The diet wars are raging more fiercely than ever. To lose weight, we’re told, we need to follow a low-carb diet. Or maybe a low-fat diet. Or perhaps a Mediterranean, paleo, or plant-based diet. Or just magically reduce calorie intake. It’s all very confusing. To the point of paralysis. To cut through the noise, it’s always helpful to go back to first principles. What do we know about the mechanisms of fat loss? And what is it about certain foods that are more ‘slimming’ or ‘fattening’ than others?

Body weight is ultimately determined by long term energy balance (1), and we can represent this as follows:

• Change in body weight = calorie intake – calorie expenditure (assuming water weight is constant).

A reduction in body weight, therefore, requires a sustained calorie deficit. In other words, calorie intake must be lower than calorie expenditure for a sufficient length of time (more than a few days). So far, so simple. The problem is, body weight regulation is highly complex and dynamic, so creating and sustaining a calorie deficit is complicated stuff (2). For example, increasing calorie expenditure by doing more exercise makes some people eat more, and therefore not lose any weight, or even gain weight (3). Decreasing calorie intake increases hunger and decreases calorie expenditure (4), which dampens the effect of the calorie restriction and makes it difficult to maintain (the drive to eat becomes irresistible). That said, the most effective (non-medical) way to lose weight is by restricting calorie intake (5). Adding plenty of exercise, particularly resistance training, will ensure that most of the weight lost is fat (6).

To complicate things further, different foods have different effects on our appetite regulation, and therefore on our calorie intake. One of the mechanisms involved here is satiety – the reduced interest in food after eating (in other words, the feeling of not being hungry). For the same number of total calories, some foods increase satiety more than others, and therefore reduce calorie intake. This is interesting, because prioritising such foods may allow us to create a calorie deficit (and lose weight) without feeling hungry all the time.

Wouldn’t it be useful, then, if someone had investigated the effect different foods have on satiety. Well, it turns out they have. Back in 1995 a group of Australian researchers tested the sating effects of 38 different foods (7).

The foods were split into six categories:

• Fruits: Grapes, apples, oranges, bananas
• Bakery products: Croissant, cake, doughnuts, cookies, crackers
• Snack foods and confectionary: Mars bar, yoghurt (flavoured), ice cream, Jellybeans, crisps, peanuts (roasted, salted), popcorn
• Protein-rich foods: Cheese, eggs, lentils, baked beans, beef steak, fish
• Carbohydrate-rich foods: White bread, wholemeal bread, grain bread, white rice, brown rice, white pasta, brown pasta, potatoes, French fries
• Breakfast cereals: Cornflakes, Special K, Honeysmacks, Sustain, All-Bran, natural muesli, porridge

Each food was fed to a group of participants on separate mornings, and the participants rated their satiety/hunger every 15 minutes for two hours thereafter. From this data, a satiety score for each food was calculated. The food portions all contained 240 Calories, so differences in satiety scores between foods cannot have been due to differences in calorie intake.

Fruits had the highest satiety scores on average, followed by protein-rich foods and carbohydrate-rich foods, then breakfast cereals, then snacks and confectionary. Bakery products came in a distant last. High-sugar, high-fat junk foods, such as Mars bar, doughnuts, cake, and croissants had the lowest satiety scores of all foods tested. Whole, unprocessed foods like steak, apples, porridge, and fish had some of the highest satiety scores. But the winner, by a country mile, was plain boiled potatoes. They were three times more filling than white bread, and nearly twice as filling as steak! Brown pasta and wholemeal/grain bread were far more filling than their refined, white counterparts, and stacked up well overall. Baked beans, despite presumably containing sugar, also scored pretty well. I should point out here (and this is really important) that nothing was added to the foods during preparation (other than the lentils, which were cooked in a fresh, tomato-based sauce, and the breakfast cereals, which were served with milk). No fats, oils, sweeteners, salt, herbs, or spices were used to enhance the flavour or texture of the foods.

At the two-hour point, participants were allowed to eat as much as they wanted from a ‘standard’ range of food and drink. It will probably come as no surprise that participants ate fewer calories at the all-you-can-eat buffet after being fed the foods with higher satiety scores, indicating that the more filling the food, the fewer the calories consumed at the next meal.

But what is it about the composition of these foods that makes them more or less sating? Thankfully, our friends in Australia answered this as well. The most important factor was the energy density (calories per gram) of the food. The lower the energy density the more sating the food. For example, the energy density of croissants (8) is five times that of boiled potatoes (9).

A close second was the ‘palatability’ of the food, in other words how pleasurable the food was to eat. The less palatable the food (which correlated to lower fat and sugar content), the higher the satiety score. Intuitively we know that Mars bars and crisps are far more pleasurable to eat than plain potatoes or steamed fish, and this extra palatability induces greater hunger and prompts us to eat more calories (10).

Lower fat content was linked with increased satiety. Does this mean we should always choose low fat foods? Not necessarily. Fat content only reduces satiety in so far as it increases energy density (11). So, we don’t need to be concerned about the fat in whole, fresh foods, such as salmon, eggs, and whole dairy, as they have relatively low energy densities. It’s the high-fat, energy-dense junk food that’s problematic. For example, salmon (a fatty fish) contains around 1.8 Calories per gram (12), whereas French fries come in at around 3.3 Calories per gram (13).

Higher protein, fibre, and water content all appeared to increase satiety. Fibre and water content exert much of their influence by increasing portion size and lowering energy density (by the way, for water content to have an effect it must be incorporated into the food, nut drunk alongside the food (11)). Protein, on the other hand, seems to have uniquely sating properties compared to carbohydrates and fat, independent of energy density (14). Indeed, high protein diets have been shown to increase weight loss (and preserve muscle mass) (15).

It would also have been interesting to see how the degree of food processing affected satiety scores. This wasn’t covered in the study, so I analysed it myself. I assigned each food to one of three processing categories: Unprocessed, Processed, or Ultraprocessed. Unprocessed included whole foods, such as fish, potatoes, fruit, and porridge. Lightly processed foods, such as brown pasta, baked beans, cheese, and muesli were considered Processed. And Ultraprocessed included the very refined, junk-type foods, like sugary breakfast cereals, white bread, crisps, and cake. The Ultraprocessed foods had a dismal average satiety score, similar to that of white bread. Unprocessed foods were nearly twice as sating as Ultraprocessed foods, and Processed foods were almost exactly in between.

Now, let’s try to distil this down into some actionable recommendations. If you’re trying to lose or maintain weight, you should try to increase satiety, by doing the following:

• Eat whole, fresh foods such as fish, meat, eggs, fruit, vegetables, beans, and wholegrains, which have low energy density, moderate palatability, and are high in protein and/or fibre.
• Avoid ultraprocessed junk food, such as sugary breakfast cereals, white bread and pasta, French fries, crisps, cookies, cake, ice cream, and confectionary, which tend to be energy-dense and hyperpalatable, and low in fibre and protein.
• Include foods with a high water content, such as porridge, soups, and stews.
• Eat food as close to its natural state as possible. If you start adding fats, sweeteners, and salt to whole foods (for example, butter on your potato, pepper sauce on your steak, or honey on your porridge), you will increase the energy density and palatability, and will likely eat more.

If that all seems a little overwhelming, just remember one, simple rule:

Eat whole, fresh foods (or real food as we call it).