How can eating between meals cause tooth decay?

Frequent snacking on carbohydrate-containing foods can be an invitation to tooth decay. The decay process begins with plaque, an invisible, sticky layer of harmful bacteria that constantly forms on teeth. The bacteria convert sugar and starch that remain in the mouth to acid that attacks tooth enamel. The longer sugars are retained in your mouth, the longer the acids attack. After repeated attacks, tooth decay can result.
This is a tricky question nowadays due to the fact that there are so many diets that recommend eating several meals a day!  The old paradigm of "3 square meals" is often not the norm. Should a person that is following the guidelines of one of these diets (or someone who eats "constantly" throughout the day) consider brushing and flossing after every one of them?  Without getting "gross", the easy answer seems logically to be yes. But, consider the fact that it may not be possible or convenient to do so. Some good news for those who don't wish to become obsessed with a "mobile" toothbrush and flossing station. I wouldn't worry about these diets/habits in regard to decay, as it takes some time for the biofilm to attach itself to teeth. The biofilm layer is usually necessary for the acid environment caused by bacteria attaching itself to the teeth to cause harm.

If good hygiene is practiced at least once in every 24-hour period of time, then decay should not be a factor of concern for the majority of people. I know this might upset some professional people, nonetheless, it's true-you don't have to become obsessed!  Bad breath, on the other hand, might be a greater concern and the recommendation for that now is either gum with xylitol or a non-alcohol based mouth rinse for the constant eating. 
Romesh Nalliah
Each snack is a source of nutrition for decay forming bacteria. When you increase the number of snacks you increase the opportunities for these bacteria to gain nutrition and form cavities. However, some weight loss programs recommend snacking as a way to increase metabolism and lose weight. In these circumstances it is important to be vigilant about maintaining your oral hygiene after snacks.

More meals/snacks = more chances for food particles to create bacteria in your mouth, which leads to decay. In addition, if you are snacking, there usually isn’t a chance to brush your teeth afterwards (after all, how many people keep a toothbrush in their car for use after the drive-thru?) I would recommend having a toothpick handy (and even a pick/flosser), and swish a little water around your mouth after snacking. It’s not a perfect solution, but it will help.

Eating between meals increases the time that your teeth are exposed to food. If the foods contain sugar or natural sugars you will be at a greater risk for tooth decay. The longer the sugar is on your teeth the greater the risk for decay. If you do snack, make sure that your snack is healthy and low in sugar. Try to brush your teeth after snacking if possible. If you cannot brush, chewing a sugar free gum and/or rinsing with water can help remove some of the sugars. The bacteria that causes decay uses the sugars to make acid. The acid then eats into the tooth causing decay.

Aaron B. Schwartz, DDS
Pediatric Dentistry
Eating between meals allows for carbohydrates to remain on teeth for extended durations. These carbohydrates are consumed by specific bacteria. These bacteria then excrete (poops out) acid which 'erodes' through the dental enamel (protective armor), initiating the caries process. 

Don't leave carbohydrates sitting on teeth, allowing the bacteria to do their dirty work!  Brush 3x/day and floss every night.
Eating between meals increases the amount of availible sugar for the cavity-causing bacteria to feed on. Having a more constant amount of sugar in the mouth makes it easier for these bacteria to destroy tooth structure, leading to tooth decay.

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