There’s this crazy clickbait ad floating around the internet and social.

It is so specific, making blanket statements that, when you’re aged 25-35 to fast for 14 hours, but when you’re 35-45 to fast for 16 hours. By the time you’re 45, they suggest fasting for over 20 hours a day. What?! Where did these advertisers get their information?

As two engineers who have researched, written about, and practiced intermittent fasting for years, this was news to us. But if there were an optimal intermittent fasting schedule by age, we’d want to know, too.

The great thing is that intermittent fasting is so popular that new studies are published monthly.

So, we take a deep dive into answering, once and for all, if there’s a “best way” to do intermittent fasting by age and share an intermittent fasting by age chart based solely on the data.

Intermittent Fasting By Age Chart Cover

Intermittent Fasting by Age for Weight Loss

According to a survey taken by the International Food Information Council, 10% of every American aged between 18-80 is intermittent fasting right now.

That’s millions upon millions of people intermittent fasting. And we’re for it! But, with this popularity, come a lot of gimmicky products and bad information. Such as this pesky ad for an “intermittent fasting according to age” tool.

Intermittent fasting according to age recommendations according to science

In this article, we’ll take a deep dive into the research to help you determine what is your best intermittent fasting window.

The health benefits of intermittent fasting are immense, from resetting metabolism to improved blood sugar levels to cellular rejuvenation and autophagy. And of course, weight loss.

While creating this intermittent fasting by age chart, it was important to know why people were fasting.

According to Google search data, weight loss is above and beyond the main reason why people intermittent fast. There are about 50 times more searches for “intermittent fasting for weight loss” than “intermittent fasting for managing diabetes,” for example.

Knowing this, we’ll analyze the research to answer, specifically, how age affects intermittent fasting for weight loss.

Intermittent and extended fasting bundle - printable pdf guide

The Proven Fasting Bundle (Printable PDF Guide)

Read by 143,364 fasters in 2022 alone, the Fasting Bundle is the structure and tools you need to succeed.

  • Intermittent and alternate-day fasting meal plans
  • Checklist for before, during, and after a prolonged fast
  • Benefits, challenges, and a beginners how-to guide

Plus, we're just an email away for support.

Factors for Weight Loss and Aging

When creating this intermittent fasting by age chart, there were a few obvious factors to consider, like:

  • does sex/gender affect intermittent fasting results?
  • what happens to metabolism as you age?
  • how do body composition and muscle mass change as you age?
  • for menopausal women, how do hormones affect weight change?

It turns out that the science of each of these facts is fascinating, and we go into depth below on all of them.

But, spoiler alert, it turns out that essentially none of them affected intermittent fasting success. Ethnicity, sex, and age don’t affect results. And vice versa, aging doesn’t heavily affect metabolism or muscle mass until around the age of 60.

This means, if you’re between the ages of 18-60, you can intermittent fast the same way… With one caveat: lifestyle changes do affect how you should approach intermittent fasting.

So while your body doesn’t necessarily have to change much between ages 20 and 60, in terms of how it affects intermittent fasting success, your lifestyle likely will a lot.

So, let’s begin!

How does sex/gender affect intermittent fasting?

Initial studies show… not much. Particularly when it comes to weight loss.

Diets are notoriously challenging to study — or should I say people are notoriously challenging to study. That’s because self-reported diets may not be accurate, but more and more researchers are trying to address how the sex differences could affect intermittent fasting results.

The most comprehensive studies I’ve seen are out of the University of Illinois, Chicago. Participants were studied while alternate-day fasting (which in their case, meant eating 500 calories on Days 1, 3, 5, etc. over one meal, and eating normally on Days 2, 4, 6, and so forth) for 8 weeks.

While white males aged 50-59 showed the “best” results, they didn’t particularly stand out. Ninety percent of all 121 participants, no matter the age, race/ethnicity, or sex, lost weight. The average (60% of participants) lost between 4-8% of their original body weight in the 8 weeks. Not bad!

Anecdotally, Ryan and I can attest to this as two people of different sexes and races. We’ve not only intermittent fasted for years but we’ve done extended 3-5 day fasts too, all to similar (and excellent) success.

Most recently, we did a 5-day fasting-mimicking diet with ProLon and tracked our weight, inches, and metabolism throughout — with similarly spectacular results. Plus the experience was awesome (full ProLon review here with results and a 20% discount code, RYANANDALEX, to try it yourself).

So while this suggests that an intermittent fasting chart doesn’t need to be divided as men vs. women, I wasn’t as convinced that age also doesn’t matter. I believe it’s more nuanced than that, especially when considering how metabolism changes with age and after working with thousands of fasting clients over the years.

So, onto the next topic: metabolism and aging.

How Aging Affects Metabolism

If you’ve followed our blog and journey for a while (and if not, subscribe to our weekly newsletter!), Ryan and I have taken a deep dive into metabolic health these past few years. It all started when a nifty little device called Lumen hit the market, making it possible for people to track their metabolisms in real-time at home for the first time.

