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A doctor has explained the science behind Slimming World’s controversial rule around one food so people can understand it, as they’ve been left baffled for years
As we usher in a new year, many individuals are keen to shed the extra pounds gained over the festive period. Remember, you’re perfect just as you are, but if you’re aiming to adopt a more mindful eating approach, establishments like Slimming World are well-known for setting you on the right path.
However, one of Slimming World’s rules has left dieters scratching their heads in the past. Many have labelled it ‘odd’, with others claiming it lacks any logical sense. But a doctor has now shed light on the scientific reasoning behind this rule. So, what is this elusive rule?
It’s the banana rule. For those unfamiliar with Slimming World, there are certain foods you can consume freely – and bananas fall into this category.
However, if you opt for a mashed banana, it will cost you some of your ‘syns,’ despite containing the same number of calories.
Slimming World captioned a video: “What’s the deal with mashed bananas? Dr Sarah Bennett, one of our expert nutritionists, is here to set the record straight”.
The video sparked a flurry of comments about the banana rule, including: “I can eat 20 bananas and not have to syn. But mash one and it’s five syns. Make it make sense” and “You don’t swallow a banana whole, you mash it with your mouth to swallow, so how does that work?”
Dr Sarah Bennett clarified that while mashed and whole bananas do contain the same number of calories, there is indeed a scientific rationale behind the dietary plan’s decisions.
She explained: “Mashed, blended, and juiced fruit goes down easier, and it has less volume, so it’s less filling. Also, the process of feeling full starts long before the food reaches our stomach.
“Chewing releases satiety hormones, as do the later stages of digestion. By mashing the banana, you reduce the impact of both of these steps, so you don’t feel as full. That’s when you might find yourself reaching for other snacks later”.
Dr Sarah Bennett clarified that as a Slimming World member, “you can still eat mashed banana,” noting that “nothing is banned”. However, “when it comes to weight loss, eating whole fruit has a bunch of benefits”.
Research conducted in 2025 examining whether whole fruit or fruit juices were superior found that scientists discovered “fruit consumed in solid form provides greater satiety due to delayed gastric emptying and related physiological reactions”.
They also revealed that the “synergistic effects of polyphenols and fibre in whole fruit benefit the gut microbiome by acting as prebiotics and producing short‐chain fatty acids that reduce inflammation”.
In summary, there are more “health benefits in whole fruit than fruit juice”.
A senior dietician at The British Heart Foundation notes: “Whole fruits generally contain less sugar than foods like cake, biscuits and chocolate, and come without the added fat.
“Because of their water and fibre content, they will fill you up for relatively few calories.
“A medium banana, for example, contains less sugar than a standard 50g bar of milk chocolate and half the calories, making fruit a good swap for sweet treats.”
