If you’re on a 16:8 intermittent fasting schedule and ravenous, it may seem counterintuitive to push your fast a little longer. But this can be what moves you from sugar burning to fat burning and gets you to the point where you’re burning stored fat and experiencing less hunger throughout the day.
The 16:8 schedule is so popular that 16:8 has almost become shorthand for intermittent fasting itself. Sixteen hours of fasting with an 8-hour eating window is a very doable schedule and a great starting point if you’re new to fasting and just learning how, or you have a high metabolism or little weight to lose. But it’s also the schedule I see referenced the most by people talking about how IF didn’t work for them. They didn’t lose any weight — or they even gained weight — on 16:8. And while I can’t armchair diagnose that the 16:8 schedule is why they didn’t lose weight like they’d hoped, I can see some shortcomings for those trying to lose weight.
If you’re struggling with a 16:8 IF schedule or if you’d like to explore why a shorter window may be more effective in reaching your goals, read on.
8 hours is a long eating window
A few years ago, I tried intermittent fasting with a long eating window. I saw no immediate difference in my weight or health. After a few weeks, I decided not to bother with it anymore.
Think about it. Eight hours is a full workday or a full night’s sleep worth of eating time. This can really trip you up if you combine this very long feasting window with intermittent fasting’s biggest selling point, eat whatever you want in your window.
I didn’t see results and dropped out, which was treating IF like a diet, not a lifestyle. I was expecting it to work like a weight loss diet: A short-lived period of discomfort and restriction that’s rewarded by rapid weight loss. I didn’t give it the time it needed to make a difference, and I didn’t attempt to tweak it until it worked for me. That was a mistake.
I absolutely believe there are people who’ve earnestly tried this lifestyle to no avail (because everyone is different, right?). But the common thread I’ve seen in so many but it didn’t work for me stories is the 16:8 schedule coupled with the mantra I can eat whatever I want. Throw in not being acclimated to fasting yet and still getting ravenous during the fast, and the 16:8 may be kryptonite to your weight loss goals.
If you have an 8-hour feasting window and break your fast around 11:00 in the morning, eat lunch, then snack in the afternoon, then eat dinner and dessert before finally closing your window at 7:00 in the evening, is it really all that surprising when excess weight doesn’t budge?
You hang onto glycogen longer
You don’t start reaching ketosis easily and regularly until you’ve burned through your liver’s glycogen stores. When that point is reached is different for everybody, but you get there by fasting regularly for shorter periods like 16:8 for several days or a few weeks. These stores don’t get fully replenished every day, so eventually, your liver has little to no glycogen stored and you get into ketosis as soon as you’ve burned through yesterday’s glycogen. Or, when you’re ready, you can go for somewhat longer fasts (20–72 hours — please don’t go any longer without medical supervision), burn through your glycogen stores and get into ketosis sooner.
The process of burning through your glycogen stores will be even slower if you take days off from fasting, such as skipping Saturdays or whole weekends. As Gin Stephens, author of the intermittent fasting books Delay Don’t Deny, Feast Without Fear, and Feast, Fast, Repeat likes to point out “Saturday isn’t a special occasion; it happens every week.”
Now that I’m at my goal weight, I do enjoy a somewhat longer eating window on weekends, but I’m still fasting 18–23 hours daily.
Likewise, if all bets are off treat-wise the second you open that luxurious 8-hour window, you may be replenishing your glycogen stores in a way that keeps ketosis out of reach for you. This means you’ll continue fueling your body on glycogen instead of keytones, and not reach the energy and mental clarity of fasting ketosis that everyone succeeding at IF is so excited about.
16:8 means less time in ketosis
It’s believed that once your liver has burned through its stored glycogen, the average time it takes to reach ketosis or fat-burning mode is around 12 hours. Theoretically, a 16-hour fast provides about four hours of ketosis — assuming you’ve already burned through your excess glycogen stores. This is why a lot of people do find that the 16:8 schedule helps them lose weight and improves their overall health. For example, my spouse (age 59), who didn’t have a lot of weight to lose, easily dropped down to his college weight and lost his middle-age tummy on an easy 16:8 schedule.
But if you’ve got more than a few pounds to lose, lead a sedentary lifestyle, are menopausal, or are looking to make faster progress, consider spending some time each day in heavy ketosis, which the 16:8 schedule doesn’t touch.
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The joy of heavy ketosis
Heavy ketosis — when the body fuels itself entirely on stored fat rather than glycogen — is believed to begin around the18-hours fasted mark for most people. If you stop your fast at 18 hours, you’ll spend an extra two hours beyond the 16-hour mark in ketosis but you’ll still never enjoy the benefits of heavy ketosis. That’s because all ketosis stops the minute you break the fast, and if you break it at 18 hours you leave no time for heavy ketosis to work its magic.
As I write this, I’m 22 hours and 20 minutes fasted, and definitely in heavy ketosis. I always know when I’m in heavy ketosis because I feel mentally sharp, physically energetic, and deeply contented. Based on how I feel, I know that for me heavy ketosis usually starts around the 17-hour mark — most likely because as someone who has fasted every single day for several months my glycogen stores are long gone, I’ve become a fat-burner, and I reach heavy ketosis easily.
It is this ketosis rocket-fuel energy and sense of well-being that keeps me committed to intermittent fasting. If I hadn’t lost any weight but still felt this good, I’d be just as committed to my IF lifestyle.
Do the math
It’s easy arithmetic: On average, the total length of your fast minus 12 hours is the time you’re spending in ketosis, assuming you’ve already burned through your liver’s glycogen stores. The total length of your fast minus 18 hours is the time you’re probably spending in heavy ketosis. That’s the amount of time your body is using only your fat stores for fuel, which is how you lose fat while intermittent fasting. If your goal is fat loss, you must give your body enough fasted time to actually tap into its fat stores.
A shorter eating window keeps me out of trouble. On a typical workday, I open my window by sitting down to dinner and close it right after dessert when I’m full. Closing my window keeps me away from the bowl of ice cream, the buttered popcorn or the cookie jar, ensuring I’ll get into the sweet spot of ketosis earlier the next day.
I rarely think about food when I’m in heavy ketosis. While I have an occasional mild hunger pang or two, it’s nothing compared to the gnawing hunger I experienced back when I was eating several times a day. You know that feeling of “I must eat immediately to stop this unbearable hunger”? I no longer experience that.
16:8 was training wheels for me
The second time I started IF, I began with a 16:8 schedule and struggled mightily to work my way up to 18-hour daily fasts. Then I learned about clean fasting from Gin Stephens and was able to easily shift to OMAD (one meal a day), fasting longer with no struggle.
I wouldn’t have lost much weight, if any, by staying on a 16:8. I had several issues working against me: menopause, hypothyroid, and spinal nerve root damage to name a few. By shifting to OMAD, I lost 36 pounds in about six months. While I cut back on carbs a bit for the first few months, I didn’t need to stick with that long term to continue losing weight or to maintain my weight loss once I reached my goal.
If you’re struggling with IF on a 16:8, try the counterintuitive route of fasting a bit longer. You may be pleasantly surprised at the results.
Note: I’m not a medical professional, just a regular person who has succeeded with intermittent fasting. Nothing you read here should be taken as medical advice.