LIBIDO: Facts and Fantasy

With the exception of prolonging life, nothing has fascinated, intrigued and indeed mystified man more than the fantasy of enhancing his sexual prowess. Human history is rife with the symbolisms of sexuality and fecundity and the list of pills, lotions and potions to stimulate libido is long and tired. It is only in the recent past, however, that the basic mechanisms have been elucidated, which have allowed us to address the issue of libido and in fact sexual dysfunction per se more rationally. Sexual dysfunction is an umbrella term that encompasses an array of conditions that afflict both genders. Of these, erectile dysfunction (ED) is the condition most exhaustively covered by the mass media--thanks in no small part to the willingness of the ex-senator Bob Dole to bring ED out of the closet, as it were. The "courage" shown by Mr. Dole is largely responsible for mature emphasis on, and awareness of, sexual health.

Erectile dysfunction is the dreaded word that can change lives for good and exact a heavy emotional price. To the sufferers of ED, it is most disconcerting that it sneaks up over the years and is hardly discernible. Its underlying cause may just as well be psychological as organic or physical. Conservative estimates are that roughly 150 million men suffer from ED worldwide, particularly those over the age of 60.

What is ED, or impotence, and how does it ensue? Essentially ED is the inability to attain or sustain penile engorgement necessary for normal coitus and could be either primary or secondary. Primary ED, which is quite rare, is indicative of severe psychopathology; in secondary ED, on the other hand, the coitus cannot be successfully completed, despite initial engorgement. In rare cases, biogenic factors such as low testosterone and disorders of hypothalamic-pituitary-gonadal axis complicate the presentation of primary ED. In secondary ED, approximately 70 percent of cases are psychological that include guilt, fear of intimacy or depression.

The mechanisms that contribute to ED are numerous and complex (see sidebar) and may be presented either concurrently or sequentially.( n1) A cursory look at the flow sheet shows that any one of the steps involved may precipitate and/or aggravate ED. It also suggests that the erectile process is so tightly orchestrated that, in turn, is controlled by a number of biochemical steps. Thus the ineffable "mind-set" for an intimate encounter is central to sustained engorgement of the penile tissue. It is for this reason that in the early stages of marketing Viagra, Pfizer's direct-to-consumer advertisement blitz forcefully underscored the importance of romantic context in its efficacy. The requisite context stimulates nerves, which begin to fire and, ultimately, initiate the cascade of reactions that cause penile tumescence (swelling).

Whereas the hydraulic physiology of the erectile process was discerned much earlier than the biochemical steps involved, it is only recently that the role of blood flow has been fully understood. This was impelled by the recognition that the erectile process is not a result of muscle contractions and relaxation but is due to blood engorgement. The more the blood flows through the male reproductive organ (MRO), the firmer the penile engorgement. The "gatekeepers" for the blood flow are smooth muscle cells in the spongy tissue of MRO. When smooth muscles are constricted, the blood flow is restricted to a trickle--not unlike squeezing a garden hose. When these same cells relax, however, the blood flows as if the pressure on the hose were released. Consequently, as the blood from the arteries gushes to fill the two expandable reservoirs inside the MRO, called corpus cavernosa, rigid and sustained tumescence results.

What triggers the blood to fill corpus cavernosa? In the early 1990s it was demonstrated that in healthy individuals blood flow is triggered when nerve endings release nitric oxide (NO), a relatively short-lived neurotransmitter, which is the same substance that relaxes smooth muscle cells as well.( n2) Quickly thereafter it was found that NO initiates the cascade of reactions to engorge the MRO and cause tumescence. Briefly, nerves in the pelvic area respond to stimuli from the brain to produce NO, which dilates the blood vessels throughout the region to supply blood for engorgement. Since NO is indispensable in this process, it was logical to investigate whether its decreased amounts contributed to ED. Indeed, it turned out that nitric oxide synthase, the enzyme that produces NO, is the culprit in impotence. It should be noted that aging alone does not necessarily precipitate impotence. Attendant organic problems--such as vascular disease, high blood pressure and damage to the cardiovascular tree, to name but a few--may suppress the release of NO and contribute to ED.

