Background

Sexuality is an essential human experience involving pleasure; sexual identity; affection; sociocultural, emotional, and cognitive factors; intimacy; and physical experiences. Sexual sensations may be aroused by several types of stimuli: fantasies, erotic thoughts, fondling, masturbation, and coitus. Sexual response is expressed by a succession of phases that manifest themselves physiologically in a sequential manner, with interconnected stages that complete the cycle of human sexual response [1]. The word libido is usually used to refer to sexuality. Libido is a Latin term, considered the word to be equivalent to designating one’s sexual drive. Psychoanalysts brought this scientific meaning closer to our daily experience, defining libido not only as a sexual desire but also as something concrete in the human experience: sex [2].

As the knowledge about human sexuality advances, differences between specifically masculine and feminine characteristics of response to sexual stimuli become better identified [3]. These differences are associated with biopsychosocial factors, particularly, sexual hormones (estrogens vs. androgens), sexual education (repressing vs. permissive), and environment (controlling vs. stimulating). In this context, for sexual desire to settle in a positive manner within the individual, his/her health should be in good condition. Sexual health is not only limited to the absence of disease or dysfunction but also involves a permanent balance of different sides involved in the sexual act and a wide range of factors, including the physical, mental, and emotional characteristics, as well as social wellbeing in all sexual behaviors. This can also be described as the integration and coordination among mind, emotions, and body, which control the social aspect of life [4].

The physiology of human sexual response consists of four phases that take place in both genders and may display some distinct specificities among them. This begins with (1) excitement, which corresponds to vaginal lubrication for women and penile erection for men. This is characterized by two phenomena, vasocongestion (increased blood flow in organs and body areas) and myotonia (local or general muscular contractions); (2) plateau, which corresponds to continuous excitement; (3) orgasm, which corresponds to maximum excitement, with extensive vasocongestion and myotonia, and is perceived by the number of movements in the pelvic muscles, represented in males by ejaculation; and (4) resolution, the phase in which relaxation and the involution of phenomena take place, which is more “evident” in men [5, 6]. Sexual desire would be the first phase of human sexual response, involving physiological modifications that take place after a positive sexual stimulus. This phase is associated with (1) central nervous system (CNS) areas, such as the limbic system, hypothalamus, and neocortex; (2) hormones, such as gonadotrophin releasing hormone (GnRH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), testosterone, and estrogen; (3) neurotransmitters, such as serotonin, dopamine, norepinephrine, and oxytocin; and (4) sexual stimuli [6]. Any disturbance resulting from the imbalance caused by a dissatisfaction can lead to sexual dysfunction [4].

The sexual response cycle may be negatively influenced by different factors. The etiology of sexual dysfunctions is multifactorial and may be related to (1) organic causes diseases such as diabetes, high blood pressure, atherosclerosis, obesity, neurological diseases, menopause, andropause, erectile dysfunction, cancer, endocrine alterations, surgical treatment, and use of drugs, and (2) psychological causes: individual factors (personality, low self-esteem, history, religious beliefs, depression, fatigue, anxiety, stress, fears, frustration); interpersonal factors (poor communication, low confidence, fear of intimacy with others), and psychosocial factors (performance anxiety, sexual traumas, surreal response expectations, among others) [5].

