16:8 Worked for Me. 18:6 Broke Me. Here's the Data.

i did 16:8 for eleven months. lost body fat, maintained muscle, felt sharp in the mornings, and had data i was happy with. then i pushed to 18:6 because the content i was reading suggested more fasting hours meant more benefit. within eight weeks i had lost four pounds of muscle, my morning cortisol had spiked measurably on two consecutive blood panels, and i was waking at 3am three nights a week.

the extra two hours of fasting was not a small change. it was a different protocol with different physiological effects, and i had treated it like a simple extension.

this is the full story, the data, and the specific protocol i came back to and have held for the past year.

The Mechanism Most Fasting Content Skips

Insulin is suppressed when you are not eating. During the fasted window, your body has access to stored fat for fuel and cellular repair processes including autophagy are upregulated. That part you have probably heard.

Here is what most guides omit.

Cortisol is highest in the first 90 minutes after waking. This is called the cortisol awakening response and it is a normal part of waking up. It mobilizes energy, sharpens cognition, and prepares your system for the day. The problem with very long fasting windows, anything approaching 18 or more hours, is that cortisol stays elevated longer in the morning because your body is managing blood glucose stability under stress. The cortisol awakening response has nothing to pull it down.

You feel alert. You feel focused. You feel like the fast is working. But your system is in a mild sympathetic state that, over weeks, starts to look like chronic low-grade stress on every biomarker that tracks it.

The threshold at which this becomes a problem is individual. For me it was the eighteenth hour. The sixteen-hour window avoids this for most healthy adults because the cortisol awakening response has resolved before you are pushing into metabolic stress territory.

My 11-Month 16:8 Data

Eleven months of consistent 16:8. Last meal by 7pm, first meal at 11am. Training at 8am fasted.

Starting body composition from a DEXA scan: 182 pounds, 18 percent body fat. After eleven months: 177 pounds, 14.5 percent body fat. Lean mass change: gained one pound of lean mass while losing four and a half pounds of fat. Net weight down five pounds. Composition significantly better.

Morning cortisol at month-six blood panel: 12.4 micrograms per deciliter. Middle of the normal range for a healthy adult. HRV morning average: 51. Sleep quality on Garmin: 81 average over eleven months. The best numbers I had produced in three years of tracking.

The fasted training works for strength maintenance if your protein target for the day is adequate. I was eating 160 to 170 grams of protein between 11am and 7pm. If that number drops below one gram per pound of bodyweight in an eight-hour window, muscle maintenance is harder. The window is not the problem. The total protein is what people underestimate.

What Happened at 18:6

I pushed to 18:6 in month twelve. Last meal still at 7pm. First meal pushed to 1pm. Training stayed at 8am, now five hours into the fast.

By week four: morning workouts felt harder. Not from fatigue. Something more like irritability and reduced focus. I attributed it to a difficult work period and kept going.

By week eight: blood panel. Morning cortisol: 19.8 micrograms per deciliter. Top of the standard normal range. Not clinically elevated but a 60 percent increase from my six-month panel on 16:8. HRV morning average had dropped from 51 to 43. I was waking at 3am three to four nights a week, a classic sign of nocturnal cortisol elevation.

DEXA at month eight of the new protocol: lean mass had decreased by four pounds. Body fat had dropped another percentage point. To people reading that from the outside it looks like a win. It was not. I had lost muscle, which is the one outcome time-restricted eating is supposed to preserve.

The two extra fasting hours had moved me from a fat-loss protocol into a mild catabolic state. I stopped the 18:6 experiment at week ten.

The Protocol That Holds

I came back to 16:8 and added two specific adjustments based on what the 18:6 data showed.

Adjustment one: I moved training to 9am instead of 8am to shorten the fasted training window to two hours from wake. The cortisol awakening response has largely resolved by the ninety-minute mark for most people.

Adjustment two: I added four grams of leucine on training days at 10am, one hour before breaking the fast. Leucine is the branched-chain amino acid with the most direct mechanistic role in muscle protein synthesis signaling. It does not meaningfully break the fasted state for metabolic purposes while providing the anabolic trigger that prevents the muscle-catabolism window from widening. The Thorne Leucine I use is essentially flavorless in water.

Current data: lean mass stable at the post-16:8 high. Morning cortisol: 13.1 on last panel. HRV: 49. Sleep recovering toward 80 average. I have held this protocol for fourteen months since coming back to it.

The Direct Recommendation

If you are new to time-restricted eating: 16:8 is the starting protocol. Last meal at 7pm. First meal at 11am. Morning training is fine fasted. If you strength train, prioritize one gram of protein per pound of bodyweight in your eating window before anything else.

If you are already on 16:8 and considering extending: I would not push past 17 hours without a blood panel baseline. Morning cortisol and HRV are the two markers worth tracking. If cortisol goes above 18 on a fasted-morning panel and your HRV drops 5 or more points, the extended window is not the right experiment for you.

Nothing I'd link unless I'd buy it with my own money tomorrow.

Want the full fasting protocol with blood panel timing and specific lab markers? The Stack: twentystack.substack.com/p/the-stack

Tell me your training schedule, whether you are trying to lose fat or maintain muscle, and what time you wake up. I will give you the specific eating window I would run in your situation. The window timing relative to training changes the answer more than most guides acknowledge.