(By the way, we share a ton of our metabolism data in our articles, like our popular ‘Stages of Fasting by Hour.’ And of course, here’s our experience with Lumen with discount code RYANANDALEX if this interests you).

As engineers, we went all in, tracking how our metabolism changed daily, after meals, after workouts, and even while I was pregnant and breastfeeding.

All of this taught us how nuanced metabolism is, plus how much things like nutrition, activity level, stress, and sleep quality can affect it.

So when considering how age can affect intermittent fasting when it comes to weight loss, the conversation wouldn’t be complete without some commentary on metabolism.

The largest study that has looked at metabolism and aging included over 6,400 participants ranging from 8 days old to 95 years old across 29 countries.

To sum up the findings, gender/sex also didn’t play a large role in metabolism.

The study found that peak metabolism is reached around age 20, but more or less holds steady for decades until age 60. After the age of 60, both total energy expenditure (TEE) and basal metabolic rate (BMR) decline by about 0.7% each year.

Not what you were expecting? It’s actually great news that our metabolisms don’t need to slow down until age 60! If you feel like you’re gaining weight earlier than that, it’s not a biological reason but a lifestyle reason.

This brings us to the next topic how muscle mass and body composition change as we age.

Muscle Mass, Aging, and Intermittent Fasting

The truth is, that most people start losing muscle mass after the age of 30. This is mostly attributed to things like sedentary office jobs and a hectic, multi-kid household, although natural aging processes like sarcopenia start around 40 years old.

But it doesn’t have to be so. If you counterbalance aging with strength training workouts, you won’t have to lose 3-5% of your muscle mass each decade like the average person. This could equate to 1-2 lbs. of muscle mass each year.

The situation is even more serious for seniors, with 60-80-year-olds losing 12% of muscle mass each decade, and when over the age of 80, 30%.

And lost muscle mass doesn’t just directly impact weight loss and metabolism, but also your risk of injury or falls, particularly for older adults.

In terms of intermittent fasting, the answer is the same: muscle mass won’t be affected if you’re strength training.

Fasting for 24 hours has been shown to naturally stimulate human growth hormone by 100%-2000%. This hormone is vital for muscle growth, body composition, cell repair, and metabolism.

The key is to make sure you’re consuming enough protein during your feeding window and following a well-tailored workout plan that incorporates strength and resistance training. (Here are two free ones for you that work great: our 12-Week Dumbbell Workout Plan and our Bodyweight Workout Plan.)

If not, then your body may equally choose to lose weight by cutting muscle instead of cutting fat.

Knowing all of this, aging doesn’t have to impact muscle mass, nor does intermittent fasting. So, in our intermittent fasting by age chart, while we don’t need to worry about it until age 60, this is only the case if we’re not exercising properly.

Menopause, Intermittent Fasting, and Weight Loss

We’ve saved the best for last: is there an optimal intermittent fasting schedule for menopausal women?

First off, the studies show that intermittent fasting can be beneficial for women going through menopause. One of the main hormones affected and potentially changed by menopause, particularly in pre-menopausal women, is insulin sensitivity. Since intermittent fasting helps rebalance insulin resistance, this is a powerful tool when combatting postmenopausal weight gain.

Further studies have shown that most sex hormones affected by menopause (like testosterone, progesterone, and estradiol) were unchanged with intermittent fasting. The only exception was DHEA (dehydroepiandrosterone) which decreased by about 13% after two months of fasting.

DHEA is linked to adrenal health and progesterone and testosterone production, but it turns out that high levels of it have been linked with breast cancer.

Overall, from the studies we’ve researched, the benefits of intermittent fasting for menopausal women, including weight loss and improved insulin sensitivity, point to being extremely beneficial.

But, none of this says which intermittent fasting schedule should be followed by age. Let’s take what we’ve learned and put it into a chart at last.

Intermittent Fasting By Age Chart

Intermittent Fasting by Age Chart

Ages 0-18

* Intermittent fasting is not recommended

It’s a fairly blanket statement that children and adolescents shouldn’t intermittent fast. The worry is that, if they do, they won’t receive enough nutrients to grow if they can only eat within certain hours.

In reality, our 2-year-old is essentially “on” a 14/10 schedule (14-hour fast with a 10-hour eating window) because she sleeps 13 hours a night. So fasting isn’t innately “bad” for kids and happens every night’s sleep.

However, doctors agree that intermittent fasting is not appropriate as a weight loss tool in children and adolescents. Instead, other strategies should be considered first like creating a more active lifestyle, modifying eating habits, and cutting out soda.

Ages 18-60

* Consider the IF schedule that best fits your lifestyle
* Add quarterly extended fasts

I know, what a big age grouping! But from all of the research studies and reviews we’ve read, there’s no reason why this age group shouldn’t be lumped together.

Between the ages of 18-60, your metabolism remains the same. Your muscle mass remains the same too as long as you exercise and strength train. Intermittent fasting doesn’t affect the hormones of menopausal women.

So if you’re between the ages of 18-60, no matter if a man or a woman, your only consideration is choosing an intermittent fasting schedule that makes the most sense for your lifestyle and therefore you’ll be the most successful at.

