Doctor explains science behind controversial Slimming World rule

The new year has arrived, prompting countless individuals to tackle any extra pounds gained during the festive period. Whilst you’re absolutely fine as you are, if you’re considering a more mindful approach to eating, organisations like Slimming World have built a reputation for helping people achieve their goals.

Yet dieters have previously been puzzled by one particular Slimming World guideline, which many have branded as ‘odd’, with some insisting it’s completely illogical. A medical professional has now shed light on the scientific reasoning behind this controversial rule.

But which guideline has sparked such debate?

It concerns bananas. For those unfamiliar with Slimming World’s system, certain foods can be consumed freely without restriction – and bananas fall into this category.

However, should you mash that banana, it will require you to use some of your ‘syns’, despite containing identical calories.

Slimming World captioned their explanatory 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 opened with numerous bewildered comments about the banana policy, 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 confirmed that both mashed and whole bananas contain identical calorie counts; nevertheless, there’s robust scientific evidence supporting the weight management programme’s approach.

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,” adding that “nothing is banned”. However, she noted that “when it comes to weight loss, eating whole fruit has a bunch of benefits”.

A 2025 study comparing whole fruit and fruit juices found that “fruit consumed in solid form provides greater satiety due to delayed gastric emptying and related physiological reactions”.

The research also highlighted 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, whole fruit offers more “health benefits than fruit juice”.

A senior dietician at The British Heart Foundation stated: “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.”