I knocked a boiled egg against the table, and the crack cut through the silence.
People turned and laughed. We were staying at a hotel for a week of leadership training, a program I respected because of the commitment it required. Both the morning session and my eating window started at 8:00.
I took the eggs from the breakfast buffet because I was hungry (and pragmatic). I told everyone it was my time to eat, don’t mind me, and then started breakfast in front of them.
I still understand why that rule is appealing. With 16:8, you eat during an 8-hour window and fast for the other 16. Once you pick your hours, you have fewer decisions to make. That can help if you want to eat less.
The problem is expecting that one rule to deliver all the benefits you’ve heard about fasting.
People have asked for my opinion twice in the last two weeks. My answer always starts with their goal. Someone who wants to lose weight needs a different approach than someone focused on living longer. At 47, with an unstable shoulder and a back that acts up if I stop training, I have my own priorities.
Give the eating window a job
If a daily eating window helps you eat fewer calories without worrying about food all morning, it’s a useful tool. You can see if it’s working by watching your eating habits and your weight.
You can also lose weight without it. In a year-long trial, Liu and colleagues gave 139 adults with obesity the same calorie target. Those who also ate between 8 a.m. and 4 p.m. lost 8.0 kg; those cutting calories without the window lost 6.3 kg. The researchers’ statistical analysis estimated an additional 1.8 kg of weight loss with the window, though the difference wasn’t statistically significant (P = 0.11) [1].
That P value needs some explanation. If the eating window adds no real benefit, a difference this big could still show up by chance in about 11 out of 100 similar studies. Researchers usually use 5 out of 100 as their cutoff. In this case, they couldn’t be sure the extra benefit wasn’t just due to chance.
Maruthur and colleagues tested the timing more directly by providing the same calories to both groups for 12 weeks. An early 10-hour window produced no detectable extra weight loss or improvement in blood-sugar control compared with usual eating [2]. The 41 participants had obesity and prediabetes or diet-controlled diabetes, so this answers a question about that schedule in that population.
As a doctor, I think a daily eating window is helpful if it makes it easier to manage how much you eat. I wouldn’t add it to a diet that’s already working, just to get extra weight loss from changing meal times.
There is a stronger weight-loss result for a more demanding schedule. In 4:3 fasting, you eat normally on 4 days of the week and cut your usual calories by about 80% on the other 3, with at least one normal eating day between them. Catenacci and colleagues assigned 165 adults with overweight or obesity to that approach or daily calorie restriction. After a year, those assigned to 4:3 had lost 2.89 kg more [3].
The estimate isn’t exact: the 95% confidence interval, which shows the range of results that fit the data, goes from 0.14 to 5.65 kg of extra weight loss. Both groups had the same weekly calorie goal, coaching, and activity targets. In reality, the fasting group ended up eating fewer calories, which might explain why they lost more weight.
It comes down to “it depends.” If your goal is weight loss and that approach fits your life, go for it. Skipping breakfast until noon is a different kind of commitment and needs separate evidence.
Alternate-day fasting has also been studied on its own. In a review of 99 trials, it was the only fasting method that showed a clear advantage over daily calorie restriction, with an average difference of 1.29 kg. However, this advantage disappeared in the 17 trials that lasted at least 24 weeks [4].
I came back to fasting for a different reason
I already knew that eating less could help me lose weight. Later, I became interested in autophagy, the process where cells break down and reuse old or damaged parts. Learning about autophagy and longevity led me back to fasting with a 16:8 schedule.
That’s a good reason to put up with hunger. You hope something helpful is happening in your body while you wait to eat.
There’s a lot of science behind autophagy. Yoshinori Ohsumi won the 2016 Nobel Prize in Physiology or Medicine for discovering how autophagy works in baker’s yeast [5]. Figuring out how many hours of fasting trigger this process in people is a different research challenge.
Researchers can measure the process as it moves through human white blood cells. They call this autophagic flux. In 2017, Pietrocola and colleagues used a laboratory test that compared cells with and without a substance that blocks their breakdown machinery. Among volunteers fasting for up to 4 days, one type of white blood cell showed signs of activated autophagy [6].
The biggest nutrition trial using this method followed 121 adults with obesity for six months. They fasted for 20 hours on three days each week. An analysis after data collection found a difference compared to standard care, but the fasting group’s improvement wasn’t statistically significant. Some of the difference may have come from the comparison group getting worse [7].
Krista Varady, a fasting researcher at the University of Illinois Chicago, puts the uncertainty about timing plainly:
“We just don’t know the hours of fasting; we don’t know the level of nutrient deprivation with carbs or protein that’s required to induce it” [8].
