A groundbreaking study reveals that metabolic health, not age or testosterone, is the key factor in maintaining sexual and reproductive function in men.
For decades, the narrative surrounding men’s sexual health has been intertwined with the seemingly unstoppable march of time. A decline in vitality, performance, and fertility has often been accepted as an inevitable consequence of getting older, a simple function of the number on a birthday cake. But what if this long-held belief is fundamentally flawed? What if the primary culprit isn’t age itself, but a more subtle, and crucially, a more controllable factor?
A paradigm-shifting study, presented at the Endocrine Society’s annual meeting, ENDO 2025, is challenging these old assumptions. Research led by Dr. Michael Zitzmann, a professor and doctor of medicine at University Hospital in Muenster, Germany, suggests that the key driver behind changes in men’s sexual and reproductive systems isn’t the passage of years or even declining testosterone levels. Instead, the evidence points squarely at metabolic health, particularly modest increases in blood sugar.
“Although age and testosterone levels have long been considered an impetus for men’s declining sexual health, our research indicates that these changes more closely correlate with modest increases in blood sugar and other metabolic changes,” Dr. Zitzmann explained. This finding is not just a fascinating piece of data; it’s a call to action. It reframes the conversation from one of passive acceptance to one of proactive empowerment. As Dr. Zitzmann puts it, “This means that men can take steps to preserve or revive their reproductive health with lifestyle choices and appropriate medical interventions.”
A Deeper Look at the Evidence
To arrive at this conclusion, the researchers conducted a comprehensive long-term study, known as the FAME 2.0 study. Beginning in 2014, they followed a group of healthy men, initially 200 strong, ranging in age from 18 to 85. The participants were carefully selected to exclude those with pre-existing conditions like diabetes, heart disease, or cancer, ensuring the focus remained on the natural aging process in a healthy population. Over the course of six years, until the study concluded in 2020 with 117 remaining participants, the team meticulously tracked a range of biological markers.
They analyzed hormonal profiles, including testosterone. They assessed semen parameters, looking at factors like sperm count and quality. They evaluated erectile functioning through validated assessments. And critically, they monitored metabolic health, using Body Mass Index (BMI) and the HbA1c test, which provides a picture of average blood sugar levels over the preceding two to three months.
The results were both surprising and illuminating. Over the six-year period, the participants’ hormone levels and most semen parameters remained largely stable and within normal ranges. This finding alone challenges the idea that a steep drop-off in these areas is a universal part of male aging. However, two specific areas showed a marked decline: sperm movement (motility) and erectile function.
When the researchers cross-referenced this decline with the metabolic data, a clear pattern emerged. The men experiencing reduced sperm motility and poorer erectile performance were those who had even minimally elevated blood sugar levels. Strikingly, these levels were still below the 6.5% HbA1c threshold typically used to diagnose diabetes. This suggests that the damage begins long before a person would be considered diabetic, in a ‘pre-diabetic’ or even ‘high-normal’ range that often goes unnoticed.
The Blood Sugar Connection: How Does It Work?
While the study highlights this strong correlation, it’s helpful to understand the underlying biological mechanisms that explain why blood sugar has such a profound impact on male sexual health.
Erectile function is fundamentally a vascular event. It relies on healthy, flexible blood vessels that can relax and allow blood to flow into the penis. Persistently elevated blood sugar is known to be toxic to the delicate lining of these blood vessels, a condition called endothelial dysfunction. It makes them stiffer and less responsive, directly impairing the ability to achieve and maintain an erection. Furthermore, high glucose can lead to neuropathy, or nerve damage. Since erections are initiated by nerve signals from the brain, any interference with this communication pathway can lead to performance issues.
Similarly, sperm cells are highly vulnerable to their environment. High blood sugar can increase oxidative stress throughout the body—a state of imbalance where harmful molecules called free radicals overwhelm the body’s antioxidant defenses. This oxidative stress can damage the sperm cell’s membrane and DNA, and critically, it can impair the mitochondria, which are the tiny engines that power the sperm’s tail. This directly leads to reduced motility, meaning the sperm can’t swim effectively, a key factor in fertility.
Redefining the Role of Testosterone
Another fascinating insight from the study concerns the role of testosterone. In the popular imagination, testosterone is the master hormone of masculinity, directly governing sexual performance. The study, however, adds a crucial layer of nuance. The researchers found that while testosterone levels did correlate with the participants’ self-assessed libido, or sexual desire, they did not have a direct impact on erectile function itself. In other words, testosterone might influence how much a man wants to have sex, but it’s his metabolic health that more directly influences his physical ability to perform.
This distinction is vital for both patients and doctors, as it suggests that simply prescribing testosterone may not address the root cause of erectile issues if the underlying problem is metabolic.
A New Path Forward for Men’s Health
The ultimate message of this research is one of hope and agency. The decline of sexual and reproductive health is not an inevitable fate tied to age, but a physiological process that can be influenced and managed. By shifting the focus to metabolic health, men are given a clear, actionable path to maintaining their wellbeing long into their later years.
“We’re hopeful that the information gleaned from this study will help doctors and their patients formulate effective male sexual health maintenance plans,” Dr. Zitzmann concluded. “We now know that it’s in our power to retain sexual and reproductive wellbeing in men, even as they age.”
This means that the same advice given for preventing heart disease and diabetes—maintaining a healthy weight, engaging in regular physical activity, eating a balanced diet low in processed sugars, and getting regular medical check-ups—is also the best advice for preserving sexual and reproductive function. It’s a powerful reminder that the body is an interconnected system, and that caring for one aspect of your health pays dividends across the board.