Sixteen-eight fasting drops testosterone. That is not a scare tactic, not a supplement ad, not a rumor that spiraled on Reddit. A controlled trial on resistance-trained men tracked it over eight weeks: total testosterone dropped by roughly 21% over those eight weeks. A statistically significant decline — not a measurement error, not a fluke.
A fifth of their testosterone disappeared over those eight weeks. Every gram of their muscle stayed.
Does 16:8 Fasting Reduce Testosterone in Men?
Sixteen-eight fasting reduced total testosterone by approximately 21% over eight weeks in resistance-trained men, a statistically significant decline. Despite this hormonal shift, the fasting group maintained all of their muscle mass and gained strength at the same rate as the control group, while losing six times more body fat.
— Moro et al. 2016 · Journal of Translational Medicine · n=34
Fat-free mass held steady. Bench press climbed by three kilograms. Leg press climbed by seven. The hormone that every gym conversation treats as the direct line between blood chemistry and muscle tissue dropped by a fifth, and the tissue never flinched.
While all of that was happening, the fasting group lost six times more body fat than the group eating the same calories on a normal schedule. Sixteen percent of their fat mass gone, compared to less than three percent in the control group. The testosterone decline did not arrive as damage. It arrived alongside the study’s strongest outcome.
THE BLOOD TEST
Testosterone: −21%
IGF-1: −13%
THE BODY
Fat mass: −16.4%
Muscle: maintained
Bench press: +3 kg
The explanation lives in one number: 486 ng/dL. That is where testosterone landed after the drop. Lower than the starting point, yes, but well inside the range where muscle tissue has everything it needs to hold on and grow. A hormone shifting within its normal range is a different event from a hormone collapsing below what the body requires. Plenty of testosterone to work with. Simply less than before.
The researchers themselves pointed this out in their discussion. Anabolic hormones went down. Body composition and strength refused to follow. What the lab measured and what the gym measured told two separate stories about the same eight weeks.
These were 34 experienced lifters, all men, all in their late twenties, all eating enough protein to support muscle on its own. Whether the same paradox holds for beginners, for women, or across years instead of weeks is a question nobody has answered yet with this kind of control.
What a blood test tracks and what a training log tracks turned out to be answering different questions entirely. If you have been weighing whether the timing of your meals matters more than the total, the answer from this trial is that both moved the body, but they moved different dials.
The testosterone finding is one of twelve from the same study. The full eight-week picture covers inflammation, insulin, thyroid hormones, and every body composition marker the researchers could measure, and not all of them shifted in the direction you would expect.