Deep dive: Zinc and fertility

Zinc is a trace metal—it’s found in small, but measurable amounts in both animal and plant cells and tissues. Along with other trace elements, it is an important part of human nutrition and physiology. In this article, Heights team member and dietitian-in-training, Ruby Dalziel, will look at the ways in which zinc is essential for normal fertility and reproductive function.

Editor’s note: This article has been written to provide further information and context for healthcare professionals. It is not intended to be used in commercial communications or directed at existing or potential consumers. By continuing to read, you confirm that you are a healthcare professional.

TL:DR

Zinc, despite being a trace mineral, is an essential component in the diet, as it cannot be stored or endogenously produced by humans, yet it has numerous important functions, and is found in every cell in our bodies. 

For example, some of the highest concentrations of zinc in the male body is found in the prostate gland, which produces seminal fluid to transport sperm towards an egg. It also plays a key role in the production of testosterone and sperm motility. 

In women, zinc is thought to support the production of eggs, the release of sex hormones, and the growth and development of a healthy foetus. With this in mind, we should not overlook zinc’s importance for heterosexual couples looking to conceive. 

What is zinc?

Zinc is the second most abundant trace metal found in the human body behind iron. Despite this, the body cannot produce or store its own, and it is lost every day through physiological processes, such as faecal matter, while also being naturally excreted in sweat, urine and semen. Menstruating women also lose zinc each month.

Zinc found within our diet originates from the soil. The levels of zinc in soil fluctuates between land and different countries. The presence of zinc in the soil affects the plants that we eat (and therefore animals higher up in the food chain such as meat) and subsequently impacts our personal zinc status. Zinc is also found in seafood such as shellfish like oysters and shrimp.

The best foods for zinc

Here is a comprehensive list of foods high in zinc:

• Crab

• Oysters

• Shrimps

• Chickpeas, lentils, and beans

• Pumpkin and sesame seeds

• Almonds and cashews

• Pine nuts

• Dairy 

• Eggs

• Dark chocolate

What is the importance of zinc for the body?

Zinc is a trace element found and used in every cell in the human body. It’s indispensable. It is found predominantly in the tissues and bones, but there is also a high concentration in the prostate, with any surplus being transported in the blood plasma before excretion. Humans are unable to store or produce zinc in response to zinc availability in the diet, so an adequate and regular intake of dietary zinc is necessary to balance the loss through excretion and to maintain normal zinc homeostasis. For this reason, zinc is known as an essential nutrient.

Zinc is understood to be essential for sex hormone regulation and the formation of quality germ cells (sperm and eggs) in humans. It is found in particularly high concentrations within the sperm and semen in post-pubescent men, and it is essential for the protection of the fertilised egg directly after conception. It does this by providing the egg with a hardened membrane.

Zinc’s importance for the male reproductive system is more thoroughly researched than its importance for the female reproductive system. However, zinc does seem to play an important role in conception for both men and women and this will be further considered in this article. We also know that zinc (specifically zinc oxide nanoparticles) holds antibacterial properties and can kill bacteria in the reproductive tracts of both men and women, promoting the healthy functioning of the reproductive organs. Zinc also has other important roles in the human body, contributing to the normal function of processes such as:

• The production of DNA

• Collagen synthesis

• Enzyme reactions

• Positive influence on cognitive functions

• Smell and taste reception

• Growth and development

• Protein synthesis

• Immune function

• Antioxidant properties

• Blood sugar level support

• The production of stomach acid to digest protein

The recommended amount of zinc per day ranges from 4.7mg to 18.6mg, depending on sex, age, and other physiological factors, such as during pregnancy or lactation.

How does zinc impact fertility for men?

Zinc was first discovered in sperm in the 1920s. Further research has demonstrated its importance for men, especially those wishing to conceive. This is due to zinc’s functions in the production of testosterone, sperm motility, and in the makeup of seminal fluid. 

Testosterone

Testosterone is a sex hormone present in both men and women (produced in the testes and ovaries), although its functions differ between the two sexes. In men, testosterone is important for the production of sperm, and also:

• Sex drive

• Muscle and bone growth

• Deepening of the voice and hair growth

• The development of male sex organs

• Mood

A low level of testosterone in men can impact all of the above, and more. 

In men, if testosterone levels fall below the normal range, one is diagnosed with hypogonadism (the underproduction of sex hormones and/or sperm). Along with ageing, the cause of this can be as simple as a nutritional deficiency in zinc, which contributes to maintenance of normal levels of testosterone. Exactly why zinc impacts testosterone levels isn’t fully understood, but it has been suggested that the mineral may affect the cells in the testes that produce testosterone.

Despite zinc supplementation being a worthwhile consideration for men looking to increase their levels of testosterone, there is still more research required to demonstrate that this alone is a suitable intervention for hypogonadism. Testosterone replacement therapy is a recommended and popular option for men lacking normal levels of testosterone, which can be monitored by blood tests.

Sperm and seminal plasma

Zinc is found in high concentrations in the sperm itself; at the end of spermatogenesis (the formation of sperm in the testes), zinc is highly concentrated in the tail of mature spermatozoa and is involved in sperm motility. However, zinc is notably most concentrated within the seminal plasma (the fluid that carries sperm within the ejaculate) that is in part released from the prostate, which is nourished by high levels of zinc. The seminal plasma is nearly 100 times more concentrated with zinc than in the blood serum, suggesting the importance of zinc for not only the production of motile sperm but the successful transport of sperm within the zinc-packed ejaculate to an egg.

