An 8-year-old girl came into my dental clinic one day with a mouth full of cavities, so I asked her parents -- as I always ask any parents when their child has a lot of cavities -- what their daughter liked to eat and drink.

After talking a little while, the culprit soon became clear: the girl's parents told me she always drank diet soda and fruit juice.

They were surprised to learn that diet soda and many fruit juices are common causes of tooth decay. After all, fruit juice sounds healthy and diet soda doesn't even have sugar.

In this day and age, I think most people know that regular soda, is loaded with sugar, is bad for their teeth. What people often don't realize is that other seemingly harmless beverages, including sugar-free drinks like diet soda, are also bad for their teeth.

For instance, people often assume that fruit drinks are OK because they see the word "fruit" and think it must be good for them. The truth is that many fruit juices are loaded with sugar.

On a recent trip to the grocery store, I saw organic apple juice that contained 31 grams of sugar in each 8-ounce serving. Compare that to the roughly 40 grams of sugar you'll find in most sodas.

I recommend looking at the nutrition label on these drinks closely before you buy them for yourself of your children. If there's more than 10 grams of sugar per serving, I'd stay away from that product, or at least not drink it regularly.

Generally speaking, people know that sugar causes tooth decay. That's why they often feel like they can drink as many sugar-free diet sodas as they want without any repercussions for their teeth.

It's true that diet sodas aren't as bad as regular sodas, but they still can cause significant damage to someone's teeth, especially when people drink them too often. While diet sodas don't have sugar, they contain other elements found in regular soda that are bad for your teeth, particularly acids.

Many diet sodas have phosphoric acid, which is used to add flavor. Phosphoric acid can erode the protective enamel on your teeth and pave the way for decay. It also reduces the amount of calcium in people's bones and teeth.

Some diet sodas don't have phosphoric acid, but they still may contain citric acid, which also can erode tooth enamel.

When it comes to dental health, there is at least one carbonated beverage that you can drink guilt-free: plain sparkling water. Simply adding carbonation to water doesn't pose a threat to your teeth.

The trouble begins when flavoring agents or sweeteners are also added to the mix. Although these additives may not contain sugar, they make sparkling water more acidic and can create problems for your teeth.

I don't want to tell people they should never drink regular or diet sodas, fruit juices or flavored sparkling waters. That's just not realistic.

The important thing is to consume these drinks in moderation. That way, when you come to see me for a checkup, I can send you home with a smile on your face instead of a dental filling in your mouth.

Chiyo Shidara is a dentist at Contra Costa Regional Medical Center & Health Centers. Healthy Outlook is written by the professional staff of the county health department. Send questions to Dr. David Pepper at theairdoctor@gmail.com. For health information, go to www.cchealth.org.