That’s it, and honestly, it’s great news. It means you can’t go wrong.

And if you want an exact answer, the only way to do that is to track your metabolism in real-time. Which you can do at home with Lumen. This is what we do.

I can’t tell you how many times I sit down to breakfast, only to see that I’m still in fat-burn mode and should wait an hour or so longer before eating to maximize my fasting benefits. Or, there are other times when I feel a pang of hunger, go to measure my metabolism, and see that my body has already switched to carb-burn mode. Then I know I can stop my fast a few hours early and eat breakfast.

Using a metabolism tracker like Lumen is really the only way to truly know what your body is up to and how it’s reacting to a fast. So, it’s pretty awesome that this is an option for us. We recommend checking out our full experience and Lumen review, it’s pretty life-changing. (And we have a discount code for our readers too, RYANANDALEX for $50 $100 off).

We also recommend considering an extended fast of 3-5 days, depending on how you do it, once a quarter. In our article, “Stages of Fasting By Hour,” we share how many of the best longevity and health benefits happen only during extended fasting.

We’ve done it two different ways, with a 3-day water fast and a 5-day fasting-mimicking diet. Both are empowering and mind-blowing, and we personally prefer the 5-day fast. Read on and decide for yourself!

Ages 60-80

* Consider the 16/8 IF schedule
* Add quarterly extended fasts with doctor supervision if over the age of 70

Studies show that after the age of 60, your body changes quite a bit. Muscle mass decreases at a faster rate, despite exercise, and metabolism slows down with it.

Older adults have been the latest test subjects for intermittent fasting and the health benefits hold: lower body fat, improved lipid profiles, and reduced risk of cardiovascular disease.

The 16/8 intermittent fasting schedule is specifically recommended for this age group because studies show that it’s the least likely plan to affect caloric intake. And receiving enough nutrients and food over the age of 60 is what will keep you strong.

With all of this being said, as long as your caloric intake is enough, any IF schedule that suits you should work just fine.

From our research, as you enter your 70s, it’s a good idea to discuss your plan with your doctor. Intermittent fasting can get in the way of medicines that need to be taken with food. Plus, many clinical trials only study those up until their seventies. The thought behind this is simply due to increased muscle loss at this age, risk of dizziness, and avoiding malnutrition.

This is the same reason why extended fasting is only recommended with a doctor’s approval if over the age of 70.

None of this is to say that intermittent fasting doesn’t still boast a bevy of health benefits at this age. But as you know, doctors are less likely to recommend something that isn’t fully studied. From the dozens of studies I’ve read on fasting, none test subjects past the age of 80.

Closing Thoughts on Intermittent fasting by Age Chart

Were you hoping for a more concrete answer for the intermittent fasting by age chart? At first, we were too, but I think the reality is better this way. If you’d like to learn how we track our metabolism to optimize our fasting schedule, head over to our article titled, ‘The Best Intermittent Fasting Window to Lose Belly Fat.’

Studies don’t show that one fasting schedule is more effective than another for any age group, with the exception that the 16/8 fasting schedule seems to be the easiest for people to do. Plus, people are usually able to meet their daily calories on that one, since intermittent fasting is not about calorie restriction but about managing the timing of your food.

From all we’ve read, nutritionists and doctors agree that the most effective intermittent fasting schedule, whether 14/10, 16/8, alternate day fasting, or 5:2 fasting, is the one that works best for you.

If you want to take it a step further, get yourself a Lumen and take the guesswork out of your fasting effectiveness.

This is because intermittent fasting is not one-size-fits-all, as much as those Instagram ads would want you to believe. With our Lumens, I see that our fasting windows can change daily, depending on how we slept, what we ate, how much we exercised, and our stress levels.

With all of this being said, we highly recommend experimenting, taking thorough notes, and finding the best intermittent fasting schedule for you. The health benefits are there, and unless you’re under the age of 18, pregnant or breastfeeding, underweight, or have a specific health concern, there’s no reason not to try it.

Feel free to download the Fasting Bundle below to help you plan, track, and succeed with both intermittent and prolonged fasting.

For more resources and guides about fasting, such as our guide to intermittent fasting and sleep, head over to our Fasting Page.

Intermittent and extended fasting bundle - printable pdf guide

The Proven Fasting Bundle (Printable PDF Guide)

Read by 143,364 fasters in 2022 alone, the Fasting Bundle is the structure and tools you need to succeed.

  • Intermittent and alternate-day fasting meal plans
  • Checklist for before, during, and after a prolonged fast
  • Benefits, challenges, and a beginners how-to guide

Plus, we're just an email away for support.

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As Seen In Feature Bar Ryan and Alex Duo Life

Hey we're Ryan and Alex

The creators of Ryan and Alex Duo Life. We are a husband-wife duo and “lifestyle engineers.”

After eight years working in the corporate world as engineers, we left our high-powered jobs to tackle our true passion — helping couples engineer their best lives.

The synergy of our engineering minds and ten years of health coaching experience produced Ryan and Alex Duo Life. Our mission is to help you transform your bodies, minds, and relationship as a couple.

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