Apples may help to prevent obesity. Reuters

"An apple a day keeps the doctor away" is an old saying that should be extended to dieticians and nutritionists. Apples have zero fat, sodium and cholesterol and are packed with antioxidants and vitamins -- and a new study suggests the fruit could be a powerful tool against obesity. As the Centers for Disease Control and Prevention notes, 34.9 percent of American adults, or 78.6 million, are obese.

Researchers from Washington State University tested nondigestible compounds, dietary fiber and polyphenols -- complex chemicals found in fruits and the reason why your apple or banana turns brown -- and discovered these compounds survived the gauntlet that is your digestive system and remained intact inside the colon. The survival of these compounds is important as they interact with all the bacteria in your colon. The bacteria ferment the nondigestible compounds, which provides a boost to the production of good bacteria, notes the researchers' report.

The study involved mice and while there was a change in gut bacteria levels in obese mice, that same effect may be reduced, or not appear at all, in humans.

Gut flora, all the bacteria inside your digestive tract, are important for a host of health reasons and affect your immune system and allergies. Gut bacteria also influence weight loss and gain.

Not all apples are created equal for improving the growth of good bacteria. Researchers tested Granny Smith, Braeburn, Fuji, Gala, Golden Delicious, McIntosh and Red Delicious apples and found Granny Smith apples to have more nondigestible compounds than other varieties.

"The nondigestible compounds in the Granny Smith apples actually changed the proportions of fecal bacteria from obese mice to be similar to that of lean mice," lead researcher Giuliana Noratto, a food scientist at WSU, said in a statement.

If the results hold true for humans, apple consumption could help prevent chronic inflammation, which can lead to diabetes, and other metabolic disorders linked to obesity.

Check out the original research, which was published in the journal Food Chemistry.