**2. Effects of obesity on female reproduction**

#### **2.1 Hormonal effects of obesity**

Menstrual irregularities occur more frequently in women who are overweight. This is due to a functional alteration to the hypothalamic-pituitary-ovarian (HPO) axis from various factors. Firstly, obesity induces a hyperinsulinemic state, separate to polycystic ovarian syndrome (PCOS). Hyperinsulinemia leads to a suppression in serum hormone binding globulin (SHBG), which results in an increase in bioactive androgens. These androgens are subsequently aromatized within body fat to estrogen, which suppresses gonadotrophins produced by the pituitary [4].

Elevated androgens in PCOS also lead to an increased deposition of visceral fat, exacerbating insulin resistance and hyperinsulinemia, further stimulating androgen production and perpetuating the cycle of pituitary suppression [5].

Additionally, adipokines, cytokines produced from adipose tissue, are known to impact on ovulation. Obese women have higher levels of circulating leptin, a cell-signaling protein from adipose tissue, than normal weight women, and this can therefore mean a chronic downregulation of the receptor in the hypothalamus, [6] resulting in suppression of the HPO axis activity. A study of eumenorrheic obese women demonstrated that the amplitude of luteinizing hormone (LH) pulses was significantly reduced compared with normal weight women, again pointing to a central defect [7].

Obese women remain subfertile even if they are ovulatory. Two studies in large cohorts of Danish women showed a decline in fecundability ratios with increasing BMI [8, 9]. Another large American cohort [10] along with a Dutch cohort of over 3000 women [11] also demonstrated a linear decline in spontaneous conception rates with rising BMI.

Consequently, there are other factors at play that affect fertility in overweight and obese women.

#### **2.2 Effects of obesity on oocyte quality**

Data show that being overweight or obese can have profound impact on oocyte quality. A study of over 45,000 assisted reproduction transfers demonstrated that a higher BMI resulted in a lower likelihood of successful pregnancy when autologous oocytes were used but not when oocytes from lean donors were used [12]. This has been demonstrated in other research as well [13].

Studies also suggest that obese women in in vitro fertilization (IVF) cycles require higher levels of gonadotrophin stimulation and longer treatment to achieve an oocyte retrieval [14]. This is also the case in superovulation for intrauterine insemination cycles [15]. Obese women also have a greater risk of cycle cancelation, lesser oocytes collected, and lesser oocyte maturity than normal weight women [16–18].

Several mechanisms are believed to impact on the oocyte quality in obese women.

Obesity is an inflammatory state where women have higher circulating levels of C-reactive protein (CRP), which is an inflammatory biomarker. Adipose tissue produces many proinflammatory adipokines including leptin, tumor necrosis factor alpha (TNFα), and interleukin (IL) 6. The reproductive tissues, like all tissues, are negatively affected by inflammation. The follicular environment is altered in an obese woman. Follicular fluid contains higher levels of insulin, triglycerides, leptin, and markers of inflammation such as lactate and CRP [6]. Leptin affects steroidogenic pathways in granulosa cells, thus affecting estrogen and progesterone production. This could therefore have impact downstream on endometrial receptivity and implantation [6].

**57**

fashion.

*Reproductive Consequences of Obesity*

granulosa cell apoptosis [21].

and embryo development [21].

trophectoderm lineage [20].

effect, altering their development.

would be too.

a BMI > 25 kg/m2

**2.3 Effects of obesity on embryo quality**

*DOI: http://dx.doi.org/10.5772/intechopen.80897*

In obese mouse models, the ovaries demonstrate more apoptotic follicles and the oocytes themselves are smaller and less likely to be mature. These oocytes reveal high rates of meiotic aneuploidy due to fragmented and disorganized meiotic spindles and chromosomes that are not properly aligned on the metaphase plate [6]. Independent from this, obesity alters mitochondrial function in the oocyte. In an obese mouse model, the mitochondria have a disordered architecture with fewer cristae, more vacuoles, and evidence of swelling [19]. These abnormal mitochondria show evidence of metabolic stress, which leads to a compensatory increase in mitochondrial DNA copy number in obese mice [6, 20]. Obese mice also demonstrate evidence of endoplasmic reticulum (ER) stress where their cumulous-oocyte complexes have increased ER stress markers and increased

