Variation of Leptin During Menstrual Cycle and Its Relation to the Hypothalamic–Pituitary–Gonadal (HPG) Axis: A Systematic Review

Introduction

The physiology of energy homeostasis and the control of appetite have greatly changed since the discovery of leptin, an adipose tissue hormone that was first purified in 1995.¹ Recently, a wide range of physiological functions of leptin have been described, and leptin was found to exert a regulatory control upon insulin sensitivity, immune function, reproductive function and a wide range of neuroendocrine axes: ACTH-cortisol, TRH-TSH, prolactin, and GnRH. Leptin signals about the body-fat stores to the hypothalamus and other neuroendocrine centers, and as a feedback response, the neuroendocrine systems modify their function accordingly.^2,3

Many reviews described the role of leptin in human reproduction comprehensively, showing a key role of leptin in the complex interaction between the nutritional status and the reproductive system.^4,5 The association between leptin and reproduction was first described by observing that ob/ob gene deficient female mice were obese and sterile.⁶ Humans with congenital leptin deficiency also have obesity and infertilely and showed failure in puberty development due to hypogonadotropic hypogonadism.⁷ Both obesity and undernutrition were found to affect fertility in humans.^8,9 Furthermore, leptin play a permissive action for puberty both in girls and boys with obesity, specifically by affecting the hypothalamic–pituitary–gonadal axis.¹⁰

Leptin has a role in the physiology of female reproduction in particular, as it showed gender-related differences at birth, which persist throughout life with a circulating level two to three times higher in females than in males.⁵ Serum leptin in women with hypothalamic amenorrhea was lower than in healthy control women, and administration of leptin restored the menstrual cycles and fertility.¹¹ Recently, several studies showed the involvement of leptin in the development of several pregnancy-related diseases such as gestational diabetes.^12–14 All these findings led to the development of the hypothesis for leptin as an important player in the female reproduction system via its stimulatory effect on the hypothalamic–pituitary–gonadal axis.^3,15

The role of leptin in the menstrual cycle and female reproduction was first suggested in the late 1990s; several studies noticed a significant variation in serum leptin during the female menstrual cycle,^16–20 but this variation was absent in postmenopausal women.²⁰ Subsequently, many studies showed inconsistent leptin behavior throughout the menstrual cycle, and some reported steady increments in leptin levels from the follicular phase to reach a peak in luteal phase,^21–24 and others showed a peak during the preovulatory phase²⁵ while a stable level of leptin across menstruation was also described.^26–28 However, this discrepancy highlights a certain role of leptin signaling during the menstrual cycle, but the mechanism of this role is still to be explored.

Currently, there is no review that explores specifically the variation of leptin during the menstrual cycle and its relation to the hypothalamic–pituitary–gonadal axis. In addition, the effect of body weight on leptin during menstrual cycle is not fully elucidated. Thus, the aim of this review is to examine the variation of leptin during the menstrual cycles and its relation to the hypothalamic–pituitary–gonadal axis and to body weight. This review was carried out according to the guidlines provided by the Preferred Reporting Items of Systematic Reviews and Meta-analyses (PRISMA) statement.²⁹

Methods

Information Sources and Search Strategy

Medline/PubMed and Google Scholar databases were searched for studies of leptin during normal menstrual cycle since 1995 (the time of leptin discovery) until October 2020. The search strategy used keywords related to leptin and menstrual cycle (adipokines[Title] OR adipocytokines[Title]) OR adipose tissue hormones[Title]) OR leptin[Title]) AND menstrual cycle[Title/Abstract]) OR Endometrial Cycle[Title/Abstract]) OR ovarian cycle[Title/Abstract]) AND “humans”[MeSH Terms]). Further searching through ISI and Scopus and by manual screening the bibliographic references of the selected studies did not yield any additional papers. No language restriction was applied, and the retrieved references were imported into EndNote X5.

Study Selection

The titles were screened for relevance to the aims and scope of the current review; those studies falling within the scope and were further screened by abstract to determine the eligibility according to inclusion and exclusion criteria. The following inclusion criteria were applied: (a) prospective observational studies that had been run over one or more complete normal menstrual cycles; (b) the study population were adult healthy women with regular menstrual cycles; (c) the primary outcome of the studies was the variation of leptin levels during different phases of the menstrual cycle, with or without investigation of the relationship between leptin and the body weight (BMI); (d) leptin and other hormones assays were conducted in blood or saliva samples during different phases of the menstrual cycle. The current review excluded: (a) studies evaluating irregular menstrual cycles; (b) menstrual cycle in females with associated gynecological diseases or comorbidities, eg, polycystic ovary syndrome (PCOS); (c) studies with less than one complete cycle duration; and (d) randomized clinical studies RCT, review, systematic review, meta-analysis, case report, comments, editorials, letters and animal studies. After the preliminary eligibility screening evaluation, the full articles of the selected studies were obtained (Figure 1).

