(a) Meta-regression model for mean sperm concentration by fertility and geographic groups, adjusted for potential confounders. (b) Meta-regression model for mean total sperm count by fertility and geographic groups, adjusted for potential confounders. Graphic: Levine, et al., 2017 / Human Reproduction Update

By Clive Cookson
25 July 2017

(Financial Times) – The sperm count of men in the western world has fallen by more than half over a period of 40 years, according to an international study described by its authors as “an urgent wake-up call” about declining male health.

“Decreasing sperm count has been of great concern since it was first reported 25 years ago,” said senior author Shanna Swan of Icahn School of Medicine at Mount Sinai, New York. “This definitive study shows  …  that the decline is strong and continuing.”

The researchers, led by Hagai Levine of Hebrew University of Jerusalem, screened 7,500 sperm studies carried out worldwide between 1973 and 2011. They chose for meta-analysis 185 studies that met high standards of design and laboratory practice.

The results, published in Human Reproduction Update, show declines of 52.4 per cent in sperm concentration and 59.3 per cent in total sperm count among men in North America, Europe, Australia, and New Zealand. Subjects were not selected according to their fertility status. […]

Other fertility experts who were not involved in the project echoed the authors’ concern. “The extent of the decline in sperm counts in the western world revealed in this study is shocking,” said Professor Daniel Brison of Manchester university. […]

Although the study does not examine likely reasons for the decline, “previous studies have associated low sperm count with environmental and lifestyle influences, including prenatal chemical exposure, adult pesticide exposure, smoking, stress and obesity,” said Dr Levine. [more]

‘Urgent wake-up call’ for male health as sperm counts plummet


Background

Reported declines in sperm counts remain controversial today and recent trends are unknown. A definitive meta-analysis is critical given the predictive value of sperm count for fertility, morbidity and mortality.

Objective and rationale

To provide a systematic review and meta-regression analysis of recent trends in sperm counts as measured by sperm concentration (SC) and total sperm count (TSC), and their modification by fertility and geographic group.

Search methods

PubMed/MEDLINE and EMBASE were searched for English language studies of human SC published in 1981–2013. Following a predefined protocol 7518 abstracts were screened and 2510 full articles reporting primary data on SC were reviewed. A total of 244 estimates of SC and TSC from 185 studies of 42 935 men who provided semen samples in 1973–2011 were extracted for meta-regression analysis, as well as information on years of sample collection and covariates [fertility group (‘Unselected by fertility’ versus ‘Fertile’), geographic group (‘Western’, including North America, Europe Australia and New Zealand versus ‘Other’, including South America, Asia and Africa), age, ejaculation abstinence time, semen collection method, method of measuring SC and semen volume, exclusion criteria and indicators of completeness of covariate data]. The slopes of SC and TSC were estimated as functions of sample collection year using both simple linear regression and weighted meta-regression models and the latter were adjusted for pre-determined covariates and modification by fertility and geographic group. Assumptions were examined using multiple sensitivity analyses and nonlinear models.

Outcomes

SC declined significantly between 1973 and 2011 (slope in unadjusted simple regression models −0.70 million/ml/year; 95% CI: −0.72 to −0.69; P < 0.001; slope in adjusted meta-regression models = −0.64; −1.06 to −0.22; P = 0.003). The slopes in the meta-regression model were modified by fertility (P for interaction = 0.064) and geographic group (P for interaction = 0.027). There was a significant decline in SC between 1973 and 2011 among Unselected Western (−1.38; −2.02 to −0.74; P < 0.001) and among Fertile Western (−0.68; −1.31 to −0.05; P = 0.033), while no significant trends were seen among Unselected Other and Fertile Other. Among Unselected Western studies, the mean SC declined, on average, 1.4% per year with an overall decline of 52.4% between 1973 and 2011. Trends for TSC and SC were similar, with a steep decline among Unselected Western (−5.33 million/year, −7.56 to −3.11; P < 0.001), corresponding to an average decline in mean TSC of 1.6% per year and overall decline of 59.3%. Results changed minimally in multiple sensitivity analyses, and there was no statistical support for the use of a nonlinear model. In a model restricted to data post-1995, the slope both for SC and TSC among Unselected Western was similar to that for the entire period (−2.06 million/ml, −3.38 to −0.74; P = 0.004 and −8.12 million, −13.73 to −2.51, P = 0.006, respectively).

Wider implications

This comprehensive meta-regression analysis reports a significant decline in sperm counts (as measured by SC and TSC) between 1973 and 2011, driven by a 50–60% decline among men unselected by fertility from North America, Europe, Australia, and New Zealand. Because of the significant public health implications of these results, research on the causes of this continuing decline is urgently needed.

Temporal trends in sperm count: a systematic review and meta-regression analysis

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