It's common knowledge for most people to brush after meals; however, to keep your teeth at their healthiest, it is far better to brush your teeth before eating rather than after in many cases.
Anytime you eat or drink anything that is sugary, the bacteria in your mouth feed on these sugars to make acids. These bacterial acids cause the overall pH in your mouth to drop below normal levels and make your tooth enamel "softer" and more vulnerable to abrasion. The same is true when you consume acidic foods or beverages (i.e. citrus drinks, salad dressing, soda etc.)
For most people the pH at which your tooth enamel begins to demineralize is about pH ~5.5. The process of bringing your oral pH back to normal after eating something sugary is far from immediate and typically takes about 30 minutes with healthy saliva, so you should not rely on normal saliva flow alone to reverse these drops in pH. Consider chewing xylitol gum or usingan alkaline mouth rinse to help reblance your oral pH quickly.To the right, I embedded a chart of what your oral pH looks like after meals.
The risk of brushing immediately after meals is that you'll end up scrubbing away acid-softened layers of your enamel with your toothbrush, which is never good. Additionally, when you drink or eat things that are already highly acidic your teeth are at an even greater risk for toothbrush abrasion. Research has shown that avoid amplifying the damage caused by acids you should wait 60 minutes to engage in brushing after significant acidic exposures. Though you should still perform a slight cleaning with floss or other tools to remove stuck foods.
All things considered you'll be more effective at preventing cavities by brushing before eating or drinking since you can remove as many bacteria as possible before "feeding" them (an exception is brushing before bedtime, which you definitely should do). Reducing the total number of bacteria in your mouth that can produce acid before you eat will limit how acidic your oral pH becomes and will introduce fluoride into the dental plaque fluid to reduce the total damage bacteria can do to your teeth as well. Some studies have shown that, in areas of the mouth where bacteria may not have been brushed away for a few days, the pH can remain acidic for several hours so keeping your teeth clean is very important.
Sodium lauryl sulfate in many toothpastes is responsible for alterations in taste that cause people to avoid brushing before meals, so finding an SLS-free toothpaste is key, though you can always use a wet toothbrush to clean but you won't have the benefit of the fluoride. Also don't forget that you should always use a low abrasive toothpasteand to only use pea sized amount of toothpaste. It is imporant to understand that just by brushing before meals, you aren't leaving food to hang around on or between your teeth after a meal. Immediate use of an alkaline rinse (ideally containing fluoride to promote remineralization), stimulation of saliva production with xylitol gum or mints, as well as a light cleaning to flush out stuck foods and sugars are all important steps to maintain a healthy mouth throughout the day.