Diet Soda and Weight

Mon, 25 July 2011 - 6:59 p.m. MT
Credit: Jeff Venables

Diet Soda and Increased Waistline? Weighing the Current Evidence

By Jeff Venables

The media claims. In late June and subsequently through much of July, media reports surfaced that consuming diet soda leads to dramatic weight gain. There was nothing ambiguous about the headlines—for example, this one used by Marie Claire UK: “Diet Fizzy Drinks Could Make You Fatter” (June 29, 2011). Others were equally alarming: “Diet Drinks Make You Fat” (Daily Express UK, June 29, 2011), and “Why Thin People Don’t Drink Diet Pop” (Vancouver Sun, July 2, 2011).

Millions of people rely on diet soda as a tool for weight management, and have for decades. A closer look at these claims is merited. What is the nature of the underlying research, and what is actually being reported in the studies? Ultimately, how should we interpret the findings? Frankly, this hasn’t always been done responsibly by all of the magazines and newspapers thus far reporting them.

Underlying studies. The source of the diet soda controversy, this time around at least, is the presentation of compelling data given at a conference of the American Diabetes Association in San Diego in late June. Over two days, talks were given that seem to implicate diet soft drinks specifically, and artificial sweeteners in general, in significant increases in body weight over time.

Both studies originated from the University of Texas Health Science Center at San Antonio, and are as-yet unpublished and lacking peer review. In at least the first, however, the data is abundant and noteworthy in its scope. Researchers looked at 474 elderly people over the course of 10 years, using data from the San Antonio Longitudinal Study of Aging. They found that people who drank two or more diet soft drinks a day had the largest waistline increases over the 10-year period, a surprising five times more than that of non-diet soda drinkers.

In the second University of Texas study, 20 control mice were fed regular chow supplemented with corn oil while the experimental group, also composed of 20 mice, was fed chow containing aspartame. The aspartame rodent group showed 37% higher glucose levels in the blood after three months. Additionally, their insulin levels were lower, by 27%. These results are among the first to suggest a direct link between aspartame consumption and increased diabetes risk.

What can we truly conclude from these two studies? Regarding the first, it’s important to stress that the cause of the weight gain in the subjects remains unknown. And this is where the media “scoop” falters. All of the attention-grabbing headlines are ill-advised in that a close read of the scientific reports—and in several cases even just the news articles themselves—reveals a lot of theorizing by the researchers as to what is going on here causally. Inevitably, the media has taken up the theorizing on this complex issue and printed and reprinted it as established fact.

What goes better with fries than a soda? There are many mechanisms that may explain why diet soda drinkers are more prone to weight gain. Carbonated soft drinks are widely available—arguably foisted on people—as part of fast food value meals. If you frequent fast food establishments, you’re in a situation where you’re being offered soda frequently, maybe every day. You may well learn over time to at least make that soda a diet drink. Now you are a person who quite possibly consumes significantly more diet soda than someone who stays away from fast food. People who stay away from fast food tend to be skinnier than those who eat it every day. The real problem with your waistline, in this scenario, is that you’re eating at a fast food place daily, not that you’re consuming diet soda. But the data can’t reflect this.

In fact, the researchers controlled for age, sex, education, neighborhood, diabetes status, leisure activity level, and smoking status. However, they did not track how many total calories people consumed. Before we draw too many conclusions specific to diet soft drinks, we’d do well to consider overall eating habits. In a fries-and-a-diet-soda context, the soda is not the main problem.

A tenuous direct link? Still, the mice study offers better insight into cause. Doctors and dieticians theorize that metabolically, the sweeteners in these beverages (and foods) cause a craving for sweets that leads to indulgence in sugary foods. A related theory is that diet drinks functionally do lead to weight gain because the consumer is not getting the calories from the drink, however sweet, and seeks satisfaction of the craving elsewhere. The idea is that artificial sweeteners could have the effect of triggering appetite but, unlike regular sugars, don’t deliver something that will squelch the appetite. Still another theory is that people who consume lots of diet soda overestimate daily caloric allotment, thinking consciously but incorrectly about how many calories they are “saving” by drinking diet soda. One rodent study does not yet make the case, but it does provide some evidence that sweeteners have metabolic consequences that may be undesirable. In any event, there is really no evidence right now that indulging in a full-sugar drink is the better choice.

Cui bono? Who benefits from a popular revolution away from artificial sweeteners? There is every indication so far that the scientists behind these two studies are genuinely concerned about sugar-free sweeteners and that the concern is based on good-faith interpretations of hard data. It may be tempting to suspect the sugar lobby or the people behind the manufacture of high fructose corn syrup when something that saves soda drinkers hundreds of calories daily gets disparaged in the media. Neither study was funded by these industries, but it does not seem entirely inappropriate to wonder. On the LA Times blog (July 1, 2011) that ran the initial story of the 10-year study, a conspicuous ad ran in the middle of the piece beckoning the reader to get the facts and the myths about high fructose corn syrup. The link was to a site called, which looks a lot like a propaganda site for the corn industry. Ironically, the LA Times piece is praiseworthy for its handling of the subject, including a too-rare caveat about correlation not implying causation.  It is likely that the ad buy occurred because the people behind high fructose corn syrup simply saw an opportunity in the related subject matter and took advantage of it. But it does indicate that propaganda is nevertheless alive and well indeed.

Without resorting to corporate conspiracy, the “news” on diet soda and weight gain gets distorted simply because the media can’t resist reporting the speculation of the researchers as if it were fact. It’s just a better looking headline that way. The Vancouver Sun article, for example, admits that stating diet soda “makes” people fat isn’t true—the correlation/causation caveat appears here as well. But then the article undermines this sensibility with: “The study, of 474 people over 10 years, found that the more of the so-called diet drinks people drank, the fatter they got.” This implied direct causation is simply not accurate.

If not a villain, still not the best choice. A lot more research is needed before conclusions can be drawn on the effects of diet soda on weight gain, appetite, and metabolism. Still, no one is saying drinking a lot of diet soda is a good choice. If you’re looking for an easy way to cut calories, the flavor of these beverages has come a long way in recent years. Diet drinks certainly seem like the wiser choice when compared to their sugary brethren. But let’s remember that diet soda is at best an empty carrier, with zero amounts of beneficial nutrients. And it is true that the long-term health effects of these drinks continue to deserve scrutiny. There are animal studies, for example, suggesting a link between vascular problems and caramel-containing products. In February, a large study of over 2,500 New Yorkers found that people who drank diet soda every day had a 48% higher risk of stroke and heart attack as compared to non-soda drinkers. We face the same uncertainty about cause here, though, as well.

With regard to any carbonated soft drink, less is better. Dariush Mozaffarian, a Harvard epidemiologist who studies how various foods contribute differently to weight gain, sees artificial sweeteners as a good short-term option “to bridge people away from refined sugars,” but sums it up best by warning that consumption of high or even moderate amounts of these drinks in the long term should be avoided as much as possible.

Jeff Venables is the editor of Running & FitNews and a frequent contributor to the AMAA Journal and the American Running website.

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