What Affects Testosterone Levels: 9 Key Factors

Testosterone levels are shaped by a web of overlapping influences, from how much you sleep to how much body fat you carry. No single factor works in isolation. Age-related decline gets the most attention, but diet, exercise, stress, environmental chemicals, and underlying health conditions all push levels up or down in ways that interact with each other. Understanding these nine key factors gives you a clearer picture of what is and is not within your control.

Age

Testosterone peaks somewhere in the second or third decade of life and then gradually drops with each passing year.¹ The decline is real, but it is not as dramatic as many men fear. The fraction of testosterone that is biologically active (the portion not bound tightly to proteins in the blood) tends to fall earlier and more steeply than total testosterone.² That matters because bioavailable testosterone is the portion that actually does work in your tissues.

The machinery behind this decline involves the cells in the testes that produce testosterone, called Leydig cells, and the signaling chain that runs from the brain down to those cells. As you age, both the signaling and the cells themselves become less efficient.³ But the speed of that decline is not fixed. Comorbid conditions, body fat, medications, and genetics all bend the curve, meaning two men of the same age can have very different levels.¹ That variability is part of why researchers debate how much of “low T” in older men is truly aging versus accumulated lifestyle and health problems.

Body Fat

Excess body fat does not just correlate with lower testosterone; it actively drives levels down. Fat tissue contains an enzyme called aromatase that converts testosterone into estrogen.⁴ The more fat tissue you have, the more aromatase activity there is, and the more testosterone gets shunted into estrogen.⁵ This creates a vicious cycle: lower testosterone makes it easier to accumulate more fat, which further lowers testosterone.

Breaking that cycle through fat loss is one of the most reliable ways to raise testosterone without medication. The effect is proportional: men who lose a substantial amount of body fat tend to see meaningful increases, while modest weight loss produces modest hormonal improvement. This is also one reason why obesity can make age-related decline look worse than it otherwise would be. A lean 55-year-old and an obese 55-year-old are dealing with different hormonal realities, even though their calendars say the same thing.

Exercise

Physical activity raises testosterone, but the type and timing of exercise matter. During a single session of either strength training or endurance training, testosterone can spike substantially. One study in well-trained men found increases of about 27% during strength training and 37% during endurance training, though levels returned to baseline within two hours and stayed there.⁶ Those acute bumps are temporary.

The chronic picture looks different. A meta-analysis of training programs in younger populations found that resistance training produced a meaningful increase in resting testosterone concentrations over time, while endurance training alone did not.⁷ This does not mean cardio is useless for testosterone; losing body fat through any form of exercise helps via the aromatase mechanism described above. But if raising resting testosterone is the goal, lifting heavy things over weeks and months appears to have an edge over running alone.

There is a wrinkle here for people who overtrain. Very high volumes of endurance exercise, especially when combined with inadequate food intake, can suppress the hormonal axis. Marathon runners and ultraendurance athletes sometimes show lower resting testosterone than sedentary controls, which is the opposite of what you would expect from an active lifestyle. The dose makes the medicine.

Diet and Calorie Intake

What you eat and how much you eat both matter, but calorie balance has a larger effect than most individual nutrients. Severe calorie restriction lowers testosterone significantly, even in lean men. One study found that men practicing long-term caloric restriction had substantially lower total and free testosterone than men who maintained weight through exercise or ate a typical Western diet, and this held true even after accounting for body fat.⁸ A more extreme example comes from military personnel: U.S. Marines placed under short-term severe energy deficit saw their average testosterone drop from about 17.5 to about 9.8 nmol/L, and those whose levels fell the farthest lost the most lean mass.⁹

On the macronutrient side, dietary fat seems to play a role. A systematic review and meta-analysis of intervention studies found that low-fat diets modestly reduced total testosterone, free testosterone, and dihydrotestosterone compared to higher-fat diets.¹⁰ The effect sizes were small, but they were consistent across different measures. One observational study of middle-aged men found that higher saturated fat intake was associated with higher testosterone after adjusting for age and energy intake, but the association disappeared once other lifestyle factors were accounted for.¹¹ That result is a useful reminder: what looks like a fat-type effect in simple analyses may be tangled up with overall diet quality, body weight, and activity levels.

