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A clinical update on diet and fertility

10 May 2021
by Lisa Simon

Adequate attention to diet is an important part of conception for both partners, says Lisa Simon.

The World Health Organization (WHO) defines infertility as “a disease of the reproductive system defined by the failure to achieve a clinical pregnancy after 12 months or more of regular, unprotected sexual intercourse.” Infertility affects up to one in seven couples in the UK.1 It is imperative that health care professionals (HCPs), including dietitians, understand the importance of optimising fertility and pregnancy outcomes to support preconception health as well as promote future health outcomes for parents and their children.

Our understanding and awareness of the links between diet and fertility can be traced back to Hippocrates2 who advocated special diets for both men and women in order to optimise fertility. This mirrors the current evidence base which recognises that each partner plays an equal role in conception and fertility, with male infertility affecting up to one-third of couples. It is important, therefore, that both partners optimise their nutritional status prior to conception.3

The ever-growing awareness of the importance of nutrition on preconception health is reflected in the growing academic literature, media and social media attention. The Lancet published a series of papers in 20184 suggesting context-specific interventions for preconception health. These detail and recommend the role of HCPs in promoting preconception interventions, including supplementation and food fortification, to improve both parental nutritional status and long-term outcomes for mum and baby.

The aim of this article is to discuss current evidence around specific nutrients and dietary factors to consider within dietetic practice which are important in terms of optimising fertility during the preconception period. Practical take-home messages will assist dietitians working with those planning pregnancies or trying to conceive.

Fats

There is evidence to suggest that women who consume diets higher in monounsaturated fat have higher fertility rates.5 In addition, higher intakes of full-fat dairy, plant proteins, non-haem iron and monounsaturated fats are associated with lower rates of ovulation infertility (OI)6. Whilst some observational research into large population groups7 suggests low-fat dairy may have a negative impact on female fertility, this is not backed up across the academic literature. The current literature is not conclusive in its associations between fat intakes and fertility7 and caution should be applied when suggesting dietary changes. The lack of clarity within the literature demonstrates that large, well designed randomised control trials (RCTs) are needed before firm recommendations can be made.

For men, diets rich in anti-inflammatory omega-3 fatty acids have been shown to improve sperm health, quality and motility, whereas high intakes of saturated and trans fats adversely affect sperm quality.8

Take home message for clinical practice

As dietitians, when discussing fertility and diet, we should be encouraging women to replace saturated fats with monounsaturated, including olive oil, olives, avocados and nuts, while also being aware of portion sizes. For men, we should encourage the replacement of saturated fat with polyunsaturated fats, including vegetable oils, seeds, nuts and oily fish.

Protein

There is a body of evidence suggesting that eating more plant sources of protein and fewer animal sources may help improve ovulatory infertility. An observational study of 18,555 women observed that intakes of plant protein reduced risk of OI by 50%6. There is a growing societal and environmental interest in plant-based eating, and the observed health benefits, implications for fertility outcomes and potential links with reduced childhood obesity are frequently discussed within clinical practice.7 A systematic review9 found that whilst poorly planned vegan and vegetarian diets carry an increased risk of vitamin and mineral deficiencies, there are potential benefits from a well-planned, whole food, plant-based diet, and no known risks to foetal or maternal health with appropriate planning.

Take home message for clinical practice

Dietetic advice should be tailored to include recommendations of more plant-based, minimally processed sources of protein, such as tofu, tempeh, beans, lentils, chickpeas, nuts and seeds.

Vitamin D

Accumulating data suggests that vitamin D may be important for fertility, with receptors located in the ovaries, placenta and endometrium, as well as in testicles and sperm.10 A recent meta analysis concluded that adequate serum vitamin D levels are associated with more positive pregnancy tests, clinical pregnancies and live births in women undergoing artificial reproductive technology (ART). However, there is no association between miscarriage and vitamin D status.10 There is a gap in the current literature for more RCTs specifically designed to assess Vitamin D status and outcomes of ART.

In men, Vitamin D deficiency has been shown in observational studies to be associated with low serum testosterone concentrations and poor semen quality, including reduced sperm number, movement and morphology. However, RCTs have not shown improvements following supplementation.12

Take home message for clinical practice

Dietary advice for optimising vitamin D intakes should be considered across dietetic practice, especially for those who are trying to conceive, pregnant or breastfeeding. This is especially important during the winter months, when the sun’s UVB rays are not strong enough to enable our bodies to make vitamin D. Dietary sources include oily fish, egg yolks and fortified foods. For those following plant-based diets, fortified plant milk and yoghurts can be useful sources of vitamin D, as can mushrooms exposed to sunlight. Current guidelines11 recommend that everyone should consider taking a 10mcg supplement between October and March, and vulnerable groups (including pregnant and breastfeeding mothers) should consider year-round supplementation.

