The extraordinary decline in male fertility over the last century has coincided with a major shift in our dietary polyunsaturated fatty acids intake away from omega-3’s (found in seafood) towards omega 6’s (found in vegetable seed oils). In this post I cast my net wide and haul in a boat-load of evidence linking PUFAs to sperm function.
In a previous post I looked at some of the emerging research around the omega 6 to 3 ratio:
Two of those papers point to the link between maternal nutrition and the health of the offspring. In our seafood videos (part 3) we go into considerable detail around the mechanism by which females accumulate DHA in their gluteofemoral (hip and thigh) fat stores laid down in puberty, and the effects of seafoods on maternal and infant brain function and that all important IQ. Clearly seafoods play a pivotal role in female reproduction.
You would be forgiven for thinking, with all that focus on females and reproduction, that men are somehow let off the (fish) hook when it comes to omega 3’s and fertility. Not a bit of it. I’m not quite sure why I didn’t look into this earlier, but once I started to fish around in the research I realised that the omega 6 to 3 balance plays a significant role in sperm production and function too.
How seed oils contributed to the omega-6 imbalance
The major omega-6 PUFA (polyunsaturated fatty acids) in our diets is linoleic acid (LA), which accumulates in fat tissue. The graph below is an updated version from Stephan Guyenet. Each point represents the findings of a different study on the percentage of LA in the fat stores. As you can see it has nearly doubled since the 1950’s. That pushes the omega 6 to 3 ratio even further out of wack. It’s extraordinarily linear, and driven by the widespread adoption of industrial vegetable (seed) oils in spreads, condiments, baked and fried foods.
This graph should give us all pause for thought. For the whole of evolution, right up until 1955 it is reasonable to assume that the % LA in fat tissue had been ~10% as this is very close to the level found in the tissue of our nearest relatives the primates. The remarkable increase in the last 60 years represents an unparalleled dietary experiment with us as the guinea pigs. So it’s about time we started discussing the results…
According to a paper in the American Journal of Clinical Nutrition in 2011, titled ‘Changes in consumption of omega-3 and omega-6 fatty acids in the United States during the 20th century‘ not only has this increased consumption of LA pushed up the omega 6 side of the ratio, but competition for the elongation enzymes has almost certainly contributed to falling levels of the long chain omega 3s, the all important DHA and EPA, making the imbalance in the 6 to 3 ratio even greater.
The dramatic fall in sperm count in the last 100 years
Over roughly the same period we have seen a drop in male fertility. The graph below shows the downward trend in sperm count over the 20th century.
Further evidence of this rapid decline is documented in a study published in 2013 tracking the fertility of 26,609 men in France between 1989 and 2005. It found a reduction from 73.6 million sperm/ml in 1989 to 49.9 million/ml in 2005. That amounts to a staggering reduction in sperm production of 1.5% per year. Studies in Europe in the last two decades have identified approx 15-20% of otherwise healthy young men have sperm counts lower than the WHO infertility threshold of 20 million sperm/ml [ref].
This decline in fertility is on top of the fact that humans produce some of the poorest quality sperm in the animal kingdom. For example, whereas domesticated livestock (bulls) and lab animals (rats) produce more than 90% viable, healthy sperm, the typical level among healthy young men is only 5 -15%. This striking difference suggests that human sperm production is markedly more susceptible to environmental influences. [ref]
Is there a link between PUFAs and male fertility?
The inverse relationship between the two graphs above proves nothing in itself. Indeed I have been aware of both of the above trends (increasing omega 6 and decreasing sperm count) for years, but never made any connection between them until I started looking into omega 6 to 3 ratio and male fertility. So lets dive in and take a look at some of the key points from a recent review paper: Dietary fatty acids affect semen quality: a review V. Esmaeili et al, May 2015, Andrology [Full free text]
Male infertility is affected by a range of factors including genetic and specific diseases such as varicocoele. However, genetics and changes in disease patterns do not account for the decreased fertility observed over the last eight decades, which must therefore, be due to environmental factors which may include diet, occupation, and environmental toxins.
Among the dietary factors that have been looked into, carbohydrates and proteins have little effect on male fertility. Although some effect from minerals and vitamins has been reported, the most significant, nay pivotal aspect of sperm biology can firmly be laid at the door of polyunsaturated fatty acids.
Along with neurons and photoreceptor cells in the eye, sperm have some of the highest concentrations of DHA in the body. Unlike the brain and eye, however, the testes require a continuous supply of PUFAs as sperm are constantly being manufactured, whereas DHA in the brain has a much slower turnover. Sperm then are uniquely vulnerable to dietary insufficiency.
Testes have a high capacity for converting the short chain PUFAs LA and ALA, (alpha linolenic acid) into their all important long chain counterparts AA (arachidonic acid) and docosahexaenoic acid (DHA). This would suggest that the increase in dietary LA shown in Guyenet’s graph above could have major effects on sperm omega 3 to 6 ratios.
As we showed in our seafood videos, one of the functions of DHA in the plasma membrane of cells is to impart flexibility. In the case of sperm cells, it makes sense that DHA is concentrated in the tail, and may explain why sperm motility is reduced if there is insufficient dietary omega 3 PUFA. In simple terms, the less DHA the slower the sperm swims.
However, a second site where DHA accumulates is in the acrosome in the head of the sperm. This site is directly involved in fusing with the egg cell. It has been shown that an omega-3 deficient diet in mice prevented the second stage of sperm formation as the acrosome could not form. This provides a plausible mechanism by which low levels of DHA in the diet may lead to reduced sperm count as well as the reduced sperm motility just discussed.