Micronutrients also play supporting roles. Zinc is the most studied in this context, and a systematic review concluded that zinc deficiency reduces testosterone while supplementation improves levels in deficient men.¹² The practical implication is straightforward: if you are zinc-deficient, fixing that deficiency helps. If you are already getting enough zinc, piling on more does not keep pushing testosterone higher. Vitamin D follows a similar pattern in the research, where correcting a deficiency matters more than supplementing beyond sufficiency.

Sleep

Testosterone follows a daily rhythm, peaking in the early morning and falling through the day. Sleep is when most of that overnight rebound happens, so disrupting sleep disrupts the cycle. A meta-analysis of the available research found that total sleep deprivation (staying awake for 24 hours or longer) significantly reduces testosterone, while partial sleep deprivation (getting less sleep than usual but still some) showed no statistically significant drop in the pooled data.¹³ The suppression from total sleep deprivation was consistent whether men stayed awake for 24 hours or 40 to 48 hours.

That finding might seem reassuring if you are just getting six hours instead of eight, but the meta-analysis was working with the specific experimental designs available, and many partial-deprivation studies were small. Chronic mild sleep restriction over weeks or months is harder to study in a controlled way, so the absence of a statistically significant effect in short-term partial deprivation studies does not mean long-term short sleep is harmless. What the evidence does say clearly is that pulling all-nighters tanks testosterone, and repeated total sleep deprivation is an easy-to-avoid hit.

Alcohol and Substance Use

Alcohol directly damages the Leydig cells that produce testosterone.¹⁴ The mechanism has been worked out in detail in laboratory studies: when alcohol is metabolized inside these cells, it disrupts the chemical environment they need to synthesize testosterone.¹⁵ Moderate drinking on occasion does not appear to produce lasting suppression in most men, but heavy or chronic drinking has well-documented effects on both testosterone production and fertility.

Anabolic steroids deserve a separate mention because their effect is paradoxical. Taking exogenous testosterone or its synthetic relatives floods the body with androgens, which feels like the opposite of a problem. But the brain’s signaling system responds by shutting down its own production signals, and the testes atrophy as a result. When the exogenous supply stops, the man is left with a suppressed hormonal axis that can take months or even years to recover.¹⁶ This is one of the most common and least discussed causes of low testosterone in younger men who present to clinics.

Chronic Illness and Metabolic Health

Diabetes, insulin resistance, and metabolic syndrome are strongly linked to lower testosterone. In men with both type 1 and type 2 diabetes, low testosterone tracks inversely with insulin resistance: as insulin resistance worsens, testosterone falls, and the relationship holds even after adjusting for body fat.¹⁷ This relationship runs in both directions. Low testosterone makes insulin resistance worse, and insulin resistance further suppresses testosterone, creating a feedback loop that is difficult to break without addressing both problems.¹⁸

Inflammation is another route through which illness suppresses testosterone. When the immune system mounts an inflammatory response, even a mild one triggered experimentally, testosterone drops within hours. Interventional research has shown that introducing a low dose of bacterial endotoxin into healthy lean men produces a transient inflammatory state followed by a clear decline in testosterone, without any change in the brain hormones that normally drive production, suggesting the effect is directly on the testes.¹⁹ Chronic inflammatory conditions, ranging from autoimmune diseases to obesity-related low-grade inflammation, could therefore chip away at testosterone over time.

Stress and Social Context

The relationship between psychological stress and testosterone is more nuanced than “stress lowers T.” Acute competitive situations actually raise testosterone. In both men and women, match competition in soccer substantially increased testosterone levels.²⁰ And the testosterone response to competition is not random. Among men, the change in testosterone during play was positively correlated with teammate ratings of their playing ability and with their own sense of social connection to the team.

The outcome of a competition also matters. Research on social competition found that clear wins maintained or slightly raised testosterone, while narrow wins paradoxically lowered it in men with high baseline cortisol.²¹ That finding suggests the hormonal response to winning or losing depends on context and the individual’s stress state. The popular narrative that “winning raises T and losing drops it” is an oversimplification.

