Why have average testosterone levels in men been declining over recent decades?

Why Testosterone Levels Are Declining in Men

Testosterone levels in men have been declining over recent decades due to a combination of rising obesity rates, environmental and lifestyle factors, and chronic disease burden—not simply aging alone.

Primary Drivers of Declining Testosterone

Obesity and Metabolic Dysfunction

• Elevated body mass index is the single strongest modifiable factor associated with lower testosterone levels in contemporary men, with the decline persisting even after controlling for BMI 1
• Excess adipose tissue increases aromatization of testosterone to estradiol, which creates estradiol-mediated negative feedback that suppresses pituitary LH secretion and reduces endogenous testosterone production 2, 3
• Obesity-associated secondary hypogonadism is now recognized as a distinct clinical entity that can be partially reversed through weight loss of 5–10% 2
• The metabolic syndrome—characterized by insulin resistance, dyslipidemia, and central adiposity—is strongly linked to lower testosterone concentrations 4, 5, 6

Nutritional Deficiencies

• Deficiencies in zinc, magnesium, vitamin D, and low polyphenol intake adversely affect the hypothalamic-pituitary-gonadal (HPG) axis and contribute to reduced testosterone production 7
• Below-average caloric intake in men of healthy weight decreases testosterone levels, though the same caloric deficit in obese men may have neutral or positive effects 7

Chronic Stress and Sleep Deprivation

• Mental stress and oxidative stress negatively impact the HPG axis, contributing to lower testosterone levels independent of age 7
• Poor sleep quality and insufficient sleep duration disrupt normal testosterone production, as testosterone secretion follows a circadian rhythm with peak levels in early morning 7

Chronic Systemic Illness

• Systemic illness and chronic diseases lower testosterone levels, creating uncertainty about whether low testosterone is a cause or consequence of poor health 3, 4, 5
• Type 2 diabetes, cardiovascular disease, HIV infection, chronic kidney disease, and inflammatory conditions all suppress the HPG axis 2, 4, 5, 6

Population-Level Trends

Temporal Decline Independent of Age

• Mean total testosterone levels have declined significantly across all age groups from 1999–2016, with levels in the later period (2011–2016) being lower than in the earlier period (1999–2000) even after controlling for confounders 1
• This decline is observed even in adolescent and young adult men (ages 15–39), indicating that the phenomenon is not simply age-related 1
• The trend remains statistically significant even among men with normal BMI, suggesting that factors beyond obesity are contributing 1

Age-Related Physiological Decline

• Testosterone levels begin declining in men's mid-30s at an average rate of 1.6% per year, with free testosterone falling more steeply than total testosterone 3, 5
• Approximately 20% of men over 60,30% over 70, and 50% over 80 years have low testosterone levels 3
• During aging, total and free testosterone fall while sex hormone-binding globulin (SHBG) rises, further reducing bioavailable testosterone 5

Environmental and Lifestyle Factors

External Environmental Influences

• Environmental factors—including endocrine-disrupting chemicals, pollutants, and other exposures—are suspected contributors to the population-level decline, though specific mechanisms require further validation 7, 1
• Changes in physical activity patterns, with decreased vigorous activity across the population, may contribute to lower testosterone levels 3

Bidirectional Causation

• Reverse causation must be considered: systemic illness may result in reduced testosterone levels, making it difficult to determine whether low testosterone is a cause or consequence of poor health 4, 5
• Lower testosterone levels have been linked with metabolic syndrome, type 2 diabetes, cardiovascular disease, and higher overall and cardiovascular-related mortality, but the direction of causation remains uncertain 4, 5, 6

Clinical Implications

Distinguishing Pathological from Physiological Decline

• Diagnosis of hypogonadism requires both confirmed biochemical testosterone deficiency (two morning measurements <300 ng/dL) and specific symptoms such as decreased libido or erectile dysfunction 2, 3
• Age-related decline alone does not justify testosterone therapy; treatment is indicated only when both biochemical criteria and qualifying symptoms are present 2, 3

Modifiable Risk Factors

• Weight loss through hypocaloric diets (500–750 kcal/day deficit) and structured exercise (≥150 min/week moderate-intensity aerobic activity plus resistance training 2–3 times/week) can reverse obesity-associated secondary hypogonadism 2
• Addressing nutritional deficiencies, improving sleep quality, and reducing chronic stress may help prevent or ameliorate testosterone decline 7

Common Pitfalls

• Do not assume that declining testosterone is an inevitable consequence of aging alone; modifiable factors—particularly obesity and metabolic dysfunction—are major contributors 1
• Do not overlook the role of chronic disease burden in suppressing testosterone levels; treating underlying conditions may improve testosterone without hormone replacement 2, 4, 5
• Do not initiate testosterone therapy based on population trends or age alone; rigorous diagnostic criteria (two morning measurements <300 ng/dL plus specific symptoms) must be met 2, 3