There is a gradual decline in estrogen levels and in their cardioprotective effects in women as they approach the menopausal age (1). As females attain menopause, they are prone to develop several menopause-related symptoms or diseases, e.g., hot flashes, cardiovascular disorders, dementia, osteoporosis, sexual, and urogenital dysfunctions (2–4). To control or alleviate menopause-related symptomatology and to improve quality of life, women of postmenopausal age will require hormone replacement therapy (HRT) (2, 5). However, available evidence suggests that the administration of HRT can influence the metabolism of glucose and insulin (6–8).

An increase in insulin resistance and deterioration in glucose tolerance is known to occur with advancing age, although the effect of menopause remains controversial (9). Moreover, a lack of estrogen tends to promote an accumulation of abdominal fat, which is a major cardiovascular risk factor and one of the criteria for the diagnosis of metabolic syndrome (10). Estrogen may have beneficial effects on carbohydrate metabolism while the effects of progestogen may be deleterious; however, results may differ depending on the type of progestogen used (11). Due to conflicting results from previously conducted studies, the actions of 17β-estradiol plus norethisterone acetate on glucose homeostasis in women remain controversial (6–8, 12–22). Thus, we conducted a systematic review and meta-analysis of randomized controlled trials (RCTs) to investigate the effects of 17β-estradiol plus norethisterone acetate treatment on fasting glucose, fasting insulin, glycated haemoglobin (HbA1c) and C-peptide in women.

This meta-analysis was accomplished in agreement with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statements (23).

In this systematic review and meta-analysis, we examined the effect of the co-administration of 17β-estradiol and norethisterone acetate on markers of glucose and insulin metabolism, namely glycated haemoglobin (HbA1c), fasting glucose, insulin and C-peptide concentrations, in women. Based on data derived from 14 RCTs, our results suggest that the use of 17β-estradiol and norethisterone acetate can reduce fasting glucose, HbA1c, insulin and C-peptide levels in females. In the subgroup analyses, the age and BMI of the participants, the intervention length and the presence of type 2 diabetes mellitus (T2DM) seemed to mediate the impact of this drug combination on the examined markers of glucose and insulin metabolism.

Our findings clearly point out that the co-administration of 17β-estradiol and norethisterone acetate reduces HbA1c concentrations in females. The reduction was clinical significant (WMD: -0.65%, P=0.01) and was more notable when this drug combination was administered in elderly women (WMD: -0.62%, P<0.001 for females aged at least 60 years or more), in women diagnosed with obesity (WMD: -1.04%, P=0.03 for BMI ≥30 kg/m²) or T2DM (WMD: -0.84%, P=0.01), and when the length of the intervention was equal to or exceeded 6 months (WMD: -0.52%, P<0.001). This discovery is of particular importance as it has been revealed that HbA1c concentrations increase in women with advancing age. In a cohort study which enrolled nearly 170000 adults, Alghamdi et al. (2021) reported that HbA1c concentrations increase overall by 0.35% for each ten years increase in age, however, after the age of 50, the elevation in HbA1c is of 1.118% (32). Our results were also confirmed by other cohort studies. For example, Ferrara et al. demonstrated that the use of hormone replacement therapy (HRT) is linked with a reduction in HbA1c concentrations in women suffering from T2DM (P<0.001) (33). These conclusions were drawn based on a sub-analysis of the The Northern California Kaiser Permanente Diabetes Registry which recruited over 15000 females diagnosed with T2DM (33). Similarly, Kuh et al. depicted a positive association between HbA1c levels and age and communicated that women who were prescribed hormone replacement therapy (HRT) exhibited lower HbA1c, BMI and low-density lipoprotein cholesterol values (34). Likewise, in previous publications, we have pointed out that the co-administration of 17β-estradiol and norethisterone acetate reduces serum lipids’ concentrations, enabling thus a better metabolic control of T2DM in women (35, 36).

In terms of its impact on glycemia, the combined use of 17β-estradiol and norethisterone acetate was also successful in decreasing fasting glucose levels (WMD: -11.04 mg/dL, P<0.001). Similarly to the previously mentioned results, this type of hormone replacement therapy reduced fasting glucose when the duration of the administration was equal to or exceeded 6 months (WMD: -5.11 mg/dL, P=0.02) and when it was prescribed in females with concurrent T2DM (WMD: -29.20 mg/dL, P=0.01). However, fasting glucose values also declined in women who were at least overweight (WMD: -12.32 mg/dL, P<0.001 for BMI ≥25 kg/m²) or aged <60 years (WMD: -8.43 mg/dL, P<0.001). Taken as a whole, these findings reinforce the statement that the combination of 17β-estradiol and norethisterone acetate can be helpful in the management of T2DM in women as it improves glycemic control and is associated with a significant decrease in HbA1c and fasting glucose concentrations. This is particularly important in the prevention of T2DM-related complications, e.g., cardiovascular disease (CVD), as fasting glucose values have been linked to the 10-year risk of CVD in both pre- and postmenopausal women (37). What is more, the clinical guide for the management of T2DM during the menopause issued by the European Menopause and Andropause Society (EMAS) recommended a tailored-based approach in the prescription of HRT based on CVD risk: oral estrogens should be given to females with low risk of CVD, whereas transdermal estrogens should be administered in those with obesity or other risk factors for CVD. In addition to the use of estrogens, EMAS recommends supplementing the HRT regimen with a progestogen that does not influence carbohydrate metabolism, e.g., norethisterone (38). Moreover, the BMI seems to mediate the relationship between age and fasting glucose values in females, especially in those in whom menopause occurred later in life, e.g., after 54 years of age. Zhao et al. highlighted that the age at menopause was associated with the development of T2DM, overweight/obesity and fasting glucose concentrations (39). This information was confirmed by other investigations (40, 41). Even if these actions on glucose metabolism sound promising, we still need more RCT-based evidence to use HRT in the control or prevention of T2DM (42).

