Updated 24 June 2024
Back in school, you probably learned that your tongue has receptors (‘taste buds’) for four distinctly different tastes:
- Sweet
- Salty
- Sour
- Bitter
More recently, researchers recognised a fifth taste receptor, which detects the umami, or savoury, flavour derived from glutamate and ribonucelotides. These compounds are found abundantly in mushrooms, yeasts and yeast extracts, certain vegetables, cured and fermented animal products – and most fascinatingly, in human breast milk.
Then there’s the even more recently discovered fat taste receptor (heads up: mixtures of salt and fat blunt fat taste sensitivity, causing people to overeat fat-rich foods, which explains why you can never eat just one potato chip) and the starch taste receptor, which makes potatoes, rice and bread delicious and satisfying.
Out of all the tastes that humans are able to register, bitterness is the least preferred. Food manufacturers know that adding sugar, salt, fat and umami (most inexpensively in the form of MSG) to their products is guaranteed to boost sales. The sour taste of lemon, lime and vinegar enhances enjoyment of the other tastes – think salt and vinegar chips, or lemon gelato, or salsa.
But on the whole, foods that taste bitter don’t sell well. Many compounds that are potentially toxic taste bitter, including certain salts, rancid fats and fermentation products. Alkaloids, a class of compounds produced by plants to defend themselves against predation by insects, fungi and animals, are also bitter. So are polyphenols, a broad category of secondary plant metabolites that help plants to resist predation and to cope with environmental stressors such as extreme temperatures, drought, flood, UV-B radiation, salt, heavy metals and atmospheric pollution.
The fact that plants produce chemicals to defend themselves is leveraged by certain loud voices in the bizarre world of online food cults, to argue that “plants are trying to kill us”, so we should avoid eating them and instead eat… animals that ate nothing but plants for their entire lives, but miraculously managed to survive the experience. Hmmmm 🤔.
The fact is, our ancient ancestors “mostly ate a plant-based diet” – although they were very far from being exclusive plant-eaters – and the plant foods that our ancient ancestors ate were much higher in bitter and astringent constituents than the foods we eat today. Paleolithic peoples developed quite sophisticated food processing techniques to reduce the concentration of bitter, astringent and toxic compounds, but archaeological evidence suggests they intentionally retained a low level of these constituents:
“[There is] an increasing body of archaeobotanical studies suggesting a persistent reliance on, and tolerance of, bitter- and astringent-tasting plant foods such as pulses, mustards, almonds and terebinths, from as early as the Middle Palaeolithic through to the later prehistoric periods. “
Cooking in caves: Palaeolithic carbonised plant food remains from Franchthi and Shanidar
[Cooking in Caves. That sounds like the set-up for a new reality show.]
Since the advent of agriculture, one of the primary goals of farmers has been to breed down the level of bitterness in plants. The success of this selective breeding has in turn led to declining levels of acceptance of bitter foods among consumers; the less bitter tastes we are exposed to, the less we prefer them.
A good example of this selective breeding is lettuce, which originated as a wild plant with a bitter, milky sap, but was selectively bred by the ancient Romans in order to reduce its bitterness; modern lettuce has barely a trace of bitterness unless it goes to seed. Bloody Romans.
But while an aversion to excessive bitterness served our ancestors well by protecting them from being poisoned by overconsumption of potentially toxic alkaloids and other bitter principles*, the modern-day underconsumption of bitter foods is threatening our health in a completely different way.
Many of the phytonutrients which are most beneficial to our health are bitter compounds, such as the indole-3-carbinol and sulphoraphane found in broccoli, kale and other members of the Brassica plant family, which have antioxidant and cancer-preventing properties. Polyphenols, which are both bitter and astringent, are also associated with protection against our most common and feared chronic diseases:
“Epidemiological studies have reported a negative correlation between polyphenol intake and cardiovascular disease [10], neurodegenerative diseases [11,12] and age-related deterioration of sensory organs [13]. There are reports that people who are regular consumers of astringent and bitter drinks may be less at risk of type 2 diabetes and cardiovascular diseases, such as coffee and tea [14,15]. Large-scale intake studies of polyphenols from cocoa have also shown reduced cardiovascular deaths [16] and hippocampus-dependent cognitive improvements in the elderly [17].”
Sensory Nutrition and Bitterness and Astringency of Polyphenols
People who reject bitter tastes end up consuming less health-promoting foods as a consequence. Even worse, those who avoid bitter tastes may end up craving more sweets, as the stimulation of bitter-sensing taste buds overrides our ability to sense sweetness. And conversely, daily consumption transforms the aversive experience of bitterness and astringency into pleasurable stimuli.