Sexuality and cognition share characteristics from the peripheral area of the afferent somatic nervous system, in such a way that the understanding of sexual mechanisms also requires a good knowledge of psychological/neuromodulation mechanisms [7]. Therefore, the study of sexual functions requires the knowledge of the co-existing mental activity, as the mental process largely influences sexual response. Studies have demonstrated that, in the relationship between brain control and sexual response, the limbic system plays a key role in initiating and conducting sexual behavior, triggering the cascade of neurovascular elements, including somatic and genital responses and appropriate behavior [8]. The limbic system modulates the perception of desire and excitement, as any physical damage and/or chemical may result in desire, excitement, and behavioral dysfunctions. Therefore, the physiology of sexual excitement involves activity in the autonomic nervous system, which may prevent or facilitate the response of peripheral sexual organs, which will aid in preparing the body for sexual activity [6]. Sexual stimulation is generated differently by men and women. For men, desire is focused mainly in the final goal, that is, sexual intercourse according to the need-reward mechanism [8]. Meanwhile, for women, sexual desire is characterized as being multifactorial and is strongly associated with emotional factors [9]. Cells communicate among themselves through a flow of molecules that consistently influence their behavior and activity. Two distinct classes of communicating molecules are recognized and classified according to their origin and destination: hormones and neurotransmitters. Hormones, many times, originate in endocrine glands, which reach the target cells through the blood stream, whereas neurotransmitters are usually generated locally and are bioactive in the synaptic fissure. Any interference in this communication may result in pathological conditions, given that the disorders may affect the brain field (sexual desire) and the body (excitement and erection) [10]. In the psychological/physiological plan, mental stimuli are present in the form of sensation and perception. Serotonin, dopamine, and norepinephrine are neuromodulators that intervene in mental and sexual mechanisms. Serotonin and norepinephrine intervene in the origin of mental sensations, while dopamine intervenes in the origin of perceptions. Sexual hormones and pheromones, by contrast, intervene in the origin of sexual excitement and the libido, as shown in Fig. 1 [7].

Fig. 1
figure 1

Biochemical flowchart of psychohormonal processes related to libido. Interventions of serotonin, dopamine, and noroepinephrine on mental and sexual mechanisms. Hormones and sex hormones acting on the genesis of sexual arousal and libido. Adapted7

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Hormones (androgens in men and estrogen in women) influence sexual behavior and response capabilities, which may act by means of central and peripheral mechanisms [11, 12]. Studies referring to neurotransmitters in the sexual response for both genders have demonstrated that they are released by autonomic nervous terminations, such as acetylcholine, dopamine, noradrenalin, melanocortin (VIP), and nitrous oxide (NO) [8, 9]. Serotonin is a neurotransmitter synthesized based on tryptophan and is involved in mood regulation, as well as in the phenomena of anxiety and depression. In the limbic region, serotonin is involved in the control of pleasure and motivation. Increased serotonin in the central nervous system reduces overall sexual activity, as well as sexual desire, inhibiting orgasm [8]. General mechanisms are summarized in Fig. 2. Sexual behavior may be due to androgenic and gonadotropic activities, assigning to the components flavonoids and/or saponin present in plants, as they have been reported as capable of altering androgen levels [12].

Fig. 2
figure 2

Central effects of neurotransmitters (dopamine, noradrenaline, VIP, nitric oxide, serotonin, and oxytocin) and hormones (testosterone, estrogen, prolactin, and progesterone) in desire, sexual arousal, and orgasm

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Currently, for remission of the symptoms of sexual disorders and consequent decrease of libido, the main choice is medication. Drugs such as Flibanserin––a drug approved by the US Food and Drug Administration in August 2015 for the treatment of hypoactive sexual desire disorder [13], according to studies, is being widely used to ameliorate the symptoms of these disorders [13,14,15,16].

However, in spite of the high rate of people who choose to use drugs, in describing the physiological processes of the sexual response cycle, it is known that nutritional aspects are important in this process, as there are hormonal regulators that can, most times, reverse sexual dysfunctions with simple lifestyle changes. Therefore, eating appropriately and including specific nutrients may contribute to improving one’s quality of life, promoting sexual health. Herbal medicines may aid in this process, in turn promoting an increased libido. Thus, considering that nutrition has a significant impact on physiological processes and that sexual dysfunction can be reversed, the present article describes scientifically proven actions on the libido brought about by the use of herbal medicine and evaluates the use of nutritional interventions associated with herbal medicine treatment in patients with libido related complaints.

