Meat and Fertility

Literature review
American College of Healthcare Sciences
NUT 501. Spring 2019
Craig Turczynski, Ph.D.

Abstract

Infertility is defined as the inability to conceive after 1 year of un-protected intercourse. In the U.S. it affects over 6 million couples and the causes are widespread. There are numerous mechanisms where nutritional deficiencies can alter the reproductive system. As with other organ systems in the body, when function is compromised, steps should be taken to ensure that nutritional deficiencies are not the cause. Research on how micro and macro nutrients affect reproduction is an area of recent interest. Since protein is one of the major requirements for adequate nutrition, data have been presented on consumption of meat and how that affects fertility. There were 12 papers reviewed from the fertility literature related to meat protein with one group of researchers out of Harvard contributing greater than half of the published literature on this subject. The evidence presented strongly implicate meat consumption as a causative factor in infertility.  On closer evaluation, there is the potential for bias based the existence of variables that have not been fully understood nor controlled for. The principle variable of interest for this paper is the animal feeding method and evidence for how this can alter the nutritional content of meat is presented. There were 4 pivotal references demonstrating how this variable is a significant factor in the nutritional content. In addition to eliminating potential chemical contamination, ideal production methods can alter the fatty acid and antioxidant content. The evidence for how content differences such as fatty-acids, anti-oxidants, vitamins and chemical contaminants can affect reproductive function offer a partial explanation for the negative effect of eating meat on fertility.

Introduction

Infertility affects 15-25% of couples in the United States (Thoma, et al., 2013). According to 2012 census data there were 75 million women between the ages of 15-50 and 59% of them were mothers (Monte and Ellis, 2014). Using this number of 44 million mothers, you can extrapolate that 6-11 million couples in the US are unable to conceive after a year of unprotected intercourse. This is an enormous number and the reasons for this are widespread, but nutritional factors must be considered an important factor. In fact, nutrition has only recently been evaluated more closely as evidenced by the number of papers discovered in this review that have been published in the last 10 years. Also, nutrition was rarely, if ever considered as part of the infertility work-up in many assisted reproductive technology programs as this technology was developing (Turczynski personal experience 1993-1999).

Nutrition in reproduction is a very broad topic. The study of nutritional physiology clearly indicates that there are numerous areas where deficiencies can affect the reproductive process. Put simply, adequate nutrition is a prerequisite to fertility so when difficulties with conceiving arise, it makes sense to evaluate the couple to make sure deficiencies in macro and micro nutrients are not a factor. After that, it becomes a practical consideration of which macro and micro nutrients are best, for example, the type and quantity of meat protein consumed. Meat consumption, as a form of protein, has been an area of recent research leading to some practical recommendations for women trying to conceive.

As with all areas of medicine, bias exists based on the authors experience and the political or economic environment. Peer-review is meant to reduce the prevalence of this however, politics can play a big part in the selection of review panels for major journals. It is always wise to consider the possibility of bias and to look at results with a skeptic’s eye. It is interesting how often we hear about new research findings conducted years later that contradict the prevailing standard of care, yet we take the new research as the definitive word on the subject. This holds true for areas of medicine as well as nutritional and social issues. Examples of issues where bias have or could result in misguided policies affecting millions of people include the hydrogenation of vegetable fat, overpopulation and potentially now, global climate change. This review uses this skeptical approach as the basis for analysis. The hypothesis is: The effect of meat consumption on fertility has not been adequately evaluated and allows only general recommendations when the source of the meat has not been controlled for.

In order to test this hypothesis, the author needed to go outside of the infertility literature and look at nutritional studies focusing on the production of meat. Srdnicka-Tober, et. al., (2016) did a meta-analysis of the compositional differences between organic and conventional meat. This paper, with contributions by 25 co-authors, provides a pivotal piece of information necessary for evaluating how a meat diet can affect fertility, or health in general. Seven hundred and seven papers published between 1992-2014, in all languages were considered but the final meta-analysis was narrowed down to 67 publications. The evidence base was only strong enough to compare specific fatty acid profiles, but they were also able to calculate thrombogenicity and atherogenicity indices. Daley et al., (2010) reviewed literature and reported on the fatty acid profiles and antioxidant content of grass-fed vs grain-fed beef. Their analysis evaluated the type and quantity of saturated fat, the omega 6: omega 3 ratio and the quantity of anti-oxidants, demonstrating significant differences between these two production practices used for beef. Finally, two studies on pork production methods indicate that the diet of the pig can have an even greater impact on the nutritional content of pork because the pig is a monogastric animal as compared to the multi-compartmental ruminants (Gjerlaug-Enger et al., 2014; Dugan et al., 2015).

