The uproar about Mayor Bloomberg's new ban on selling supersized sodas has focused on two issues: 1) personal freedom (as in, I have the God-given right to buy a 72-oz cola), and 2) efficacy ("It won't make a difference anyway; people are obese because of what they eat.")
While there are reasonable arguments on both sides of these issues, I haven't seen any mainstream coverage of the thing that bothers me most about the ban:
The ban excludes diet soda.
But this is completely inconsistent with the science on soda. Despite being lower in calories, diet soda is just as likely to increase risk for obesity and other negative outcomes.
Below are three major findings on the harms of diet soda consumption. Research has also found that the articificial sweeteners in diet soda interfere with blood sugar control, increase cravings for sweet snacks, and impair the brain's processing of satiety signals.
If this ban goes into place, soda-sellers may push supersized diet sodas to keep the mark-up profits and satisfy thirsty customers. And if this ban shifts consumption toward diet soda, rather than reduce overall soda consumption, it may backfire completely.
1. Key finding: Drinking diet soda is associated with weight gain. This studied followed 5,158 adult residents of San Antonio, Texas over 10 years, and found a dose-response relationship: the more diet soda one drinks, the more weight one gains over time. This is not a matter of overweight people drinking more diet soda; among people of normal weight, consuming three diet sodas a day doubled the risk of becoming obese over time.
Fowler SP et al. Fueling the obesity epidemic? Artificially sweetened beverage use and long-term weight gain. Obesity (2008) 16 8, 1894–1900.
2. Key finding: Drinking diet soda increases risk for metabolic syndrome and type 2 diabetes in kids and teens. This study followed kids over five years, using past and current diet soda consumption to predict new diagnoses. Drinking diet soda every day was associated with a 36% greater relative risk of metabolic syndrome and a 67% greater relative risk of type 2 diabetes.
Nettleton JA et al. Diet soda intake and risk of incident metabolic syndrome and type 2 diabetes in the Multi-Ethnic Study of Atherosclerosis (MESA).Diabetes Care. 2009 Apr;32(4):688-94.
3. Key finding: Drinking diet soda is associated with increased risk of stroke. This study aimed to demonstrate the increased stroke risk associated with drinking sweetened soda, but found both regular and diet soda were associated with increased risk over a 28-year study, compared to non-consumption and drinking other beverages (eg coffee).
Bernstein AM et al. Soda consumption and the risk of stroke in men and women. Am J Clin Nutr. 2012 May;95(5):1190-9.
This is just a sample of research demonstrating that diet soda is just a big a risk factor as regular soda. Those familiar with the research tend to view diet soda as potentially even more problematic than regular soda.
So while I admire the intention behind the supersized soda ban, and appreciate how it draws on both psychological and medical research, I wish Bloomberg had a better science advisor.
Kelly McGonigal is a psychologist at Stanford University. Her latest book, which is full of strategies for behavior change, is The Willpower Instinct: How Self-Control Works, Why It Matters, and What You Can Do to Get More of It.
Follow Kelly on Twitter: http://twitter.com/kellymcgonigal