A possible cause for this cellular and organelle damage in obesity is lipotoxicity. Lipotoxicity is a condition where fatty acids from the diet that exceed the storage ability of the adipocytes can accumulate in other tissues and cause toxic effects. Obese women have higher circulating free fatty acids (FFAs), which increase reactive oxygen species (ROS) that induce mitochondrial and ER stress and leads to apoptosis. Studies have shown that the oocytes of mice have significant increased production of ROS along with depleted glutathione levels, which is an important intracellular antioxidant defense against ROS [22]. Oocytes exposed to maternal obesity or to high levels of FFA in vitro have demonstrated perturbed mitochondria with reduced mitochondrial function, which then fail to support normal cleavage

The preimplantation embryo is also affected by an obese environment. Given that the early embryonic development is largely driven by the oocyte, it is not unexpected that if the oocyte is negatively affected, then the embryo development

In a mouse model, embryos of obese females have demonstrated slower preimplantation development and disordered differentiation to inner cell mass and

reached the morula stage more rapidly. Those that reached the blastocyst stage had fewer cells in the trophectoderm and demonstrated poor glucose uptake and increased levels of triglycerides along with altered amino acid metabolism com-

Much like oocytes, embryos may also be susceptible to lipotoxicity. Murine embryos that are cultured in palmitic acid, the most common FFA present in human serum, have fewer nuclei and altered IGF-1 receptor expression [26]. This negatively affects insulin sensitivity and glucose transport at a critical stage in development. This study also demonstrated that the trophoblastic cells that are exposed to the palmitic acid proliferate less and undergo apoptosis in a dose-dependent

Elevated leptin levels also have a direct negative effect on the developing embryo. In vitro studies have demonstrated that leptin has a stimulating effect on human trophoblastic cell growth and inhibition of leptin decreases that proliferation and induces apoptosis [27]. Much like its effect in the brain, tonically elevated leptin levels in an obese state may decrease the sensitivity of trophoblastic cells to its

pared with embryos from normal weight women (BMI < 25 kg/m<sup>2</sup>

In an IVF model with autologous oocytes, obese women are more likely to create poor quality embryos [23, 24]. One study noted that embryos from women with

were less likely to develop after fertilization and those that did

) [25].

#### *Reproductive Consequences of Obesity DOI: http://dx.doi.org/10.5772/intechopen.80897*

*Obesity*

**2. Effects of obesity on female reproduction**

Menstrual irregularities occur more frequently in women who are overweight. This is due to a functional alteration to the hypothalamic-pituitary-ovarian (HPO) axis from various factors. Firstly, obesity induces a hyperinsulinemic state, separate to polycystic ovarian syndrome (PCOS). Hyperinsulinemia leads to a suppression in serum hormone binding globulin (SHBG), which results in an increase in bioactive androgens. These androgens are subsequently aromatized within body fat to estrogen, which suppresses gonadotrophins produced by the pituitary [4].

Elevated androgens in PCOS also lead to an increased deposition of visceral fat, exacerbating insulin resistance and hyperinsulinemia, further stimulating androgen

Additionally, adipokines, cytokines produced from adipose tissue, are known to impact on ovulation. Obese women have higher levels of circulating leptin, a cell-signaling protein from adipose tissue, than normal weight women, and this can therefore mean a chronic downregulation of the receptor in the hypothalamus, [6] resulting in suppression of the HPO axis activity. A study of eumenorrheic obese women demonstrated that the amplitude of luteinizing hormone (LH) pulses was significantly reduced compared with normal weight women, again pointing to a

Obese women remain subfertile even if they are ovulatory. Two studies in large cohorts of Danish women showed a decline in fecundability ratios with increasing BMI [8, 9]. Another large American cohort [10] along with a Dutch cohort of over 3000 women [11] also demonstrated a linear decline in spontaneous conception

