DEAR DOCTOR K >> My beverage of choice is diet soda, but my girlfriend told me I should stop drinking it now that I'm pregnant. Is she right?

DEAR READER >> For some time now, research has shown that when pregnant women gain a lot of weight during pregnancy, they have heavier babies. Heavier babies tend to grow into heavier children, who tend to grow into heavier adults. And being overweight or obese in childhood can adversely affect a child's future health and well-being. So it's a good idea for pregnant women to try not to gain more than their doctors tell them to.

One of the ways we try to limit calories is by reaching for artificially sweetened beverages; after all, they don't have calories. But a study recently published in the journal JAMA Pediatrics suggests that drinking these beverages during pregnancy can backfire.

For the study, researchers studied more than 3,000 pregnant women. They found that those who drank more artificially sweetened beverages tended to become heavier during the pregnancy. They also had heavier babies. The finding that the babies were heavier was not entirely explained by the mother's weight and other factors that can affect weight gain (like overall calorie intake or diet quality). It seems like there is something about the sweeteners themselves that makes for heavier babies.

Now, this is just one study. But it was carefully done. In addition, it's not entirely unexpected.


There have been concerns for a while that artificial sweeteners might not be a great idea when it comes to weight control. There is some evidence that these sweeteners actually create a craving for sweets. So while the diet soda doesn't contain calories, it may prompt you to munch on cookies that do.

But some experts speculate that it's not that artificial sweeteners create a craving for sweets. They argue that people feel so virtuous for drinking diet drinks that they think they can afford to eat more calorie-rich foods.

There is even a study, one we discussed not long ago in this column, which found another way in which diet sodas may paradoxically cause you to gain weight. The study found that artificial sweeteners changed the bacteria that live in the gut. The change led the gut to absorb more of the sugar in food. It's the calories that get absorbed and enter the blood that add weight, not the calories that pass out of your body.

Whatever it is, the bottom line is that pregnant women should generally avoid diet drinks. As much as they seem like they might help limit weight gain, they don't. Instead, drink water and other unsweetened beverages. If it's the carbonation in diet sodas that you like, drink sparkling water.

It's fine to have the occasional soda or lemonade [—] as long as your overall diet is healthy (lots of fruits and vegetables, lean protein, whole grains) and you are getting regular exercise.

Dr. Komaroff is a physician and professor at Harvard Medical School.