Well-Designed Study Finds Diet Soda No Worse Than Water for Type 2 Diabetes

A clinical trial that has largely escaped public attention found something that runs counter to years of health messaging: people with type 2 diabetes who switched from artificially sweetened drinks to water did not improve their health. In fact, their blood sugar control got worse.

The study, called SODAS (Study Of Drinks with Artificial Sweeteners), enrolled 181 adults with type 2 diabetes who regularly consumed diet beverages. Researchers at the University of California, Irvine, and the University of Minnesota divided them into two groups. One continued drinking their preferred artificially sweetened beverage—Diet Coke, Diet Pepsi, or similar products—while the other switched to water or unsweetened seltzer. Both groups received three servings daily of their assigned drink for 24 weeks. The trial was funded by the National Institute of Diabetes and Digestive and Kidney Diseases, part of the National Institutes of Health, and received no sponsorship from beverage companies.

The primary measure was hemoglobin A1C, a blood test that reflects average glucose levels over roughly three months. In the group that continued drinking diet sodas, A1C improved slightly, dropping from 7.19% to 7.14%. In the water group, it worsened, rising from 7.20% to 7.44%. The researchers also tracked fasting glucose, fasting insulin, and continuous glucose monitoring data—all of which favored the diet drink group. People drinking diet sodas lost an average of two pounds, while the water group's weight remained stable. The researchers acknowledged this weight difference as statistically real but too small to have clinical meaning.

Gideon Meyerowitz-Katz, an epidemiologist at the University of Wollongong who was not involved in the research, described the study as more rigorous than most nutrition science that receives media attention. The trial enrolled 179 participants who completed it—a substantial sample size for this type of research, roughly double the size of a competing study from years earlier that had reached opposite conclusions. That older trial, which involved 81 women in a weight-loss program, found slight A1C improvement when switching to water. The authors of the new study note the two trials are not directly comparable, since one focused on weight loss while theirs did not.

The findings invite caution about how we interpret nutrition research. This study tells us something specific: for adults with type 2 diabetes whose blood sugar is already stable and controlled, switching from diet beverages to water offered no benefit and may have caused slight harm. It says nothing about people with poorly controlled diabetes, people without diabetes, or health outcomes beyond blood glucose management. The researchers themselves emphasize that maintaining consumption of artificially sweetened beverages may be a reasonable tool for managing type 2 diabetes in people whose glucose levels are already stable.

Meyerowitz-Katz suggests the most likely explanation is that diet drinks and water are essentially equivalent for blood sugar control. But he does not rule out a genuine effect: perhaps the sweetness satisfies cravings and helps people avoid sugary snacks, or perhaps artificial sweeteners have some direct metabolic benefit. The study was not designed to answer why the difference exists, only that it does. What remains clear is that the conventional wisdom—that people with diabetes should always choose water over diet soda—does not hold up under scrutiny in this population. The question of whether that finding applies more broadly remains open.