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Questioning This Week’s Health Scare Linking Diet Soda To Belly Fat

Alarming headlines ran this week claiming that diet soda causes accumulation of belly fat and an increased risk of strokes, heart attacks and diabetes. The stories were based on a press release describing a new study – one that followed 375 people aged 65 and up for nearly 10 years.

The claim: Those who reported drinking diet soda gained slightly more circumference around the middle than those who lay off the diet drinks. The researchers interpreted their findings to mean that diet soda causes people to put fat around the middle. The results were published in the Journal of the American Geriatrics Society.


TORONTO, ONTARIO, CANADA – 2015/02/08: Coca Cola Zero and Coca Cola Diet can boxes in a store, Coca-Cola is a carbonated soft drink sold in stores, restaurants, and vending machines throughout the world. (Photo by Roberto Machado Noa/LightRocket via Getty Images)

Most stories quoted the study’s the lead author, Sharon Fowler of the University of Texas, San Antonio, making a charming but uninformative sound bite comparing the body to a rainforest and suggesting that it’s the acid in the soda that’s causing the harm.

She cautioned against diet soda as a cause of escalating obesity.  Is it true? A critical read of the coverage leaves plenty of room for doubt.

The study started with 749 people age 65 and up who took part in a survey called the San Antonio Longitudinal Study of Aging (SALSA). Of those 375 who were still breathing after 9.4 years, diet soda drinkers gained 2.11 centimeters around the waist while the non-drinkers gained .77 cm. Belly fat, or more technically “visceral fat” is a serious health problem according to a number of studies linking it to diabetes and cardiovascular disease.

The press release emphasized that visceral fat is more than just a cosmetic concern:

Metabolic syndrome—a combination of risk factors that may lead to high blood pressure, diabetes, heart disease, and stroke—is one of the results of the obesity epidemic. In fact, the World Health Organization (WHO) estimates that 1.9 billion adults were overweight (body mass index [BMI] of 25 or more) in 2014. Of this group, 600 million people fell into the obese range (BMI of 30 or more)—a figure that has more than doubled since 1980.

It should be noted that some appreciable fraction of people in the study actually did die – but those were eliminated from the results. It would have been interesting to know what they ate or drank, since being dead is worse than having conditions that make you more likely to die.

One of the ways the coverage confused people comes down to Americans’ ineptitude with the metric system. You probably read over the numbers without really thinking about it, but we’re talking about a difference of 1.3 centimeters – which converts to about a half an inch. The bubbles alone would do that to my waistline.

One concern that the press didn’t address was precision of the measurements used in the study. Have you ever tried to measure your waistline with a tape measure? The amount of change over the study period – .77 cm and 2.31 cm – imply the researchers can measure people’s waists to within a tenth of a millimeter. Perhaps they use some kind of laser technique. But the abstract of the paper admits to a pretty wide margin of error, and reveals that the researchers don’t understand that they are implying more measurement precision than they really have.

The headlines imply that diet soda has some special power to channel fat to the midsection but this, too, is not well-explained since you’d expect people over 65 to add waist girth if they gained weight. The suggestion is that people do not drink diet soda distribute their fat differently. But there’s no evidence presented for this.

There’s also an unanswered question about the relationship between cause and effect. The researchers assume their results mean that diet soda makes you get fat around the middle, but how do we know that it’s actually the extra midsection centimeters that cause people to drink more diet soda?

Or there may be a common cause behind the link. Perhaps it’s that some people have a tendency to eat and drink compulsively – a problem that can make people fat and can also steer them into a diet soda habit in the hope they will do less damage than they would binging on regular soda, beer, wine or martinis.

Finally, there’s the question of how soda causes the fat accumulation. The study’s author suggests soda disturbs people’s internal rain forests. Many reporters assumed this meant she was referring to intestinal bacteria – helpful microbes which do play a role in metabolism and weight control. A study I wrote about last fall suggested that artificial sweeteners altered our intestinal bacteria in a way that could raise blood sugar. But that remains just a provocative finding that needs replication – not part of established medical knowledge.

Fowler also suggested the acid in soda was adding the centimeters. There is a dieting/health trend that involves minding the relative acidity of foods to optimize health, but is there any evidence behind it? I checked Quackwatch – a website that’s well respected for investigating the quackiness of health trends. What did they have to say? In this post, physician Gabe Merkin explains that since your stomach acid is much stronger than anything you’d eat or drink, the acidity of ordinary foods doesn’t influence the goings-on in your bloodstream or your intestines. Merkin says the only bodily fluid that changes acidity depending on your diet is your urine.

Whatever the strengths and weaknesses of this study, it may still make sense to try cutting out diet drinks, especially if you’re frustrated with your weight or blood sugar control. Switching to water or club soda can’t hurt. Still, switching to margaritas might not improve your health, or, worse, the subject of last week’s health scare – Palcohol.

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