Remember the first time you ate an olive or drank coffee? Almost certainly, you didn’t like either of these at all. But if you keep eating olives or 95 per cent cacao chocolate, or drinking unsweetened coffee, you come to relish those bitter tastes. I vividly recall biting into an olive for the very first time, under the misapprehension that it was a black grape. I spat it out in total disgust, and it took years – and dating a Greek guy – for me to try olives again, at which point I discovered they were absolutely delicious!
Fascinatingly, bitter taste receptors are found not just on our tongues, but throughout the entire digestive tract, and in numerous other tissues including our airways, brain, fat cells, muscle cells, kidneys, ovaries and testes and brain! Obviously, we can’t ‘taste’ bitterness in our small intestines, lungs, brain or nether regions, so what the heck are these receptors doing there?
It turns out that when the bitter taste receptors throughout our gut are stimulated, they set off a cascade of chemical reactions that results in delayed emptying of the stomach (causing us to feel fuller for longer) and decreased food intake – obviously a boon for people who struggle to curb overeating. How does this work? Intriguingly, when bitter taste receptors are stimulated by bitter and astringent food constituents, they activate glucagon-like peptide 1 (GLP-1).
Now where have I heard of GLP-1? Oh yes, that’s right: GLP-1 agonists (or mimetics) are the latest blockbuster weight loss drug class. It’s kind of convenient that the ultraprocessed food industry churns out highly palatable foods that are deliberately crafted to be completely devoid of bitter principles, and that consequently drive overeating and weight gain, thereby creating a rapidly-growing (in both senses of the word) market for weight loss drugs like Ozempic and Wegovy, which work by simulating the activity of hormones that are naturally released if you eat bitter foods. But it’s not like the same companies own controlling interests in all the major players in both Big Food and Big Pharma, is it? Oh, wait.
But bitter compounds found in plants don’t help prevent obesity merely by acting on gut hormones like GLP-1. Stimulation of bitter receptors inside pre-adipocytes (cells that have the capacity to develop into fat storage cells) inhibits them from developing into fully-fledged fat cells and decreases their ability to take up fat from the bloodstream for storage.
When mice were fed the bitter flavonol fraction of cocoa for just two weeks, their white fat cells (the regular fat storage cells) shrank, and began turning into brown fat cells, which burn fat and glucose to produce body heat. Healthy young women who drank a beverage rich in catechins (bitter constituents of green tea) every day for 12 weeks, had an astonishing 18.8 per cent increase in their brown adipose tissue density.
In skeletal muscle cells, bitter taste receptors control the differentiation and regeneration of muscle cells after injury.
In the airways, bitterness stimulates immune system activity, protecting against infection in the sinuses, trachea and bronchi, and relaxes muscle tone in the lower bronchial tree, which relieves asthma.
In the testes, bitter receptors seem to play a role in sperm chemotaxis and motility – that is, the ability of sperm to detect an egg and swim towards it. Rocket-fuelled wrigglers, anyone?
The impact of bitter taste receptors on female reproductive function is even more impressive. Activation of these receptors by bitter compounds enhances egg maturation, protects against premature birth, ramps up placental defences against infection, defends the cervix and vagina against infection, induces apoptosis (cell suicide) in reproductive cancer cells, and helps overcome resistance to chemotherapy for these malignancies.
Herbalists have long prescribed bitter herbs to treat liver disease and improve liver function, and it turns out that hepatocytes (liver cells) are chock-full of bitter taste receptors. Stimulation of these receptors improves the liver’s ability to regulate blood glucose levels, reduces fat accumulation within the liver, modulates cholesterol synthesis by the liver and protects the liver against toxins.
Polyphenols are poorly absorbed in the small intestine, and hence a high proportion of dietary polyphenols end up in the colon. This low absorption rate is framed as a problem by the manufacturers of dietary supplements – a problem that they claim to overcome through various absorption-enhancement technologies – but it’s actually a significant advantage. You see, polyphenols exert many of their benefits by directly modulating the gut microbiome – that is, by increasing beneficial microbial species and decreasing harmful ones.
Furthermore, gut microbes extensively metabolise dietary polyphenols, producing secondary metabolites that are absorbed through the wall of the colon into the bloodstream. These secondary metabolites reduce the risk of obesity and cardiovascular disease and help to preserve cognitive function.
How on earth do bitter and astringent plant compounds generate this suite of incredibly diverse health benefits? The general consensus is that their effects are exerted through the principle of hormesis – a beneficial type of stress, summed up by the maxim “what doesn’t kill you makes you stronger”. Hormesis causes your body to adapt in ways that improve its efficiency, and your health:
“The hormetic concept is based on the idea that low levels of stress up-regulate adaptive responses that not only precondition, repair and restore normal function to damaged tissues/organs, but also modestly over-compensate, reducing ongoing background damage.”
