It’s not uncommon for middle-aged men to experience a decrease in libido or sexual performance. For decades, men were told to blame aging or maybe low testosterone. But new research suggests the real culprit may be far more subtle and far more common: blood sugar.

A long-term study presented at the Endocrine Society’s annual meeting in San Francisco has shown that even small increases in a key blood sugar marker lead to a decline in sperm movement and erectile function, even in men who are not diabetic.

“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,” said Dr. Michael Zitzmann, a professor at University Hospital in Muenster, Germany, who led the study.

Sugar and the Body’s Intimate Systems

The research, part of the FAME 2.0 project, followed 200 healthy men between the ages of 18 and 85 over six years. None had diabetes, heart disease, or cancer. By 2020, 117 participants remained. The scientists assessed their semen quality, hormone levels, and metabolic health, particularly focusing on hemoglobin A1c (HbA1c), a marker that reflects average blood sugar over the previous two to three months.

While hormone levels and semen volume generally stayed within normal ranges, the researchers discovered that sperm motility (how well the “soldiers” swim) declined significantly in men with rising HbA1c levels, even if their values stayed below the 6.5% threshold that defines diabetes. Erectile function also declined, but only in those men whose HbA1c values crept up.

In contrast, testosterone levels, which are often blamed for age-related sexual decline, had little effect on erectile function. Instead, libido appeared to track more closely with testosterone, and even that was influenced by HbA1c levels. “This means that men can take steps to preserve or revive their reproductive health with lifestyle choices and appropriate medical interventions,” Zitzmann explained.

In essence, the sugar circulating in your bloodstream may quietly erode sexual health years before it tips over into diabetes. “We now know that it’s in our power to retain sexual and reproductive well-being in men, even as they age,” he added.

Sugar Flies Under the Radar

For many men, blood sugar is not something they think about until a diagnosis of diabetes or a stern warning from their doctor. But experts say that ignoring this invisible factor may be costing men their health and vitality long before those red flags appear.

Chronically elevated glucose, even at levels considered “normal,” can damage blood vessels and nerves that are crucial for arousal and performance. According to data from the U.S. Centers for Disease Control and Prevention, nearly 1 in 4 Americans with diabetes don’t know they have it. Millions more hover in the prediabetic zone without symptoms—except, perhaps, in the bedroom.

This silent erosion is something many physicians are starting to recognize. “Sexual health is rarely the first thing people associate with blood sugar, but chronically high glucose levels can have an impact on intimacy—for everyone,” wrote Dr. Jamin Brahmbhatt, a urologist and surgeon, in a recent CNN commentary. “Over time, this can present as erectile dysfunction, before a man is ever diagnosed with or thought to be at risk for diabetes.”

The FAME 2.0 study adds more weight to that clinical suspicion. The researchers used rigorous semen analyses, hormone assays, and questionnaires to tease out which changes were due to age alone, and which were due to blood sugar. The conclusion: age matters less than metabolic health.

That finding reframes what it means to age “well.” A man in his 70s with stable blood sugar may retain better fertility and function than a man in his 40s whose A1c has edged into the prediabetic range.

An Added Benefit of Ozempic?

The growing popularity of GLP-1 medications like semaglutide (Ozempic, Wegovy) for weight loss and blood sugar control is also shining a broader spotlight on metabolic health. These drugs, which mimic a gut hormone that regulates appetite and glucose, have shown side benefits ranging from improved sleep to, perhaps not coincidentally, better sexual function.

In a separate presentation at the same Endocrine Society meeting, researchers found that testosterone levels increased in men who lost weight on GLP-1 injections—even without any direct hormone therapy. The proportion of men with normal testosterone levels jumped from 53% to 77% over 18 months, with an average weight loss of just 10%.

The message is clear: the body’s metabolic and reproductive systems are tightly interwoven. And sugar may be pulling more strings than previously thought.