In light of the foregoing, increase in the blood flow would be expected to stimulate libido. Parenthetically, this holds true not only for men but also for women. Interestingly, research on sleep has provided evidence that women undergo similar changes in blood flow to the pelvic floor muscle during sleep cycle as do men. Thus a healthy libidinal response depends on well-functioning vasculature in the urogenital arteries. It is precisely for this reason that clinical trials are underway to evaluate the efficacy of Viagra on female libidinal response.

While Viagra has become a market dynamo in amelioration of ED, its mode of action is inhibitory rather than stimulatory. This may explain, at least partially, the side effects of Viagra, even though it works almost immediately. For obvious reasons, two pharmaceutical houses have Viagra-like drugs in preparation. Quite plausibly, Viagra's side effects will be recapitulated in these new remedies as well. It is for concerns such as these that the so-called "Viagra refugees" are clamoring for natural alternatives to boost libido.

There are "natural solutions" galore that claim to restore sexual function overnight as if by magic. In fact, in the fray of the cluttered market place of Viagra alternatives, it is difficult not to be taken by the unabashed enthusiasm and indeed, unqualified promises of a carnal Shangri-la. Thus it is difficult to differentiate products that have a fighting chance to alleviate the symptoms of ED from those with non-validated claims. For example, inasmuch as testosterone replenishment may help resolve sexual dysfunction, it is a viable option only under competent clinical supervision. Likewise, numerous herbs have been positioned to mitigate, if not outright reverse, ED symptoms. While circumstantial evidence does suggest that some of the herbs could help attenuate the severity of ED over extended periods of time, it should be noted that these are "preventive" modalities at best. That is, they are likely to be effective only if used over the long haul as part of a disciplined nutritional regimen. In other words, contrary to the received opinion, there are no quick fixes unless, of course, the underlying problem is adequately addressed.

Erectile dysfunction is a chronic condition which takes decades to develop. As such, a systemic approach in its management is a more realistic alternative. Since blood flow to the erectile tissue is the critical factor, substances that enhance circulation should mitigate ED. Citrulline, a relatively specialized amino acid in the body, is the precursor that optimizes blood flow by "pacing" the vas culature.( n3) Mechanistically, citrulline is converted to Loarginine, which is a known inducer of NO. (Fig. 1) Thus by inducing the body's own vasodilator, citrulline ensures blood flow in sufficient amounts to the pelvic floor to engorge the erectile tissue.

Since citrulline increases NO levels via arginine, the question is forced whether arginine might not be a more obvious alternative. Not so, for arginine is less prone to be absorbed efficiently to induce NO. After all, it has to compete with a surfeit of amino acids for the same transporters in the intestinal wall to enter the bloodstream. In contrast, the relative rarity of citrulline facilitates its more ready transport across the intestinal wall. Importantly, oral intake of citrulline more effectively produces arginine in situ to generate NO.( n4) Put differently, citrulline increases the plasma levels of induced arginine better than orally ingested arginine. How so? Arginine is assimilated over the digestive tract and upon entering hepatic circulation, the bulk of it is degraded. In contrast, citrulline is not cleared by the liver, rather, it is taken up by the kidney and other tissues where it is converted to arginine. Specifically, oral intake of citrulline results in 60 percent increase in the plasma levels of arginine. (Fig. 2) It is significant that citrulline sustains plasma arginine levels much longer titan intake of a similar dose of arginine supplementation.( n5)

