Why do we like foods that are bad for us?

How about a pepperoni and sausage pizza with extra cheese? Or perhaps a chocolate eclair?

No thanks, you say? You like those foods, but you’re trying to eat healthy? Well darn — why is it that everything that tastes good is bad for us?

OK, that’s a bit of exaggeration, but we do like fats, sugars, and salt, all to our detriment — they contribute to obesity, diabetes, heart disease, cancer, and more. It seems odd, doesn’t it? Why would we evolve to like foods that are bad for us?

Well hey, I have some great news! The reason we like those things is that they are actually good for us! Really, that’s true — but only if you take it in context.

Our distant ancestors didn’t have supermarkets with candy aisles. Even fruits available to them were hardly sweet at all compared to the fruits we eat today, which are the result of many, many generations of selective breeding for taste. Our ancestors hunted, but during most of our evolutionary history success was by no means assured — they occasionally brought home some meat. And that meat was not marbled with fat like the cows we eat today, which were not only bred for taste but also given cushy lives so that their muscles don’t get tough.

So we evolved over many, many generations during which fats, sugars, and the like were generally not available in quantity. Whatever little bits you were lucky enough to come across, it was to your benefit to eat. Those who liked such foods were more motivated to eat them when available, and they benefited from the nutritional boost, which ultimately translated into greater reproductive success — they really needed the calories. So the genes that programmed into their brains the taste for such foods were passed on with greater success.

To drive this home, picture yourself lost on some grassy plains, with trees here and there. You haven’t eaten much today, so you are motivated to find something. A few of the trees and bushes have fruits that you sort of recognize. You try one and quickly spit it out — bitter. After some experimentation you find one that is, well, not great — nothing like the fruits you are used to eating — but not disagreeable. The chemical laboratories in your tongue and nose steered you away from foods which probably weren’t going to work for your body, toward foods that might. That is what your senses of taste and smell are for, not to help you decide between broccoli and Twinkies. There were no Twinkies.

But that’s why we like sweet foods today: they were a good sign for us health-wise way back then. Our ancestors were the ones who did like sugars and fats; the ones who didn’t fared less well, and their genes became less common. That’s how evolution works. But now we are able to refine and concentrate sugars and fats to form the nutritional monstrosities that call to us like sirens from supermarket shelves, and in those concentrations they are by no means good for us.

If such unhealthy crap had existed in our evolutionary past (imagine giant Twinky trees on the savanna, laden with “fruit”), we would have had to evolve to deal with it.  Our bodies might have evolved to be a little more tolerant of sugar/fat bombs. Our brains might have evolved to enjoy a little bit of Twinky now and then, but to quickly lose interest, preferring foods with the nutrition our bodies need. But that didn’t happen, because there were no Twinky trees until recently. So we are defenseless, led by our senses — like moths to a flame — to obesity, diabetes, and heart disease.

Well, OK, we aren’t quite defenseless — we can, through our intellect and force of will, override our pleasure system. But that works much better for some than for others.

After I explained this to a friend, he asked “but aren’t we evolving to deal with much higher concentrations of sugars, fats, etc.?”

Unfortunately it’s not that simple. To understand why, consider what that evolution would look like. People whose genes build bodies which don’t handle these excesses well would have to be less successful reproducers, so that their genes would become less frequent in the gene pool. A key way such people would be less successful reproducers is by dying. Well, they do die, you complain! Yes they do, BUT — they would have to die soon enough to reduce the number of children they have (and raise successfully). These days we expect to have a couple of kids in our twenties or thirties and live into our eighties. Natural selection doesn’t much care if you die in your fifties of heart disease if you weren’t going to have more kids after that anyway.

To be fair, grandparents can be of some benefit in raising their grandchildren, although that’s probably far less a factor than it used to be, what with insurance and social programs that use tax money to help the needy, not to mention the fact that at least here in the U.S. grandparents don’t typically live with their grandchildren anymore. In fact, it may be that — from our genes’ point of view, which is always about reproductive success — it is better nowadays if grandparents die young and leave a larger inheritance to their descendants sooner!

So no, sorry — we are probably not significantly evolving toward bodies that run just fine on cheeseburgers and ice cream. And in fact that’s almost surely not what would happen even if we were dying soon enough to reduce our reproductive success; incremental changes to our brains that cause us to like sugars and fats less are far more likely than sweeping changes to our physiology to allow us to thrive on junk food.

Human beings are intelligent planners, capable of working out the means to attain goals. But an old part of your brain tries to “steer” you and your great planning ability toward reproductive success by dumping neurotransmitters into your noggin to control how you feel. And there is something you should know about that old part of your brain, this part that controls what you like: it’s dumb as a stone. Worse, it has no idea that the industrial revolution happened.