Methodology

A systematic review of the PUBMED, Scielo, and EMBASE databases was conducted, using the keywords “libido, food, and nutrient.” The search identified 2.798 articles. The inclusion criteria included describing the use of some supplement, herbal medicine, or food that helped in improving the libido. By contrast, the exclusion criteria were hormone replacement therapy, surgical procedures, and therapy with drugs, that is, methods that are not within the nutrition professional’s competency.

Results and discussion

After applying the inclusion and exclusion criteria, 91 articles were selected, based on their titles and abstracts. However, after analysis and discussion by the group, 19 of these articles met the previously mentioned exclusion criteria. Thus, 72 articles were selected to be read in their entirety. After having read all of the articles, 38 of these still met the exclusion criteria and were removed from this study. Therefore, 34 articles were used in the review and construction of this work, as shown in Fig. 3.

Fig. 3
figure 3

Schematic representation of the studies found through keywords “libido, food, and nutrient” in PUBMED, Scielo, and EMBASE databases

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Herbal medicine has been identified for having an effect upon the hypothalamic-pituitary-testicular axis, thus increasing the libido. This occurs through the increase in the number of several hormone and serum testosterone concentrations, which, in turn, stimulated the dopamine receptor, through mechanisms, such as vasodilation, the generation of nitrous oxide, increased androgen, and gonadotropin [12]. Several herbal medicines have been reported as capable of affecting penile erection through different mechanisms. Alkaloids, for instance, have demonstrated ergogenic properties, which lead to vasodilation in blood vessels, resulting in erection. Meanwhile, the saponin content acts to form nitrous oxide and may lead to the relaxation of the smooth muscle by means of L-arginine/nitrous oxide [12]. In addition, the presence of flavonoids in plant extracts implies its role played in the change of androgen levels (Fig. 4).

Fig. 4
figure 4

Main bioactives of the phytotherapics cited in the studies selected through the keywords “libido, food, and nutrient” in the PUBMED, Scielo, and EMBASE databases

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Herbal medicines

After reviewing studies that used different herbal medicines, those used more frequently and with positive results regarding the human libido were selected (Table 1).

Table 1 Herbal medicines used for study results
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Tribulus terrestris

This plant may be a safe alternative to improve the domains of sexual desire. Its probable action mechanism involves serum levels of free and bioavailable testosterone [17]. A study performed with 96 women who underwent treatment with Tribullus terrestris for 4 weeks revealed increased testosterone levels, in addition to synergy with FSH-LH hormones, when compared with the placebo [18]. The plant also leads to increased testosterone levels in men [19]. A systematic review showed that T. terrestris acts on the androgen metabolism, helping protodioscin - saponin that acts on erectile dysfunction, when converted into dehydroepiandrosterone (DHEA) to convert into such a hormone inside the body, improving the sexual function and increasing the activity of the superoxide dismutase enzyme [20].

Eurycoma longifólia

Belonging to the Simaroubaceae family, it is known locally as “Tongkat Ali”, a small perennial arboreal tree commonly found in tropical forests in Southeast Asia. Phytochemical studies on this plant revealed the presence of several quassinoids, derived from squalene, biphenyl neolignans, and triterpenes. E. longifólia is capable of significantly increasing testosterone serum levels [20]. An experimental study lasting 12 weeks has identified that the plant improved the quality of sexual life [21].

Ginseng

Ginseng is traditionally used as a tonic to help the body deal with many types of stress. In addition, it is often used to improve energy levels and increase blood circulation [22]. Use of ginseng has a relaxation effect that modulates the relation between nitrous oxide and the cavernous smooth muscle [23]. In this reaction, nitrous oxide is released through the vasomolecular endothelium, which leads to the relaxation of the cavernous smooth muscle through the metabolism of calcium and potassium. Both reactions are mediated via nitrous oxide-cyclic guanosine monophosphate (NO-GMPc), and the hyperpolarizing action takes place through the K channels activated by Ca2. Ginseng plays a role in brain synaptosomes, inhibiting the absorption of r-aminobutyric acid, glutamate, dopamine, noradrenalin, and serotonin, as well as aiding in the increase in the estrogen receptor expression. The central effect of ginseng, through these multiple mechanisms is postulated as one of the possible explanations for increased sexual arousal [23]. In a work in which natural aphrodisiacs were separated, dealing with their meanings and benefits, ginseng was pointed out as having active constituents, such as ginsenosides - saponin glycosides. Some studies report that ginsenosides may lead to an increased release of nitrous oxide from the cavernous tissue smooth muscles, thus increasing sexual desire [20].