By increasing the blood flow, citrulline tweaks the body's repair kit to reduce the severity of ED presentation, which should recharge the libido for a revitalized life. Hence, citrulline provides a more rational approach toward sexual dysfunction, which stands in stark relief to much-touted aphrodisiacs with exaggerated claims. Named after Aphrodite, the Greek goddess of beauty and physical love, these supposed stimulants range from anchovies to adrenalin, licorice to lard, scallops to Spanish fly and everything in between. Aphrodisiacs fan the fantasy of unlimited, even insatiable, libido. Such fantasy, however enticing, militates against not only human physiology but also the alleged functions of a multitude of aphrodisiacs. Therefore, a systemic modality that helps correct the underlying problem is a more meaningful means to restore vitality and robustness. In that regard, citrulline is not an aphrodisiac but is much more that over time could potentially rejuvenate the zest for life. Equally, by promoting circulation through the highways and byways of the cardiovascular tree, citrulline fosters better health, which is indispensable for a healthy libido.

Step 1: Sexual stimulation causes a variety of nerves originating in the brain to start firing.

Step 2: Once stimulated, these nerves release the neurotransmitter acetylcholine in the penis.

Step 3: Acetylcholine, in turn, causes the endothelial cells in the penile arteries to begin producing NO by the action of a specific enzyme called nitric oxide synthase.

Step 4: Once produced, NO triggers the release of another chemical cyclic guanosine monophosphate (cGMP), Cyclic GMP is one of the many potent vasodilators found in the human body.

Step 5: As cGMP levels increase, the smooth muscles of the penile arteries relax, the vessels dilate, and increase blood flow causes tumescence of corpus cavernosa, producing an erection.

Step 6: Even as NO continues to generate cGMP, another enzyme begins to break it down. This enzyme, known as phosphodiesterase, functions like a brake on the entire cascade from becoming excessive or permanent (priapism). Priapism can cause permanent damage to erectile tissue.

Step 7: Following climax, or cessation of sexual stimulation, the nerves stop firing and the nerve endings stop releasing acetylcholine. In the absence of acetylcholine, the endothelial cells stop NO production, without which no cGMP can be produced. The residual cGMP is degraded by phosphodiesterate. The net result is that smooth muscles once again contract as the penile tissue goes back to its non-aroused, flaccid state.

This flow sheet demonstrates how easily this balance of steps in the erectile process can be affected at any step and how erectile dysfunction could ensue. Importantly, it also suggests that increased production of NO can enhance libido.

DIAGRAM: Figure 1: The Citrulline-Nitric Oxide Cycle.

GRAPH: Figure 2: Oral Citrulline vs. Plasma Arginine

(n1)) McConnell, J. and Wilson, J. "Impotence," In Harrison's Principles of Internal Medicine, Isselbacher et al., Eds. McGraw-Hill, Inc., New York, New York, 1998, p. 263 ff.

(n2)) Burnett, A. "Nitric Oxide in the Penis: Physiology and Pathology," J. Urology: 157, 320, 1997.

(n3)) Waugh, W., Daeschner, W., Files, B, McConnel, M. and Strandjord, S. "Oral Citrulline as Arginine Precursor," J. Nat. Med. Assoc., in press, 2001.

(n4)) Windmueller, H. and Spaeth, A. "Source and Fate of Circulating Citrulline," Am. J. Physio.: 241, E473, 1981.

(n5)) Waugh, W. "Orthomolecular Medical Use of L-Citrulline for Vasoprotection, Relaxative Smooth Muscle Tone and Cell Protection," United States Patent No. 5 874 471, 1999.


By Aftab J. Ahmed, Ph.D.

Aftab J. Ahmed, Ph.D. is director research and development and business development. Marlyn Nutraceuticals, Inc. E-mail:

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jmandrix's picture

These natural aphrodisiacs help to increase libido, otherwise known as sexual desire or sex drive. The male and female libido can slump at one time or another, especially after the age of 30, with the stresses of everyday life. In fact, 40 per cent of women note a lag in their libido just above the 30 per cent for men. Day to day responsibilities of life such as daily stress and pressure, fatigue, illness, finance, work and even the kids begin to outweigh the importance of sex, not to mention leave us without the oomph for intimacy with our partners.

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