Consequently, there are other factors at play that affect fertility in overweight

Data show that being overweight or obese can have profound impact on oocyte quality. A study of over 45,000 assisted reproduction transfers demonstrated that a higher BMI resulted in a lower likelihood of successful pregnancy when autologous oocytes were used but not when oocytes from lean donors were used [12]. This has

Studies also suggest that obese women in in vitro fertilization (IVF) cycles require higher levels of gonadotrophin stimulation and longer treatment to achieve an oocyte retrieval [14]. This is also the case in superovulation for intrauterine insemination cycles [15]. Obese women also have a greater risk of cycle cancelation, lesser oocytes

collected, and lesser oocyte maturity than normal weight women [16–18].

Several mechanisms are believed to impact on the oocyte quality in obese

Obesity is an inflammatory state where women have higher circulating levels of C-reactive protein (CRP), which is an inflammatory biomarker. Adipose tissue produces many proinflammatory adipokines including leptin, tumor necrosis factor alpha (TNFα), and interleukin (IL) 6. The reproductive tissues, like all tissues, are negatively affected by inflammation. The follicular environment is altered in an obese woman. Follicular fluid contains higher levels of insulin, triglycerides, leptin, and markers of inflammation such as lactate and CRP [6]. Leptin affects steroidogenic pathways in granulosa cells, thus affecting estrogen and progesterone production. This could therefore have impact downstream on endometrial receptivity and implantation [6].

production and perpetuating the cycle of pituitary suppression [5].

**2.1 Hormonal effects of obesity**

central defect [7].

rates with rising BMI.

**2.2 Effects of obesity on oocyte quality**

been demonstrated in other research as well [13].

and obese women.

**56**

women.

In obese mouse models, the ovaries demonstrate more apoptotic follicles and the oocytes themselves are smaller and less likely to be mature. These oocytes reveal high rates of meiotic aneuploidy due to fragmented and disorganized meiotic spindles and chromosomes that are not properly aligned on the metaphase plate [6].

Independent from this, obesity alters mitochondrial function in the oocyte. In an obese mouse model, the mitochondria have a disordered architecture with fewer cristae, more vacuoles, and evidence of swelling [19]. These abnormal mitochondria show evidence of metabolic stress, which leads to a compensatory increase in mitochondrial DNA copy number in obese mice [6, 20]. Obese mice also demonstrate evidence of endoplasmic reticulum (ER) stress where their cumulous-oocyte complexes have increased ER stress markers and increased granulosa cell apoptosis [21].

A possible cause for this cellular and organelle damage in obesity is lipotoxicity. Lipotoxicity is a condition where fatty acids from the diet that exceed the storage ability of the adipocytes can accumulate in other tissues and cause toxic effects.

Obese women have higher circulating free fatty acids (FFAs), which increase reactive oxygen species (ROS) that induce mitochondrial and ER stress and leads to apoptosis. Studies have shown that the oocytes of mice have significant increased production of ROS along with depleted glutathione levels, which is an important intracellular antioxidant defense against ROS [22]. Oocytes exposed to maternal obesity or to high levels of FFA in vitro have demonstrated perturbed mitochondria with reduced mitochondrial function, which then fail to support normal cleavage and embryo development [21].

#### **2.3 Effects of obesity on embryo quality**

The preimplantation embryo is also affected by an obese environment. Given that the early embryonic development is largely driven by the oocyte, it is not unexpected that if the oocyte is negatively affected, then the embryo development would be too.

In a mouse model, embryos of obese females have demonstrated slower preimplantation development and disordered differentiation to inner cell mass and trophectoderm lineage [20].

In an IVF model with autologous oocytes, obese women are more likely to create poor quality embryos [23, 24]. One study noted that embryos from women with a BMI > 25 kg/m2 were less likely to develop after fertilization and those that did reached the morula stage more rapidly. Those that reached the blastocyst stage had fewer cells in the trophectoderm and demonstrated poor glucose uptake and increased levels of triglycerides along with altered amino acid metabolism compared with embryos from normal weight women (BMI < 25 kg/m<sup>2</sup> ) [25].

