Friday, January 27, 2012

Oysters as iconic aphrodisiacs: good or bad science?

Oysters are food's iconic aphrodisiac: sensual, suggestive, companionable, concealing succulent valuables in hard-to-crack shell-vaults, and thought to be instrumental in carrying out rewarding sexual activities. Is this icon's reputation based on fact or fiction? Intuitively, after a few thousand years of untarnished reputation, oysters must be doing something right.

By definition, an aphrodisiac is something that increases sexual desire, or libido. While some explanations emphasize sensual qualities such as untouched, one-off virginity (i.e., this oyster has never been opened), others focus on physical aspects such as silky, unguent  qualities with oceanic affects (wild, mysterious), and some explanations look to physical similarities to genitalia, and while all this could could be classified as folk-theory explanation, two complementary explanations of how oysters increase sexual desire and performance with at least a pseudo scientific character are based on transformations of the family of chemicals called amino acids: 1) the dopamine hypothesis, and 2) the  D-aspartic acid (D-Asp) and N-methyl-D-aspartate (NMDA). Sexual desire and sexual performance are considered as two distinct outcomes of eating oysters, though the concept of aphrodisiac often also implies sexual performance.

My first oyster-sexuality post is about deconstructing and reconstructing the oyster-dopamine hypothesis for increased libido and performance in humans—in the name of science. Does eating seafood such as oysters that may contain dopamine increase dopamine levels in consumers?  If so, does this affect sexual desire or performance? Is this cause-to-effect link science-based or folk theory?

Science-based theories are explanations that have been tested and have accumulated enough respectable evidence to be considered the best, truest explanation available--given current knowledge--until more information and research provide an even better explanation or proves the theory false. For millennia a common myth was that the sun revolved around the earth. Although scientists such as Copernicus and Galileo argued that planets revolve around the sun, not the reverse, not until many centuries later had sufficient evidence accumulated to prove that theory false: a slight change in stellar positions due to Earth's speed, or aberration,  parallax (another small change in stellar positions caused by Earth's changing position, and the Doppler effect, which is a slight change in color of stars due to Earth's speed. In 2012, does scientific evidence support the claim that oysters encourage and support sexual activities through increasing dopamine levels?  Will dopamine levels in humans be increased by eating oysters? Today's post addresses this question.

About Dopamine 

The production of neurotransmitters such as dopamine and serotonin is regulated by extremely complex interactions between receptor levels, metabolic needs, nutritional status and more. That said, it pays to remember that dopamine is just a bunch of chemicals, in much the same way as Francis Crick reminded us that the brain is just a bunch of neurons.  Remembering that may help simplify some processes for purposes of understanding, but in no way does such a simplification detract from the mystery and complexity of what emerges when chemicals interact. 

Broadly, dopamine marks (neurally) the  importance of stimuli.  The greater the importance, the more heavily marked a stimulus is, the more dopamine is released. In a field of almost infinite stimuli, dopamine is involved in learning, in focusing,in attending to, in appreciating, in repeating survival-oriented behaviors, in marking surprising events, and  in addiction, to name a few behaviors. Many of these  can be most simply characterized as varying states of arousal or motivation to do something. Dopamine is actually involved in almost all human bodily functions, including the simple but necessary act of breathing. Increased dopamine corresponds to greater desire to act and interact as well as higher likelihood of competent performance. Lack of dopamine corresponds to boredom, passive behavior, and incompetence. Extreme cases of too much or too little dopamine reflect in disorders such as obsessive-compulsive repetitive behaviors or apathy.

Dopamine is the feel-alive neurotransmitter, not the feel satisfied or complete. Dopamine is more of a process neurotransmitter than a product one. Typically, in most organisms, dopamine corresponds to seeking and goal-oriented functions. In oysters, elevated dopamine levels are found in larvae seeking to settle. In humans, elevated dopamine is characteristic of individuals involved in a meaningful seeking process. The process of liking something one has already acquired is more closely linked to serotonin and the reduction of drives.

The reward and seeking systems are intimately linked and inseparable. Feeling that you are on track for a possible rewarding goal is reinforced by evolutionary programs with feelings characterized as good: feeling vibrant, right, on the verge, about to be rewarded, etc.

The link between dopamine and motivated behaviors is, then,  well supported by scientific experiment. Science also supports a correlation between dopamine and feeling good. Sexual behavior is one of these motivated behaviors. A later post, in early February, will address the second hypothesis, which links sexual performance to consumption of oysters. All posts aim to facilitate science-based, ecological decision-making by consumers and producers through better understanding of how things work.

