottles), though, realize that this study shows only an association; it does not at all prove that drinking soda actually leads to metabolic syndrome. As New York University nutritionist Marion Nestle notes on her blog, the habit might be a marker for some other less-healthy behaviors that actually do cause the syndrome. So, as the authors of the study say, it's an interesting finding that needs more investigation.

The study in rats has been getting a lot of media attention, which has surprised a lot of the nutritionists I've talked to since the idea behind the research isn't new. That said, here's a rundown on the theory: Starting with their first taste of breast milk, mammals begin to associate sweet taste with calorie-dense foods. They get the message that eating something sweet means they don't need to eat as much to maintain their weight as they might of something nonsweet. But, the theory goes, what if that connection is disrupted say, by eating foods flavored with a highly sweet but noncaloric sweetener? Then they have no way of gauging through flavor alone whether something is likely to be high in calories or not. Without those cues, the animal may overeat.

In this case, the rats studied at Purdue University who were fed low-fat yogurt flavored with saccharine ate more rat chow and got fatter than those who ate the same yogurt flavored with glucose. The rats that ate the saccharine-sweetened yogurt were also less able to compensate for calories (i.e. eat less at the next meal to make up for a lot of food eaten at the last one) than those whose meals were "predictive"," or sweetened with glucose. What does it mean for humans? The authors conclude that while you can't take what you find in lab rats and assume it will apply to people, too, "it is conceivable that the widespread use of artificial sweeteners may have similar effects on us.

It's true that thanks to artificial flavorings and fats, and the sheer variety of our diets, "it's very difficult to link particular tastes and sensory experiences with what food is anymore,"says Barbara Rolls, professor and Guthrie chair in nutrition at Pennsylvania State University. "We often have no clear signal as to what the calorie content of any of our foods is." So she advises that, because we tend to eat a certain constant volume of food, we should consciously make choices that are lower in density. That means veggies and fruits as the basis of our diet, rather than foods that are more likely to be calorie-dense (like processed and fried foods).

Meanwhile, other research has shown that in humans, drinking diet soda as a replacement for regular soda actually does lead to weight loss, she says. Susan Swithers, one of the authors of the rat study, acknowledges that sweeteners may work when used as part of a conscious calorie-reduction plan even if they also unconsciously dull the sweetness-calories link. But no one is saying that artificial sweeteners are the key one way or the other: They aren't likely to be the magic bullet to get you from fat to thin any more than they're likely to be the sole reason you got fat in the first place.

I am not a rat, and I make a conscious effort to keep my diet pretty healthy. So I'm going to stick with my Diet Pepsi. But, for the sake of my wallet, I'll try to alternate glasses of soda with that original diet drink, water.