Impact of Time-Restricted Eating on Testosterone Levels
Time-restricted eating (TRE) can decrease testosterone levels, particularly when implemented with high protein intake or in an 8-hour eating window, as demonstrated in multiple studies with elite athletes and resistance-trained males.
Mechanisms and Evidence
Time-restricted eating affects testosterone levels through several mechanisms:
Hormonal changes: TRE with an 8-hour feeding window has been shown to significantly decrease both total and free testosterone levels in resistance-trained males 1.
Protein intake interaction: High-protein (≥35%), low-carbohydrate diets combined with TRE can cause a large decrease in resting total testosterone (approximately 5.23 nmol/L) 2.
Athletic populations: Elite cyclists following TRE with an 8-hour feeding window experienced significant decreases in free testosterone and IGF-1 levels despite improvements in body composition 3.
Factors Affecting Testosterone Response to TRE
The testosterone response to TRE appears to depend on several factors:
Body composition: The effect of calorie restriction on testosterone varies based on BMI:
- In overweight/obese men: Calorie restriction may increase total testosterone
- In normal-weight men: Calorie restriction tends to decrease total testosterone 4
Feeding window timing: Early time-restricted eating (eating earlier in the day) appears more beneficial for metabolic parameters than late eating windows 5.
Duration of implementation: Short-term effects may differ from long-term adaptations, with most studies showing more pronounced hormonal changes in the initial weeks 2.
Clinical Considerations
When considering TRE for patients concerned about testosterone levels:
Fasting state measurements: Testosterone should be measured in the morning while fasting, as both glucose loads and mixed meals can transiently but significantly lower testosterone levels (mean drop of 100-123 ng/dL) 6.
Monitoring recommendations: For men implementing TRE, especially those with borderline low testosterone, regular monitoring of testosterone levels is advisable.
Risk assessment: Men with pre-existing testosterone deficiency should be counseled about the potential for further decreases in testosterone with TRE.
Balancing Benefits and Risks
Despite potential testosterone decreases, TRE offers several benefits:
Body composition: TRE can reduce body fat percentage while maintaining fat-free mass 1, 3.
Metabolic health: Early time-restricted eating improves insulin sensitivity, blood pressure, and can promote weight loss 5.
Inflammatory markers: TRE may reduce inflammation markers and have protective effects on some components of the immune system, as shown by decreased neutrophils-to-lymphocytes ratio 3.
Practical Implementation
For patients interested in TRE but concerned about testosterone:
- Consider a more moderate approach with a 10-hour eating window rather than 8 hours
- Focus on consuming adequate calories within the eating window
- Moderate protein intake (avoid very high protein diets)
- Schedule testosterone testing in the fasting state, in the morning
- Consider lifestyle modifications that support testosterone production, as recommended by the AUA guideline 7
Conclusion
While TRE offers metabolic benefits, the evidence suggests it may lower testosterone levels, particularly in athletic populations and when combined with high protein intake. This effect should be considered when recommending TRE to men, especially those with existing or borderline testosterone deficiency.