1. Why Does the Same Chemical, PTC, Taste Bitter to Some People and Not Others?
A chemical called PTC (phenylthiocarbamide) shows a huge range in how people perceive it. Some people taste a strong bitterness, while others taste almost nothing. The story of this research begins in 1931, when a chemist at an American company reportedly realized he could not taste the bitterness of PTC, while his colleagues could. However, this specific story is a story recorded in later sources, rather than a confirmed fact.
In 2003, researchers found that a difference in just one gene, TAS2R38 on chromosome 7, could explain 55 to 85 percent of the difference in how people taste PTC [3]. This percentage refers to how much of the variation is explained by genetics, not the percentage of people who find it bitter. The TAS2R38 gene is the blueprint for one specific taste receptor. This raises a bigger question: if gene variants help explain differences in PTC taste, how many receptors do we have in total to handle all other bitter things?
2. How Do About 25 T2R Bitter Taste Receptors Detect Thousands of Bitter Substances?
The family of bitter taste receptors, known as T2R, was first reported in 2000. These receptors work in taste cells on the tongue and the roof of the mouth [1]. In humans, there are about 25 different types of these receptors, based on the number of genes [2]. How can 25 sensors handle thousands of bitter chemicals?
In a 2010 experiment, scientists tested 104 bitter substances against 25 human receptors. They found that just 3 receptors could detect about half of the substances. Additionally, 63 of the substances activated only 1 to 3 receptors. On the other hand, 19 substances activated up to 15 different receptors [2]. This suggests a mix of sensors: some are broad and catch many things, while others are narrow and specific. Together, this combination covers a wide range of bitter compounds. However, this result is based on only 104 substances, so it does not cover every possible bitter chemical.
3. Why Do Both Taster and Non-Taster Versions of the PTC Gene Remain in People?
Why have both the "taster" and "non-taster" versions of the PTC gene survived? A 2004 study examined 330 chromosomes and found similar frequencies of both types in Africa, Asia, and Europe. This led researchers to suggest that natural selection, specifically balancing selection, might have kept both types in the population [4].
However, a 2016 study challenged this idea. Researchers looked at TAS2R38 in 5,589 people from 105 populations worldwide. While they found signs of balancing selection in ancient times, they found no evidence of selection after humans left Africa. The current distribution of these genes is mostly explained by population history, rather than active selection [5]. This does not mean the old hypothesis was wrong, but its scope has narrowed. It seems that long historical events, not just current survival needs, shape how we taste.
4. Did Bamboo-Eating Lemurs in Madagascar Lose Their Sensitivity to Bitterness?
Bamboo contains cyanide compounds, which are toxic. Researchers from Kyoto University studied three species of lemurs in Madagascar that eat bamboo: the broad-nosed, golden, and gray gentle lemurs. In 2021, they published results on how the bitter receptor TAS2R16 reacts to the substance beta-glucoside [6].
The study found that all three bamboo-eating species had lower sensitivity to this substance compared to their close relative, the ring-tailed lemur. Interestingly, the genetic mutations that reduced sensitivity were different for each species. This suggests that each species evolved this change separately after they started eating bamboo [6]. Note that this was an experiment using cells, so we cannot say the lemurs find bamboo "tasty." A similar case was found in Japanese macaques, which have much lower sensitivity to salicin, tested in both cell experiments and behavioral trials [7]. This suggests that amino acids in the sensors changed to match the animals' specific diets.
5. How Does an Animal's Diet Change the Number of Its Bitter Taste Receptor Genes?
A 2014 study compared taste receptors across 41 mammals, 4 birds, 2 reptiles, 1 amphibian, and 6 fish. The number of working bitter receptor genes varied greatly, from 0 in the bottlenose dolphin to 51 in the western clawed frog [8].
This number correlated with the proportion of plants in the animal's diet. Since plants often contain more toxic substances than meat, the toxins in food are thought to be a major force driving the diversity of receptors. However, this is a hypothesis derived from correlation, not a proven cause [8]. Loss of taste sensors is not limited to bitterness. Cats and seven other meat-eating animals, such as the Asian small-clawed otter and spotted hyena, have broken sweet receptor genes. Dolphins and sea lions have also lost several types of taste sensors [9]. It appears that taste sensors may be lost in lineages with different diets.
6. What Do Bitter Taste Receptors Do Outside the Tongue?
Bitter receptors are not only found on the tongue. A 2026 study suggests they may act as sensors for substances inside the body, such as steroid hormones flowing in the blood [10]. This is still at the stage of possibility, and their function is not yet confirmed. It is possible that sensors that evolved to watch for poisons have taken on other jobs in the body.
7. How Can You Explore Bitter Taste Yourself with Food Like Bitter Gourd or Black Coffee?
If you want to explore this further, try eating a small amount of the same food, such as bitter gourd or black coffee, with family or friends. Talk about how it tastes to each of you. You can also just discuss tastes with people who have tried the food. There is no right or wrong way to taste, and no hierarchy of preference. Do not force anyone to eat something if they have allergies or dislike it.
When visiting a zoo, look at the descriptions of animal diets. Predict how the bitter or sweet sensors might differ between animals that eat mostly plants versus those that eat meat. For official information, the Kyoto University research news from April 16, 2021, introduces the study on bamboo-eating lemurs, and the paper abstract is available on PubMed.