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We often hear people talking about the ‘magic’ of Slimming World – and it definitely does feel like magic when you find out that you can eat the foods you love, never feel hungry and lose weight. But that magic is based on sound scientific principles – and we wanted to pull back the curtain to give you a look at the fascinating science behind Slimming World’s Food Optimising plan.

We’re really proud of the decades of research that have gone (and the research that continues to go) into the creation of our effective, easy-to-follow way to help people lose weight and feel healthier and happier. So, grab a cuppa and enjoy a slice of the science behind Slimming World, brought to you by our Special Advisor on the Science of Weight Management, Dr Jacquie Lavin:

“The approach behind Food Optimising has always been based on science: not just the physiological science of weight loss but also the psychology – how we think about (and our emotional reactions to) food, our weight and the process of losing weight.

To lose weight, we need to consume fewer calories than we use. This creates an energy deficit and the body then starts to use its energy reserves, which are stored as body fat. At Slimming World, we want to help people lose weight and to maintain this weight loss in the long term – and this means offering a plan that’s easy to stick to and enjoyable, and is sustainable in the long term.

Faddy diets may work in the short term by drastically cutting calorie intake, but a diet that isn’t practical – for instance, that needs endless monitoring or counting, leaves us feeling hungry or deprived of our favourite foods, or means we can’t eat in the same way as our friends and family – is going to be very difficult to keep up. And as soon as we go back to our old way of eating, we’ll stop losing weight and are likely to regain the weight we lost. 

So, what’s the science behind Food Optimising?

Since the beginning, Food Optimising has been based on the importance of allowing members to satisfy their appetite and avoid the feelings of hunger and deprivation that are likely to lead to giving up. 

From the very early days, Slimming World founder Margaret Miles-Bramwell OBE recognised that a huge fear for slimmers is that they’ll need to restrict and limit what they eat to lose weight – which inevitably leads to feelings of hunger, deprivation and, often, rebellion. When we rebel, we’re more likely to reach for those high-fat, high-sugar foods that may have been responsible for our weight gain in the first place, often feeling as if we’ve failed and lost control of our good intentions. This can lead to feelings of guilt and shame for many, and create a vicious cycle of yo-yo dieting. This is exactly what Food Optimising is designed to avoid.

Food Optimising has successfully embraced the scientific principles of appetite regulation in a practical way, to help members reduce their energy intake and lose weight without ever feeling hungry or deprived. 

It’s based on an understanding of how a food’s nutrient content and energy density (its calories per gram) affect satiety (our sense of satisfaction and fullness). Slimming World has been actively involved in this area of research for more than 25 years. 

There’s strong evidence that foods higher in protein and carbohydrates are far more satiating than foods high in fat. Research also shows that we feel full due to the volume of food we eat, not the number of calories we consume. Choosing low energy dense foods (those with fewer calories per gram) means we can eat a larger volume of food to help us feel full for fewer calories, thereby naturally reducing our energy intake and helping us lose weight. For example, 250g of carrots contains around the same amount of calories as just 20g of chocolate – that’s a lot of carrots! 


Discover more about the Slimming World plan – including how our flexible eating plan goes hand-in-hand with activity and support here.

What are Slimming World Free Foods?

Free Foods are at the heart of Food Optimising. They include a wide range of foods such as fruit, veg, pasta, rice, potatoes, pulses, lean meat (and some meat replacements), fish and poultry. Free Foods are low in energy density and higher in protein and carbohydrate, which helps to satisfy the appetite and keep us feeling fuller for longer. Slimming World members love that they can eat Free Foods in unlimited amounts – with no counting, weighing or measuring needed, until they feel full – as well as the flexibility to choose Free Foods in the amounts that fit with their individual needs and requirements, for a truly personalised plan.

Scientific study

Our approach, allowing members to eat an unlimited amount of low energy dense Free Foods, has been shown to be a highly effective way to lose weight while still feeling satisfied. A study (1) led by the University of Leeds and published in the Journal of Nutrition showed that eating low energy dense foods (based on Food Optimising) left participants feeling less hungry and more full, while consuming fewer calories.

Despite eating a much larger volume of low energy dense food – on average 1.2kg more than on the days they consumed high energy dense meals – the participants’ calorie intake was actually much lower. On low energy dense meal days, participants consumed an average 1,901 calories, compared with 2,958 on the high energy dense days. 

The study also compared people following traditional calorie-counting, portion-controlled diets with slimmers following Food Optimising. The Food Optimising group reported feeling more in control of their food choices and more motivated to continue losing weight – and lost nearly twice as much weight over 14 weeks.

By filling up on low energy dense foods, we can eat a larger amount of food and feel more satisfied while losing weight, which makes it easier to stay on track – and there’s never any guilt about what we’re eating or how much is on the plate. 

the science behind Slimming World - the slimming world blog

More on the blog: 5 favourite Free Food recipes

What about Slimming World Syns?

The inclusion of Syns is a key psychological component of Food Optimising’s success. It allows members to enjoy a variety of foods which may be banned on a typical diet. Besides hunger, another reason we find diets hard to follow is because we feel like we’re missing out on our favourite foods (such as chocolate, biscuits, crisps or alcohol) or everyday essentials (a bit of butter/spread, sauce or gravy with a meal). The mere suggestion that we can’t have something can automatically make us want it even more, even if we didn’t really want it in the first place! If a food is banned, we can start to crave it or just feel that it’s going to be impossible to live without it – which adds stress and anxiety around food and trying to manage our weight. 

So Syns are the stroke of genius in the Food Optimising plan, enabling you to enjoy higher energy dense foods (those foods that may not be great for helping you feel full or for your weight loss) in controlled amounts and without a shred of guilt. 

Free Foods and Syns help our members to develop a healthy balance in their diet – shifting from their old patterns of high energy dense, less-satisfying food choices to eating a higher proportion of low energy dense, satisfying and healthy foods, with the old favourites eaten in greater moderation.

You can find out more about joining Slimming World here.

A perfect balance 

A sustainable eating plan needs to be healthy and balanced. Food Optimising has been designed to align with current healthy eating guidelines, allowing members to get all the important nutrients the body needs. The whole structure of the Food Optimising plan will help you to achieve this balance: with lots of Free Foods, limited Syns and, of course, the addition of Healthy Extras, which work alongside Free Foods to ensure that you’re getting a good overall balance of nutrients, particularly supporting a good intake of calcium and fibre. In fact, research conducted by the British Nutrition Foundation found that the diet of members following Food Optimising meets UK dietary guidelines more closely than that of the average UK adult female population (2).

The result for Slimming World members is a flexible, filling and easy way of eating that’s healthy and balanced, and feels as far from a diet as it’s possible to be. Combined with powerful group support, which allows you to share problems and solutions with other members and your supportive Slimming World Consultant, this helps to develop realistic, healthy habits that you can keep up for life.”



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