An observational study published just this month in Andrology, looked at fatty acid composition and semen quality in a wide range of Norwegian men. It found that the DHA level in the sperm correlated positively with total sperm count, sperm concentration, sperm vitality, progressive sperm motility and normal sperm morphology. Higher DHA concentrations in sperm was associated with fewer head and tail deformities. Linoleic acid (short chain omega 6), however, correlated negatively.
PUFAs having multiple double bonds are particularly susceptible to lipid oxidation. The products of this oxidation can cause DNA damage – not helpful for a sperm whose goal in life is to attempt to pass DNA on to the next generation intact! However, the study found evidence that the higher the DHA, the less DNA damage was occurring, suggesting that the omega 3s were protective and reduced oxidative stress.
Yet another factor in sperm quality related to PUFAs (polyunsaturated fatty acids) is the effect of trans-fats. A study published in 2010 found that in healthy young men, increasing trans-fats in the diet led to a marked decrease in total sperm count [ref]
The omega 6 to 3 ratio has been found to be relevant to sperm production in many animal studies, but also in humans. A study published in the journal Andrology in 2010 compared fatty acid composition of sperm from fertile and infertile men. They found that fertile men had higher levels of DHA in their sperm than infertile men. The omega 6 to 3 ratio was higher in infertile men (15:1) than fertile men (6:1). [ref]
And yet it now seems that even an omega 6 to 3 ratio of 6:1 is probably sub optimal. It is widely agreed that pre-agricultural humans, like many wild animals, had a ratio closer to 1:1 [ref]. Evidence for this ratio being optimal in sperm production comes from studies in animals, for example, a 2015 paper ‘Effects of different dietary n-6/n-3 polyunsaturated fatty acid ratios on boar reproduction‘ concluded that:
Our data suggest that the ideal n-6/n-3 ratio in the diet of breeding boars is 1:1, and proper balancing of n-6/n-3 fatty acids plays an important role in male reproduction.
Similar results were found for mice, cockerels [ref], rams [ref], buffalo bulls [ref], and dogs [ref]. In the latter study five dogs were placed on long term fish oil supplementation and compared to five control dogs. The following graphs show remarkable boosts to the fertility of the fish oil supplemented dogs:
Incredibly, there are hardly any equivalent intervention studies in humans! However I managed to track down one study published in 2011 in which 238 infertile men were randomised to 1.84 g per day of EPA and DHA, or placebo for 32 weeks. The results were quite striking:
If fish oils can improve fertility, then so should fish consumption.
Fish and Fertility
A study in 2014 published in the Journal of Nutrition looked at the diets of 155 men presenting for evaluation at the Massachusetts General Hospital Fertility Centre.
They found multiple parameters of sperm quality were positively associated with fish consumption (See graphs opposite). It is interesting to note the main uptick in these graphs is amongst men eating fish more than 2.4 times per week. [ref]
Processed meat consumption had the opposite relationship.
The researchers concluded that switching two servings per week of processed meat for fish was associated with an ∼60% higher total normal count.
Contaminants in fish
In our seafood talk (second video) we tackled the issue of mercury in fish, and found that there was no evidence of mercury levels in fish causing harm to mothers or their offspring, indeed there was a positive correlation – the more fish eaten the higher the IQ of the offspring. Experimental evidence found that the benefits of fish eating easily outweighed the harms from mercury, so long as the fish eaten were not the really large mercury-accumulating species like shark, swordfish or marlin.
So what about the effect of mercury on male fertility? Methyl mercury (the form found in fish) is known to reduce male fertility. A 1998 study in Hong Kong found that sub-fertile men had more mercury (from fish) in their hair than fertile men, leading to the suggestion that the mercury was contributing to the infertility. However, that is not necessarily the case: There could have been other factors at play that increased both infertility and mercury accumulation – for example they might have had poor detox pathways, or been exposed to infections.
Evidence that fish mercury is probably not the underlying cause of the observed infertility in the Hong Kong study came in 2007 from a Swedish study [full paper], which looked at the sperm of 189 fishermen from the east and west coast of Sweden. Although the men ate large quantities of fatty fish, and had raised methyl mercury levels there was no evidence of reduced sperm count, motility or quality, nor any differences in sperm quality between the lowest and highest exposure. This contradicts the implications of the Hong Kong study, and adds to the weight of evidence suggesting fish eating promotes fertility despite the mercury.
Many of the animal studies on diet and sperm quality find an important role for vitamin E. Some studies even claimed that vitamin E is more significant that omega 3 supplementation [ref] As ever, the problem with supplements vs real foods is that biological effects are frequently modulated by a host of nutrients which come together in whole foods. Vitamin E is naturally found alongside DHA in oily fish and ALA (short chain omega 3) in walnuts. Indeed DHA requires vitamin E to function properly as I reported on in the post: Omega-6 and Omega-3 PUFAs: Five recent research papers
It’s not all about PUFAs
Having spent several days researching dozens of papers on this topic I feel the evidence is pretty convincing that more omega 3 (fish) and less omega 6 (vegetable oils) is a pretty safe bet for male reproductive health. However, it was clear that there are many other factors involved and I don’t think PUFAs are the whole story. My suspicion is that other candidates may include environmental toxins such as diesel fumes, endocrine disrupting chemicals that are now all pervasive and found in everyday items from till receipts to fire retardants in sofas, and more recently the rise of GMO soy which is known to decrease male fertility. But those topics are for another day, and for now I’ve had enough of thinking about sperm, and I want to go and eat some seafood.