Much like oocytes, embryos may also be susceptible to lipotoxicity. Murine embryos that are cultured in palmitic acid, the most common FFA present in human serum, have fewer nuclei and altered IGF-1 receptor expression [26]. This negatively affects insulin sensitivity and glucose transport at a critical stage in development. This study also demonstrated that the trophoblastic cells that are exposed to the palmitic acid proliferate less and undergo apoptosis in a dose-dependent fashion.

Elevated leptin levels also have a direct negative effect on the developing embryo. In vitro studies have demonstrated that leptin has a stimulating effect on human trophoblastic cell growth and inhibition of leptin decreases that proliferation and induces apoptosis [27]. Much like its effect in the brain, tonically elevated leptin levels in an obese state may decrease the sensitivity of trophoblastic cells to its effect, altering their development.

However, there are studies in human models that have not demonstrated a negative effect of obesity on embryo quality, showing no significant difference in the quality of transferred embryos between the different BMI groups [28–31]. Although it is worth noting that despite the quality of transferred embryos being similar, other studies have suggested a reduction in the overall quality of all embryos created in an IVF cycle [14, 24], with fewer surplus embryos cryopreserved in an obese population compared to women with a normal BMI [14]. A retrospective analysis of IVF/ICSI cycles observed that in young women, obesity led to a significant reduction in average embryo quality, cryopreservation, and also embryo utilization [24]. A large retrospective analysis of over 6500 IVF cycles demonstrated no difference in embryo quality but did comment that there were poorer outcomes in the obese women [31]. Certainly, large prospective trials are required to further elucidate the effect of obesity on the embryo.

#### **2.4 Effects of obesity on endometrial receptivity and implantation**

There are conflicting data as to whether or not obesity affects endometrial receptivity and implantation of embryos, and there are several suggested mechanisms.

Leukemia inhibitory factor (LIF) has been implicated in the regulation of implantation, and a significant negative correlation between endometrial glandular LIF and BMI has been observed [32]. It has also been suggested that a state of relative hyperestrogenemia that is seen in obese women (due to aromatization of androgens to estrogen in adipose tissue) may also have a detrimental effect on receptivity [32].

Obesity is associated with insulin resistance and hyperinsulinemia. Elevated insulin levels have been associated with a reduction on glycodelin and insulin-like growth factor binding protein 1 (IGFBP1). Low levels of glycodelin have been associated with recurrent pregnancy loss, and IGFBP1 is an integral molecule involved in adhesion during implantation [32]. Derangement in these molecules may contribute to reduced receptivity in obese women.

As noted previously, obesity is an inflammatory state and obese women have been observed to have elevations in proinflammatory cytokines (IL6, TNFα), and these inflammatory markers are thought to exert negative effects on implantation [14].

Obese women also have a different pattern of endometrial gene expression during implantation than lean women [33], which is more pronounced when examined in the context of infertility. It is postulated that this is due to all or some of the abovementioned factors and the change in the intrauterine milieu of the obese women.

Although there are several plausible mechanisms as to how obesity impacts negatively on endometrial receptivity and implantation, the data for impact on infertility are inconsistent and contradictory. The best model for discriminating between the obesity effects on oocyte/embryo and endometrium is the oocyte donation model [34]. A retrospective review of over 2500 oocyte donation cycles demonstrated a negative trend in pregnancy rates with a rising BMI and a statistically significantly lower pregnancy rate in overweight and obese women compared to normal weight women [35]. However, the implantation rates were considered similar, suggesting the difference between groups was due to an increased pregnancy loss rate in the obese women. Another study also demonstrated lower live birth rates among obese surrogates compared to normal weight women [36]. Other smaller studies have suggested no difference in outcomes in obese oocyte recipients [37].

A case-controlled trial looking at IVF with autologous oocytes observed that women with a BMI > 25 kg/m2 had reduced implantation and pregnancy rates along

**59**

*Reproductive Consequences of Obesity*

*DOI: http://dx.doi.org/10.5772/intechopen.80897*

endometrial receptivity and implantation.

