Y chromosome disappearing, sperm counts & testosterone levels dropping: What studies are telling about men's health
Y chromosome losing genes over time
Rodents offer evolutionary clues
Live Events
Y chromosome loss already affecting men's health
Effects on immunity and cancer
2017 study: Sperm counts declined sharply
Testosterone levels also showing long-term decline
Chemical exposure, temperature, obesity among possible causes
Avoiding further decline
Spiny rats' example of shrinking Y Chromosome
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Scientists are raising concerns about the long-term future of male reproductive health. The Y chromosome, which plays a critical role in male development, is losing genetic material and may disappear in about 11 million years. Separately, a major 2017 study revealed that sperm counts in men from industrialised countries have dropped sharply over the last four decades.The Y chromosome, which distinguishes biological males from females, is shrinking. Unlike the X chromosome that carries about 900 genes involved in many bodily functions, the Y retains only about 55. Scientists found that since humans diverged from monotremes like the platypus roughly 166 million years ago, the Y chromosome has lost about five genes per million years.At that rate, projections suggest it could vanish completely in about 11 million years. Though once dismissed, the theory has gained ground after researchers found species like the Japanese spiny rat and certain mole voles reproducing without a Y chromosome.In 2022, researchers discovered a duplicated DNA segment near the SOX9 gene in the Japanese spiny rat. Normally, male traits develop when a gene on the Y chromosome (SRY) activates SOX9. But in this species, the new DNA copy triggers SOX9 on its own, even in animals with two X chromosomes.When this fragment was inserted into mice, the animals still developed testes. This shows evolution can find workarounds. If humans ever lose the Y chromosome, natural selection may promote new pathways for male development. Over time, this could lead to different populations evolving in separate directions.Before any evolutionary shift happens, scientists say many men already lose the Y chromosome in some cells as they age. From around the age of 50, bone marrow stem cells can divide abnormally, resulting in white blood cells that no longer carry the Y chromosome – a condition called 'mosaic loss of Y.'By age 80, more than 40% of men show this loss in a large portion of their blood cells. In a study of 1,153 older Swedish men, those with the loss had shorter lifespans, higher risk of heart disease and cancer, and were seven times more likely to develop Alzheimer's.Kenneth Walsh from the University of Virginia bred mice with Y-lacking blood stem cells. The mice later developed heart failure and died early, indicating the chromosome loss plays a direct role.A gene on the Y chromosome known as UTY influences immune responses. Without it, some immune cells become less effective, while others produce more scar tissue. Tumors in mice grew twice as fast in the absence of the Y chromosome.In human cases, bladder cancers without the Y chromosome were found to be more aggressive, but also more responsive to certain treatments like checkpoint inhibitors. This highlights the Y chromosome's complex role in immune function and cancer biology.In 2017, an international research team led by Dr. Hagai Levine of Hebrew University of Jerusalem analysed data from 185 studies involving over 42,000 men. The analysis showed that between 1973 and 2011, sperm concentration dropped by 52%, and total sperm count fell by 59% among men in North America, Europe, Australia, and New Zealand.'The extent of the decline is a heartache,' said Levine. 'It's hard to believe – it's hard to believe for me.'The researchers observed that many men now fall below 40 million sperm per millilitre – a level associated with reduced chances of conception. In contrast, no significant decline was found in men from South America, Asia, or Africa.In addition to falling sperm counts, research shows that average testosterone levels in men have been steadily declining over the past several decades. A well-known study from Massachusetts, published in the Journal of Clinical Endocrinology & Metabolism, found that testosterone levels in men dropped by about 1% per year between 1987 and 2004, even after adjusting for age and other health factors. This means a 65-year-old man in 2004 had significantly lower testosterone than a 65-year-old in 1987. Researchers believe the decline may be linked to rising rates of obesity, lack of exercise, exposure to endocrine-disrupting chemicals, and overall lifestyle changes.Though the study did not examine causes, researchers pointed to various environmental and lifestyle factors. Past research links prenatal exposure to hormone-disrupting chemicals with poor sperm quality. Substances like pesticides, lead, and flame retardants can interfere with hormone production.Other experts suggested global warming could play a role. Dr. Harry Fisch from Weill Cornell Medicine noted that sperm counts vary by season and climate. 'I think global warming, not phthalates, is responsible,' said Fisch.Michael Dourson from the University of Cincinnati, while supporting the study's conclusions, questioned the broader health impact, noting that life expectancy has continued to rise in many of the same regions.Researchers agree that individuals can take steps to limit further sperm and Y chromosome loss. Avoiding smoking, reducing exposure to industrial chemicals, and maintaining a healthy lifestyle are seen as effective measures. Good sleep, a balanced diet, and exercise may help slow the loss.Doctors are now testing antifibrotic drugs to treat heart damage linked to Y chromosome loss. In cancer care, Y chromosome status is already being used to guide some treatments. Future health check-ups could include Y-loss scores, much like cholesterol levels.The Japanese spiny rat shows that mammals can survive without a Y chromosome, sparking interest among evolutionary scientists. But while biology may adapt over millions of years, today's middle-aged men already face health risks from losing the chromosome.This situation creates a contrast between species-level evolution and individual-level medical concerns. Researchers say that both areas will shape future debates on male health and evolution.'The impact of the modern environment on health of populations and individuals is clearly huge, but remains largely unknown,' said Dr. Levine. 'Sperm count has previously been plausibly associated with environmental and lifestyle influences, including prenatal chemical exposure, adult pesticide exposure, smoking, stress and obesity. Every man can reduce exposure to chemicals, avoid smoking, keep balanced diet and weight and reduce stress.'
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