The fasting app Zero claims autophagy increases after 16 hours. Fastic says intermittent fasting activates autophagy and can help you stay in your prime years longer. Both apps sell subscriptions [9,10].
Fasting might trigger autophagy, but no study has shown exactly how many hours are needed. As a doctor, I wouldn’t suggest using an eating window just for autophagy.
My body needs something different now
When I was 16, I would go even without water before wrestling weigh-ins.
By that point, I had spent weeks cutting calories. In the last days, I ate very little, sometimes just broth, sometimes nothing at all. On the final day, I used the sauna to sweat off the last kilogram.
The method brought the number on the scale down. It also belongs to a dangerous part of wrestling’s history. In late 1997, 3 previously healthy college wrestlers in different states died while rapidly cutting weight. All had restricted food and fluid and tried to maximize sweating; they developed fatal overheating [11]. That combination of dehydration and heat is distinct from restricting the hours you eat. Obviously.
Now, I need enough strength to keep training despite the injuries I’ve picked up. My right shoulder has dislocated several times, even after surgery, and is still unstable. I believe keeping the muscles around it strong helps keep it in place.
My back is another reason I keep lifting weights. When I go on holiday and skip the gym for a couple of weeks, I feel my body get weaker, notice more back pain and twitching, and then feel better once I get back to regular weight training.
Some of that might be mental. This is just my own observation. I’ve never actually measured muscle loss from a training pause or fasting.
Keeping my strength up is now more important to me than squeezing my meals into a shorter window.
When you read a claim about fasting and muscle, check what the researchers measured. A body scan’s fat-free mass includes water and other tissue as well as muscle. Moro and colleagues followed trained men for a year, with 10 in each group. The normal-diet group gained fat-free mass; the researchers couldn’t distinguish the 16:8 group’s change from chance [12].
The results differed between the groups, and the window group reported eating less. These findings are worth noting, but they don’t prove that fasting caused the men to lose muscle.
There’s also evidence that you can gain muscle while eating within a window. In an eight-week trial, women who lifted weights and ate about 1.6 grams of protein per kilogram of body weight each day gained 2% to 3% fat-free mass in all groups, with no clear difference between them [13]. It was a small study: only 24 of 40 women qualified for that analysis, and a supplement company funded it.
To me, these findings show it’s important to focus on what you eat during your window. My clinical advice is to make sure you get enough training and protein before you start limiting your meal times. The way I handle protein depends on how much I’m training, and I will write a refresher on protein soon.
In my Upward ARC framework, nutrition is part of the Activate step. I judge it partly by whether it lets me keep training and stay strong with the body I have.
Try this today
For weight loss, see if a daily eating window helps you eat less and fits your routine. Think about the stricter 4:3 schedule separately.
If you’re only keeping an eating window to reach a certain hour for autophagy, it’s worth rethinking that choice.
If your goal is to stay strong, make sure you have enough food and time for training. A shorter eating window should fit around those needs.
You still have to get through your day
When I used an eating window, I often crashed in the afternoon. Having a protein-rich breakfast after training worked better for me.
A review of 71 studies with 3,484 healthy adults found no clear difference in thinking skills between fasting and eating. Most fasts lasted 12 hours, but longer fasts showed small declines [14]. Your own workday is a good way to see if the schedule helps you focus or just leaves you hungry and distracted.
In 17 trials, fasting and daily calorie restriction didn’t show a clear difference in hunger or fullness, though all the studies had some bias concerns [15]. If you feel better using an eating window, that should count in your decision too.
My advice is to stop forcing yourself to stick to an eating window if you spend your mornings thinking about food, crash in the afternoon, or keep missing dinners you want to attend. I’d count business dinners just as much as family ones.
I usually finish dinner two to three hours before bed. There’s some evidence for eating earlier in the evening: a 7.5-week trial in 39 adults with overweight or obesity found that ending meals at least three hours before sleep improved the overnight drop in the lower blood pressure number. It didn’t help with the other main outcome: how well the body responds to insulin, the hormone that controls blood sugar [16]. Both groups dimmed the lights three hours before bed, so the only difference was meal timing. This doesn’t mean there’s a universal dinner deadline.
Long-term health effects are harder to pin down. A 2025 study followed 19,831 adults for about 8 years and found that eating within less than 8 hours was linked to a higher risk of death from heart and circulatory disease compared to eating over 12 to 14 hours [17]. But eating hours were estimated from just two days of food recall, so there’s a lot of uncertainty about people’s real habits and whether the short window actually caused the higher risk.