**2.5 Effects of obesity on miscarriage**

than in normal weight women [42].

be significantly lower in the obese population.

cies showed that women with a BMI > 25 kg/m2

**3. Effects of obesity on male reproduction**

**3.1 Hormonal effects of male obesity**

which ultimately affects spermatogenesis.

impact of obesity on embryo quality and endometrial receptivity.

with increased miscarriage rates [38]. Once again, large well-designed prospective studies using this model are required to further examine the effect of obesity on

The role of obesity and miscarriage is also debated. Given the recognized impact of obesity on both the embryo and the endometrium, it is a reasonable assumption that miscarriage rates would be higher in an overweight and obese population. Several studies have demonstrated ever-increasing odds of miscarriage with increasing BMI, in ovulation induction for anovulatory infertility, as well as in IVF cycles in both fresh and frozen cycles [28, 17, 39, 40]. A large meta-analysis of over 47,000 cycles confirmed that overweight or obese women have a higher rate of miscarriage compared with normal weight women [41]. This has also been demonstrated in donor oocyte cycles, with higher miscarriage rates in obese recipients

Interestingly, however, a larger follow-up study of over 2600 donor oocyte cycles by the same group [43] did not demonstrate a difference in miscarriage rates. There was a trend toward a negative impact; however, it was only when a composite measure of ongoing pregnancy rate per cycle was calculated that this was shown to

A meta-analysis looking at both spontaneous and assisted reproduction pregnan-

miscarriage <20 weeks gestation. Subgroup analysis confirmed this to be in the donor oocyte cycles but not across all patients in the studies [44]. Another study demonstrated that in a group of women with a history of recurrent pregnancy loss (RPL), obesity is a well-recognized risk factor for miscarriage in a subsequent pregnancy [45]. A striking study looking at the chromosomal make-up of miscarried specimens from patients with RPL demonstrated that obese women had a much higher rate of euploid pregnancy loss compared to normal weight women. This supports the theory of the

Historically, the impact of obesity on reproduction has largely been researched in female populations with very little examination of the impact of male obesity. There is, however, a growing body of research to indicate that obesity in the male is a cause for concern. A systematic review of 30 studies with over 115,000 participants found that obese men were more likely to experience infertility and that clinical pregnancy and live birth rates per assisted reproduction cycle were reduced.

Much like in the female, the hypothalamic-pituitary-gonadal (HPG) axis is dysregulated in the setting of male obesity. There is strong evidence of a negative effect of obesity on total testosterone, SHBG, and free testosterone [46] as well as reduced inhibin B concentrations and diminished luteinizing hormone (LH) pulse amplitude [4]. It is well understood that suppression of SHBG by hyperinsulinemia in obese men increases androgen availability for aromatization to estrogen in adipose tissue, which may then lead to negative feedback and reduction in gonadotrophin secretion [4]. Consequent to this is a decreased Leydig cell testosterone secretion,

had a significantly higher rate of

*Obesity*

effect of obesity on the embryo.

mechanisms.

receptivity [32].

women.

However, there are studies in human models that have not demonstrated a negative effect of obesity on embryo quality, showing no significant difference in the quality of transferred embryos between the different BMI groups [28–31]. Although it is worth noting that despite the quality of transferred embryos being similar, other studies have suggested a reduction in the overall quality of all embryos created in an IVF cycle [14, 24], with fewer surplus embryos cryopreserved in an obese population compared to women with a normal BMI [14]. A retrospective analysis of IVF/ICSI cycles observed that in young women, obesity led to a significant reduction in average embryo quality, cryopreservation, and also embryo utilization [24]. A large retrospective analysis of over 6500 IVF cycles demonstrated no difference in embryo quality but did comment that there were poorer outcomes in the obese women [31]. Certainly, large prospective trials are required to further elucidate the

**2.4 Effects of obesity on endometrial receptivity and implantation**

may contribute to reduced receptivity in obese women.