Figure 1 Flow chart of the literature search and selection process.

The following data were extracted from the selected studies:

• General information about the study: author(s), journal name, title, publication year, study design, and location of the study
• Age of the subjects
• Number of participants
• Duration of the study (number of cycles evaluated)
• Frequency of leptin sampling during each cycle
• Major outcomes and conclusion
• Limitations if any

Quality Assessment

Based on a six-point scale by Hayden et al³⁰ the following criteria was used to evaluate the quality of each selected study: (1) study participation (the sample is large enough and represents the population of interest); (2) study attrition (full explanation of the sample drop out); (3) determination of the factor of interest (clear definition and description of the factor of interest is provided); (4) confounding factors control; (5) outcome measurement (full explanation of the method used for outcome measurement in such a way that reduces measurement bias; and (6) appropriateness of statistical analysis. One point score was given for each criterion; the score of 0–3 points indicated low quality studies, while scores above 3–6 were considered high-quality studies.

Results

A primary search by MEDLINE/PubMed identified 388 articles; two extra articles were identified by Google Scholar search. By screening the titles and the abstracts, 358 articles were excluded for reasons of scope and/or not meeting the inclusion and exclusion criteria of the review. A total of 30 full-text articles were retrieved and analyzed for eligibility; of these, eight articles were excluded because five of them did not meet the inclusion and exclusion criteria and three articles did not achieve a minimum quality assessment score (≤3). Finally, 22 articles were included for the qualitative analysis in the current review (Figure 1).

The selected studies included women from 14 countries: Poland,³¹ Czech Republic,²⁸ Germany,^16,32–34 Saudi Arabia,^35,36 USA,^20,37 Greece,²⁴ Switzerland,²³ Spain,³⁸ Italy,^18,27,39,40 Austria,⁴¹ UK,¹⁷ Japan,⁴² Sweden,⁴³ and Finland.⁴⁴ These studies included cycling women with a wide age range (18–44 years), and the number of cases ranged from six¹⁷ to 259³⁷ women. All studies were prospective observational studies, except four clinical studies that included multiple groups.^16,39,43,44 In these four studies, leptin variation during menstrual cycle was determined on the control groups, which included spontaneous cycling women without any intervention; thus, they have been included in the present review. The majority of the studies had evaluated leptin during one complete menstrual cycle; however, two studies had evaluated leptin over three consecutive menstrual cycles^16,32 and one study over two consecutive menstrual cycles.³⁷ All studies measured leptin in the blood, except one study that evaluated leptin variation during menstrual cycle in a saliva sample.³³

Variation of Leptin During Menstrual Cycle

Of the included studies, 18 have shown significant changes in leptin level across different phases of the menstrual cycle^{16–20,23,24,31–39,41,43} (Table 1), while four studies reported no changes in leptin throughout the menstrual cycle^27,28,42,44 (Table 2). Most of the studies reported significant changes in leptin and showed a steady increment from the lowest level at the early follicular phase to its peak at late luteal phase—except two studies, where leptin peak was detected at the mid-cycle phase near the time of LH surge.^37,39

Table 1 Summary of 18 Studies That Showed Significant Variation of Leptin During Different Phases of the Menstrual Cycle

Table 2 Summary of Four Studies That Showed Stable Leptin Level During Different Phases of the Menstrual Cycle

Relationship Between Leptin Variation During Menstrual Cycle and the Hypothalamic–Pituitary–Gonadal Hormones

The relationship of leptin during menstrual cycle with at least one of the hormones of the hypothalamic–pituitary–gonadal axis was evaluated by 17 studies,^{17–20,23,24,27,31,33–39,42,44} as seen in Tables 1–2.

