Why Do We Crave Junk Food?

 by Anne Munier

Evolution influences human behaviour by holding on to traits that make us more likely to survive and have babies. Behaviours like avoiding vicious predators, jerking your hand away if you touch something hot, or cooperating with community members all helped our ancestors survive, and have been passed down through generations. Traits that lead to very *unhealthy* outcomes (like, say, having an irresistible urge to play with vicious predators) tend to get weeded out of the population, because kids who play with tigers generally don’t survive long enough to pass on their genes.

So why do so many of us feel compelled to overeat junk food? Junk food, for the sake of this article, means processed foods that are high in sugar, fat, and salt. This type of diet is harmful for our health, yet it is a rare person who craves steamed broccoli over milkshakes, pizza, or brownies.

Oddly enough, these cravings are very much shaped by human, and even pre-human, evolution.

A monkey with a sweet tooth munching on jackfruit

Millions of years ago, some early primates developed a taste for fruit, even though it tends to grow high up in trees, and it’s much easier to munch on plants that are close by. These particular primates did awfully well because fruit has much more energy (i.e. calories) than other foods, and is easier to digest. This leads to healthy primates in good condition to have lots of babies. These babies inherited their parents’ love of fruit, and likewise did better than their counterparts who couldn’t be bothered to climb for their food. And so it continued through the generations, until eventually the whole species were driven to eat as much fruit as possible.

(Fun Fact #1: Primate species that eat the most fruit have the biggest brains.)

Moving along our evolutionary story, early humans were developing ever-larger brains, which, along with our active hunter-forager lifestyles, meant we needed a ton of energy to keep going. We continued to love sugar from fruit or wild honey when we could get it (which wasn’t often), as well as high-fat foods, which are also high in calories. When we had the chance to eat more than we needed, our bodies stored the surplus as fat, allowing people to have more babies, and enhancing survival during times of food scarcity.

(Fun Fact #2: Humans tend to be better at storing fat than other animals. Even super-skinny humans have more body fat than the chunkiest chimpanzee).

Fast forward to the 21^st century -- most of us live in relative prosperity and have constant access to junk food. And while treats that our hunter-gatherer ancestors enjoyed came wrapped up with fiber, vitamins and minerals, today’s treats -- not so much.

We are now hard-wired to seek out sugar and fat, to the point that they stimulate pleasure centres in our brain. This is true for newborns and adults, across all cultures in the world. Healthy food elicits a bit of this pleasure stimulation, junk food elicits A LOT more. And the more junk food we eat, the more we develop a tolerance to it, so we need to eat more to get that same pleasure response. Sound familiar? Addictive substances, including cigarettes and drugs like cocaine, work on our brain in the same way.

Melt-in-your-mouth Cheetos have a very profitable mouth feel.

Then there are the manufacturers of processed junk foods. They want to maximize their profit by selling as much product as they can. And they have all sorts of tricks up their sleeves about how to get us (and keep us!) hooked on the stuff. For example, they figure out a food’s:

• Bliss point: the perfect ratio of sugar, fat, and salt to maximize deliciousness and over-consumption

• Mouth feel: how food interacts with your mouth -- the more quickly it melts, for example, the more it confuses our brain into thinking that we’ve consumed fewer calories than we really have

What is the result of these biological forces on one hand, and the commercial forces on the other?

The number of people who are obese is growing annually, as are associated conditions such as high blood pressure, heart disease, chronic inflammation, and Type 2 diabetes. Add to this the impacts on mental, psychological and dental health, and ballooning health care costs that affect everyone.

So, given enough time, could humans evolve to lose a taste for junk food, since it’s so damaging? Maybe, but let’s not hold our breath. Evolution acts genetically, by passing on genes to our children. But most disease related to excessive junk food affect people later in life, after they’ve already had (or not had) kids.

Is it inevitable then, that we’ll continue to eat excessive amounts of junk food, and our health will keep deteriorating? Not at all! Here are two things to keep in mind as we go forth in our junk food-laden society:

1. Evolution has also endowed us with big beautiful brains capable of critical thought. Even if things are stacked against us, we can make great (if sometimes tough) decisions about our eating and exercise habits.

2. We need to recognize that diet goes beyond will power. Our biology and powerful market forces are designed to make us crave junk food, which, sadly, is often cheaper than whole food. So let’s NOT blame anybody for “lifestyle diseases” caused by too much junk food. Rather let’s consider societal changes that could improve things -- like limiting marketing of junk food to children, or ensuring that healthy food is easily available to all of us. What ideas do you have?