16:8 Time-Restricted Eating Reduces Testosterone by 17% in Men — But Muscle Is Preserved: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

Researchers wanted to understand what happens to testosterone, muscle mass, inflammation, and cardiometabolic health when men combine 16:8 time-restricted eating with resistance training over a full year. This is one of the longest-running RCTs ever conducted on intermittent fasting in men, extending the same research group's earlier 8-week work to 12 months — enough time to see whether hormone changes are transient or sustained. For more on fasting and muscle, see our article on whether intermittent fasting destroys muscle.

Who Was Studied

Participant profile: Healthy adult males, resistance-trained with at least 5 years of training experience, lean body mass (BMI in normal range), no metabolic conditions, not taking any hormonal or performance-enhancing medication.

How the 16:8 TRE protocol worked in this study: All food was consumed within an 8-hour window each day, with meals at approximately 1pm, 4pm, and 8pm. Total daily calories were matched to the normal diet group — this was not a calorie-restriction study. The fasting window ran from approximately 8pm to 12pm the following day (16 hours).

The resistance training protocol: Three sessions per week for both groups, consistent in volume and intensity throughout the 12 months. Neither group was permitted to change training habits outside of the study protocol.

What the Researchers Found

Testosterone

This is the headline finding: 16:8 time-restricted eating reduced testosterone by nearly 17% over 12 months compared to normal diet controls in lean, healthy, resistance-trained men.

Free testosterone also decreased in the TRE group. Importantly, SHBG (sex hormone-binding globulin) did not change significantly, which suggests the reduction was driven by changes in total testosterone production rather than shifts in binding protein concentrations.

IGF-1 (Insulin-like Growth Factor 1)

IGF-1 — a growth hormone that promotes muscle growth — was also significantly lower in the TRE group compared to the ND group at 12 months. This is consistent with the body's broad downregulation of anabolic signalling during prolonged periods of reduced feeding frequency.

Inflammatory Markers — Dramatically Improved

These are three of the primary drivers of systemic inflammation and are associated with cardiovascular disease, metabolic syndrome, and accelerated ageing. All three improved significantly with 12 months of TRE.

Cardiometabolic Markers

• Fasting glucose: significantly improved
• Fasting insulin: significantly improved
• HOMA-IR (insulin resistance index): significantly improved
• Total cholesterol: significantly improved
• LDL cholesterol: significantly improved
• HDL cholesterol: significantly improved
• Fat mass: significantly reduced

What Did Not Change

• Fat-free mass (muscle mass): no significant difference between TRE and ND groups — muscle was fully preserved despite the testosterone reduction
• Muscular strength: no significant difference — training performance was maintained
• SHBG (sex hormone-binding globulin): no significant change

What the Researchers Concluded

The researchers concluded that 12 months of 16:8 time-restricted eating combined with resistance training produces a significant reduction in testosterone and IGF-1 in lean resistance-trained men, while simultaneously improving inflammatory markers, insulin sensitivity, lipid profiles, and fat mass — without any loss of muscle mass or strength.

They noted the testosterone reduction may reflect a "hormetic adaptation" — the body reducing anabolic hormone production in response to reduced feeding frequency — rather than a sign of harm, given that body composition and performance were unaffected.

What This Means If You Fast

• The testosterone drop is real but context matters. A 17% reduction in testosterone sounds alarming, but this group of men preserved full muscle mass and strength over 12 months. The functional outcomes were unaffected by the hormonal change.
• Lean, trained men are the most affected. The reductions seen here were in lean, resistance-trained males. Research on overweight men (including a 2026 RCT on 4:3 IF during resistance training) found no significant testosterone change, suggesting body composition and metabolic status modulate the response.
• The inflammation benefits are striking. Reducing IL-6, IL-1β, and TNF-α simultaneously over 12 months is a meaningful health outcome. Chronic low-grade inflammation is a driver of most major diseases. Fasting appears to suppress it significantly even in already-healthy men.
• Muscle mass does not depend solely on testosterone. This study is important evidence that the relationship between testosterone and muscle mass is not as direct as commonly assumed. Other anabolic drivers — adequate protein, progressive resistance training, sleep, and growth hormone — appear to compensate. Our article on building muscle while intermittent fasting covers this in more detail.
• The 8-hour eating window is the key variable. Both groups trained equally and ate similar calories. The only difference was timing. This makes the study a clean test of feeding window effects independent of calories or exercise.
• Consider your priorities. For men whose primary goal is maximal testosterone optimisation (athletes, bodybuilders in specific phases), this data is worth discussing with a sports medicine physician. For most men whose goals are fat loss, longevity, inflammation reduction, and metabolic health, the tradeoff appears strongly favourable.

Study Limitations

• Very small sample size (n=20 total, n=10 per group) — results should be interpreted cautiously and replicated in larger trials
• Males only — no female data; findings should not be applied to women
• Lean, resistance-trained men — results may not generalise to sedentary, overweight, or metabolically unhealthy men
• Self-reported dietary adherence — actual eating window compliance was not independently verified
• Single facility — all participants from the same institution in Italy, limiting generalisability
• No measurement of bioavailable testosterone — only total and free testosterone were reported
• Possible confounding from training adaptations — 12 months of consistent resistance training itself affects hormone levels; the interaction with TRE is difficult to fully isolate

Source

Moro T, Tinsley G, Pacelli FQ, Marcolin G, Bianco A, Paoli A. Twelve Months of Time-restricted Eating and Resistance Training Improve Inflammatory Markers and Cardiometabolic Risk Factors. Medicine & Science in Sports & Exercise. 2021;53(12):2577–2585. doi: 10.1249/MSS.0000000000002738. PMID: 34649266

Frequently Asked Questions

Does intermittent fasting lower testosterone in men?

Based on this 12-month RCT, yes — 16:8 time-restricted eating reduced total and free testosterone by approximately 17% in lean, resistance-trained men after 12 months. However, muscle mass and strength were fully preserved despite this reduction.

How much did testosterone drop with 16:8 fasting in this study?

Testosterone fell by 3.12% at 2 months and by 16.81% at 12 months in the TRE group, compared to no significant change in the normal diet group. Both changes were statistically significant (p = 0.01 at 2 months, p = 0.0004 at 12 months).

Did the testosterone reduction cause muscle loss?

No. Fat-free mass (muscle mass) was preserved in the TRE group and was not significantly different from the normal diet group after 12 months. Muscular strength was also maintained. This is the key clinical finding — the testosterone reduction did not translate into functional decline.

Does SHBG change with intermittent fasting?

In this study, SHBG did not change significantly. The reduction in total testosterone was driven by decreased production rather than shifts in binding protein levels.

Should men be concerned about doing intermittent fasting long-term?

The concern about testosterone is understandable, but this study suggests the functional consequences may be minimal for men whose goals are health and body composition rather than elite performance. The substantial improvements in inflammation, insulin sensitivity, and lipid profile over 12 months represent significant health gains. Men on testosterone therapy or with hypogonadism should consult their endocrinologist before starting a fasting protocol.

Related Research and Articles

• Does intermittent fasting destroy muscle? Myth vs. fact
• Can you build muscle while intermittent fasting?
• Does fasting boost human growth hormone (HGH)?
• Intermittent fasting and inflammation: the research explained
• Should you work out in a fasted state?
• When is the best time to exercise during intermittent fasting?
• Intermittent fasting benefits: the complete science-backed guide

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