Effect of Obesity on Leptin Variation During Menstrual Cycle

Among the selected studies, only four studies investigated the effect of obesity on leptin variation during menstrual cycle by comparing normal weight women with overweight/obese women.^35,36,38,42 Two studies showed significant fluctuation of leptin level with the lowest values during the early follicular phase and the highest during the luteal phase in both normal weight and overweight/obese groups.^35,36 In contrast, one study detected leptin variation during the menstrual cycle in the normal group with higher levels during preovulatory and luteal phase, while overweight women showed stable leptin level.³⁸ Another study reported no changes in leptin levels during the menstrual cycle in the normal weight group, while leptin showed significant variation during the luteal phase in the obese group.⁴²

Discussion

The role of leptin in the reproductive function in humans is well documented. Leptin is considered to be the link between body fat storage and the hypothalamic–pituitary–gonadal axis. Many human and animal studies suggest that leptin has also been implicated in the physiology of puberty, menstrual cycle, menopause, pregnancy, and lactation.⁴⁵ However, the role of leptin during the menstrual cycle showed some controversy. In addition, the mechanism describes the effect of obesity on leptin level during the menstrual cycle needs further review and analysis.

The majority of the studies evaluating leptin in normal-weight, regularly menstruating females showed a significant variation during the menstrual cycle (Table 1). Only a few studies reported a small nonsignificant variation in leptin level across menstruation^27,28,42,44(Table 2). This insignificant variation could be attributed to the small sample size and method of determination of the menstrual phases in these studies, which is based on menstrual rhythm only. However, some of these studies reported a significant variation in leptin during the menstrual cycle in the obese group,⁴² while others showed high leptin levels in the ovulatory cycle group compared to the nonovulatory cycle²⁷ and a positive correlation with LH level.⁴⁴

The pattern of leptin variation during menstrual cycle is almost similar among all studies; leptin showed a high concentration during preovulatory and luteal phases when compared to the follicular phase with a steady increment starting from the follicular phase through the ovulatory phase and reaching a peak during the luteal phase (Table 1). However, two studies detected a peak of leptin just before ovulation, which coincides with LH surge (preovulatory peak),^25,39 but this peak was not significantly different from the high luteal leptin level reported by these studies.

To control for the effect of adiposity on leptin variation during menstrual cycle, only normal weight women with BMI <25 were included in most of the studies. However, some studies had further demonstrated significant leptin variation during the menstrual cycle in overweight/obese women similar to the pattern observed in normal weight women.^35,36,42 The presence of leptin variation in obese women indicates factors other than adiposity are implicated in leptin variation during menstruation. Corpus luteum formation during the luteinization phase could be possible cause of elevated leptin, and corpus luteum has been found to produce leptin in human⁴⁶ and bovine models.⁴⁷ Further support came from comparing ovulatory and anovulatory cycles; decreased leptin concentration was found during the anovulatory cycle (ie, without ovulation and corpus luteum) compared to the corresponding higher leptin level during the luteal phase of the ovulatory cycles (with ovulation and corpus luteum formation).^25,27 In addition, the LH surge during ovulation stimulates the production of many inflammatory mediators within the ovarian follicles.⁴⁸ Because leptin is considered a pro-inflammatory mediator,⁴⁹ the high serum leptin during preovulatory and the luteal phases might be related to inflammatory responses associated with ovulation.

Leptin produces a regulatory role on the menstrual cycle exerted at the hypothalamic–pituitary–gonadal axis both directly and indirectly.⁵⁰ Since the GnRH neurons have no leptin receptors, the action of leptin on the hypothalamic–pituitary–gonadal axis was found to be mediated through intermediate neurons.⁴⁵ Intermediate neurons connected to the GnRH neurons on the hypothalamus and control the GnRH secretion through release of neuropeptides, such as proopiomelanocortin (POMC), neuropeptide Y (NPY), and kisspeptin.^5,50,51 In addition, leptin could induce LH secretion directly from anterior pituitary.^52,53 As further proof for the permissive action of leptin on the menstrual cycle, menstruation was restored by administration of recombinant leptin in females with hypothalamic amenorrhea.^11,54

The literature on the relationship between leptin and female sex hormones during the menstrual cycle is inconsistent. Leptin was found to be positively correlated with estrogen and progesterone during the menstrual cycle in many studies.^{17,19,24,33,35,37,39} In contrast, other studies reported no relationship of leptin levels with sex hormones during different menstrual phases.^{18,20,23,34,36,38} However, the absence of correlation between leptin and sex hormones reported by some studies does not mean an absence of leptin’s role during the menstrual cycle. These controversial results might highlight a complex feedback loop controlling the interrelation between leptin and the hormones of hypothalamic–pituitary–gonadal axis, instead of a simple unidirectional relation.

Conclusion

This review clearly reports a significant variation of leptin hormone across the menstrual cycle, with a significant interplay with other hypothalamic–pituitary–gonadal hormones. Further studies on the molecular mechanism of leptin action exerted upon the hypothalamus, pituitary, and the ovary are recommended using both animal and human models.