Bitter and sweet and grapefruit

As you may have noticed, I’ve been very interested in the genetics of bitterness and sweetness, and in the masking effects of bitterness on sweet and vice versa. One interesting result comes from a study of the genetics of perceived bitterness of common beverages, by Valerie B. Duffy and her colleagues.•

While the study also addresses the bitterness of alcohol and espresso coffee, what caught my eye today was the discussion concerning grapefruit juice. 

I don’t eat grapefruit or drink grapefruit juice for reasons other than not liking it—it affects the metabolism of a number of medications, none of which I am on but which I could potentially be on, for example coumadin—but I do find it bitter, and not sweet, and I don't like it.

Turns out that I have the gene changes that correspond to “Het” in the diagram below, from the article. As you know, proteins are made from amino acids strung together in a chain. The code for indicating which amino acid goes where is in your DNA. Single changes in the code can make for different amino acids in the protein, which makes the protein work less well, or even not at all.

Some people have only the amino acid cysteine at position 299 in protein chain that makes up their bitter receptor TAS2R19, and are indicated as Cys299 in the graph—both parents gave them the DNA code that gives this version of the protein. Some people have only the amino acid arginine at that position, because they inherited that version of the DNA from both of their parents—they are indicated as Arg299 in the graph. Arginine at position 299 makes for a less effective receptor, so they don’t find grapefruit juice bitter. As it turns out they tend to find it sweet instead, and like it much more than do the people who have the code for cysteine at position 299.

As for me, “Het” stands for heterozygote, which means that I inherited one version of the DNA from my mother, so I make half my proteins with the arginine at position 299, and one version from my father, so I make the other half of the proteins with cysteine at position 299. (I know which parent gave me which variant because my mother adored grapefruit, and my father thought it was way too bitter, and didn’t like it at all.)

What interested me as a heterozygote is that I never have found grapefruit to be sweet, and when I had to eat it as a child I heaped sugar on it to make it bearable. As you can see from the graph, there is no real difference between Cys299 and Het in liking—that’s what is meant by p=.115—whereas there is a large difference between Cys299 and Arg 299 (the lower the p value the greater the difference), and some difference between Het and Arg299. 

What is not marked on the graph, and what intrigued me is the possible consequence of a difference in sweetness perception in conjunction with bitterness perception. In other words, for the hertozygotes, is it the combination of a slight increase in bitterness perception coupled with a slight decrease in sweet perception that makes for a greater dislike?

And another question: is there a competition between bitterness and sweetness, so that people who have at least one copy of the more effective receptor (like me) may not perceive the bitterness as strongly, but it is sufficiently strong to dampen the sweetness of the grapefruit, and to make it less delicious.

* Hayes JE, Wallace MR, Knopik VS, Herbstman DM, Bartoshuk LM, Duffy VB. Allelic Variation in TAS2R Bitter Receptor Genes Associates with Variation in Sensations from and Ingestive Behaviors toward Common Bitter Beverages in Adults. Chemical Senses. 2011;36(3):311-319. doi:10.1093/chemse/bjq132.