The purpose of this paper was to review the literature on the effect of meat protein consumption on fertility and to scrutinize the data for bias, pointing out which variables have not been controlled for. Literature was located by doing a search on google and PubMed using phrases like “fertility and protein” and “infertility and nutrition”. Key papers were read, and the bibliographies were reviewed drilling down on references specifically addressing meat consumption and infertility. Meat production methods were also searched on google and PubMed using terms like grass-fed, pastured, organic, feeding and meat, pork, beef, etc.

Results

The author will summarize some of the key findings in the papers and then present on overall combined result at the end of this section. Meta-analysis articles were incorporated into combined result because these authors further analyzed the combined data. Review articles were more closely evaluated for the specific papers related to meat consumption and the results from those individual papers were used in combined result, instead of the review article.

In Charvarro, et al., (2008), there was a total of 26,971 pregnancy or pregnancy attempts with a 12.7% incidence of infertility and 20.2% of those infertility events defined as ovulatory. This resulted in a 1.6% overall incidence of ovulatory infertility in the study group with most of the cases being a diagnosis of PCOS. Meat intake was positively associated with ovulatory dysfunction and the consequences of adding one additional serving of meat per day resulted in a 32% higher risk of ovulatory infertility. Furthermore, consuming 5% of energy as vegetable protein instead of animal protein resulted in a 50% lower risk of ovulatory infertility.

Overall, this was a very strong correlation, statistically significant, from a well published group out of Harvard. While some weakness can be attributed to the collection of data such as pregnancy, pregnancy attempt and diet being self-reported, a correlation between animal protein intake and ovulatory dysfunction is strong.

In Salas-Huetos, et al., (2017), a systematic review revealed that the components of what is considered a healthy diet for men such as whole foods, seafood, fruit, vegetables, chicken, whole grain and low-fat dairy was associated with better fertility parameters. This was related to higher levels of omega-3 fatty acids, antioxidants such as folate, Vitamin D, E, C, Beta-carotene and lycopene. Whereas, higher consumption of processed food, processed meat, full-fat dairy and cheese, alcohol, caffeine and sugar are associated with a negative influence on fertility parameters. From this paper, I was able to extract 4 studies that had specific results related to meat. Mendiola et al., (2009), Eslamian et al., (2012), Afeiche et al., (2014a) and Afeiche et al., (2014b) concluded that intake of processed meat products was associated with poorer sperm quality parameters.

In Fontana, R. and Della Torre, S. (2016), the authors have done a very thorough and thought-provoking analysis looking at the mechanisms by which macro-nutrients influence cellular and endocrine function. Their paper does not have specific information related directly to meat consumption and is therefore not used for the final combined analysis. They do however, present significant indirect evidence useful in confirming the hypothesis.  Specifically, there is plenty of evidence demonstrating that the fatty acid profile a person consumes has numerous potential mechanisms for modulating reproductive function. Overall, a higher intake of poly-unsaturated fatty acids with a greater proportion of these being omega 3 was beneficial to metabolic and hormonal characteristics. The lower ratio of omega 6 fatty acids such as arachidonic acid can influence prostaglandin synthesis and subsequently affect ovarian steroid production, pain and inflammation. The authors conclude with Hippocrates prophetic quote on how the precise amount of nourishment and exercise is the safest way to health.

In Nassan et al., (2018), the total meat/fish intake per day ranged from 0-4.12 and averaged 1.2 servings per day. Egg consumption range and average was 0-4, .44 servings/day and vegetable protein range and average were 0.04-7.56, 1.29 servings/day. Processed meat intake was associated with lower fertilization rates (negative observation) and fish intake was associated with lower peak estrogen concentrations during hyperstimulation (possibly a positive observation). Total meat/fish intake, egg consumption or vegetable protein consumption was not associated with assisted reproductive technology success rates, but fish intake was significantly associated with a higher birth rate. This same effect was not duplicated with fish oil supplementation.

Using statistical analysis, the authors estimated the effect of substituting one type of protein for all other types of protein, both animal and plant derived. Substituting fish for all other proteins consistently increased the chance of live birth and this substitution effect was strongest when fish substituted for processed meat.

Interestingly, unprocessed meat was positively correlated with live birth rates when data was evaluated in categories but not when evaluated as a continuous variable. Another example of additional variables possibly influencing the results was that women with higher meat consumption also had higher BMI, total fat, carbohydrate and protein intake, higher total energy intake, higher alcohol consumption and greater adherence to a “western” dietary pattern. Making it difficult to accept the conclusion that the effect was due to eating meat alone.

Gaskins and Chavarro’s (2018) review paper was useful for pulling out 2 additional studies not previously reported here on the meat consumption and fertility. Jeong et al., (2010) assessed the risk of contaminants such as hormones on human health and concluded that levels are negligible and not likely to pose a risk when utilized according to approved veterinary practices. Antibiotics and anti-microbials on the other hand could affect the gut microflora of the person consuming the meat and need to be further investigated. This is an important observation since we know gut microflora can have profound affects on human health, however, it only reinforces the need to use healthier meat production methods to eliminate this possible health effect. Briga, et. al., (2015) reported on the development of 2659 human embryos cultured in-vitro from 269 patients and observed a negative influence of the consumption of red meat on the ability of the embryo to form a blastocyst. Consumption of red meat and BMI also had a negative effect on implantation and pregnancy.