So first, dopamine is a feel good, feel right, goal-directing neurotransmitter. When a normal human smells, sees, or tastes something previously associated with a pleasurable experience, dopamine neurons in the brain start firing in rapid bursts. This burst-firing communicates to other areas of the brain that reward can be expected and initiates goal-directed behaviors to attain that reward. To most people, the physical aspect of oysters may be pleasantly arousing for a variety of reasons: because oysters are remembered as delicious, just the sight of oysters can trigger the reward system (expectation of future reward): oysters resemble unctuous, vital sexual organs, also cuing the reward system insofar as sex is linked to pleasure in a specific person. Considerable research supports the link between sexual imagery and arousal, though I have so far not found specific experiments linking sexual arousal to the sight of oysters. So although not falsified, the sight of raw oysters as a cause for increased dopamine is still folklore.

How it works

The chemical sequence for dopamine synthesis in humans fairly roughly parallels that for oysters: dopamine is created from a precursor called L-DOPA, which in humans is created from another precursor, tyrosine. The parallel stops here. Tyrosine is an amino acid that can be either synthesized in the liver of humans or acquired through eating foods such as oysters.

Amino acids are the building blocks of proteins. In humans, dietary protein are broken down though digestive processes into the amino acids it uses to assemble 50,000 different proteins needed for overall functioning.

Oysters as raw material for dopamine production in the brain

Physiologically, oysters come through smelling if not like roses, at least like oysters, and all good. Raw oysters are great foods: complete proteins, rich in the two key amino acids that are precursors of dopamine: tyrosine and phenylalanine. Eating raw oysters supplies a healthy functioning body with all the proteins that will be broken down to provide most of what is required  physiologically for normal desired sexual outcomes. The key precursor of dopamine is the amino acid tyrosine, potentially available to brain and body through eating raw oysters. Tyrosine is classified as a non-essential nutrient, since it can be synthesized in both brain and body in addition to being acquired through diet. However, the main amino acid from which tyrosine is synthesized is acquired only from diet: namely, phenylalanine. Both phenylalanine and tyrosine are found in oysters. Once tyrosine is acquired from food or synthesized in the body or brain from phenylalanine, it undergoes several further transformations before it becomes dopamine. First, tyrosine is converted to L-DOPA, then to dopamine. Both norepinephrine and  epinephrine are derived by metabolyzing dopamine. For dopamine synthesis to occur, a suite of chemicals must be present in addition to tyrosine: oxygen, vitamins B3, B6, and C, folic acid, iron, and copper. Oysters are among the largest food sources of zinc, copper, and iron, and are also good sources of vitamins A, B1, B2 and C.

Within the brain system: while dopamine cannot pass the blood-brain barrier (BBB), chemical precursors necessary for the synthesis of dopamine such as the amino acid Phenylalanine, which will later be converted to Tyrosine then L-Dopa can pass the BBB.  Dopamine in the brain and elsewhere is produced from tyrosine, a non-essential amino acid that's manufactured from phenylalanine, an essential amino acid. Amino acids are chemical molecules that join together to form proteins. Phenylalanine can be obtained only from certain foods, one of which is oysters.

Tyrosine, once acquired through food or synthesized by the brain and kidneys, is converted to dopamine in two metabolic steps: first to L-DOPA, second. L-DOPA is converted to to dopamine.  From food, tyrosine can be obtained in small amounts from  oysters. Foods high in Phenylalanine, which is metabolized in the body to tyrosine, include many fin and shellfish.  If the amino acids required for dopamine synthesis are present in the brain in sufficient amounts, along with other facilitating chemicals, and if certain environmental stimuli cue the brain that dopamine should be produced, then normal brains will produce dopamine and release it into a synapse where it can be received by one of five or more dopamine receptors. 

But in order for tyrosine or phenyalanine to pass through the blood brain barrier (BBB) and become available to the brain for synthesis for the reward function of the neurotransmitter dopamine to take effect, a complex of factors must be in place. The need for dopamine release must be cued. If cued, then oysters can support dopamine production and release by providing most of the required chemicals. While oysters may not initiate sexual desire or guarantee sexual performance, all evidence supports the claim that oysters are a food that optimizes many of the physiological aspects of sexual engagement, and as such, a best choice for dining with a sexual motive. The rest of this post explains how this happens. The following (planned) post will discuss the ecological sustainability aspects of oysters harvested in the Greater Santa Barbara Ecosystem.

Net: oysters are a best choice for dining with the objective of feeling good, with or without a sexual ulterior motive. In a healthy individual, oysters support biochemical processes that can result in elevated levels of dopamine. Dopamine signals goal-directed actions. But eating oysters does not guarantee increased dopamine levels.

Thwarting Processes

The body and brain both use amino acids such as tyrosine and phenylalanine to synthesize several catecholamines, among which dopamine is but one, the others are adrenaline and noradrenalin, involved in fight or flight response. The latter are released by the adrenal medulla of the adrenal glands, typically as a response to stress. There will be competition in the body at the cell level whereby cells will try to pull out amino acids before they even reach the BBB. So not all amino acids acquired in food will make it to the brain.

