A 2017 review and meta-analysis found sperm counts among Western men (i.e. men in Australia, Europe, New Zealand, and North America) declined 50—60% between 1973 and 2011, with an average decline of 1.4% per year. The meta-analysis found no indication the decline is leveling off. The amount of decline among men in North America and men in Australia/Europe is similar. The decline in sperm count among men in South America, Asia, and Africa is less than men in Western countries, though the amount of decline in these regions is uncertain. Reasons for the decline are not known with certainty, but it may be associated with chemical exposure or maternal smoking during prenatal development or pesticide exposure or lifestyle changes during adulthood.
Although it is possible for men to father children into old age, the genetic quality of sperm, as well as its volume and motility, all typically decrease with age. In other words, older sperm are less likely to result in a successful pregnancy and, moreover, the cumulative fragmentation of sperm DNA over time increases the likelihood that a small fraction of men will pass on achondroplasia and transmit multiple genetic and chromosomal defects. For example, the percentage of sperm with highly damaged DNA, comet extent, DNA break number, and other comet measures has been found to be significantly higher in men aged 36–57 years than in those aged 20–35 years. Advancing paternal age has been implicated in a number of possible health effects. One particularly well-studied connection is the link between advancing age and autism. For example, one study of 943,664 children less than 10 years old, found that, with confounding variables controlled, the risk of autism increased with increasing paternal age. In men with a normal level of sperm production (normozoospermia), the percentage of sperm DNA fragmentation is positively correlated with age, and inversely correlated with progressive sperm motility. No age related effects on sperm were noted in separate control groups recruited in different geographical locations, indicating that dietary habits, lifestyle or ethnicity could play a part in the quality of sperm.
While advanced age can be a possible factor in sperm motility and health, the sperm of men below 20 years of age has likewise been linked to an increase in birth defects such as neural tube defects, hypospadias, cystic kidney, and Down syndrome.
Sperm are heat-sensitive, and cannot endure high temperatures. Increases of 2–3 °C are associated with increased DNA fragmentation. The body has compensatory mechanisms, like the cremaster muscle relaxing and letting the testicle hang further away from the warm body, sweating and a countercurrent exchange of blood cooling inflowing blood. However, despite these compensations, there are activities that should not be performed too often, in order to prevent infertility due to heat;
- sauna sessions
- bathing for a long time in hot water
- Long-time tanning bed sessions
- Placement of a laptop computer over the groin for extended use
Fever raises the body temperature, which can strike sperm quality. In the same way, sperm quality can be lower in the summer.
Contrary to widely held beliefs, no evidence supports that wearing constrictive underwear, or "briefs," decreases fertility. Even with an elevation in temperature of 0.8–1° caused by wearing constrictive underwear, no changes in sperm parameters, no decrease in spermatogenesis, and no changes in sperm function are observed. [dubious ]
A blow from outside does not affect the sperm quality of already produced sperm cells. Furthermore, the testes are well protected in the scrotum, for example by the tunica vaginalis, making the testes slide away from external pressure rather than being malformed from it; however, a hard enough hit can close or crush the capillaries that supply the sperm producing tissue, resulting in permanent or temporary and partial or total inability to produce sperm in the affected testicle.
There is suspicion that many toxic substances, including several types of medication and hormones, and also constituents of the diet, influence sperm quality. While a few chemicals with known effects on fertility have been excluded from human consumption, we cannot know if others remain undiscovered. Many products that come into direct contact with spermatozoa lack adequate testing for any adverse effect on semen quality.
A 2008 report demonstrated evidence of the effects of feminizing chemicals on male development in each class of vertebrate species as a worldwide phenomenon; these chemical are suspected of reducing the sex ratio and sperm counts in humans. Ninety-nine percent of over 100,000 recently introduced chemicals[vague] are poorly regulated.
At least three types of synthetic toxins have been found in the semen of student volunteers: polychlorinated biphenyls (PCBs), DDT, and hexachlorobenzene. DDT and hexachlorobenzene are associated with decreased semen quality, while PCBs are associated with decreased fertility overall. Leaks of dibromochloropropane (DBCP) have caused sterility in men. Soldiers that were exposed to dioxins and dioxin-like compounds during the Vietnam war have given rise to children with an increased rate of birth defects.
- Depo-Provera, Adjudin, and gossypol are examples of substances used as male contraceptives or in chemical castration. Recent studies have found that THC present in cannabis can confuse the movements of intact sperm, reducing their ability to achieve fertilization.
- Selective serotonin reuptake inhibitors (SSRI) may cause low sperm count.
- Many antibiotics, e.g. penicillin and tetracycline, suppress sperm production.
