Data from the Office of National Statistics (ONS, 2004) indicates that there is a continuing trend towards childbearing in later years. Unfortunately, a plethora of research has found that the chance of conceiving declines with age (Rowe, 2006) and is associated with a higher risk of obstetric complications (Hansen et al., 1986). Data from the ONS shows that total fertility rate was 2.43 children per women in 1970 and has currently declined to 1.80 children per women. Furthermore, increasing numbers of women are putting off having a family until they are in their thirties. The average UK woman now has her first child at 29.5 years compared to 26.3 years three decades ago (ONS, 2004).

The World Health Organization defines infertility as the failure to conceive a child after 1 year of unprotected intercourse (Johnson and Everitt, 2000). Although female infertility can be attributed to medical and environmental factors, individual studies suggest that external, dietary factors can play a fundamental role (Grodstein et al., 1995; Crha et al., 2003; Westphal et al., 2004). Steures et al. (2006) recently reported that dietary advice given to 126 couples unable to conceive naturally for at least a year was just as effective as using intra-uterine insemination (IUI) with ovarian hyper stimulation. Of the 127 given the IUI medical treatment, 33 per cent conceived and of those who were provided with dietary advice, 32 per cent conceived. This literature review therefore sets out to review the role of key dietary factors and their associations with female fertility.

Amenorrhea, anovulation and luteal phase dysfunction are just some of the medical problems that rise in female alcohol abusers (Mello, 1988). Is has been speculated that alcohol can disrupt the hypothalamic–pituitary–gonadal axis in women, prolonging time to conception (Mello, 1988). Research undertaken by Teoh et al. (1990) found that alcohol increased plasma estradiol and prolactin levels, which may subsequently increase the risk of spontaneous abortion. Grodstein et al. (1995) interviewed 1,050 women from infertility clinics and found that endometriosis was 50 per cent higher in subjects that consumed any level and form of alcohol. A follow-up study designed by Jensen and colleagues (1998) recorded and measured lifestyle factors in 430 Danish couples over six menstrual cycles until conception occurred. After adjustment for confounders, the odds ratio decreased with increasing alcohol intake from 0.61 among women drinking 1-5 alcoholic beverages a week to 0.34 among those drinking more than ten alcoholic beverages a week. Similar findings were reported by Eggert et al. (2004) who undertook a large prospective study of 7,393 randomly selected Swedish women over an 18-year period. Alcohol consumption was extracted from self-estimated postal questionnaires and rates of hospitalization were examined. High levels of alcohol intake were associated with an increased risk of infertility examinations.

Finally, in addition to delaying fertility, if conception does occur and alcohol intake remains to be high there is an increased risk of the child developing a fetal alcohol related disorder (Barr and Streissguth, 2001). As evidence within this field is relatively consistent, Public Health campaigns may need to be devised in the future – to deliver the message that high levels of alcohol consumption can decrease fertility and if conception does occur; can impinge upon the health status of the developing child.

In healthy women the production of reactive oxygen species (ROS) is generally in equilibrium with antioxidant levels. However, when this balance is disrupted and there is an overabundance of ROS, oxidative stress takes place (Agarwal et al., 2005). ROS can influence oocyte maturation, fertilization and embryo development and fuel the age-related decline in fertility (Agarwal et al., 2005).

A few studies have investigated the effect of antioxidant supplementation on female reproduction. Crha and colleagues (2003) measured ascorbic acid levels in the follicular fluid of 76 women with fertility problems, after half were given 500mg supplements. Ascorbic acid levels were found to be statistically significantly higher in the follicles of women that had taken vitamin c supplements when compared to the control group. Vitamin C supplementation was also found to impact upon the number of successful pregnancies (58 vs. 32 per cent; case vs control) in non-smoking women. Oyawoye et al. (2003) also found that total antioxidant capacity was higher in the follicular fluid of subjects whose oocyte had been successfully fertilized. Such findings support the theory that ROS play a key role in the reproductive function of women. Furthermore, in a study comparing selenium status in two groups of women (18 with one successful pregnancy, 26 with a recurrent history of miscarriage) venous blood and hair scalp samples were taken. Evidence of selenium deficiency in women with recurrent miscarriages was evident from the hair samples but not the blood venous samples (Al-Kunani et al., 2001).

