It's impossible to directly compare the health effects of diet and regular soda, since we have far more research on the regular kind.

The studies we do have suggest regular soda is linked with obesity and weight gain, while diet soda hasn't been strongly tied to any negative health outcomes.

For that reason, Aaron Carroll, a pediatrician and professor of pediatrics, says it's fine to drink diet soda occasionally — but never the regular varieties.

Just how bad is your Diet Coke habit?

Probably not as unhealthy as you think. And swapping it for the regular stuff won't do you any favors.

"If I have to choose between diet drinks and those with added sugar, I'll go with the diet," Aaron Carroll, a professor of pediatrics at the Indiana University School of Medicine, writes in his new book, "The Bad Food Bible: How and Why to Eat Sinfully."

The existing science on diet soda hasn't gotten the golden seal of approval that comes after extensive studies in humans. The research we do have is mostly in mice, but so far it suggests that the artificial sweeteners in soda are not overwhelmingly bad for our health.

On the other hand, extensive studies about human consumption of regular soda reveal an overwhelming link to two unhealthy outcomes: weight gain and obesity.

Comparing these two scenarios makes the choice between diet soda and regular soda clear, according to Carroll.

"There's a potential — and likely very real — harm from consuming added sugar. There is likely none from artificial sweeteners," he writes.

Still, dozens of seemingly damning scientific studies on diet soda and the artificial sweeteners it contains have been published over the last decade. Research has linked saccharin to cancer, and found evidence that people who regularly consume diet drinks are heavier than people who don't.

But Carroll says every one of those studies was riddled with enough errors not to be taken seriously. The way they were communicated to the public made it look like they were far more conclusive and severe than they really were, he wrote.

Research tying artificial sweeteners to cancer was done in rats who were vulnerable to cancer

Flickr/Global Pandora In the 1980s, any product containing saccharin, the zero-calorie sweetener sold under the brand name Sweet'N Low, was required to carry a frightening warning label: "This product contains saccharin, which has been determined to cause cancer in laboratory animals."

Out of roughly 50 studies of rats fed saccharin — in amounts far beyond what a normal person would consume on a daily basis — only one experiment found that it seemed to cause cancer. But the rats in that study were a specific kind of rat that often become infected with a bladder parasite, Carroll explains in his book. The parasite leaves them extra vulnerable to cancer.

Carrol writes that the conclusion people should have drawn from all those studies is: "Rats are more vulnerable to side effects from saccharin than are people, for whom there's no clear evidence of risk."

Instead, artificial sweeteners everywhere have gotten a bad rap. Many were shocked by the recent news that President Donald Trump drinks roughly a dozen Diet Cokes per day. Of course, doing anything to excess is probably not a good idea. But dozens of further studies have failed to find any link between saccharin and cancer in humans, despite lingering skepticism.

"Public perception, once turned against any food, is very hard to change," Carroll writes.

The study linking diet drinks to dementia also found a link between regular soda and dementia

Other recent studies link diet drinks to even scarier-sounding health outcomes, including research published this spring suggesting that diet soda could cause dementia.

But that study was only one of a pair of studies looking at beverage consumption and brain issues. The second found a connection between drinking regular soda and brain shrinkage. More importantly, both studies fall into a category of research commonly found in food and beverage science called "observational" research.

Observational studies follow large groups of people over long periods of time and don't control for the variables they are testing. As a result, these kinds of studies can tell us if there's a connection between two things, but they can't tell us if one thing necessarily causes the other. In many cases, a link researchers observe between two things is later found to be caused by an external thing that no one accounted for.

The jury is still out on whether any external factor played a role in the studies about drinks and dementia. For now, there is no definitive research that tells us that sugary drinks cause brain shrinkage or that diet drinks cause dementia.

Plenty of research links regular soda to disease

Plenty of large, well-done studies link sugary drinks like soda and juice to weight gain, obesity, diabetes, and heart disease in human beings.

La Piazza Pizzeria/Flickr

One of them was a large review of 50 years' worth of studies published in the American Society for Clinical Nutrition. It found "strong evidence for the independent role of the intake of sugar-sweetened beverages, particularly soda, in the promotion of weight gain and obesity in children and adolescents."

Another paper written by seven experts in public health, nutrition, and economics describes the links between sugary drinks and America's obesity problem even more strongly:

"The science base linking the consumption of sugar-sweetened beverages to the risk of chronic diseases is clear," the authors concluded.

With this in mind, Carroll appears justified in his choice of diet soda over regular.

"When it comes to sugar and artificial sweeteners, the evidence is as strong as can be: the former is much worse for you than the latter," he wrote.