Bitter Taste Rejection--A Marker for Dietary Trophic Levels

(Comparative Anatomy and Physiology Brought Up to Date--continued, Part 7J)

Bitter Taste Rejection:
A Marker for Dietary Trophic Levels

This section is based on the excellent paper by Glendinning [1994]. The primary interest here is on how bitter taste rejection threshold levels vary according to dietary trophic levels (based on possible evolutionary adaptation). However, the article by Glendinning also provides hard information that addresses some of the myths about bitter taste promoted in fruitarian/raw circles, and that material will be examined here as well.

Fruitarian/raw vegan myth: bitter = toxic

One of the enduring myths of fruitarianism/raw veganism is that any food that tastes bitter absolutely must be toxic. Glendinning [1994, pp. 1217-1218] notes (with citations) that all naturally occurring toxins taste bitter to humans at some concentration level, i.e.:

Proposition A:
Natural toxin implies bitter taste.

However, the claim that is promoted in raw circles is the logical converse, i.e., that:

Proposition B:
Bitter taste implies natural toxin.

This section will focus on proposition B.

Lack of proof for the claim that bitter implies toxic. Glendinning [1994] defines the bitter rejection response as the aversive reaction that occurs when one eats bitter foods. He then asks whether the claim that bitter implies toxic is valid, and reports that (p. 1217):

The bitter rejection response has not been evaluated rigorously in terms of its value in avoiding poisons. For the bitter rejection response to be an effective poison detector, there should be a predictable relationship between the threshold concentration for bitterness and that for toxicity. If the bitter threshold is substantially higher or lower than the toxicity threshold for many compounds, the effectiveness of the bitter rejection response as a poison detector would be uncertain. To the author's knowledge, no investigator has explicitly tested for such a relationship across a series of bitter compounds.

Evidence that the claim "bitter implies toxic" is false. The hypothesis is then presented that perhaps the bitter taste threshold co-varies with the level of toxicity of the bitter compounds, i.e., that proposition B above (the fruitarian/raw claim) may be true. Glendinning then goes on to summarize the evidence that indicates the claim is false:

Genetic variation. The bitter taste threshold in mice varies according to the autosomal gene Soa. In particular, the bitter taste sensitivity of mice is not a function of the toxicity of the item being tasted.
Inter-strain and inter-species studies with mice found wide variation in bitter taste sensitivity, and no relationship between bitter taste rejection and toxicity levels of the items tested.
A comparison of the bitter taste threshold with the toxic level of the same compounds, in both mice and humans, found no relationship between the bitter taste and toxicity levels. (See figure 2, p. 1220 of Glendinning [1994] for the comparison plots--very interesting!)
Several mammalian species have been observed freely feeding on toxic plants, apparently indifferent to the toxicity. The inference here is that the toxins in these plants failed to produce the bitter rejection response.

Interested readers are strongly encouraged to read the original article for the citations and details. Glendinning [1994, p. 1218] concludes:

Taken together, the above studies indicate that bitter taste sensitivity does not accurately mirror the body's systemic reactivity to naturally ocurring or synthetic [toxic] compounds in several species of mammal.

Fallacious "proofs" that "bitter implies toxic" as cited in raw circles. The reality that there is no legitimate scientific proof for the fruitarian/raw claim provides yet another example of advocates' claims failing to meet the logical requirement of burden of proof. Instead, the claim may be presented with bogus "proofs" such as the following.

A simple "proof" that one encounters is the claim, "Your taste buds evolved to reject bitter because toxic items are bitter." The preceding seems attractive, but it is actually a claim that proposition A somehow "proves" its own converse, proposition B. This is logically invalid.
A more extensive crank science "proof" that one encounters is when the fruitarian/raw "expert" references a phytochemical database, picks out a toxic chemical (in this case, a bitter chemical would likely be chosen) that is present in the food and then announces that the food contains toxins, therefore there is "scientific proof" that the (bitter) food must be "toxic."

Space does not permit a full exploration here of the massive logical fallacies in the above crank science "proof." However, a short reply to the above is that practically every chemical is toxic in a large enough dose. If one is going to examine toxins in foods, one must consider dosages (harmful dosage of chemical vs. amount in food) and bioavailability as well. The crank science approach (common among fruitarian extremists) of equating the presence in a food of a chemical that is toxic (in isolation, and in large doses) as "proof" the food is "toxic" is intellectually dishonest, and borders on pathological fear-mongering.

