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Fatty diet 'reduces sperm quality'

Men wanting to become fathers should “put down that English breakfast” the Daily Mail has today declared, as research has linked a diet high in saturated fat to a reduced sperm count. The study used surveys to...

Men wanting to become fathers should “put down that English breakfast” the Daily Mail has today declared, as research has linked a diet high in saturated fat to a reduced sperm count.

The study used surveys to examine the diets of 99 men attending a US fertility clinic, comparing how their food intake related to the quality of their sperm samples.

Researchers found that men with a higher fat diet had both lower sperm counts and sperm concentrations. The third of men with the highest total fat intake had 43% lower total sperm count and 38% lower sperm concentration than men with the lowest third. Researchers said this association was driven mainly by saturated fat intake. Higher omega-3 intake was also linked to improved sperm quality.

While these results have received a lot of attention on the internet, the design of this small study means it cannot prove a cause and effect relationship between diet and sperm quality. This is because it did not follow the participants over time to see how their diet related to later sperm quality. To confirm or refute the results, larger studies that follow more men over time are required.

This study cannot, and does not, prove that a high-fat diet causes lower fertility. However, a balanced diet has many known health benefits and the study does reinforce the idea that a healthy diet is essential for good health and wellbeing.


Where did the story come from?

The study was carried out by researchers from the Harvard Medical School in Boston and was funded by grants from the National Institute of Environmental Health Sciences and National Institute of Diabetes and Digestive Kidney Diseases in the US.

It was published in the peer-reviewed medical journal Human Reproduction.

Media reports generally over-stated the implications of the results of this small cross-sectional study. However, both the Daily Mail and BBC did include quotes from experts in the field warning of the numerous limitations of this type of study, balancing out the initial attention-grabbing headlines.


What kind of research was this?

This study looked at the association between diet and semen quality using a cross-sectional analysis, meaning it looked at both factors at a single point in time. A cross-sectional study can never prove a cause and effect relationship, as without observing factors over time it is not possible to know which event came first.

In this study, for example, it was not possible to tell whether men with low sperm counts happened to have had a bad diet or whether eating a bad diet actively caused their sperm counts to reduce. A cohort study, which follows people over a set time, would shed more light on whether differences in diet are likely to be causal. It might also be technically possible to conduct a trial looking at men’s sperm quality before and after modifying their diet, although such trials can be harder to perform than studies with a cohort design.

The research authors suggest there is a lack of research on modifiable factors that influence male fertility. As a result, they aimed to investigate a suspected association between dietary fats and semen quality among men attending a fertility clinic.


What did the research involve?

A total of 173 men were recruited from couples attending the Massachusetts General Hospital Fertility Center in the US. The men were attending the centre in order to use their own semen to conceive using assisted reproductive technology.

Dietary information was assessed using a validated 131-item food frequency questionnaire that asked men how often, on average, during the previous year they had consumed specific amounts of each food, beverage or supplement listed in the questionnaire.

Semen was collected after a period of 48 hours of ejaculation abstinence. Sperm counts, concentration and percentage motility (the percentage of the sperm that were mobile) were measured within 45 minutes of collection using standard laboratory techniques.

Reference standards from the World Health Organization were used to assess sperm concentration and motility. The way the sperm looked (morphology) was also assessed.

Fatty acid levels in the sperm and seminal fluid were also analysed in a sub-set of 23 of the men based on the availability of stored semen.

There were 99 men with complete data on semen quality and dietary habits. They were divided into three groups according to the proportion of their total calories that were derived from fats. Low, intermediate and high fat intake groups were defined as consuming 26%, 32% and 37% of their total calories from fat respectively. Participants were also grouped depending on the major fat categories they consumed (saturated, monounsaturated, polyunsaturated, and omega-6 and Omega 3). The analysis took account of differences in:

  • total energy intake
  • age
  • abstinence time
  • BMI
  • smoking
  • intake of alcohol and caffeine


What were the basic results?

Men in the study were largely Caucasian (89%) and had an average age of 36.4 years. The majority were also overweight or obese (71%) and reported never smoking (67%). Overall, 41% of the men had semen categorised as normal, 12% had a low sperm concentration, 53% had less than half their sperm able to move and 32% had abnormal sperm morphology. The main findings were as follows:

  • The third of men with the highest total fat intake had 41% lower total sperm count (95% CI -14% to -62%) and 38% lower sperm concentration (95% confidence interval [CI] -10% to -58%) than the third of men with the lowest.
  • Researchers reported that the association between total fat intake and sperm count and concentration appeared to be driven by saturated fat intake.
  • Men in the highest third of saturated fat intake had 41% lower sperm concentration then men in the lowest (95% CI -14% to -60%). This was after statistical adjustment for total energy intake, age, abstinence time, BMI, smoking, intake of alcohol and caffeine.
  • Higher intake of omega-3 polyunsaturated fats was related to more favourable sperm morphology. The third of men with the highest intake of omega-3 fatty acids had 1.9% (95% CI 0.4% to 3.5%) higher rates of normal sperm morphology than men in the lowest third.
  • In the subsample of 23 men, higher levels of saturated fatty acids in sperm, and to a lesser extent in seminal fluid, were associated with lower sperm concentration and motility. Conversely, higher levels of omega-3 were related to higher sperm concentration and motility.
  • There were no results reported on whether the men tested were able to conceive a child with their partners.


How did the researchers interpret the results?

The researchers conclude that: ‘In this preliminary cross-sectional study, high intake of saturated fats was negatively related to sperm concentration whereas higher intake of omega-3 fats was positively related to sperm morphology’.

They that say further studies with larger samples are required to confirm these findings.



This small, cross-sectional study has attempted to find any associations between higher fat intake and lower sperm count and concentration. It also examined whether higher intake of omega-3 polyunsaturated fats was associated with a higher proportion of sperm showing ‘normal’ appearance and physical characteristics.

However, given that the study assessed diet and sperm quality at the same point in time, it cannot prove there is a cause and effect relationship between the two. It is not possible to know, for example, whether a bad diet causes low sperm counts or whether men with low sperm counts tend to have both a bad diet and lower sperm count due to some unknown factor linking the two. The small number of participants involved also increases the risk that the results have been influenced by chance.

The study has other limitations that should be considered when interpreting and assessing the implications of its findings. These include:

  • Dietary intake is notoriously hard to measure accurately in studies of this kind. The method used in this study is adequate but still asked participants to recall food and drink consumed in the past year. This long recall timespan is likely to be subject to inaccuracy.
  • Most men in this study were overweight, but lower sperm concentration was only found in those who had higher fat intakes. This is interesting as previous studies may have grouped men according to their weight and missed such an association, as they did not measure dietary fat intake.
  • There were no results reported on whether the men tested were able to conceive a child with their partners. It is possible that even men with the lowest quality sperm may have been able to conceive naturally or with the assistance of reproductive technology.
  • All the men in the research were attending a fertility clinic in order to use assisted reproductive treatment, although the specific reasons for this are unclear. This means we cannot tell how typical their sperm was compared to that of the general population or how well the results apply to other men not seeking this type of help.
  • The researchers made multiple comparisons, increasing the risk they would discover a link by chance alone.

This study cannot, and does not, prove that a high-fat diet causes lower fertility. Larger, prospective cohort studies that follow more diverse groups of men, which assess their diet and sperm quality over time, are needed to confirm or refute these findings. However, this study does reinforce the general perception that a healthy diet may is essential for good health and wellbeing. There are many existing reasons to limit dietary saturated fat if you are a man or a woman as it is known to be linked to many diseases including cardiovascular disease.

Analysis by Bazian

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