Because tyrosine can be synthesized in the liver from phenylalanine by the enzyme phenylalanine hydroxylase, it is not classified as an essential amino acid. But tyrosine cannot be synthesized in the brain. It must enter the brain by means of a neutral amino acid transporter, which also transports phenylalanine, tryptophan, methionine and the branch-chained amino acids. So there is competition for transport at the Blood-Brain Barrier (BBB). Amino acids all compete for the transporter, so a large quantity of one of the other amino acids in the blood stream could affect how much tyrosine successfully enters the brain. If tryptophan wins, then a relaxation/demotivating process takes effect. If tyrosine wins, the next order of brain business is action. Then, in order for dopamine synthesis to be initiated, dopaminergic neurons have to have been signaled that dopamine production is needed. Signals come from the environment or cognitive sources, such as a major threat in the case of the environment, or for cognitive species, expectation of reward, imagination of pleasure, remembrance of something exquisite that functions to motivate to further action, etc.

Non-brain dopamine 

Dopamine is synthesized within the kidney in the proximal tubule from circulating L-dopa, via the enzyme L-amino acid decarboxylase. Effects include variability of salt retention, which can affect blood pressure. Plasma or blood dopamine also varies in response to stress.  Dopamine may be further processed into norepinephrine by dopamine beta-hydroxylase. As indicated above, all of these catecholamines are involved in arousal and motivated action.

Catecholamines are water soluble, so they circulate in the blood in addition to being found in the brain. All catecholamines cause physiological responses that prepare for some activity.  In humans, effects that will be experienced are increased heart rate, blood pressure, changes in blood glucose levels in the autonomic nervous system. Under specific conditions, other cognitive or environmental cues inform the brain that sexual activity is now in play. Some of these could be relevant to further sexual activity.

In  humans, blood dopamine is short-lived. If dopamine is injected by IV, its half life is so brief, minutes at most, that the injected dopamine will not produce most of the effects attributed to dopamine and increased sexual performance.  For a human to experience significant increases in sexual desire and performance, dopamine would have to be IV-dripped continuously over the entire period of sexual motivation and performance.

If dopamine is taken by mouth, it is rapidly degraded in the intestine and blood and it does not penetrate from the blood into the brain. Therefore, the precursor L-DOPA is usually administered when dopamine deficiency becomes a medical problem. L-DOPA may be converted to dopamine in blood and in the brain.  Certain chemicals can block the conversion of L-DOPA in blood to dopamine, making it more likely that the L-DOPA may arrive and be transported across the blood-brain-barrier for conversion to the neurotransmitter dopamine.  The amino acids tryptophan and tyrosine both cross the blood-brain barrier in the same pathway. If tryptophan crosses the barrier, it will have a calming rather than arousing effect.


Dopamine the messenger is especially important to oysters in their larval stage, when they swim about more or less freely looking for the right place to settle down. Stimuli in desirable environments cue the larvae that this is the right time and place to settle down, in the same way that ripe peaches in an orchard signal humans that now is the optimal moment to pick and eat nutritious fruit. For oysters, such stimuli cues include chemicals secreted by fellow oysters already inhabiting that area (proving that it is indeed habitable and that oysters are social animals), chemicals such as ammonia exuded by certain bacteria, light cues, temperature, currents, salinity, surface tension, presence of adults or spat (newly settled larvae), shell-matrix proteins, mantle cavity and tissue fluids, etc. 

Cues trigger the transformation of the L-DOPA chemical in the larvae into dopamine, which communicates to the organism’s integration center (equivalent of other brain organs in humans) other settlement behaviors. The final settling of larvae is cued by waterborne chemicals secreted from conspecific adult oysters.

 Most dopamine in oysters is produced mainly in the larvae stage to direct settlement activities, prior to metamorphosing into an oyster as we know it. The message carried by dopamine in oyster larvae is"Settle down here!" If the object of humans is greater sexual interest and performance, the message will not translate across species. Eating dopamine-loaded larvae will not result in more dopamine in the consumer.

In humans, cues to release dopamine vary hugely, since dopamine is associated with arousal and goal-driven behaviors, which vary enormously across individuals. For example, risk-taking and insight both signal dopamine release, probably not normally linked to eating oysters.Humans, as the blog title suggests, occupy the cognitive niche, any almost any stimuli present imagined or remembered can be a cue.

Reward in the Cognitive Niche

At its simplest, reward-related behavior means moving towards in order to obtain an object, whether food or sex or other, as distinguished from not moving at all or moving away from so as to avoid contact. At its most basic level, dopamine marks stimuli that have some importance in the survival of an organism. Whatever is marked garners more attention than that which is unmarked and unremarkable. Both seeking and reward-related behaviors organize and mobilize chemical processes towards goals that relate directly or indirectly to survival, mating, and reproduction.

