Scientists compared the sugar levels in orange juice and milk London – Naeem Arzu Washing down a sugary breakfast with a glass of milk can help prevent tooth decay, says a group of US dentists. According to Christine Wu , professor of dentistry at the University of Illinois, dry sugar-coated cereals contain high levels of carbohydrates.
Naturally occurring bacteria in the plaque on the surface of tooth enamel breakdown the refined sugar to produce plaque acid, which gradually corrodes the teeth.
A study conducted by Wu’s former student, Shilpa Naval, on 20 adults involved eating 20 grams of dry Froot Loops cereal, which contains around 29 grams of sugar per serving, and then drinking different beverages.   
The pH (plaque acid) was measured before eating, at two and five minutes after eating, and then between two and 30 minutes after drinking a liquid.
A pH level of seven is neutral, anything below this is acidic and anything above is alkaline.
Test results showed that the pH level of the teeth rapidly after just the cereal alone, and remained at pH 5.83, showing that the bacterial plaque acid level had significantly gone up.
Subjects who drank apple juice showed a pH level of 5.84 after 30 minutes, whilst those who had consumed water had pH levels at 6.02.
Those who had consumed milk showed a higher pH level, ranging between 6.4-6.7.
Though fruit juices are considered a healthy drink first thing in the morning, they contain high sugar levels which are unhealthy for teeth.  Milk on the other hand has a fairly neutral pH level and helps neutralise plaque acid, which help prevent tooth corrosion and cavities.