March 19, 2015 7:02 AM EDT

Diet soda gives you a sugar rush far stronger than the granulated stuff in a sugar bowl ever could–and for no calories. But research is mounting that low- and no-calorie sweeteners may not be great choices for dieters. A recent study found that over nine years, diet-soda drinkers gained nearly triple the abdominal fat–3 in. (8 cm)–as those who didn’t drink diet soda. Though scientists are still puzzling over how this may happen, here’s what they think is going on.

1. Not all sweetness satisfies.

When you consume enough real sugar, your brain gets the message, and a sense of satiety–or fullness–takes over. “Regular sugar has caloric consequences,” says Dr. Helen Hazuda, professor of medicine at the University of Texas Health Science Center at San Antonio and senior author of the new study. “Your body is used to knowing that a sweet taste means you are ingesting energy”–that means calories–“and that if you don’t burn them off, it’s going to convert to fat,” she says. But the most popular artificial sweetener in diet drinks, for instance, is about 200 times sweeter than sugar without triggering a feeling of satiety.

2. That can lead to overeating.

Bad things can happen when you strip sweetness of its power to satisfy: the link between eating and the role of calories in your body starts to crumble. According to a recent study, when rats ate yogurt mixed with an artificial sweetener, they consumed fewer calories and gained more weight than rats that ate sugar-sweetened yogurt, suggesting that the no-calorie sweeteners interfere with a natural ability to regulate incoming calories. This–no surprise–can lead to sugar cravings and weight gain.

3. It may also mess with your microbes.

A recent study in Nature found that artificial sweeteners changed the colonies of gut bacteria in mice in ways that made the animals vulnerable to insulin resistance and glucose intolerance, which are metabolic disorders that can lead to weight gain and increase the risk of Type 2 diabetes.

4. It might be bad for your heart.

In a study based on dietary questionnaires of 9,500 people, those who said they drank one can of diet soda a day had a 34% higher risk of metabolic syndrome–a cluster of risk factors that can lead to heart disease and Type 2 diabetes–than those who didn’t drink diet soda. The study stopped short of drawing a cause-and-effect link, but the association surprised the authors, who called for more research.

Sources: Journal of the American Geriatrics Society; Behavioral Neuroscience; American Journal of Clinical Nutrition; Nature; Circulation

This appears in the March 30, 2015 issue of TIME.

Write to Mandy Oaklander at mandy.oaklander@time.com.

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