Not so, according to recent research by Sharon Fowler and colleagues at the University of Texas Health Science Center. Her study of nearly 750 people indicates that contrary to public expectations, consumption of diet sodas might actually be causing weight gain. At the very least, these no-calorie alternatives are not helping people lose weight.
Fowler’s study is especially noteworthy because all subjects in her cohort were over 65 years of age. Seniors typically experience significant age-related shifts in body composition, and consequently they have the highest risk of cardiometabolic disease.
On top of this, adults over 50 in the US drink more diet soda than regular soda - the only demographic that does so. One third of adults over 65 years report drinking diet soda, according to a 2013 Gallup survey.
Dr. Fowler’s study, titled The San Antonio Longitudinal Study of Aging (SALSA), tracked 749 individuals (Mexican-American and European-American) over 65 years old, across a ten-year period. The participants were categorized according to average diet soda intake at baseline, and at follow-ups: non-users (<0.05 per day), occasional users (>0 but <1 per day), and daily users (≥1 per day).
The investigators measured waist circumference, height and weight (for BMI) at baseline, and at each follow up. Full follow up data was available for 466 participants.
The results were remarkable.
Between follow-up visits, diet soda drinkers gained significantly more girth than non-users (2.11cm vs. 0.77cm increase in waist circumference). This effect was significant, even after controlling for all other factors that might explain the increase (age, diabetes mellitus status, leisure time physical activity, smoking status, demographic factors, follow up length).
A Dose-Response Relationship
When they broke the data down further, the UT researchers found a positive dose-response relationship between diet soda intake and waist circumference, with daily drinkers having almost four times the increase in waist size than non-drinkers.
Over the course of the trial, this amounted to less than an inch for non-users, 1.8 inches for occasional users, and 3.2 inches for daily users (Fowler SP, et al. Am J Geriatr Soc. 2015; 63(4): 708-15).
There was no such pattern of pronounced weight gain associated with regular (sugar-sweetened) soda intake, although frequent soda drinkers’ waistlines also grew somewhat during the trial.
Interestingly, diet soda drinkers in this study had higher education levels and socioeconomic status, were more likely to be European American, less likely to smoke, and tended to do more exercise than non-users. In spite of these apparent health advantages, diet soda drinkers also had significantly higher BMI and larger waistlines than those who didn’t drink diet soda.
Over time, trends in BMI and waist circumference showed a diverging pattern: BMI peaked by 75 years of age, and then declined rapidly in both men and women. Meanwhile, waist circumference continued to increase steadily before reaching a plateau around 80 years old in men; this same pattern was seen in women, but was even more striking. Women’s waistlines started out smaller, but caught up to their male counterparts by 80 years old, indicating greater gains in abdominal fat.
The “diet soda effect” appears to be influenced somewhat by initial baseline BMI.
In those with a healthy BMI (<25), diet soda consumption had little effect on waistline circumference. In people who were already obese (BMI >30), there was a significant gain associated with diet soda consumption. Fowler’s group observed an intermediate effect in those categorized as overweight, BMI 25-30).
Abdominal Fat in a Bottle
An expanding waistline indicates excess abdominal fat, which is a predictor of cardiometabolic disease. A larger waist circumference (particularly >35” for women, >40” for men) is associated with greater risk of type 2 diabetes, hypertension, cardiovascular disease, and cognitive impairment. These chronic diseases are among the leading causes of death in the US, particularly in this age group, and impose a significant healthcare burden.
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