We've all heard the saying that an apple a day keeps the doctor away, but now it seems that not just any apple will do — it's the Granny Smiths that have the real power.
US researchers found that Granny Smith apples are fantastic obesity-fighters thanks to high levels of dietary fibre and polyphenols.
"We know that, in general, apples are a good source of these nondigestible compounds but there are differences in varieties," said lead researcher Giuliana Noratto a food scientist from Washington State University.
"Results from this study will help consumers to discriminate between apple varieties that can aid in the fight against obesity."
Noratto and her colleagues assessed the compounds from various apple types and found Granny Smiths trumped Braeburn, Fuji, Gala, Golden Delicious, McIntosh and Red Delicious for the number of indigestible compounds they contained.
"The nondigestible compounds in the Granny Smith apples actually changed the proportions of faecal bacteria from obese mice to be similar to that of lean mice," Noratto said.
Nondigestible compounds are good for the health because they promote good bacteria that "clean out" the digestive system.
They stay whole — even after being chewed and subjected to stomach acid and digestive enzymes — so they're still intact when they reach the large intestine where bacteria ferments them, which helps keep the gut healthy.
In obesity, bacteria levels in the colon are often imbalanced which increases inflammation, which in turn is associated with diabetes.
"What determines the balance of bacteria in our colon is the food we consume," Noratto said.
Eating a healthy diet helps the colon get a good balance of bacteria, which stabilises metabolic processes to reduce inflammation.
Milena Katz, spokesperson for the Dietitian's Association of Australia, told ninemsn that Granny Smiths tend to be more sour so they have a lower glycaemic index — thus keeping you fuller for longer.
"You need two pieces of fruit a day and a Granny Smith is a great option," she said.
"But oranges and pears also have a lot of insoluble fibre, especially if you eat the pear skin and the white bits of the orange."
Katz says one apple a day won't cure obesity: the message from this study is that eating high fibre foods is important for health and preventing weight gain.
"Eating good quality yoghurt with a probiotic (and) eating five serves of vegetables is a good general recommendation," she said.
"If people actually ate about 40g per day of fibre, you are going to start feeding your gut the right amount of nourishing nutrients."
The study was published in the journal Food Chemistry.