Although these investigations seem to report similar findings, that antioxidants reduce the number of ROS and fuel oocyte maturation, further research needs to be undertaken. There is a lack of randomized controlled trials within this field. These studies are regarded as the “gold standard” and would enable scientists to draw firm conclusions about associations between antioxidant levels and infertility, providing the investigators with sufficient statistical power.

Caffeine consumption and its inter-relationship with subfertility have been investigated in some detail. It has been proposed that caffeine affects the eggs ability to implant in the uterus (Pollard et al., 1999; Vogel and Spielmann, 2005) and, subsequently, delays the time to conception. The European multicenter study is one of the largest studies to investigate caffeine intake and infertility to date. In this study consumption of caffeinated beverages was monitored in a randomly selected sample of 3,187 women recruited from Denmark, Germany, Italy, Poland and Spain (aged 25-44 years). Findings were consistent in all countries; women consuming higher levels of caffeine took longer to conceive. Effects were most prominent in women drinking more than 500mg of caffeine per day, and smokers (Bolumar et al., 1997). Grodstein and colleagues (1995) also examined caffeine use in 1,050 women with infertility. Women were categorised according to the principal cause of infertility: ovulatory factor, tubal disease, cervical factor, endometriosis or idiopathic infertility and relative risks with caffeine determined using regression analysis. It was identified that women consuming more than 7g of caffeine per month had a higher risk of tubal infertility and endometriosis. Wilcox et al. (1988) found that caffeine has a dose-response effect. Fecundity was assessed in 104 women attempting to conceive and data divided into five levels. Fecundability of the group consuming some of the highest caffeine intakes fell to 20 per cent of that of the lowest group. Based on the research that is available at present, safe intakes should not exceed 300mg caffeine per day, or 4.6mg per kg⁻¹ bodyweight day⁻¹ (65kg woman, Nawrot et al., 2003). Further research, however, is required to determine safe caffeine intakes for women that are planning a pregnancy.

Studies investigating associations between micronutrient intakes and fertility levels are sparse. One case report documented by Jameson (1976) reported that seven women with intestinal malabsorption were infertile. It was speculated that this may be attributed to low serum zinc concentrations. When zinc levels were restored, one case conceived. It was concluded that zinc therapy may play a role in cases of unexplained fertility. This theory is further supported by Soltan and Jenkins (1983) who identified that zinc concentration was lower in 48 subjects with primary infertility, copper was, however, statistically significantly lower. Hidiroglou (1979) discussed the implications of trace element deficiencies in ruminants. It was concluded that selenium, manganese, iodine and zinc deficiency may all result in subfertility. Further knowledge, however, is undoubtedly required to determine the role which these trace elements play in fertility. As most of the research undertaken in this area was three decades ago, some of which was within animal populations, further investigation is imperative. Studies need to focus specifically on individual micronutrients in relation to time to conception. Interventions studies may be a useful way to obtain new data within this field in the future.

As women are leaving it later to have children, many find that they are subfertile, or infertile. Therefore, important messages need to be conveyed to women of childbearing age – not only that fertility declines with age but also the fact that diet throughout the childbearing phase of the life cycle can impact upon fertility status. Women should be advised to monitor their alcohol and caffeine intakes and ensure that their antioxidants and mineral intakes are optimal.

Research indicates that dietary factors can influence time to conception, and subsequently, fertility rates. Evidence from previous studies appears to be consistent when studying alcohol and caffeine; both dietary factors reducing the likelihood of a successful conception. Investigations studying the effects of antioxidants, micronutrients and associations with female fertility are limited and there is a clear need for the application of randomized control trials within this field, to support what little research has already taken place. As rates of infertility are undoubtedly rising (ONS, 2004) clear messages identifying associations between diet and fertility need to be conveyed to the public sectors and targeted at women of childbearing age to prevent future disappointment and reduction in fertility levels.