Sensory Nutrition and Bitterness and Astringency of Polyphenols
Exercise is the textbook example of a hormetic stress: it’s a controlled form of stress that we intentionally subject our bodies to, in order to improve our cardiorespiratory fitness, increase our muscle and bone strength, and sharpen our reflexes so we’re less prone to falls.
Cold and heat exposure, in the form of cold plunges and saunas, are also hormetic stressors that are increasingly popular among people with an interest in health optimisation and lifespan extension.
Yet some of the most ardent advocates of these forms of hormetic stress take a diametrically opposed view to the form of hormetic stress that is most readily available to just about everyone, regardless of income and fitness level: daily consumption of plant foods that contain bitter and astringent substances.
Contrary to the incoherent ramblings of big, tough men who are afraid to eat plants because “plants are trying to kill us”, our bodies respond to intake of bitter compounds found in plants by upregulating our innate defences against infection, reducing fat storage and dialling up the ability to burn fat and glucose as fuels, repairing damaged muscle cells, enhancing our ability to breathe deeply and even boosting our capacity to reproduce.
One pathway by which these hormetic effects are induced, is that many potentially harmful bacteria secrete bitter substances. Tissues that are particularly subject to bacterial invasion, such as the airways, gastrointestinal tract and genitourinary system, have evolved to detect the early signs of infection by peppering their surfaces with bitter taste receptors. When these receptors are activated by bacterial byproducts, they alert the immune system to ramp up its defences.
These same bitter taste receptors are also activated by alkaloids, polyphenols and other bitter and astringent compounds made by plants, ensuring that our mucous membranes are in a constant state of readiness to defend against infection.
Another pathway is xenohormesis:
“Xenohormesis is a biological principle that explains how environmentally stressed plants produce bioactive compounds that can confer stress resistance and survival benefits to animals that consume them. Animals can piggyback off products of plants’ sophisticated stress response which has evolved as a result of their stationary lifestyle. Factors eliciting the plant stress response can judiciously be employed to maximize yield of health-promoting plant compounds. The xenohormetic plant compounds can, when ingested, improve longevity and fitness by activating the animal’s cellular stress response and can be applied in drug discovery, drug production, and nutritional enhancement of diet.”
Xenohormesis: health benefits from an eon of plant stress response evolution
Isn’t that mind-boggling? Whatever doesn’t kill plants makes you stronger… but only if you put on your big girl or big boy pants and eat them.
The bitter truth is that disorders and diseases ranging from chronic overeating, obesity, and cardiometabolic disease to cancer, neurodegenerative disease, asthma and infertility – all of which have increased dramatically in recent decades – may all be united by a common thread: the lack of stimulation of our bitter-sensing taste receptors (both those in our mouths and those scattered throughout the rest of our bodies) by the modern hyperpalatable diet of ultraprocessed foods, and animal foods in quantities that are inconsistent with our species’ evolutionary history.
For years, I have been counselling my clients to increase their intake of bitter foods. This advice was based on my personal and clinical experience that incorporating more rocket, watercress, dandelion greens, green tea, unsweetened cacao and other bitter foods into the daily diet, decreases the preference for overly sweet foods. I discovered this completely accidentally, after noticing that the more dark, leafy green vegetables I ate, the less desire I had to eat anything sweet.
When I hesitantly suggested that my clients try this out, they experienced exactly the same phenomenon. Furthermore, those with a Mediterranean, Asian or Indian background reported that their parents or grandparents had always included bitter foods in their diet, whether that be wild greens and herbs, bitter melon, olives or citrus peel, and that many of these foods had a long tradition of folk medicine usage.
In recent years, research on the many other benefits of incorporating bitter and astringent tastes into our diet has given me even more reasons to sing the sweet praises of bitter foods! So don’t be a chump who’s afraid to eat plants because they’re “trying to kill you”. Incorporate bitter and astringent foods and beverages into your daily diet, and you’ll reap the manifold benefits of their hormetic effects on health and longevity.
Are you struggling with sweet cravings and overeating? Need dietary guidance that is personalised to your unique situation? Apply for a Roadmap to Optimal Health Consultation today.
* It’s important to note that there is no strict correlation between bitterness and toxicity:
“If the bitter rejection response accurately predicts the potential toxicity of foods, then one would expect the threshold for the response to be lower for highly toxic compounds than for nontoxic compounds. The data revealed no such relationship. Bitter taste thresholds varied independently of toxicity thresholds, indicating that the bitter rejection response is just as likely to be elicited by a harmless bitter food as it is by a harmful one. Thus, it is not necessarily in an animal’s best interest to have an extremely high or low bitter threshold.”
Is the bitter rejection response always adaptive?
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