Iron

Iron deficiency is the most common nutritional deficiency worldwide. Women of reproductive age are at increased risk before conception due to menstrual losses, insufficient dietary intakes, or previous multiple pregnancies. It is important that iron status is determined prior to conception for both men and women, as deficiency in both sexes can lead to fertility problems, including defective spermatogenesis, reduced libido, oxidative damage to sperm and OI.13 There may be some merit in considering the type of iron consumed, with observational data from the Nurses’ Health Study II demonstrating that women who consumed higher amounts of non-haem iron are at decreased risk of OI.5

Take home message for clinical practice

Dietetic advice should be tailored to encourage dietary sources of non-haem iron, such as legumes, grains, nuts, tofu and dried fruit, along with guidance on how to optimise absorption. This includes ensuring a source of vitamin C with every meal, soaking grains and legumes when possible before cooking to break down phytates and avoiding drinking tea, coffee and wine with meals. Calcium supplements should be taken in between meal times.

Zinc and folate

The link between folate deficiency in women and the risk of neural tube defects is well documented. The guidelines for women who are trying to conceive are a daily supplement of 400mcg of folic acid,14 although some women who are at an increased risk of having a pregnancy affected by neural tube defects (e.g. diabetes, previous neural tube defects in pregnancy) may benefit from a higher dose of 5mg.14 Folate is also an important nutrient in terms of male fertility as it is needed for the synthesis of DNA in sperm.15

Zinc is important for male and female fertility; in men, it is required for spermatogenesis and motility and in women it plays a role in hormone balance and ovulation.16 Dietary sources include wholegrains, nuts, seeds (especially sesame seeds) and beans, as well as oysters and lean red meat. As men lose zinc in each ejaculate, their requirements are slightly higher than women (9.5mg vs 7mg) and they should be encouraged to consume adequate sources daily.

Take home message for clinical practice

Advice should be given on how to increase dietary folate and zinc intake, including supplementation as appropriate. Healthy eating messages to include plenty of fruit and vegetables, particularly dark green leafy varieties, fortified cereal products, nuts and seeds are particularly important in this patient group. Iodine Iodine is essential in the production of thyroid hormones which are essential for adequate brain and neurological development. Iodine is seen as a key nutrient for the first 1,000 days of life: from conception to just before the child’s second birthday and deficiency or excess may lead to thyroid disorders which then impact further on fertility.

In men, thyroid hormones affect reproductive function by altering serum testosterone levels and the regulation components of semen such as calcium, zinc and magnesium. Low or deficient iodine levels have been associated with lower semen concentration and longer duration of time to pregnancy compared to men with optimum levels. Additionally, excessive intakes have also been linked to decreasing semen quality parameters.17

In women, it is important that iodine stores are sufficient prior to conception due to the role of iodine in egg maturation. In addition, the foetus is reliant on maternal iodine stores until around week 18 of pregnancy. There are currently no specific recommendations for the preconception period, with general iodine requirements for adults being 150mcg daily. During pregnancy and lactation this increases to 200mcg daily.18

Take home message for clinical practice

Women should be encouraged to build up their iodine stores several months prior to conception. Sources include fish – with white fish generally containing more than oily fish – and dairy products. For plant-based eaters, it is important to ensure that plant milks are fortified with iodine and there are a growing number of manufacturers who are now doing this. Seaweed is a concentrated source, especially kelp, however the content is variable and can be excessive, therefore this is not a recommended regular source and should not be consumed more than once a week, particularly during pregnancy.18 For those who do not consume regular or sufficient iodine-rich foods, a supplement should be considered, ensuring that it does not exceed 150mcg. This should be taken in the form of potassium iodide or potassium iodate. Seaweed or kelp supplements should be avoided as the iodine content can vary considerably from that shown on the label. Those who have been consuming low amounts of iodine for a prolonged time and those with existing thyroid conditions should always liaise with their GP before taking supplements.18

Weight

Maternal pre-pregnancy weight is a strong predictor of fertility and pregnancy outcomes. Having a low body mass index (BMI) can result in a disrupted menstrual cycle and a compromised immune system, meaning an increased risk of infection and inflammation. An older systematic review found a higher rate of complications, such as interuterine growth restriction, neural tube defects and small for gestational age infants, as well as increased rates of preterm births and neonatal deaths and chronic disease later in life due to foetal programming.19

Obesity is associated with a three-times higher risk of OI, with a reported U shape association with BMI and risk of anovulatory infertility. This is largely related to alterations in hormone production; with higher levels of circulating oestrogen and testosterone, secretion and bioavailability. There is also a reduced ability of the egg to implant into endometrial tissue and an increased risk of leptin resistance in the child.20 A cohort study of 33,000 women found that women with pre-pregnancy obesity had increased risks of gestational diabetes, hypertension, preterm birth, caesarian section and large for gestational age infants.21

Take home message for clinical practice

Weight loss for those who are living with obesity of 5-10% can increase fertility. However, weight loss to reach a healthy BMI will provide the most benefit. The National Institute of Clinical Excellence (NICE)22 promotes the role of the MDT to support with weight management during the preconception period, particularly those with a BMI above 30kg/m2. However, there are several barriers to implementing clinical guidelines. Many do not seek preconception advice or are unaware of the availability of preconception care. There may also be a lack of knowledge from practitioners, lack of time during consultations or a lack of resources.23 It is also important, as health care professionals, to approach the subject of weight sensitively, without bias or judgement. Identifying barriers is essential if solutions are needed to overcome them.