In addition, in vitro studies have observed altered sperm function by the following medications:
- Many psychoactive drugs, including many antidepressants, many antiepileptics (e.g. lithium), and propranolol
- Opioid analgesics
- Calcium channel blockers
- Phosphodiesterase inhibitors (e.g. caffeine, theophylline, pentoxifylline)
- Calcium chelators (e.g. EDTA)
Also, numerous products that are intended for exposure to spermatozoa have only a general assumption of safety based on the absence of evidence of actual harm.
- Anabolic steroids use and use of other hormones can reduce sperm quality. Changes in hormone homeostasis affect the spermatogenesis.
The body also has natural variations in hormone concentrations, giving sperm quality natural fluctuations as well.
- Drinking over 1 litre of cola a day might decrease sperm quality by up to 30% (study claims there is correlation, but not causation)
- Soy products decrease sperm quality due to the high content of a type of phytoestrogen called isoflavones. Theoretically, this exposure to high levels of phytoestrogen in men may alter the hypothalamic-pituitary-gonadal axis. A few studies on animals have shown that such a hormonal effect may be significant and decrease fertility. On the other hand, most studies have shown that isoflavone supplements have little to no effect on sperm concentration, count, or mobility, and cause no changes in testicular or ejaculate volume.
- A review in 2010 concluded that there is little evidence for a relationship with semen parameters and increased BMI.
- Folate (vitamin B9) may protect sperm cells from aneuploidy.
- Gossypol has been associated with reduced sperm production. It is present in crude cottonseed oil, and potentially the organ meats from animals poisoned with it
- Recently, a review of epidemiological/observational studies providing the most comprehensive analysis to date of the associations between diet or nutrient intake and the risk of infertility. It suggests that diet modifications may be useful in modulating male fertility and fecundability. Healthy diets (i.e. the Mediterranean diet) rich in such nutrients as omega-3 fatty acids, some antioxidants and vitamins, and low in saturated fatty acids (SFAs) and trans-fatty acids (TFAs) are inversely associated with low semen quality parameters. In terms of food groups, fruit, seafood, poultry, eggs, cereals, leafy vegetables, dark chocolate, and low-fat dairy products have been positively related to sperm quality. However, diets rich in processed meat, fried foods, potatoes, full-fat dairy products, caffeine, alcohol and sugar-sweetened beverages have been inversely associated with the quality of semen in some studies. The few studies relating male nutrient or food intake and fecundability also suggest that diets rich in red meat, processed meat, tea and caffeine are associated with a lower rate of fecundability. This association is only controversial in the case of alcohol. The potential biological mechanisms linking diet with sperm function and fertility are largely unknown and require further study.
- One study found that following a typical Dutch diet, rich in fruits, vegetables, potatoes, meat, full-fat dairy products, seafood and pastries, increased sperm count by 50%. Men who consume a Dutch diet have sperm counts nearly twice as high as men who don't. 
Environmental mutagens that are associated with decreased semen quality include the following:
- Plutonium, widely spread from nuclear weapon tests, accumulates in the testes, where it disrupts zinc metabolism, in turn causing genetic damage.
- Ethylene oxide, a chemical sterilizer, is associated with decreased semen quality.
Other environmental agents associated with decreased semen quality include:
- Cadmium, causing damage to Sertoli cells, thereby impeding spermatogenesis.
- Lead, causing reduced spermatogenesis and abnormal spermatozoa.
- Mercury, being highly damaging to spermatogenesis.
- Many pesticides, causing decreased semen quality as well as sperm chromosome anomalies.
- Polybrominated diphenyl ethers (PBDEs).
- Many solvents, such as benzene, toluene, xylene, styrene, 1-bromopropane, 2-bromopropane, perchloroethylene, trichloroethylene.
Both a too short period of time since last ejaculation and a too long one reduce semen quality.
A period of time of less than one day reduces sperm count by at least 20%.
Longer periods of abstinence correlate with poorer results – one study found that couples where the man had abstained for more than 10 days before an intrauterine insemination (IUI) had only a 3% pregnancy rate. An abstinence period of only 1 or 2 days produce the highest pregnancy rates per IUI cycle compared with longer intervals of ejaculatory abstinence. This increase in pregnancy rate occurs despite a lower value of total motile spermatozoa. Daily sexual activity increases sperm quality in men minimizing DNA damage in the sperm—because it is speculated to result in less storage time where damage may accumulate.
Masturbation vs intercourse
Semen samples obtained via sexual intercourse contain 70-120% more sperm, with sperm having a slightly higher motility and slightly more normal morphology, compared with semen samples obtained via masturbation. Sexual intercourse also generates a 25–45% increase in ejaculate volume, mainly by increased prostate secretion.
However, the single factor or factors for the intercourse advantage have not yet been isolated. It cannot be explained by presence of visual perception of physical attractiveness alone during stimulation, although there may be a slight correlation. Neither do any substantial fluctuations in sex hormones explain the intercourse advantage. It is hypothesized that sexual intercourse subdues an inhibition from the central nervous system, but what, in turn, is the subduing factor is still not completely known.
