Excuses from Evolution
- October 2, 2006 3:17 PM |
- By Quirks
Not since a British cook accidentally poisoned half the royal court by serving them the wrong part of the potato (potatoes are part of the nightshade family, and eating the stems can make you sick), has a vegetable had such a muddled relationship with us as broccoli has today.
President George H. Bush famously banned it from the White House. Nutritionists espouse its value as a food. Children, the world over, hide it under their plates. But some new research suggests why you’re either going to love or hate this vegetable. It might be in your genes.
Broccoli is part of a family of vegetables called the cruciferous vegetables. These veggies are rich in a number of compounds. Unfortunately, among those is a group called the glucosinolates. I say unfortunate, because glucosinolates are actually toxins. They partially block the thyroid’s ability to take up iodine. For most people this is no big deal, we get enough iodine in our diet to counter any effects from this toxin. But, if you’re living in an area where iodine is limited, eating broccoli could be very hazardous to your health.
From an evolutionary perspective then, it would make sense for humans in low-iodine areas to develop some way of avoiding these glucosinolates. One of the best ways available to our species is through our taste buds. If we can distinguish, through taste, vegetables that contain these toxins and those that don’t, then we’re more likely to survive.
Previous research has identified the receptors in our taste buds that are sensitive to these toxins. And we also know that while some people have these sensitive receptors, other people don’t. Which means that some people can pick out the toxin (it tastes bitter to them), and other people can’t.
At least, these sensitive people can taste the toxin when they get a pure sample. But, it’s not like we get pure samples of toxin in the real world. In the real world, the toxin comes mixed with all the other flavour compounds in our food.
Which is where this new study comes in. Dr. Paul Breslin from the Monell Chemical Senses Center wanted to know if people who were sensitive to these toxins could pick them out from the background of other flavours in broccoli and other bitter vegetables. He took two groups, sensitive and non-sensitive, and fed them different bitter vegetables. Then he had the subjects rate the bitterness of the foods.
As you might expect, the sensitive people rated broccoli, turnip and other vegetables as more bitter than the ratings by the non-sensitive people. What was curious though, was this wasn’t just simply sensitivity to bitterness. There are plenty of bitter foods, bitter melon for one, that don’t contain these glucosinolate toxins. And both sensitive and non-sensitive people rated bitter melon equally bitter.
It seems that some people really are experiencing broccoli and its cousins as a bitter unpleasant food. They’ve been genetically primed to avoid it. Other people don’t have the aversion. For them, broccoli is a pleasant plant.
If broccoli contains a toxin you might wonder why we don’t all just find it bitter and avoid it. Here’s the real evolutionary lesson in all this. As I mentioned before, this toxin is only a problem if you’re in an area where iodine is limited. Otherwise, this toxin has a secondary effect - it can act as an anti-cancer agent. So, from the species’ point of view, it makes sense to have both sensitive and non-sensitive people in a population. When you’re in an iodine-limited environment, the sensitive people will dominate, as they’re best able to deal with the toxic nature of the compound. When you’re somewhere that’s iodine-rich, then the non-sensitive people will have the advantage because they’ll get the anti-cancer benefits of eating broccoli. This maps onto what we actually see when we start looking for the genes behind the sensitivity in the real world.
But the next time you’re serving up broccoli and your guest turns up their nose, don’t scorn them. They aren’t complaining about your cooking - they just evolved that way!
— Pat Senson
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