Think of this old part of your brain as the firmware in a computer. I call this part of my brain Dumbo, and the corresponding part of a woman’s brain Morona. Dumbo and Morona only get updated by evolution, a glacially slow process that — as we saw above — doesn’t necessarily lead us someplace we’d like to go, because humans want more out of life than just lots of descendants. Dumbo and Morona do not reason; they execute encoded heuristics that contribute to reproductive success. The encoding that is there is almost entirely from a time when our lives were not very much like they are today, so there is a huge gap between the environment we were programmed to live in and the one we really do live in. Dumbo thinks I am a hunter-gatherer, and that if I run across a bit of sugar or fat it’s an opportunity not to be missed. So that’s how he steers me.

Quite a lot of human unhappiness is the result of Dumbo and Morona being in serious need of an update; stay tuned for more about that.

But for now, hand me another slice of that pizza, would you?


For those especially interested in evolution…

I should be honest here and say that there are other ways (besides dying early) in which evolution could operate on our desire to eat foods that are bad for us, but they don’t change the story. I’ll go through a couple of those here.

1. You could become undesirable during mate selection and have trouble finding a mate. This kind of selection is called “sexual selection.” So if a person’s genes contribute to their eating a diet which makes them less desirable as a mate, then they will have fewer choices in the mating game.

2. You could have trouble performing the tasks required to raise a family. So if a person’s genes contribute to their eating a diet which makes them sick (diabetes leaps to mind), that could theoretically impact their reproductive success.

Eons ago, these would have been very important factors — had junk foods been available. Couples didn’t use birth control to limit themselves to a couple of kids; dying early might very well reduce the number of children you left behind. Getting a disease like diabetes was much more likely to kill you or leave you incapacitated and unable to take care of your family. Modern medicine (and the insurance that pays for it) can inform us of the danger of such a diet and significantly ameliorate its effects. For example, a diabetic might be given insulin — paid for by insurance — and still work and raise a family.

Long ago, a less desirable mate might have meant less children — after all, what we find attractive in a mate is largely about reproductive potential (more on this in later posts!). But modern medicine goes a long way toward allowing everyone who wants a child to have one — even if, for example, they don’t have optimal hormone levels or hips wide enough for a safe delivery. A graduated income tax and other “progressive” policies go a long way toward allowing everyone who has a child to raise it successfully. And birth control drastically reduces the number of children we have from our true reproductive potential. And frankly, a lot of those heuristics are horribly out of date anyway. So really, not getting as attractive a mate has little effect on reproductive success these days.

Eons ago, you couldn’t eat junk food — it didn’t exist. Now that it does, the effects on reproductive success are pretty limited, so evolution doesn’t have a lot to work with.

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6 Comments

  1. Sherry
    Posted February 21, 2014 at 5:38 pm | Permalink | Reply

    Very interesting. Since I lean more towards being a Creationist (except for Genesis 4:16, 17) I think most of our tastes today came out of the Middle Ages.

    • Posted February 22, 2014 at 1:54 am | Permalink | Reply

      Hi cuz! Wow, interesting — I would have guessed that Creationists would say that Yahweh gave humans those tastes when he created them. Of course, I would wonder *why* a god would do such a thing, given all the suffering it causes. Perhaps it’s considered good for humans to struggle against temptation?

      But it sounds like you believe that our tastes are not innate at all, that we acquired them culturally. Right?

  2. Sherry
    Posted February 23, 2014 at 9:11 pm | Permalink | Reply

    Culturally, yes, definitely. And I was mostly referring to European and American tastes, because I’m pretty sure Asians are totally different. And I don’t believe the Lord would give us tastes when He created us because that would fall under the “free will” umbrella. We have “free will” to choose how to behave, believe and eat! BB&E! Hmm, I like that!

    • Posted February 23, 2014 at 11:29 pm | Permalink | Reply

      Oh yes, I certainly agree that our broader taste preferences are cultural; there is no “pizza gene”! It’s just the underlying sensory apparatus in the tongue and the olfactory bulb — and the part of the brain that processes their sensory input — that are determined by our genes. That includes our general taste for sugars, fats, and salt — but not how we feel about chicken masala vs. kung pao chicken.

  3. Posted March 23, 2014 at 3:29 am | Permalink | Reply

    Here’s an interesting article on kids’ heightened preference for sugar and salt:
    http://www.npr.org/blogs/thesalt/2014/03/19/291406696/why-a-sweet-tooth-may-have-been-an-evolutionary-advantage-for-kids?utm_source=howtogeek&utm_medium=email&utm_campaign=newsletter

  4. Posted December 25, 2015 at 4:51 am | Permalink | Reply

    And here is an article about how junk foods interfere with the mechanism in our brains that would otherwise cause us to stop eating:
    http://www.scientificamerican.com/article/how-sugar-and-fat-trick-the-brain-into-wanting-more-food/?WT.mc_id=SA_HLTH_20151215

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