Chronic psychological stress is a different animal. Sustained elevation of cortisol, the primary stress hormone, can suppress the reproductive axis. The brain essentially deprioritizes reproduction when it perceives ongoing threat. This is hard to quantify in humans because chronic stress rarely exists in isolation; stressed people often sleep less, drink more, eat poorly, and exercise less, each of which independently affects testosterone.

Environmental Chemicals

Endocrine-disrupting chemicals are synthetic compounds that interfere with hormonal signaling. Bisphenol A (found in some plastics and can linings), phthalates (found in personal-care products and flexible plastics), and alkylphenols are the most studied, and all are widespread in the environment.²² Laboratory studies have demonstrated that these chemicals can harm the male reproductive system, and population-level evidence associates higher exposure with disrupted sex-hormone levels.²³

The challenge is quantifying how much they matter in everyday life. Occupational exposures, such as workers in plastics manufacturing, can be high enough to produce measurable hormonal changes. For the average person, the contribution of endocrine disruptors to testosterone levels is probably small compared to factors like body fat, sleep, and metabolic health, but it is not zero. Reducing unnecessary exposure through steps like avoiding microwaving food in plastic containers and choosing fragrance-free products is a low-cost precaution, even if the individual benefit is hard to measure.

Genetics Set the Baseline

All of the factors above operate against a genetic backdrop that varies enormously between individuals. Large genetic analyses have identified many locations in the genome that influence testosterone levels, and the genetic determinants are largely sex-specific; the genes that predict testosterone in men have almost no overlap with those that predict it in women.²⁴ Your genes do not just set a starting level, either. They also influence how sensitive your body is to the testosterone it produces. Variation in the androgen receptor gene, for example, modifies the relationship between testosterone and how energetic a man reports feeling, with some receptor variants amplifying the effect and others muting it.²⁵

This helps explain why two men with the same testosterone number on a lab report can feel completely different. One may have receptor variants that make his tissues highly responsive, so a moderate level is plenty. Another may have variants that blunt the signal, making the same number functionally insufficient. Genetics is the factor you cannot change, but knowing it exists helps you interpret everything else: the same lifestyle change will not produce identical hormonal results in two different people.

Seasonal and Daily Rhythms

Testosterone is not static even within a single day. Levels are highest in the early morning and decline through the afternoon and evening. This daily rhythm is robust in younger men but flattens with age, to the point that men 45 and older do not show as much morning-to-afternoon variation and can be reliably tested later in the day.²⁶

There is also a seasonal rhythm, though the exact pattern depends on the population studied. A large Italian cohort showed a peak in total and bioavailable testosterone from August through October, with a low point in March.²⁷ A Norwegian study found the lowest total testosterone in summer months with the longest daylight hours and highest temperatures, but free testosterone peaked in winter.²⁸ The discrepancy may reflect differences in latitude, climate, and lifestyle between Mediterranean and Arctic populations. What both studies agree on is that the seasonal swing is real, even if modest, and could meaningfully affect a single lab test. If your testosterone was borderline-low on a March blood draw, it might be worth retesting in autumn before drawing conclusions.

When Measurement Itself Muddles the Picture

Many men receive a low testosterone diagnosis based on a single blood test, but a single result is a snapshot of a moving target. Besides the daily and seasonal rhythms, testosterone fluctuates with recent meals, recent exercise, acute illness, medication use, and even how much sleep you got the night before your draw. Guidelines generally recommend confirming a low result with a second test on a different morning, and most clinicians test in the early morning to catch the daily peak.

For men over 45, the morning-only rule is less important because the daily rhythm is less pronounced.²⁶ But regardless of age, a single number without context is easy to misinterpret. A man who was up all night, just finished a hard workout, or is fighting a cold may get a reading that looks low but does not reflect his typical hormonal state. The brain orchestrates testosterone production through a tightly regulated feedback loop, coordinating signals from the hypothalamus, pituitary gland, and testes.²⁹ Any transient disruption at any level of that loop can temporarily alter the number on a lab report. What matters clinically is the pattern over time and whether symptoms match, not a single snapshot.