Advanced glycation end-products (AGEs)

AGEs are compounds that form inside the body when sugars attach to protein molecules. This reaction can either occur quite slowly, the Maillard reaction, or when there is a lot of oxidative stress or reactive oxygen species in the body and a lot of inflammation.

In terms of fertility, there are receptors for AGEs (RAGEs) in the uterus and ovaries, and in women who are overweight, an accumulation of AGEs in the uterine tissue can have a negative impact on fertility. Not only does it take longer to conceive, but the uterus becomes inflamed by the accumulation of AGEs and inhibits the implantation of a fertilised egg into the endometrial tissue.24 Women with PCOS have also been shown to have high circulating levels of AGEs25 and there is evidence that the accumulation of AGEs at the level of the ovarian follicle could trigger early ovarian aging.26 

In men, high consumption of dietary AGEs doesn’t seem to have an effect on sperm motility but they do cause a reduction in the quality of the DNA within the sperm.27

Take home message for clinical practice

Both sexes should be advised on how to reduce their intake of AGEs. This includes the avoidance of grilled, barbequed, toasted and fried foods and using alternative cooking methods where water is added, such as casseroling, slow cooking, steaming, poaching and microwaving. This does not allow the brown layer to form on foods, which are the AGEs forming. Additionally, using acidic marinades such as lemon juice and vinegar will reduce the formation of AGEs when the foods are cooked, and avoiding highly processed foods, sugar sweetened beverages and overall consumption of free sugars will also reduce the AGE content of the diet.28

Oxidative stress and antioxidants

Oxidative stress is generated by free radicals and, when compared to fertile men, up to 25% of infertile men have been found to have significant levels of free radicals in their semen.29 The injury induced may lead to cell death, abnormality and motility loss,30 as well as damage to DNA, abnormal fertilisation, reduced implantation and poor embryonic development.31

Preventing cell damage by free radicals is important, and tobacco, recreational drugs, alcohol, pollutants and infections are particularly damaging. It is thought that approximately 40% of male infertility is caused by environmental and host factors eg weight.32

Research on male fertility often focuses on the neutralising effects of antioxidants. Lycopene, a carotenoid found in red fruit and vegetables, is of particular interest as it preferentially accumulates in the testes, as well as the adrenal glands, liver and prostate, and in these tissues it has a ten-fold greater concentration than in other tissues.31 This is possibly due to the large number of lipoprotein receptors, or higher metabolic/oxidation rates in these tissues.31

Unlike other carotenoids, lycopene cannot be converted into vitamin A in the body efficiently, however it remains a powerful antioxidant. Absorption is affected by several factors, being reduced with advanced age, smoking and alcohol, and increased when cooked and eaten alongside other fats due to its lipophilic nature. Lycopene traps free radicals, alleviates oxidative stress and prevents damage to lipids, protein and DNA. However, it is important to consume a variety of carotenoids, as this gives a more marked effect than any individual compound.31

In addition to foods containing carotenoids, we should encourage the consumption of foods containing other nutrients that exert antioxidant effects, including vitamins C, B6 and E, as well as Zinc and Selenium.

Take home message for clinical practice

Men and women should be encouraged to eat a range of fruit and vegetables, ensuring adequate amounts of yellow, orange and red varieties. Advice should be given on ensuring these vegetables are cooked and eaten with an unsaturated fat source to increase absorption.

Summary

In summary, for those working with families considering pregnancy or undergoing a fertility journey, we should be promoting a varied, balanced diet ensuring that we highlight the importance of addressing the diet and lifestyle of both partners, not just the person who will carry the child. Encouraging partners to participate in dietary counselling offers benefits and improved conception outcomes for couples trying to conceive. Every fertility journey will look different, and positive dietary changes for non-biological parents or other family members offer benefits for family health as well as role modelling opportunities beyond the conception period. Dietary counselling should include encouraging wholegrain carbohydrates and a variety of colourful fruit and vegetables. Within our practice, we should consider promoting a higher ratio of plant protein to animal protein, reduction of dietary AGEs, as well as ensuring that women are aware of the importance of consuming sufficient iodine, zinc, folate, iron, antioxidants as well as supplementing folic acid prior to conception.

References

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