Scrutiny of all the studies leaves 3 original papers evaluating meat consumption on female fertility factors and 4 original papers evaluating male fertility. The male fertility papers however, specifically looked at processed meat only and some studies had other dietary differences between the study subjects. These studies are summarized below in Figure 1.

The evaluation of meat production methods on the nutritional content of the meat revealed how significantly the nutritional composition of the meat can be altered based on how it is produced. In the Srdnicka-Tober, et. al., (2016) meta-analysis they were able to detect significant differences between organic and conventionally raised meat for fatty acid concentrations when all meat types were analyzed together. Organic meat had similar overall saturated fat, lower mono-unsaturated fat and higher poly-unsaturated fat. Organic meat also showed lower levels of specific saturated fatty acids myristic and palmitic acid. Individually, organic chicken meat was lower in saturated and mono-unsaturated fat and higher in poly-unsaturated fat. Organic chicken meat was also specifically higher in omega 3 poly-unsaturated fat. Pork meat raised organically was lower in mono-unsaturated and higher in poly-unsaturated fat. When they switched to an unweighted analysis, additional differences were detected. Organic meat contained significantly higher concentrations of ALA, DPA and omega 3 PUFA, and had a lower omega 6: omega 3 ratio. The calculated thrombogenicity index for organic meat was also lower. By calculating the total fatty acid intake that people in the EU consumed on average, they were able to demonstrate a much healthier overall profile of fats when consuming organic vs conventional meat.

The most consistent finding of the review by Daley et al., (2010) was that grass-fed beef was lower in total fat content. In addition, grass-based diets changed the saturated fat make-up towards higher levels of cholesterol neutral stearic acid and lower levels of cholesterol raising myristic and palmitic acid. There is also a higher level of conjugated linoleic acid, trans-vaccenic acid, precursors to CLA and omega 3. Finally, grass-fed diets result in higher levels of precursors for Vitamin A & E and antioxidants glutathione and superoxide dismutase. In-fact, grass-fed beef is known for having yellow colored fat as result of higher beta-carotene levels. Gjerlaug-Enger et al., (2014) demonstrated in their original work feeding rapeseed to pigs, as compared to conventional feeding that the rapeseed diet resulted in 41% higher levels of alpha-linolenic acid in the meat. They also observed a 20% higher level of EPA, DPA and DHA. The omega-6: omega 3 ratio improved from 6.6 to 4.7. Likewise, the review by Dugan et al., (2015) reinforced the observation that feeding regimes can create high omega 3 pork.

Discussion

The observational work of Westin A. Price (1939) discovered that primitive tribes understood the need for preparing the young mother and father to-be, nutritionally, before and after conception. The diets of these people frequently consisted of meat and fish, especially during winter months when fruits and vegetables were in short supply. Wild game and fish were prized for their bone marrow and organ meat and the wisdom that was passed down generation to generation was that these foods were important to fertility. It was even known that it was better to postpone getting pregnant until the cows that supplied milk to the people had an opportunity to eat the fresh grass in the spring.

The recommendations from fertility experts, based on the data presented in this review, are that eating meat is related to poor fertility and that meat consumption should be limited. This recommendation should be taken with caution since none of the fertility data has controlled for the production method used. Also, after more closely evaluating the references, we observed that 4 of the 7 original studies related to meat and fertility have come from the same research group out of Harvard. Furthermore, there is a concern about using processed meats in this review since it is not possible to know if the results are due to the processing methods such as curing and smoking or from the meat itself. Taking out the studies reporting on processed meat, would leave just two studies showing a positive relationship between meat consumption and infertility.

The studies that investigated the differences in nutritional content of meat produced by conventional compared to organic, grass-fed or modified diet fed animals clearly show that these are completely different products. Most importantly they showed a difference in fatty-acid, fat-soluble vitamin and anti-oxidant concentrations. Concentrations and ratios of these components were shown to have a differential effect on fertility (Fontana and Della-Torre, 2016). These authors also make the same argument about the reproductive effects of a vegetarian/vegan diet that the author of this review is making about a meat diet; that the reproductive effects have not been fully elucidated. Additional studies on the effect of eating meat need to be conducted after controlling all other variables. The meat products used in the study need to be produced using the cleanest and healthiest methods available and the nutritional content of the product should be analyzed.

Conclusion

Taken together the data show that good fertility, just like good health of other organ systems in the body, really depend on a nutritionally sufficient and chemically clean diet. At this point, the published data do not eliminate the fact that this diet could include meat, as long as the source of meat comes from a healthy production system.

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