An apple a day may keep obesity away, particularly if it is of the Granny Smith variety, according to a new study from Washington State University.

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Researchers say that Granny Smith apples may protect against obesity and its related disorders by balancing gut bacteria.

The research team, led by Giuliana Noratto of the School of Food Science at Washington State, publish their findings in the journal Food Chemistry.

Apples have many health benefits, according to previous research. Last year, Medical News Today reported on a study suggesting that eating an apple a day may be just as beneficial as daily statin use for preventing vascular mortality.

A 2011 study also claimed that apples and pears may reduce the risk of stroke by more than 50%.

But according to the team involved in this latest research, very few studies have looked at how the bioactive compounds in apples that are not absorbed during digestion - such as polyphenols (a type of antioxidant) and dietary fiber - affect the friendly gut bacteria that boost immunity and aid weight maintenance.

Furthermore, Noratto notes that few studies have looked at how the effects of these bioactive compounds vary by apple variety.

Non-digestible compounds in Granny Smiths boost growth of good gut bacteria

With this in mind, Noratto and her team analyzed how the bioactive compounds of seven different varieties of apples - Granny Smith, Braeburn, Fuji, Gala, Golden Delicious, McIntosh and Red Delicious - affected the good gut bacteria of diet-induced obese mice.

The researchers found that, compared with all other apple varieties, Granny Smiths appeared to have the most beneficial effect on good gut bacteria.

The balance of gut bacteria among obese people is usually impaired. But when comparing the feces of obese mice fed Granny Smiths with the feces of lean mice, the team found that the proportions of bacteria - specifically the presence of Firmicutes, Bacteroidetes, Enterococcus, Enterobacteriaceae, Escherichia coli and Bifidobacterium - were very similar.

The researchers explain that Granny Smiths are high in non-digestible dietary fiber and polyphenols, and low in carbohydrates. Even after chewing and exposure to stomach acid and digestive enzymes, the compounds are unscathed when they reach the colon. The bacteria present in the colon then ferment the compounds, producing butyric acid that spurs on the growth of friendly gut bacteria.

Around 34.9% of American adults are obese and, as a result, are at increased risk of obesity-related conditions, such as heart disease, stroke, type 2 diabetes and even cancer.

Noratto says their findings may lead to strategies that prevent obesity and its associated disorders.

An imbalance of gut bacteria - determined by the food we eat - can cause chronic inflammation that leads to diabetes. But the researchers say that re-establishing the balance of gut bacteria through consumption of Granny Smith apples may reduce such inflammation, as well as promote satiety.

Commenting on the findings, Noratto says:

"We know that, in general, apples are a good source of these non-digestible compounds, but there are differences in varieties. Results from this study will help consumers to discriminate between apple varieties that can aid in the fight against obesity."

Our Knowledge Center article on the health benefits of apples looks at some of the other ways the fruit can be good for us.