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A growing body of evidence is showing that it might not just be added sugar, but also artificial sweeteners, that are contributing to the rapidly rising rates of obesity and diabetes in America.

An important new long-term analysis of 375 seniors in Texas has shown a clear relationship between the number of diet sodas consumed on a daily basis and abdominal obesity – the belly fat that serves as the most obvious indicator of underlying metabolic syndrome and, with it, increased risk of diabetes and cardiovascular disease.

The study, which was conducted by researchers in the University of Texas Health Science Center in San Antonio, followed a group of Mexican-American and European-American seniors for an average of 9.4 years, with three follow-up visits during that time.

The study found that the increase in waist circumference (3.16 inches over nine years) for participants who drank one or more diet sodas per day was nearly four times the increase that non-soda drinkers experienced (0.8 inches). Occasional diet soda consumers (0-1 sodas per day) fell about mid-way between those, with an average waist increase of 1.83 inches over the course of the study.

Interestingly, the authors found that diet soda drinkers were similar to those who don’t consume diet drinks with respect to age and sex, but had higher education levels and were more likely to live in the suburbs and to be of European descent, less likely to smoke and more likely to expend more energy during leisure times – all elements we would normally associate with greater health-promotion behavior.  Even so, diet soda drinkers had higher rates of obesity from the start of the study (45 percent obese) than non-drinkers (27.8 percent). The researchers adjusted for these differences in the study, but noted that these are precisely the people who are most likely to replace sugar-sweetened soda with diet drinks in an effort to lose weight.

These results are consistent with a growing body of research in both humans and animals, showing that frequent consumption of diet soda or artificial sweeteners is associated with greater body mass index (BMI), obesity and metabolic syndrome.

Why, precisely, that is happening is still unclear. Previous research had indicated that artificial sweeteners could be a major cause of both obesity and diabetes by changing the balance of bacteria in our guts, favoring the microbes that induce glucose intolerance (see previous SugarScience Alert).

Regardless of the biological causes, this study raises concerns about the safety of drinking not just sugar-sweetened sodas, but also substituting diet sodas, especially for people already at risk for obesity, heart disease and diabetes.

Laura A. Schmidt, Ph.D., M.S.W., M.P.H., is a professor in the UCSF School of Medicine. She has dedicated her career to intervening on the social determinants of health and to understanding how lifestyle risk factors, such as alcohol and poor diet, influence chronic disease and health inequality.