Here's a possible explanation for the old saying "an apple a day keeps the doctor away." Danish researchers looked into the question of why apples are good for us by analyzing the microbial content of the digestive systems of rats. The study animals were put on a diet rich in apples, apple juice, and apple purée and their gastrointestinal flora was compared to the microbial content of animals on a regular rat diet. They reported that rats eating a diet high in pectin¸ a component of dietary fiber in apples, had increased amounts of beneficial bacteria - the kind known to improve intestinal health. The investigators concluded that as a result of eating apples regularly, the friendly bacteria "help produce short chain fatty acids that provide ideal pH conditions for ensuring a beneficial balance of microorganisms." They also found that the good bacteria produce butyrate, a chemical that is an important fuel for cells of the intestinal wall. More research is needed to see if pectin has the same effects in humans. The study was published Jan. 20, 2010 in BMC Microbiology.