Bitter taste as trophic marker

Tolerance to bitter as a dietary adaptation. Turning our attention from fruitarian myths to more serious matters, Glendinning [1994] presents the hypothesis that the bitter taste might serve as a trophic marker. The argument is based on evolutionary adaptation. Inasmuch as plants contain many (unavoidable) bitter constituents, it could be suggested that animals with a plant-based diet would be likely to develop some degree of tolerance for the bitter taste, i.e., they will develop some degree of adaptation to it, hence have higher bitter taste rejection thresholds.

Similarly, those animals whose diet almost never includes the bitter taste, i.e., carnivores, would rarely encounter the bitter taste and have a low tolerance for it, hence have very low bitter rejection thresholds. Omnivores would fall in between the two extremes in bitter taste tolerance. Glendinning [1994] hypothesizes that the bitter rejection thresholds would follow the order, from lowest to highest threshold (p. 1219):

Carnivores
Omnivores
Herbivores--grazers (i.e., grass feeders)
Herbivores--browsers (i.e., forb, shrub, and tree feeders)

Comparison of bitter rejection thresholds in mammals. Glendinning then examined the bitter rejection thresholds of 30 mammal species to the chemical quinine hydrochloride (QHCL), using data from published studies. QHCL was chosen because it is widely studied, and sensitivity to QHCL has been found in a number of studies (see Glendinning for citations) to be correlated to sensitivity to many other bitter compounds. Next, Glendinning computed average QHCL taste thresholds by trophic group. The numbers he obtained [Glendinning 1994, pp. 1222, 1225] are as follows:

TROPHIC GROUP		QHCL Taste Threshold
Carnivores		2.1 x 10^-5 M
Humans		3.0 x 10^-5 M
Omnivores		3.0 x 10^-4 M
Herbivores	Grazers	6.7 x 10^-4 M
	Browsers	3.0 x 10^-3 M

Note: in above table, M = moles/liter.

Note that the results obtained agree with Glendinning's hypothesis about the order of bitter rejection threshold values that would be expected based on the trophic level of foods consumed. Also of interest is that the number for human tolerance of QHCL lies between the numbers observed for carnivores and omnivores, and specifically is closest to the average carnivore value observed.

The significance of Glendinning [1994] is that the study provides an actual analysis of bitter taste tolerance data for a wide variety of species, and reaches a result which suggests, or points to the possibility based on the data obtained (which is of course not proof by itself) that humans may be carnivores/omnivores. This coincides with the extensive evidence already given previously that humans are faunivores.

No doubt certain fruitarian extremists will react to the above and claim that humans reject the bitter taste because our natural food is (nearly exclusively) sweet fruit. However, as discussed in previous sections, there is no legitimate scientific evidence for the claim that humans evolved as fruitarians (whether strictly so or not).

Additionally, many wild fruits are quite bitter, and food shortages or competition for food would likely make the consumption of bitter fruit necessary at times--implying that "fruitarian humans" should have some tolerance for bitter. If humans were strict fruitarians, one would expect our QHCL tolerance level to be similar to the chimp. However, the QHCL tolerance level for chimps is 1.0 x 10^-4 M [Glendinning 1994, p. 1226], a whole order of magnitude larger than the number for humans. Finally, many of the same fruitarian extremists claim (at times with intense emotion) that bitter foods must be toxic, the very claim that Glendinning neatly assesses and discredits.

GO TO NEXT PART OF ARTICLE

(Insulin Resistance and The Carnivore Connection Hypothesis)

Return to beginning of article

SEE REFERENCE LIST

SEE TABLE OF CONTENTS FOR:
PART 1 PART 2 PART 3 PART 4 PART 5 PART 6 PART 7 PART 8 PART 9

GO TO PART 1 - Brief Overview: What is the Relevance of Comparative Anatomical and Physiological "Proofs"?

GO TO PART 2 - Looking at Ape Diets: Myths, Realities, and Rationalizations

GO TO PART 3 - The Fossil-Record Evidence about Human Diet

GO TO PART 4 - Intelligence, Evolution of the Human Brain, and Diet

GO TO PART 5 - Limitations on Comparative Dietary Proofs

GO TO PART 6 - What Comparative Anatomy Does and Doesn't Tell Us about Human Diet

GO TO PART 7 - Insights about Human Nutrition & Digestion from Comparative Physiology

GO TO PART 8 - Further Issues in the Debate over Omnivorous vs. Vegetarian Diets

GO TO PART 9 - Conclusions: The End, or The Beginning of a New Approach to Your Diet?

Back to Research-Based Appraisals of Alternative Diet Lore

Beyond Veg home | Feedback | Links