Research Reviews

Research Review: Bitter blind?

Why you don’t like broccoli… genetically speaking

by Helen Kollias

Pretty much everybody has heard of red/green color blindness, but I bet you haven’t heard of “bitter blindness”. Yup, just as there are people who can’t tell the difference between the colors red and green, there are people who can’t taste a certain type of bitter flavor. And just like color blindness, it’s genetic.

Tasters and non-tasters

Way back in high school, I had a biology class where we had to look at different genetic traits. We had to go through a bunch of questions. Questions like:

•    Is your ear lobe attached or not? (Nope.)
•    Can you roll your tongue? (Yup.)
•    Can you taste phenylthiocarbamide (PTC) or 6-n-propylthoiuracil (PROP)? (Uhmm… I have no idea.)

To answer the last question, we put a PCT-laced strip of filter paper on our tongues, then waited for a reaction.

Reaction #1: “What is this supposed to taste like?” or “This tastes like filter paper.”

Reaction #2: “Bleah – this tastes terrible!” or “It’s really bitter.”

After our teacher (a “non-taster”) had a good laugh at the “tasters” he explained what the reactions meant. People who had reaction #1 were “non-tasters”, while people who had reaction #2 were “tasters”.

Things you wish you learned in high school biology

Being able to taste PTC or PROP doesn’t really matter much in your day-to-day life, because you aren’t going to run into either of them outside of the odd genetics class. But if you can taste PTC or PROP you can also taste other thiourea compounds –- and those you definitely have run into.

So, what things have thiourea compounds? Well, broccoli, Brussels sprouts, cabbage, kale and other Brassicaceae family members do.

For many of you a light bulb might have just gone off: You don’t like broccoli, Brussels sprouts, cabbage and you’ve never tried kale. After years of being tortured by being forced to eat them, you now find out you could have a genetic predisposition to dislike these vegetables!

Why didn’t this come up in biology class?

A specific taste receptor determines why some people can taste bitterness in broccoli and some can’t. A specific gene, TAS2R38, produces different versions of the TAS2R38 taste receptor. “Tasters” have a version of the TAS2R38 taste receptor that can detect thiourea compounds and “non-tasters” have a version of the receptor that can’t.

broccoli Research Review: Bitter blind?

What is a gene?

A gene used to be defined as DNA that codes for a protein. But here’s a more user-friendly way of thinking of a gene: Think of DNA as a book and genes as recipes in that book. Each gene or recipe contains instructions on how to make something, most often a protein. Most often, genetic variations are subtle and caused by changes in the gene (or recipe, in our analogy).

Let’s say, for example, that you’re making a berry Super Shake, but you’ve run out of blueberries. You decide to use more strawberries to make up the difference. What happens? It’s still a berry Super Shake, but it tastes a little different and looks a little different (more pink). Yet it’s still a Super Shake.

With the TAS2R38 gene, you have some recipes that can make a thiourea-detecting receptor and some recipes that can’t. In real life, things are a little more complicated than my recipe analogy, of course: in some cases, genes may code for more one protein and sometimes they make other things, but thinking of genes as recipes really helps with the basic concept, which is that genes are instructions for making proteins.

Research question

Here’s a study that looked at whether people who can taste thiourea (PROP) eat less broccoli (and fewer vegetables in general):

Bell KI, Tepper BJ. Short-term vegetable intake by young children classified by 6-n-propylthoiuracil bitter-taste phenotype. Am J Clin Nutr. 2006 Jul;84(1):245-51.

When designing the experiment, the researchers decided to use children instead of adults, because they felt that children would be more likely to decide on food choice based on taste alone and it would be easier to tell children’s preferences from facial expressions. Yes, you read right, one of the measures of this study was facial expression after eating the vegetable. (More on this later.)

Methods

The participants

The participants were:

  • 65 preschoolers
  • 3.5-4.5 years old at the start of the study
  • 33 girls
  • 32 boys

Children with allergies, recent illnesses, or taking medication that alter food taste were excluded. Parents gave consent for the study.

Initial findings:

  • 63% were “tasters”
  • 37% were “non-tasters”

Determining taster or non-taster

Just like in my high school biology class, researchers gave the preschoolers a thiourea compound taste — in this case, PROP — to figure out who was a taster and who was a non-taster. The preschoolers drank 10 mL of PROP in spring water (0.56 mmol/L PROP). Then researchers asked them, “Do you taste anything?”

  • If children said no and their facial expression confirmed this response then they were “non-tasters”.
  • If children said it tasted “yucky,” “gross,” “poo-y” or some other synonym with the supporting facial expression then they were “tasters”.

Once the researchers had established who were “tasters” and “non-tasters”, they tried a couple of tests to see whether tasting ability affected children’s behaviour.

Test 1: The “quasinaturalistic, free-choice snack test”

So, you may ask, what is a “quasinaturalistic, free-choice snack test? Is it like that Mary Poppins song?” Despite the name, it’s simple. The children were given a variety of vegetables at snack time and they got to choose what to eat –- that’s it.

Vegetable choices:

  • Raw broccoli*
  • Cucumber*
  • Pitted ripe black olives *
  • Mini carrots
  • Red bell peppers

*bitter tasting vegetables

What did they find?

  • “Non-tasters” eat more vegetables during snack-time than “tasters”
  • 92% of “non-tasters” ate some vegetables, compared to only 68% of “tasters”
  • 32% of “tasters” ate no vegetables at all
  • “Non-tasters” ate more bitter vegetables compared to “tasters”, but there was no difference in broccoli consumption

Test 2: Hedonic test (aka the “facial expressions while eating various vegetables” test)

The second test then examined how much the children liked or disliked the vegetables from the first test.

In the hedonic test, researchers gave each child a vegetable to eat. They asked the child how the vegetable tasted, and watched for facial expressions. Using a 5-point facial hedonic scale (5 = super good; 1= super bad) the researchers recorded the children’s verbal and facial responses. (Can’t you just see a researcher with a clipboard writing “y-u-c-k-y-f-a-c-e…”?)

First, no surprise — the “non-taster” children liked raw broccoli more than the “taster” children in this test. But there was no difference in preference for the rest of the vegetables. Genetically speaking, it seems that even “taster” kids can’t get a note from their doctors to avoid the rest of the produce aisle.

Conclusion

The take-home message from this study is that people are different. More specifically, preference or aversion to broccoli does have a genetic basis.

Does this mean people who are “tasters” should give up on eating vegetables? No.

As Test 2 suggests, if you hate broccoli you have many other choices. (For that matter, if you do like broccoli you also have many other choices.) Try as many vegetables as possible and find ones you (and your children) enjoy. Go to the grocery store and pick up a green vegetable you never tried. Have you tried Swiss chard? Bok choy? Asparagus? Arugula? Try cooking vegetables in different ways too. You might not like something raw, but find it pleasant when cooked properly — for example, what about a cream of broccoli soup or broccoli stir-fry?

Additionally, our sense of taste changes as we age; we become more able to tolerate bitterness. If you hated broccoli as a child, it doesn’t necessarily mean you’ll still hate it as an adult. After all, your three-year-old would probably find the taste of your beloved morning coffee disgusting.

People want the best diet or the best workout program. Surprise! There is no universal best for everyone for their entire lives. There are guidelines that you should follow, but within those guidelines you have to figure out what works for you. If you don’t like broccoli you don’t have to eat it –- but eat some vegetables!

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