Home or in clinic
The sperm quality is better if the sample is collected at home than in the clinics. Collecting the sperm at home gives a higher sperm concentration, sperm count and motility particularly if the sperm is collected via sexual intercourse. If the semen sample is to be collected by masturbation, a specimen from the early stages of the ejaculate should be into a clean new and unused, sealed collection cup.
For semen that has been ejaculated, the quality deteriorates with time. However, this lifetime can be shortened or prolonged, depending on the environment.
Sperm outside the body generally has a life expectancy which is considered to depend on pH, temperature, presence of air and other factors, and is unpredictable but smaller than the life expectancy inside the human body. For instance, sperm donors who collect the sample outside the clinic are advised to have handed in the sample no more than one hour from collection, and to keep it, if not at body temperature, then at least at room temperature.
In a non-harmful environment outside the body, such as in a sterile glass container the number of motile sperm decreases by approximately 5-10% per hour. In contrast, in a latex condom, the quality decreases by 60-80% per hour, rendering the sample unusable in a relatively short time.
Tobacco smoking lowers the sperm quality, perhaps by decreased ability to attach to hyaluronan on the egg cell. Wright et al. have reviewed evidence that smoking is associated with increased sperm DNA damage and male infertility. Smoking cannabis can decrease sperm quantity.
Long-term stress is also suggested. The practise of tucking can reduce both the sperm count and sperm quality. Meta-analysis indicates that mobile phone exposure affects sperm quality negatively.
Higher levels of intelligence are also correlated with higher levels of sperm quality in three key indicators: sperm concentration, sperm count and sperm motility. Men who scored high on a battery of intelligence tests tended to have higher counts of healthy sperm, while low scorers tended to have fewer and more sickly sperm. It is conceivable that intelligence might tip off a man's overall health to women looking for a mate with healthy genes, explained University of New Mexico evolutionary psychologist Geoffrey Miller at a talk at Harvard University. "Though the connections between brains and sperm were 'not awesome, they're there and highly significant,' Miller said. All things held equal, good sperm and good brains go together."
Lack of exercise, as well as excessive exercise, are minor factors. In professional sports, semen quality parameters tend to decrease as training requirements increase. The effect differs substantially between different professional sport types. For example, water polo appears substantially less harmful to semen quality than triathlon.
A longer duration of sexual stimulation before ejaculation slightly increases sperm quality.
Males carrying Robertsonian translocations in their chromosomes have significantly higher degree of sperm cell apoptosis and lower concentration. Sperm cells also have decreased forward motility, but normal morphology.
A semen analysis typically measures the number of sperm per millilitre of ejaculate, and analyzes the morphology (shape) and motility (ability to swim forward) of the sperm (the typical ejaculate of a healthy, physically mature young adult male of reproductive age with no fertility-related problems usually contains 300–500 million spermatozoa, though only a couple of hundred survive in the acidic environment of the vagina to be candidates for successful fertilization). Also usually measured are the concentration of white blood cells, the level of fructose in the semen, and the volume, pH, and liquefaction time of the ejaculate.
Hamster zona-free ovum test
A man's sperm are mixed with hamster eggs that have had the zona pellucida (outer membranes) removed, and the number of sperm penetrations per egg is measured. No strong correlation has been found between hamster egg penetration rates and the various semen parameters and the role of the hamster egg penetration test in the investigation of the causes of infertility should be evaluated further. However, a negative result on the hamster test correlates with a lower probability of the man's partner becoming pregnant.
Antisperm antibodies test
Presence of antisperm antibodies may be responsible for sperm agglutination, reduced sperm motility, abnormal postcoital test. Several tests are presently available including Sperm Immobilization test, Sperm Agglutination tests, Indirect immunofluorescence test, Enzyme-linked immunosorbent assay, Radiolabelled Antiglobulin Assay. One of the most informative and specific tests is Immunobead Rosette Test which can identify different antibody classes involved (IgG, IgA, IgM) and location on the sperm cell (head, body or tail).
Hemizona test is a test to evaluate sperm zona-binding capacity. In this test, the two halves of human zona pellucida is incubated with patient's capacitated sperm and control fertile donor's sperm.
- PCR-based detection of the pathogens in the semen of patients with asymptomatic genital infection.
- Biochemical markers like Creatine kinase, Reactive oxygen species.
When performing cryopreservation of semen, it is the sperm quality after reviving the sample that is of importance, because many sperm cells die in the process.
To be of use in assisted reproductive technology, the sample should after thawing have more than 5 million motile sperm cells per ml with a good grade of mobility. If the grade of mobility is poor, 10 million motile cells per ml is required.
When a sperm sample is prepared for intrauterine insemination, it is washed at a facility such as a fertility clinic or a sperm bank. Some sperm does not survive the washing process, as is also the case when freezing the sperm.
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