Insulin levels were also affected by the combined use of 17β-estradiol and norethisterone acetate. Our results suggest that, following the use of this drug combination, females exhibit a decline in insulin concentrations (WMD: -1.34 mIU/L, P=0.001). The decrease in insulin values was more notable in elderly women (WMD: -1.62 mIU/L, P=0.001 for age ≥60 years), women suffering from T2DM (WMD: -2.22 mIU/L, P<0.001) or overweight/obesity (WMD: -1.36 mIU/L, P=0.002) or when the length of the intervention exceeded or was equal to 6 months (WMD: -1.21 mIU/L, P=0.005). However, C-peptide concentrations only decreased in females diagnosed with overweight/obesity (WMD: -0.20 nmol/L, P<0.001 for BMI ≥25 kg/m²) or T2DM (WMD: -0.25 nmol/L, P<0.001). C-peptide concentrations are employed in clinical practice to evaluate the function of the beta cells of the pancreas, as this substance is generated in equimolar quantities to insulin as thus can reflect cardiometabolic health (43). These findings are in line with the results of previously published investigations which confirmed that HRT reduces fasting glucose, insulin and C-peptide values (44). In addition, Palla et al. pointed out that anthropometric indices indicative of an increased BMI, e.g., body perimeter and/or waist-to-hip ratio, are correlated with insulin and C-peptide levels, particularly in females with excessive body weight and other cardiometabolic ailments, e.g., metabolic syndrome (45). Thus, these women might be more sensitive to the action of 17β-estradiol and norethisterone acetate due to their excessive concentrations of insulin and C-peptide. It seems that 17β-estradiol can influence the levels of pro- and anti-inflammatory cytokines during the menopause (46). An assessment in a murine model reported that IL-10 concentrations increased, whereas IL-1β and TNF-α decreased, following the administration of this drug in aged rat hearts. Consequently, this compound was able to decrease insulin resistance and enhance insulin signalling which might explain the results of our meta-analysis (46).

Our study had several strengths and limitations. To our knowledge, this is the first meta-analysis to examine the impact of 17β-estradiol and norethisterone acetate on glucose metabolism in females. As the data was derived strictly from RCTs and the reported data is robust. In addition, we examined the inter-study heterogeneity and performed subgroup analyses based on several factors that could have influenced our findings, e.g., length of the intervention, age or BMI of the participants, presence of T2DM.

Several limitations must be also noted. The studies were conducted in multiple countries throughout the globe and thus the subjects had different demographics, clinical findings and most likely genetics. In addition, different brands of HRT drugs might have been administered based on the local availability. The studies conducted by Darko et al. (15) and Kernohan et al. (13) have wide confidence intervals due to their sample sizes, thus, their results do not provide a precise representation of the population mean. In addition, we combined just six studies that have selected C-peptide as an outcome variable and the findings of these investigations display significant differences.

Moreover, the inter-RCT heterogeneity was sometimes high and potential confounders apart from the ones explored in the subgroup analyses might have influenced the final results. The present study is the first systematic review and meta-analysis of randomized controlled trials (RCTs) which investigated the effects of 17β-estradiol plus norethisterone acetate treatment on fasting glucose, fasting insulin, glycated haemoglobin (HbA1c) and C-peptide concentrations in women based on articles published till June 14th, 2022. Future studies need to confirm the potential benefits of this drug combination in the prevention and/or management of cardiometabolic disorders.

Evidence to date highlights that the co-administration of 17β-estradiol and norethisterone acetate in females reduces fasting glucose, HbA1c, insulin and, in some instances, C-peptide concentrations.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

WC and LZ carried out the concept, design and drafting of this study. WC and LZ reviewed literature, searched databases, screened articles, and extracted data. WC and LZ performed the acquisition, analysis, interpretation of data, and revision. All authors reviewed the manuscript for editorial and intellectual contents. All authors approved the final version of the manuscript.