Body mass index (BMI) is a mathematic calculation that provides a measure for assessing the health risks associated with body weight. It is defined as the body weight divided by the square of the body height, and is universally expressed in units of kg/m2, resulting from weight in kilograms and height in metres (Pic. 1).
Numerous research studies confirm that body size below and particularly above a defined range increases the risk for health problems. Commonly accepted BMI ranges are underweight: under 18.5 kg/m2, normal weight: 18.5 to 25, overweight: 25 to 30, obese: over 30. Each category is associated with specific related conditions and prognosis:
BMI is not accurate for certain people—athletes with high muscle mass, pregnant women, people under age 18, and the very elderly. However, the correlation between BMI and cardiac health is strong.
Obesity, which is roughly defined as a body mass index (BMI) of more than 30 kg/m2, has been reported to be negatively associated with the reproductive capacity of both women and men.
Most obese women are not infertile; however, obesity and its negative impact upon fecundity and fertility are well documented. Obese women are three times more likely to suffer from infertility than women with a normal body mass index (BMI). Furthermore, obesity as such disturbs menstrual cycle and ovulation. A large questionnaire study demonstrated that menstrual cycle irregularity and anovulation were correlated with being overweight or obese. Obese women had a rate of menstrual disturbance 3.1 times that of women with normal weight. In addition, studies have reported that women with obesity have decreased pregnancy and increased miscarriage rate. Also, impaired fertility has been demonstrated even in obese women with normal menstrual cycle. Obesity is also common in women with polycystic ovary syndrome (PCOS), which is associated also with menstrual irregularities, hyperandrogenism (elevated levels of male sex hormones) and infertility.
Concerning men, it has been found that high BMI decreases sperm quality such as sperm count, concentration, and semen volume rather than sperm motility (overall or progressive). Being overweight decreases the quality of total sperm count and semen volume, obesity decreases the quality of total sperm count, sperm concentration, and semen volume, while changes of sperm motility do not show significant statistical difference. As sperm quality and spermatogenesis are vital for male fertility, these results demonstrate the detrimental effect of overweight and obesity on male fertility.
For women, being underweight and having extremely low amounts of body fat are associated with ovarian dysfunction and infertility. Underweight women (BMI < 19 kg/m2) have a four-fold longer time to pregnancy than women with a normal BMI. Specifically, underweight women required an average of 29 months to conceive as compared to 6.8 months in women with a normal weight profile.
Indeed, as for the initiation of menses, a minimum of fat mass is necessary, for maintaining ovulatory function, and therefore, fertility. Conditions of energy deficit, such as eating disorders (ED), malnutrition and strenuous physical activity, are associated with subfecundity and infertility.
Men who are underweight are also at risk of infertility. Men who are underweight tend to have lower sperm concentrations than those who are at a normal BMI. As the majority of the available literature focuses on the impact of obesity, more research is needed into the effects that being underweight may have on male fertility.
High BMI/Overweight and obesity
The effect of obesity on life expectancy and future health is complex. While there is clear association of obesity with an increased lifetime risk of many chronic, so-called civilizational diseases such as hypertension, coronary artery disease, stroke, and diabetes, patients with higher BMI have a better outcomes and survival rates in the case of some serious diseases such as pneumonia or certain tumours. However, it is well-documented that in the general population, obesity is correlated with a shorter life expectancy and worse quality of life in older age.
While most obese people are fertile, being overweight or obese is associated with several conditions and health complications that have a negative effect on fertility.
The prognosis of underweight patients depends on the underlying cause, but low body weight is generally associated with poorer outcomes of any other disease. Underweight individuals are at increased risk of dying, mainly due to external causes of death. This prompts at screening and counselling this risk group for modifiable risk factors for external causes of death, e.g. frailty or alcohol or drug abuse. Furthermore, among underweight individuals, smokers may be regarded as a vulnerable population. An overall improvement in diet, exercise and lifestyle is needed to achieve a healthy body weight and sufficiently decrease the risks of complications.
Being underweight has a detrimental effect on fertility of both males and females, as their reproductive functions decline together with the depletion of metabolic reserves, body fat and energy. Severe body mass loss, and subsequently severely low BMI, scan cause infertility, which, however, can be reversed by gaining weight back.