Another analysis of the same survey and death records found the lowest risk of death from any cause was around 11 to 12 hours [18]. The authors used different rules and statistical methods. Both papers used overlapping data, and neither one proves what a safe eating window is.
In a newsletter in May 2025, I was too sure about preserving lean mass and left out the mortality finding that was already in my research notes. So here’s the full picture.
I don’t need a fixed eating window to manage my calories anymore. On training mornings, I drink my usual warm water, work out, and shower before breakfast. I usually have eggs or muesli with Greek yogurt, plus a big protein shake. On other mornings, I might write for about two hours before eating.
Reply and let me know what you hoped fasting would do for you. I read every message.
Stay healthy.
Andre
PS: If you know someone trying to pick a fasting schedule, feel free to share this with them. This is how this newsletter grows, and it’s the only way I want it to.
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References
[1] Liu, D., Huang, Y., Huang, C., Yang, S., Wei, X., Zhang, P., Guo, D., Lin, J., Xu, B., Li, C., He, H., He, J., Liu, S., Shi, L., Xue, Y., Zhang, H. (2022). Calorie restriction with or without time-restricted eating in weight loss. New England Journal of Medicine, 386(16), 1495-1504. https://doi.org/10.1056/NEJMoa2114833
[2] Maruthur, N. M., et al. (2024). Effect of isocaloric, time-restricted eating on body weight in adults with obesity: A randomized controlled trial. Annals of Internal Medicine, 177(5), 549-558. https://doi.org/10.7326/M23-3132
[3] Catenacci, V. A., et al. (2025). The Effect of 4:3 Intermittent Fasting on Weight Loss at 12 Months. Annals of Internal Medicine, 178(5), 634-644. https://doi.org/10.7326/ANNALS-24-01631
[4] Semnani-Azad, Z., et al. (2025). Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: Systematic review and network meta-analysis of randomized clinical trials. BMJ, 389, e082007. https://doi.org/10.1136/bmj-2024-082007
[5] Nobel Assembly at Karolinska Institutet. (2016, October 3). The Nobel Prize in Physiology or Medicine 2016. https://www.nobelprize.org/prizes/medicine/2016/press-release/
[6] Pietrocola, F., et al. (2017). Autophagy, 13(3), 567-578.
[7] Bensalem, J., et al. (2025). The Journal of Physiology, 603, 3019-3032. https://doi.org/10.1113/JP287938
[8] Inverse. (2021, updated 2024). Stop trying to trigger autophagy. https://www.inverse.com/mind-body/stop-trying-to-trigger-autophagy
[9] Zero. (n.d.). Fast your way to autophagy. Accessed September 5, 2026. https://zerolongevity.com/blog/fast-your-way-to-autophagy/
[10] Fastic. (n.d.). Intermittent fasting app. Accessed September 5, 2026. https://fastic.com/en
[11] Centers for Disease Control and Prevention. (1998). Hyperthermia and dehydration-related deaths associated with intentional rapid weight loss in three collegiate wrestlers, North Carolina, Wisconsin, and Michigan, November-December 1997. MMWR Morbidity and Mortality Weekly Report, 47(6), 105-108.
[12] Moro, T., et al. (2021). Twelve months of time-restricted eating and resistance training improves inflammatory markers and cardiometabolic risk factors. Medicine & Science in Sports & Exercise. https://pmc.ncbi.nlm.nih.gov/articles/PMC10115489/
[13] Tinsley, G. M., et al. (2019). Time-restricted feeding plus resistance training in active females: A randomized trial. American Journal of Clinical Nutrition, 110(3), 628-640. https://doi.org/10.1093/ajcn/nqz126
[14] Bamberg, C., & Moreau, D. (2025). Psychological Bulletin, 151(9). https://doi.org/10.1037/bul0000492
[15] Elsworth, R. L., et al. (2023). The effect of intermittent fasting on appetite: A systematic review and meta-analysis. Nutrients, 15(11), 2604. https://doi.org/10.3390/nu15112604
[16] Grimaldi, D., et al. (2026). Arteriosclerosis, Thrombosis, and Vascular Biology, 46(4), e323355. https://pubmed.ncbi.nlm.nih.gov/41674465/
[17] Chen, M., et al. (2025). Diabetes & Metabolic Syndrome, 19(7), 103278. https://doi.org/10.1016/j.dsx.2025.103278
[18] Mao, Z., et al. (2025). Aging Cell, 24(11), e70230. https://doi.org/10.1111/acel.70230