ReviewThe effect of excess body fat on female and male reproduction
Introduction
Obesity represents a familiar concept both in the medical community and the general public. It is typically defined by using the weight and height measurements to calculate the body mass index (BMI), with patients having measurements of ≥30 kg/m2 being classified as “obese.” However, this is not without limitations. Given the possibility for BMI to misrepresent changes in age, muscle mass and race, alternate measures have been proposed. These include waist circumference (WC), skinfold thickness, body impedance, and body composition. With benefits and drawbacks for each of these [1], both the scientific literature and clinical practice continue to rely on BMI for obesity classification.
Reproductive health can be assessed via many potential outcomes. The most profound are those related to fertility and infertility. The 2017 International Glossary on Infertility and Fertility Care defines fertility as the “ability to achieve a clinical pregnancy” [2], while infertility as the “disease characterized by the failure to establish a clinical pregnancy after 12 months of regular, unprotected, sexual intercourse” [2] or “therapeutic donor insemination” [3]. Infertility is considered a disease of the reproductive system and disability by the World Health Organization (WHO) [4]. It can be further defined as primary (inability to have a first live birth) or secondary (failure to have a subsequent live birth after prior births) [5]. These are distinct from measures of fecundability and fecundity, which are the probability of achieving conception and capacity to have a live birth, respectively. In population reports, the total fertility rate (TFR) is often utilized, representing the average number of births per woman [2]. What remains uncaptured by these indices are other facets of reproduction, which may encompass disorders, such as polycystic ovary syndrome (PCOS), abnormal uterine bleeding, endometriosis, erectile dysfunction or reproductive malignancies.
Both metabolic and reproductive health represent complex and diverse topics. The present review aimed to provide the current body of evidence regarding their association.
Section snippets
Global prevalence of excess body fat
It is commonly accepted that overweight and obesity have reached a pandemic level. Notably, even though all countries around the world have been affected, heterogeneity with regards to socio-economic factors, body size preferences, local environment factors and regional disparities suggest that the etiology of this pandemic is substantially complex [6].
A recent analysis of data by the Global Burden of Disease suggests a 50% and an 80% increase over the last 40 years for overweight and obesity,
Reproduction
The realm of reproductive health in mammals expands grosso modo from puberty and its onset, gamete, and sex steroid production to mating, pregnancy and lactation. It is commonly accepted that these events require a significant and also specific amount of energy other than that spent for essential biologic processes (e.g. cardiovascular function, digestion). More specifically, the idea of a minimal fat threshold permissive for pubertal onset has long been known both in mice and in humans [18,19
Male infertility
Data from observational studies in the USA, Norway, and Denmark in 2006–2007 suggested that male BMI is inversely associated with fertility and fecundity [[96], [97], [98]]. One of them, examining the effect of both male and female BMI, suggested a dose-response association and a cumulative result of male and female obesity [98], whereas another suggested a J-shaped association of male infertility in relation to BMI [98].
Many studies assessed the association between obesity and semen
Lifestyle
Obesity has been associated with adverse reproductive outcomes, in pregnancies achieved either spontaneously or after IVF [49]. Obese women have lower birth and higher miscarriage rates after IVF compared with counterparts of normal weight [156]. Additionally, obese women may need higher doses of clomiphene citrate or FSH for ovulation induction or controlled ovarian hyperstimulation (COH) [156]. Finally, obesity is associated with pregnancy complications, such as gestational diabetes mellitus
Conclusions
Although, historically, energy deficit and low body weight have been associated with infertility, the opposite side of the spectrum, obesity and overweight, has comparable adverse effects on reproduction. The main pathophysiologic mechanisms include increased oxidative stress, lipotoxicity and disturbances in the concentrations of adipokines that directly affect the gonads. Both natural conception and ART outcomes are affected by obesity. Clinical evidence is contradicting, with some, but not
Funding
There was no funding support for this work.
Declaration of competing interest
The authors have no conflict of interest to declare.
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