Proof that diet soda actually leads to reduced calorie counts in a meal

Some consumers go to fast food outlets and order high-calorie meals like cheeseburgers and fries before choosing a diet soda to go with their meal. But are they really saving on calories? Or do those low-cal drinks encourage them to buy more fattening foods?
Some studies suggest that people may experience the “Big Mac and Diet Coke effect” where they justify ordering bigger burgers, larger fries, or a dessert because they consider some part of their meal healthier. Meanwhile, others have shown that ordering diet pop can encourage better habits, or at least reduce the overall caloric load.
According to a new study from the University of British Columbia’s Sauder School of Business, on average those diet drinks really do result in a significantly lower calorie count.
For the study titled “Do Consumers Order More Calories in a Meal with a Diet or Regular Soft Drink? An Empirical Investigation Using Large-Scale Field Data,” researchers used data from Canada-wide consumer surveys conducted between 2000 and 2007 that included peoples’ “food away from home” meal consumption.
Because McDonald’s makes its nutritional information (including calorie counts) widely available, and because its offerings are consistent across the country, the team homed in on the consumer data involving McDonald’s meals specifically, then merged it with the fast food chain’s calorie information.
In total, they examined roughly 9,000 McDonald’s meals involving more than 2,000 people. Of those, roughly 64% of those surveyed ordered regular soft drinks, 20% ordered diet pop, and the remaining 16% went back and forth between the two. The researchers also focused on more calorie-rich meals (lunches and dinners) and on larger drinks (medium or large) because they would more clearly demonstrate the caloric gulf between those who ordered diet and those who chose sugary sodas. Because the data spanned seven years, they could also see patterns in buying behaviours.
“For a particular person, we might see five, six, seven occasions, and we could see what their individual patterns were,” says UBC Sauder Professor Emeritus Charles Weinberg, who co-authored the study with Amazon Inc. Research Scientist Sina Ghotbi and University of Guelph Associate Professor Tirtha Dhar. “So, we could control for individual effects.”
After crunching the numbers, the researchers found no evidence that consumers use diet drinks to justify other indulgences. In fact, after controlling for drink size and consumer demographics, they found that people who bought diet drinks had significantly lower overall calorie counts: 298 fewer calories for those who ordered a large diet drink — or 18 fewer tsp. of sugar — and 156 fewer calories for those ordering a medium (about 10 tsp.). What’s more, the total food calories they ordered did not go up when they ordered a diet drink.
The research marks the first field study to examine whether “Big Mac and Diet Coke” behaviour leads to greater calorie consumption. “We’re looking at the actual behaviour, not why people engage in it,” says Weinberg. “So, we’re focused on what people actually do.”
At first blush, the question might seem lighthearted, but in reality it’s serious business, especially given that the obesity crisis in the United States has worsened dramatically — from 12% of U.S. adults in 1990-1991 to 42.4% in 2017-2018 — and the consumption of sugary, high-calorie sodas is considered to be a major factor. What’s more, 85 million Americans — or 37% of the population — dine at a fast food restaurant per day.
Weinberg emphasizes that from a health perspective, drinking water with a meal is still the healthiest option, but for consumers who are set on soda, diet pop might represent a healthier alternative than sugary drinks.
As a result, Weinberg says health authorities may want to rethink their advice on soft drinks. It’s not that they should necessarily advise people to drink diet sodas, but they may not want to caution against it — and making diet soft drinks more available could form part of an effective harm-reduction strategy.
“People like going to fast food restaurants, and in the obesity crisis one of the main culprits is people having added sugars in their meals,” says Weinberg. “If you’re a person who likes to have carbonated soft drinks and you’re not willing to switch to water, this seems to be an intermediate step you can take.”
In the meantime, he says consumers should understand the impact of their decisions.
“People make choices. There are going to be people who say, ‘I prefer the taste of regular soda and I want to enjoy it,’” says Weinberg. “It’s not like you have to give up every pleasure. But you should know in terms of calories what that pleasure is, and what it’s costing you, then make the choice.”