Although also found outside the brain in plasma, dopamine is most famously known as the brain  neurotransmitter, one that elicits well-rewarded human behaviors. In this context, dopamine is known as a brain chemical manufactured in the brain for messenger functions, although dopamine is also synthesized outside the brain in the kidneys.

Over evolutionary time behaviors that have promoted the survival of species, such as eating healthy food, mating, reproduction, as well as many social practices, have been reinforced in more cognitive species by the mechanism of the reward of feeling good, and in less cognitive species such as oysters, as biological programs that are initiated automatically by specific cues, discussed later in this post.

In humans, Evolution reinforces or rewards survival-promoting practices with feelings of pleasure and vibrancy such as the simple satisfaction of having consumed something delicious and nutritious, or the slightly more complex event of imagining consuming the same thing, to the probably more complex "totally alive" feelings associated with being hot on the search for something,which includes the desire to know, acquire, have, or interact with promising objects and their associated stimuli.  Two common examples are the enticing scents of a ripe peach and the even more complex experience of having inklings of being on the right track.  We smell something promising to be delicious, experienced in either a recent or remote past as life-sustaining, and we want it. We feel we are on track, and are motivated to continue the search. Eating nutritious, fresh food is a behavior that promotes survival and is reinforced without thinking. Instinct is an example of survival programs packaged for automatic execution without cognition. In species occupying the cognitive niche, the variables that could possibly influence survival, mating, and reproduction are mind-blowingly extensive.

What should emerge from the above discussion is that oysters create their own dopamine, which triggers settlement behaviors. Once settled, other chemical reactions trigger metamorphosis of larvae into oysters as we know them. Probably, oysters don’t so much feel desire or wanting, which are human terms describing certain kinds of behaviors. But like oysters, humans create their own dopamine, which will also be triggered by stimuli, either in the environment or because of the way human brains work, by the brain itself, remembering, anticipating, imagining, and so on. The dopamine in humans seems to be involved more complexly in a huge range of behaviors than those found in oysters.

Conclusion: Dopamine will not necessarily increase merely through eating oysters

Is dopamine found in oysters? It is, and not only in oysters. Research indicates that all animals that actively seek out resources can be said to engage in reward-seeking behaviors that involve dopamine. That includes worms to humans.  Certain cues in the environment evoke automated responses that have been programmed into organisms over evolutionary time, such as eating when hungry and stopping when full. Dopamine appears to be instrumental in initiating many goal-oriented behaviors in many species.

Is this mind-blowing news? It is insofar as the cognitive process of naming elements and processes can be considered wonderful, and insofar as scientific evidence continues to confirm Darwin’s hypothesis that we are all descended from a common ancestor. Common descent implies that some characteristics will be shared across phylla, and dopamine is such a broadly shared chemical. As a chemical messenger, dopamine can be likened to an electric current that is triggered by switching a lever to the on state, which results in a light bulb providing illumination. But turning on the on-switch in your house will not affect the lights in your neighbor’s house. And having lights on doesn't directly impact what activities then will occur in your house.

If you were to be swallowed then digested by a whale, the dopamine you made in your own brain would not be conserved as the messenger chemical functioning as it did in your brain: organizing or initiating focus, coordinating muscle movement. The whale would not benefit from an increased sense of vibrancy by suddenly acquiring more dopamine derived directly from consuming you and your elevated levels of dopamine. Things don't work like that. Dopamine in oysters may initiate a particular goal-directed activity such as settling down on hard sediment but it won't affect the oyster's libido.

Net: each human has to synthesize his or her own dopamine in his or her own brain for it to work as a neurotransmitter/messenger in the brain, since dopamine does not pass the blood brain barrier (BBB). 


The name of this blog is “Reward in the Cognitive Niche.”The concept of niche distinguishes ways and means organisms function. Reward in the cognitive niche differentiates reward in niches that can not be classified as cognitive from those that can. Humans occupy the cognitive niche, where conscious manipulation of data is characteristic to survival, which in turn, distinguishes some of the reward-related behaviors humans engage in from those engaged in by oysters, as well as functions specific to humans also not found in oysters, such as imagining and insight. Oysters are not good at consciously manipulating data for their own ends, but humans are.

 As a reminder: my mission in writing this blog is to facilitate and make more rewarding good decision-making in consumers committed to both a high quality of life and ecological sustainability. The next post will address the ecological sustainability of farmed oysters in the Greater Santa Barbara Ecosystem, probably post is around January 30.The following  post will address the second folk theory of oysters as aphrodisiacs: the  D-aspartic acid (D-Asp) and N-methyl-D-aspartate (NMDA) theory of oyster-consumption increasing libido/performance

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