There's no way around it -- junk food tastes amazing. Really, REALLY amazing.
We've all pondered why healthy food doesn't taste as good, and it turns out it's because plant foods are readily available in nature, and the bitterness from some of these foods tells our brain they might be poisonous.
But why exactly are doughnuts, cookies, chips, cupcakes, pizza and burgers so damn appealing? And can't we teach our brains to like healthy food the same way we do junk food?
According to Russell Keast -- a professor in sensory and food science and the director of the Centre for Advanced Sensory Science at Deakin University -- the answer to why we love junk food is simple, but at the same time, very complex.
"In a way, it's quite straightforward -- salt, fat and sugar. Those are the three nutrients that are really important in terms of our hedonic drive, or our liking," Keast told The Huffington Post Australia.
"If we're thinking about our species and how we came to be -- the caveat here is evolutionary thought -- it's that we evolved in nature. In nature we've got plants everywhere, and there was no need for us to develop the desire to eat the plants. However, our bodies also required certain things.
"Salt, as an example. Presumably life evolved out of the ocean. Seawater has a whole lot of sodium. The origins of life, these cellular objects, were bathed in the salty solution. Now, our bodies and cells bathe in a salty solution. Since we've moved out of water and onto land, we need to replenish the sodium."
Therefore, if we came across food that contained sodium, it was imperative for our species' survival to have a mechanism to know to eat that food.
"And that's where the appetite of nature of salt presumably comes from," Keast said.
"We find the food in nature that has a physiologically relevant level of sodium (not too much that would be harmful, and not too little because we don't want to waste energy consuming too much of a food). If salt is there, we consume it, hence the liking."
The same goes for sugar and fat.
"Things like sugars are readily available forms of energy. Again, it's really important for a species to know that if the food has free sugars in it, these are great energy for us," Keast explained.
"We consume them and it provides us with, in this case, the fuel to be able to escape or go to the next food source. Sugar is very important.
"Fat is an incredibly dense form of energy and, again, that's very important. If we came across it, there were cues that told us to consume these foods.
"Those three independent nutrients really drive our liking."
The problem with our intense liking for salt, sugar and fat is that they are readily available nutrients, and we eat far too much more of them than is actually required.
"The other important thing which comes up in modern day society (and something we're very good at) is combinations," Keast told HuffPost Australia.
With our ability to combine, create and consume foods, our brain is amazed. It's going, 'What? We've got fat, salt and sugar all in one food? That's just superb'.
"We are yet to find a cupcake tree or doughnuts in nature, so our primitive brain is amazed at today's food supply because through 4 billion years of evolution, that cupcake wasn't sitting there for species and our evolutionary pathway.
"But now, with our ability to combine, create and consume foods, our brain is amazed. It's going, 'What? We've got fat, salt and sugar all in one food? That's just superb'.
"That's really what we've got to today and why combinations of foods can be so desirable. These things are really innate to us and key for our survival."
WHOA. Just when you thought you had that sugar addiction out of control. This is potentially why foods high in sugar can affect the reward centres in our brains, and why people can feel 'addicted' to junk foods.
"Obviously it's to a lot lower degree [than drugs], but for some people who may have these really strong desires to consume these foods, certain studies have shown that those areas of the brain which are responsible for pleasure 'light up' more in those people who are seeing, smelling or eating those attractive foods," Keast said.
"These are similar pathways or brain areas that are affected by drugs."
At this stage, more technology and research is required to find out exactly why this occurs and what it means, but watch this space.
"All of these have to be taken with a grain of salt, to use an appropriate analogy. We'll learn a lot more in the next 20 years about how the brain works," Keast said.
To add more salt to the wound that is this junk food phenomenon, certain people have a stronger hedonic drive or liking for junk food.
"Some people, and there are a lot of individual differences, may be more driven and find it harder to resist these foods. Their hedonic drive to consume these burgers, doughnuts or chocolate to excess is really strong," Keast said.
"Other people, less so. There's also different psychological reasons why people may have stronger desires or wants for those foods, such as emotional or stress eating. That certainly comes into it."
On top of this, when we eat lots of fat, sugar and salt on a regular basis, we tend to adapt to these tastes and need more to feel satisfied.
"This is where it starts to get a more complex," Keast said. "Certainly, what happens in the taste system is we tend to habituate or adapt to diets. Studies have shown that if we consume more fat, our responses to fat actually reduces. That's the body saying, 'well, I don't need all of the mechanisms to identify those particular nutrients, so I won't produce as many mechanisms'.
"Now, part of that may also be a problem simply because, if our response to those nutrients is diminished, we're not recognising them at the level we once were. We're requiring higher levels of the food to get that same satisfaction that we used to get.
"The habituation, which is common to all sensory systems, may actually be part of the problem -- if you habituate too easily, you may be still wanting the level of satisfaction you used to get, and the only way to get that is to consume more, which becomes a problem."
By now you're probably asking: is there a way we can teach our brain to reduce our liking for junk food and increase our liking for healthy food?
"This is a really difficult question and I wish it were easier," Keast told HuffPost Australia.
"We've done a lot of work where we've reduced the fat in people's diet and increased the ability to taste fat, and we've increased the satiety (the fullness they feel from the nutrient). We haven't been able to, to this date, increase the pleasure they experience from the food."
Essentially, there's a disconnect between our response of being able to recognise the nutrients, and the liking of the food.
"The holy grail is asking, can we increase liking of a food, while decreasing salt, sugar and fat? Even if we can significantly, from a health perspective, reduce the level of salt, sugar and fat to have positive health outcomes for population, while still maintaining liking of foods which reduce salt, sugar and fat, that would be a big gain.
"But it's that last bit of the liking which appears to really resistant to change. It may be that it's not just related to activation of systems, it's more embedded in memory. We know how much we like something and it's really hard to change that.
"There's things which will take 20 to 50 years to start to work out. I think a lot of it is mired in our brain, and our understanding of that is incredible rudimentary at this stage."
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