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The more you read about junk food, the more depressing it gets. As the waistband of the global obesity crisis tightens and health policy leaders scratch their heads as to how best to get a handle on it, junk food giants continue to pour millions into the science of creating new and irresistible concoctions that fuel overeating.

The science of junk food makes for compelling, sometimes sickening, reading. It’s crucial to start conversations like this, to increase awareness of what’s going on behind the scenes, to help spark change. 

Steven Witherly is a food scientist who has devoted more than 20 years to studying why some foods are more enjoyable or even addictive than others. The food science theories outlined below are covered in greater detail in his book - Why humans like junk food.  It’s a fascinating read.  

Why it’s easy to overeat junk food…

It’s no accident that the foods that breed obesity and chronic disease are often the hardest to resist. Junk food manufacturers are in the booming business of trying to make you want and eat more. And they’re using every food science trick in the book to achieve this.

Here we take a look behind the scenes at some of the food science theories that influence your food choices and make you want more…

Dynamic contrast

Food scientists know that people prefer foods with sensory contrasts – texture and taste contrasts, how quickly foods meltdown in your mouth, even temperature changes and visual contrasts too. Think light and dark, sweet and salty, crunchy and smooth. The brain also has a craving for ‘novelty’ – it triggers endorphin release, which produces a ‘thrilling’ effect.  Junk food manufacturers understand this and so produce foods with exceptionally high dynamic contrast to intensify the food pleasure experience. Think of a crunchy sweet outer shell that you bite into to reveal something soft and gooey inside.

Vanishing caloric density

Dr Robert Hyde’s ‘vanishing caloric density’ theory states that foods that quickly disappear in the mouth are more rewarding and encourage overeating. Think how easy it is to keep eating popcorn, cheese puffs, chocolate, French fries and ice cream. It seems the brain is easily confused by foods that have a fast meltdown response in the mouth and perceives that fewer calories are being eaten than actually are, which makes it very easy to eat more.

Sensory specific satiety (SSS)

Sensory specific satiety (SSS) is a very important theory in food pleasure – it states that as we eat food, the pleasure response to the sensory properties decreases within minutes. If the brain can identify a single strong taste or aroma it quickly becomes bored with it so junk food manufacturers produce foods either with highly complex tastes and aromas (such as those triangular-shaped tortilla chips) or that are deliberately bland (lightly salted crisps / vanilla ice cream), specifically to reduce SSS and to make sure you can eat plenty in one go. The triangular tortilla chips have a complex aroma profile, yet not one particular aroma dominates. This lack of a specific easily identifiable aroma means you can continue to eat them without sensory burnout.

Evoked qualities

Every single eating experience lays down another food memory in parts of our brain that control our perceptions of taste, texture, aroma and calorie content.  And so food cravings are often triggered by sight, smell and caloric memories of past food experiences. Junk food manufacturers try to initiate these cravings by using flavours that are universally loved, without us consciously knowing it. The triangular-shaped tortilla chips again are a classic example of a food containing high-evoked qualities. They contain many different familiar flavours such as corn, cheese, garlic, tomato and onion, all on one tortilla chip to evoke past memories of not just one but all your favourite foods and so promote strong cravings.

Energy density theory

Food pleasure is a combination of sensory factors and caloric stimulation by protein, fat and carbohydrates. Adam Drewnowski has found that high energy density food is associated with high food pleasure. The brain prefers high calorie foods to low calorie ones – in fact brain scans have demonstrated a reduced pleasure response when study subjects looked at a plate of vegetables compared to a higher calorie alternative. And given the choice of two foods exactly alike in sensory terms, the brain, with instructions from the gut and fat cell, will always choose the higher calorie original. Fats are energy dense and the brain prefers fat and fat-based flavours – you will find that many snack foods aim to achieve as close to a 50% fat content as possible, to vastly increase their desirability.

Post-ingestional conditioning

Humans quickly become conditioned to prefer the taste of protein, fat and carbohydrate simply by consuming them regularly. What’s interesting though is that sweet and high fat foods condition more readily and this is part of the problem with junk foods that are unusually rich in taste-active components such as sugar, fat, salt and MSG and have high caloric density – these readily create potent food preferences and become addictive.

Brain neuroscientist Ann Kelley has commented that foods high in sugar and fat may even change the brain in the same way that many recreational drugs can cause addictive behaviour. Obesity may be an addiction to junk or highly palatable food.

Salivary response

Saliva is essential for us to taste properly and so foods are more pleasurable when they are moist and promote the production of saliva. Junk food manufacturers know this and you will often find specific ingredients added to induce the production of more saliva. Acids keep saliva flowing so that food never dries out in the mouth and taste experience is enhanced. Look out for ingredients on junk food labels such as lactic acid and citric acid as these are often added to stimulate plenty of saliva.       

Super normal stimulus or super size me

Energy dense foods become even more desirable if larger than expected. Big portions, like supersized ice cream sundaes, thrill and excite the palate and people just eat more. Brian Wansink, author of Mindless Eating and marketing professor at Cornell University found that portion size may be even more important than taste as a driver of what and how much you eat. He found that giving cinemagoers their popcorn in larger buckets made them eat 34% more!

Many of the most commonly craved junk foods are exaggerated combinations of salt, fat and sugar that were completely inaccessible to our caveman ancestors; perhaps we have evolved to crave these rare nutrients. And hence we respond to supernormal versions of food with supernormal eating responses (overeating).


Casein is a major milk protein that is often added to junk foods. After digestion, morphine-like molecules called casomorphins are created, which many food scientists believe can make food more addictive. Casomorphins have the added effects of making a food more memorable by lengthening the time between food ingestion and digestion. They also override our fat satiety system – which means you can eat casein-containing fatty foods for longer before feeling full, thus encouraging overeating.

High glycemic starch

The brain finds rapid absorption of sugar (glucose) and the equally fast reduction in hunger more rewarding than slow release sugars. In addition, the fast insulin increase that accompanies refined sugar intake acts on the brain in such a way that encourages overeating. And through insulin’s complex interaction with leptin, serotonin, and other neurotransmitters, a very potent food memory is formed. The brain wants to remember junk food since it perceives it to have greater short-term survival value than other foods. 

Use your knowledge to spark change

If you’ve ever opened a cylindrical tube of crescent-shaped crisps and wondered why they’re almost impossible to leave alone, or have chosen to pick up a giant-sized bar of chocolate when you only nipped out for petrol, hopefully you’ll now be somewhat the wiser. And whilst it can be depressing to learn that the odds are stacked against you when it comes to resisting or even eating just a small portion, understanding what’s going on behind the scenes can be actually be incredibly empowering. Suddenly, knowing what’s really going on means the whole process becomes conscious (rather than unconscious or mindless) and this is a vitally useful step towards helping you to get back in control of your eating. Awareness is the first step to making any change. It’s only when you become aware that you can make a decision to change and do things differently. Slow down, build in pause points and stop to think before you next reach for a bumper bag of junk. You’ll be amazed at just how much of a difference this simple mind shift can actually make. 

Mindless Eating - Brian Wansink
Why Humans Like Junk Food - Steven Witherly

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