gested no difference in outcomes in obese oocyte recipients [37].

receptivity and implantation of embryos, and there are several suggested

There are conflicting data as to whether or not obesity affects endometrial

Leukemia inhibitory factor (LIF) has been implicated in the regulation of implantation, and a significant negative correlation between endometrial glandular LIF and BMI has been observed [32]. It has also been suggested that a state of relative hyperestrogenemia that is seen in obese women (due to aromatization of androgens to estrogen in adipose tissue) may also have a detrimental effect on

Obesity is associated with insulin resistance and hyperinsulinemia. Elevated insulin levels have been associated with a reduction on glycodelin and insulin-like growth factor binding protein 1 (IGFBP1). Low levels of glycodelin have been associated with recurrent pregnancy loss, and IGFBP1 is an integral molecule involved in adhesion during implantation [32]. Derangement in these molecules

As noted previously, obesity is an inflammatory state and obese women have been observed to have elevations in proinflammatory cytokines (IL6, TNFα), and these inflammatory markers are thought to exert negative effects on implantation [14]. Obese women also have a different pattern of endometrial gene expression during implantation than lean women [33], which is more pronounced when examined in the context of infertility. It is postulated that this is due to all or some of the abovementioned factors and the change in the intrauterine milieu of the obese

Although there are several plausible mechanisms as to how obesity impacts negatively on endometrial receptivity and implantation, the data for impact on infertility are inconsistent and contradictory. The best model for discriminating between the obesity effects on oocyte/embryo and endometrium is the oocyte donation model [34]. A retrospective review of over 2500 oocyte donation cycles demonstrated a negative trend in pregnancy rates with a rising BMI and a statistically significantly lower pregnancy rate in overweight and obese women compared to normal weight women [35]. However, the implantation rates were considered similar, suggesting the difference between groups was due to an increased pregnancy loss rate in the obese women. Another study also demonstrated lower live birth rates among obese surrogates compared to normal weight women [36]. Other smaller studies have sug-

A case-controlled trial looking at IVF with autologous oocytes observed that

had reduced implantation and pregnancy rates along

**58**

women with a BMI > 25 kg/m2

with increased miscarriage rates [38]. Once again, large well-designed prospective studies using this model are required to further examine the effect of obesity on endometrial receptivity and implantation.

## **2.5 Effects of obesity on miscarriage**

The role of obesity and miscarriage is also debated. Given the recognized impact of obesity on both the embryo and the endometrium, it is a reasonable assumption that miscarriage rates would be higher in an overweight and obese population.

Several studies have demonstrated ever-increasing odds of miscarriage with increasing BMI, in ovulation induction for anovulatory infertility, as well as in IVF cycles in both fresh and frozen cycles [28, 17, 39, 40]. A large meta-analysis of over 47,000 cycles confirmed that overweight or obese women have a higher rate of miscarriage compared with normal weight women [41]. This has also been demonstrated in donor oocyte cycles, with higher miscarriage rates in obese recipients than in normal weight women [42].

Interestingly, however, a larger follow-up study of over 2600 donor oocyte cycles by the same group [43] did not demonstrate a difference in miscarriage rates. There was a trend toward a negative impact; however, it was only when a composite measure of ongoing pregnancy rate per cycle was calculated that this was shown to be significantly lower in the obese population.

A meta-analysis looking at both spontaneous and assisted reproduction pregnancies showed that women with a BMI > 25 kg/m2 had a significantly higher rate of miscarriage <20 weeks gestation. Subgroup analysis confirmed this to be in the donor oocyte cycles but not across all patients in the studies [44]. Another study demonstrated that in a group of women with a history of recurrent pregnancy loss (RPL), obesity is a well-recognized risk factor for miscarriage in a subsequent pregnancy [45].

A striking study looking at the chromosomal make-up of miscarried specimens from patients with RPL demonstrated that obese women had a much higher rate of euploid pregnancy loss compared to normal weight women. This supports the theory of the impact of obesity on embryo quality and endometrial receptivity.
