How diet affects fertility: a summary of the latest evidence

How Diet Affects Fertility: A Summary of the Latest Evidence

The topic of diet and fertility has attracted a lot of attention in recent years, with 3 systematic reviews published in the last two months.

Infertility affects 48 million couples worldwide and 186 million individuals globally, and it can have a negative impact on their physical, mental, and financial well-being.

While there are many causes and treatments for infertility, one of the modifiable lifestyle factors that may influence fertility is diet. For example, diet can affect hormone levels, ovulation, menstrual cycles, inflammation, oxidative stress, and embryo and sperm quality.

Yet despite growing evidence that diet is associated with reproductive outcomes in women and men, there is no official guidance regarding which dietary approach to follow for optimal fertility.

Preconception diet and fertility in women

In this week’s post, we first summarise the main findings from an extensive scoping review, published last week that included 36 observational studies on preconception nutrition in women and fertility.

The review examined the effects of different foods, nutrients, and diets on a variety of fertility outcomes in women who were trying to conceive naturally or with assisted reproductive technologies (ART), such as IVF.

Findings revealed that:

• Eating less energy-dense foods, reducing carbs, choosing plant-based proteins over animal ones, cutting down on trans fats, and limiting junk food may improve your fertility. These results must be interpreted with caution as many of these factors may play a role in fertility through interactions of modifiable dietary factors including total energy intake, diet composition, nutrient intakes (such as antioxidants) as well as other characteristics of food.
• Eating more seafood, dairy, and soy may or may not help your fertility. The evidence is not very clear or consistent for these foods.
• The evidence was also less clear for the Alternative Healthy Eating Index (aHEI-2010) and the Fertility Diet. These are two scoring systems that measure how well you follow certain dietary guidelines or recommendations. They did not show strong or consistent associations with fertility outcomes.
• Following a ‘profertility diet’, a Mediterranean-type diet, or a ‘healthy diet’ may boost your chances of getting pregnant. These diets are rich in fruits, vegetables, whole grains, legumes, nuts, seeds, olive oil, fish, and moderate amounts of dairy and meat. They also limit processed foods, red meat, and trans fats.
• But the most convincing evidence was for the Mediterranean diet (MD), which showed consistent benefits for pregnancy rates and other fertility outcomes in both natural and ART pregnancies. This diet may work by reducing inflammation, which can harm reproductive health.

The review also identified some limitations and gaps in the existing evidence. For example:

• The studies varied widely in terms of how they measured and compared different dietary components and patterns, how they controlled for other factors that may affect fertility (such as obesity), and how they reported their results. This made it difficult to draw definitive conclusions or causal relationships from the observational data.
• Few studies stratified the results by different causes of infertility, such as PCOS or endometriosis. This limits the generalisability and applicability of the findings to different groups of women who may have different nutritional needs and preferences.
• The studies were mostly conducted in developed countries (The West, Europe, and Scandinavia), which may not reflect the dietary patterns and practices of other regions or countries that have different environmental, cultural, or religious factors that affect food production and consumption.

A Closer Look at the Mediterranean Diet – in both men and women

A second systematic review which included a meta-analysis was published in Nutrition Reviews last month. This review looked specifically at the Mediterranean diet (MD) on fertility markers for both women and men with infertility.

High adherence to the MD compared to low adherence was associated with –

• Live births: Odds ratio (OR), 0.656; 95% CI, 0.231–1.86, I² 83.2% (2 studies)
  
• Pregnancy rate: OR, 1.192; 95% CI, 0.329–4.325, I² 93.8% (3 studies)
  
• Sperm concentration >15 x 106/mL: OR 2.862; 95% CI, 1.583–5.174, I² 32.97% (3 studies)
• Sperm count > 39 x 106/mL: OR, 2.944; 95% CI, 1.276–6.789, I² 65.8% (2 studies)

Note the I² measures the heterogeneity of studies, and refers to the variability or inconsistency of the results across studies. I² ranges from 0% to 100% with higher values indicating more heterogeneity. There is no definitive threshold for deciding what level of I² indicates significant heterogeneity but most would agree that a value above 50% represents substantial heterogenicity and a value > 75% indicates high heterogenicity. The I² , however, should be interpreted with caution when the number of studies is small or the effect size is large.

The authors concluded that the evidence of the role of MD on female and male fertility parameters is of low or very low quality, and their meta-analysis could NOT confirm that high adherence to the MD increases the chances of live births.

The glimmer of hope here were the favourable results in sperm quality (with low heterogeneity), but the authors note more studies are needed.

The famous phrase ‘more studies needed’ really points to the biggest problem of all – the lack of randomised controlled trials (RCTs). Most of the studies included in these reviews are observational, which have inherent limitations and cannot establish causality.

More high-quality studies are needed, especially randomised controlled trials, and better methods to assess adherence to dietary interventions.

But all easier said than done given the challenges of conducting RCTs in this area. Some of them are:

• Ethical issues: It may not be ethical to randomly assign people to different diets that may have potential benefits or harms for their fertility, especially if they are trying to conceive or undergoing assisted reproductive technologies.
• Practical issues: It may be difficult to recruit and retain participants who are willing to follow a specific diet for a long period of time, and to monitor their adherence and compliance. Although somewhat reduced by randomisation, it may also be challenging to control for confounding factors, such as lifestyle, environmental exposures, genetic variations, and other dietary components
• Methodological issues: It may be hard to define and measure the exposure (diet) and the outcome (fertility) in a standardised and valid way, and to account for the heterogeneity and complexity of both. It may also be difficult to determine the optimal dose, duration, timing, and frequency of the dietary intervention, and to balance the trade-offs between internal and external validity.
• Statistical issues: It may require a large sample size and long follow-up time to detect meaningful effects of diet on fertility, especially if the outcome such as pregnancy is influenced by many factors. It may also be challenging to analyse and interpret the results, especially if there are multiple comparisons, interactions, or subgroup effects.

These challenges do not mean that RCTs are impossible, but they do require careful planning, design, conduct, analysis, and reporting of the studies. Nor do they mean that these observational studies are worthless. In the field of evidence-based nutrition, a level of uncertainty is inevitable and RCTs should be complemented by other types of evidence, such as observational studies, mechanistic studies, animal studies, and systematic reviews.

As you can see, there is a lot of complexity and uncertainty involved in studying diet and fertility. The scoping review we discussed in this blog post did a great job of summarising and synthesising the current evidence on this topic, but it also highlighted some gaps and limitations that need to be addressed in future research.

We hope that this blog post helped you understand some of the main findings and implications of the review, as well as some of the challenges and opportunities for further investigation.

Be sure to check out our free Webinar on Diet and Fertility  with transcript and access to a download resource of the Mediterranean diet pyramid.