Male infertility—positive horizons

A meta-analysis in 2016 concluded that in general, zinc supplementation can significantly increase the percentage of normal sperm morphology, sperm motility, and semen volume but really interestingly, the analysis reported that after zinc supplementation, the sperm quality of infertile men was significantly increased.

How does zinc impact fertility for women?

Zinc was first discovered as a critical nutrient in the female reproductive system in the 1940s. It is important to mention that while research is still limited, and most has been conducted on small female mammals, (not female humans), zinc is understood to play a critical role in the female reproductive system.

Along with the importance of zinc preconception in developing oocytes (maturing egg cells), supporting the production and secretion of 2 essential female hormones for ovulation, it is also necessary at the very moment of conception. Zinc continues to play an important role during pregnancy for successful foetal growth and for breastfeeding mothers. 

Sex hormones and preconception

For cisgender women who experience menstrual cycles (e.g. those not taking any oral contraception), zinc levels preconception are important for the maintenance of hormonal balance. Zinc is thought to stimulate the pituitary gland in the brain, which releases a number of sex hormones in the body, including follicle-stimulating hormone (FSH) and LH (luteinising hormone). FSH and LH are essential in the menstrual cycle as FSH stimulates the maturation of follicles (to release an egg) and LH encourages ovulation (the release of an egg to be fertilised around day 14 of a 28-day cycle).

Germ cells

Zinc is understood to support the regulation of the menstrual cycle by nourishing healthy ovarian follicles and maturing oocytes, therefore promoting ovulation. 

Fertilisation

Following sperm fusion to the egg, a phenomenon known as a zinc spark occurs. This is a quick release of zinc, essential for inducing egg activation in addition to facilitating zona pellucida hardening, which is a thin membrane surrounding an egg. This hardens to prevent another sperm from penetrating an egg following successful fertilisation, promoting foetal development.

During pregnancy

Once a woman is pregnant, along with other nutrients, zinc recommendations increase, as the trace element is essential for the successful implantation, growth, and development of a foetus.

Risks of zinc toxicity

It is important to bear in mind that zinc is a trace element, and therefore the body does not require intake in large doses per day.

For men and women of reproductive age (19+), the upper level of zinc per day is 40mg, but there is no need to be taking more than:

• 11mg for males per day, and

• 9mg per day for females

How is zinc absorbed into the body optimally?

Zinc is exclusively absorbed in the small intestine. It is important to stress that zinc absorption does not only depend on an adequate diet, but is also greatly affected by its bioavailability. While the advice is clear here; look for high zinc foods and ensure the zinc is also highly bioavailable, i.e. not paired with any compounds that may inhibit its absorption, this can feel like a minefield. Without a dietitian or nutritionist to advise the best foods to be buying, and in what quantity to ensure your body is getting the zinc it needs, supplementation can offer some clear reassurance. A high quality supplement containing zinc in a well researched form (there are many different forms of all nutrients), you can be sure how much zinc you are obtaining and take comfort in your body’s requirements being met.

The small intestinal lining cells, known as enterocytes, transport zinc from the lumen of the small intestine, to the portal blood, where zinc is taken to the liver. Here, zinc is predominantly bound to albumin (a protein), which is how the metal is distributed throughout the body.

Foods of animal origin, specifically the organs and flesh of mammals, fish, eggs, and dairy products, are the richest source of bioavailable zinc in the diet. Some plants also contain high levels of zinc, but phytates also found in plants can inhibit zinc absorption. Phytates are particularly high in white rice and beans.

How to improve chances of fertility other than food?

While we understand that zinc is an important component of a healthy diet, especially for couples wishing to conceive, we recognise that a holistic approach is important too. Here are some of our tips advised by governing bodies to support your conception journey:

• When looking to conceive, the NHS advises that it is important to maintain a ‘healthy weight’. This is different for each individual but aim to do 150 minutes of moderate-intensity activity a week or 75 minutes of vigorous-intensity activity a week. 

• Be conscious of food intake if increasing your exercise routine. The balance is key, especially for menstruating women—the body will not allow ovulation to happen if one is energy deficient in general. 

• It can be helpful to keep note of the length of menstrual cycles, so it is easier to predict ovulation. The NHS advises that you should be expecting ovulation between 12-16 days before a period begins.

• Cisgender women may also keep track of LH in the urine with LH testing kits (these can be bought online) and offer a clear picture of where you are in your cycle.

• It’s recommended by reputable pregnancy sources such as Tommy’s that you try to reduce caffeine intake to 1 small cup a day maximum when trying to conceive. 

• Men and women should quit smoking and aim to reduce alcohol intake—the chances of conceiving if you are having unprotected sex every 2-3 days is almost halved if you are a smoker.

• Consider food processing cooking methods such as heating, sprouting, soaking, or fermenting legumes to increase the bioavailability of zinc.

• Use mineral or canola oil during sexual intercourse for lubrication. Saliva is not recommended as it can impact sperm motility and viability. 

Despite the diet and a healthy microbiome playing a crucial role for maintaining zinc homeostasis, conception is not a simple journey for many couples, and therefore, we hope that our practical tips for other lifestyle factors to consider alongside the diet can offer a more holistic approach to the journey too.

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