Obesity may be defined as a body mass index (BMI) of 30 or higher. It is a medical condition in which excess body fat builds up to the extent that the health of the individual becomes negatively affected. It is becoming increasingly common in both men and women in Western developed nations, including Australia and the US, and exerts significant financial pressure on health care systems.

Obesity is usually not caused by a single factor, although the genetic make-up of a person can influence the probability of developing the condition. The most significant contribution to the rise in obesity is thought to be the changes in behaviour and environment seen in modern society as a result of technological advances. In particular, the reduction in physical activity levels has been put forward as a major contributing factor in the development of the disease.

Many health conditions are associated with obesity. Well-known conditions include type 2 diabetes, coronary heart disease and sleep apnoea Infertility.

However, obesity can also affect the fertility of both men and women by contributing to problems with ovulation and general sexual dysfunction. These are discussed later in this article.

Fertility can be measured in various ways. One of them is fecundability, which is the probability of a woman becoming pregnant for a given menstrual cycle. When a couple experiences infertility, it usually takes some time before the cause of the problem can be identified, as there are several possible direct biological causes of infertility. These include:

• Disorders of the female reproductive system, in particular blocked or damaged fallopian tubes
• Disorders of ovulation
• Low sperm count
• Poor quality of sperm

In many cases (about 1/3), the cause of infertility cannot be identified. These are diagnosed as ‘unexplained infertility’.

Environmental and lifestyle factors such as smoking can adversely affect fertility in males and females. Health issues such as weight and fat distribution of an individual can also impact on fertility.

It is now known that obesity is associated with a general decline in overall fertility, with a significant relationship existing between excess body fat and problems with reproduction. It is therefore becoming increasingly that deimportant in Western nations to understand this association because of the steadily rising rates of obesity in these societies.

Little research has been carried out on male fertility compared to the vast amount of research done on female fertility. However, some studies indicate that the degree of male obesity has a direct effect on oestrogen levels, with increased fat deposits corresponding to higher oestrogen and lower testosterone levels. These elevated oestrogen levels in men tend to suppress fertility because they prevent the synthesis of androgens (hormones required for the development and maintenance of normal male reproductive function). In addition, metabolic syndrome, a condition that can arise from obesity, is also associated with symptoms of erectile dysfunction, hence compounding the adverse effects obesity has on fertility.

Fertility issues arising from obesity in women include:

• Irregular menstrual cycles
• Problems with ovulation
• Increased androgen levels
• Polycystic ovarian syndrome (PCOS)
• Decreased success rates with assisted reproductive technologies (ART)
• Increased risk of miscarriage

These associations between obesity and reduced fertility are particularly significant when excess fat is deposited around the abdominal area. Changes in circulating sex hormones are thought to be largely responsible for decreasing fertility. The overall effect of these changes is usually an increased production of androgens, leading to a condition called hyperandrogenism (excessive androgen production). This condition is evident in obese women who experience amenorrhoea.

Obesity is also often associated with increased insulin production and insulin resistance. These two factors are believed to contribute to hyperandrogenism in obesity because insulin is important for the regulation of sex hormone production. This can be seen in obese women with PCOS, where excess weight gain leads to excessive insulin production, promoting increased androgen secretion and abnormal follicles in the ovary. This results in an overall disruption in ovarian and menstrual activity, causing fertility problems.

Obese women who turn to assisted reproductive technologies (ART) also face additional obstacles compared to non-obese patients. Obesity is linked to decreased success of fertility treatments such as in vitro fertilisation (IVF).

The decrease in fertility associated with obesity can come about as a result of interactions between physiological and psychological factors. Psychological factors include a decreased sex drive and increased sexual dysfunction, which can lead to obese individuals not having sexual intercourse as frequently as non-obese people, even if they are living with a sexual partner within a relationship. Obesity can therefore affect a person’s sexuality, which impacts on their overall fertility.

Weight loss can dramatically improve fertility in obese people. It is often the first step in fertility treatment when an obese patient seeks help from assisted reproductive technology (ART). Even a loss of 5-10% can significantly improve ovulation and pregnancy rates.

Weight loss improves fertility in obese people both physiologically and psychologically. Psychological state and mood has been found to increase significantly after weight loss, and a reduction in insulin and androgen levels has been found in women. This may lead to an 80% improvement in menstrual function and pregnancy rates of 29%.