Maca (Lepidium meyenii)

Maca is the edible root of L. meyenii, a crucifer (Brassicaceae family) grown exclusively in the Central Peruvian Andes. This plant is suspected to have a compound with a portion similar to the human testosterone molecule, which may have exercised its androgenic effects through actions performed on the testosterone receptor in target organs, not affecting the level of testosterone or gonadotropin [24]. The plant contains arginine, a constituent that improves male sexual performance. Steroids, such as campesterol, stigmasterol, and sitosterol, are described in a study that lasted 21 days, proving its role in improved sexual performance. The maca root may lead to a relief in sexual dysfunction induced by serotonin re-absorption, including a beneficial effect on the libido. There may also be a dose-related effect, but no specific action mechanism has been studied. However, it has been noted that the plant has no effect on the gonadal hormone serum levels or on the serum levels of LH, FSH, prolactin (PRL), 17-alpha hydroxyprogesterone, thyroid stimulating hormone (TST), and 17-beta-estradiol.

Ginger (Mondia whitei)

Extracts from the plant root may potentiate androgenic effects and the action of norepinephrine on the different channels. Ginger modulates physiological activities involved in penile erectile dysfunction, aiding in erection and increased libido [22]. Studies have indicated that the plant’s aphrodisiac properties may be measured by means of several mechanisms––one of which by increased testosterone levels that cause increased libido. Another mechanism consists of decreasing the stimulation of the α-adrenergic enzyme on the cavernous tissue, consequently maintaining penile erection. Lastly, it has shown the capability to increase the amount of sperm as well as its motility. Although further clinical trials on the plant are required, it has the potential to be a supplement designed to increase general and sexual well-being [25].

Low-energy, low-fat, high-protein diet

A study involving 31 men, subjected to a modified meal replacement program and to a diet including high nutritional quality, whole foods, reported an improved sexual function. Over half of the patients with severely compromised erectile dysfunction reported an improvement of 5 to 10% in sexual desire, due to weight loss. Weight loss, in the short run, induced by caloric restriction, influenced the increase in testosterone. As visceral adiposity and insulin resistance are associated with the production of inflammatory cytokines and with the aromatization of testosterone into estradiol in the adipose tissue, weight loss leads to the release of pituitary gonadotropin and to the production of testicular androgens, in turn improving the sexual function. The strong points in the study are the comprehensive evaluations, parallel dietetic approaches, and relatively long duration [26].

Conclusion

As noted, sexual stimulation is caused differently in different genders since men focus on the necessity-reward mechanism, while women are more often associated with emotional factors. Herbal medicine, in general, has revealed properties that contribute to increasing the activity of specific hormones that act on the libido. Some herbal medicines stood out, including T. terrestris, for increasing testosterone serum levels, and E. longifólia, which also leads to an increased biosynthesis of several androgens; ginseng, which increases energy levels and stimulates smooth muscle relaxation with nitrous oxide; Maca (L. meyenii), which improves sexual performance, in addition to having androgenic effects; and Mondia whitei (ginger), which improves the libido and erection. In addition to these, one study demonstrated the effectiveness of a hypocaloric, hyperproteic, and hypolipidemic diet on the libido, improving sexual and erectile functions, as well as increasing testosterone levels. Although studies in literature that investigate the action of herbal medicine are still being performed on animals, the need for more studies performed on human are evident. Herbal medicine previously studied in human beings have demonstrated positive effects on the libido, indicating a promising field of action for nutrition as a support in treating sexual dysfunctions.