Is intermittent fasting good for you? If so, what is the science behind intermittent fasting, and what are the health benefits?
This past week, we spent hours combing through research studies — on humans, rats, and monkeys, oh my! — to get your questions (and ours) on intermittent fasting answered.
Amongst all of the fluff, conflicting research, and heated opinions about whether or not intermittent fasting healthy, we made a breakthrough.
It’s not safe for everyone, and there is a “right” and “wrong” way to go about intermittent fasting and specific ways to maximize your results (i.e., weight loss or muscle gain).
No one else will tell you this stuff. Seriously, we know after reading hundreds of websites and studies.
intermittent fasting health benfits
We are two engineers who have been intermittent fasting since 2018. When we couldn’t find an in-depth article that explained the science and health benefits of intermittent fasting, we researched to cover our bases.
This article is a product of that research, and we’ll answer the following questions. For additional well-researched content backed by personal experience, check out the articles on our fasting page.
Table of Contents:
What is the science behind intermittent fasting?
Intermittent Fasting Health Benefits:
Lose weight without changing my diet?
Increase my muscle mass?
Boost my metabolism?
Increase my longevity?
Reduce my risk for cancer?
Is intermittent fasting safe for everyone?
The Science Behind Intermittent Fasting
Intermittent fasting is a way to control how, when, and where from your body creates energy.
In simplified terms, after every meal, your body gets to work processing food, with the ultimate goal of supplying energy. In your stomach, acids and enzymes digest your food, breaking down carbohydrates into simple sugars called glucose.
Glucose now floods the bloodstream through your stomach and small intestine. To regulate this spike in blood sugar levels, your body counters by producing a hormone called insulin.
Insulin’s job is to lower blood sugar levels by turning glucose directly into energy, or if there is excess, storing it in the liver, muscles, or fat cells for later energy use.
How does intermittent fasting work?
The premise of intermittent fasting is that when your body is in a “fasted state,” for enough time, insulin levels will go down.
The measure of ‘enough time’ is scientifically described as between 12-36 hours, depending on your diet, insulin sensitivity, and exercise level.
When insulin drops far enough, these two things happen.
1. Fat cells will begin to release their stored sugar for energy, thus burning fat through lipolysis in adipose tissues and fat oxidation in the mitochondria.
2. Your body will increase its insulin sensitivity, benefitting your metabolism while reducing the risk of diabetes and heart disease.
As intermittent fasting gains in popularity, more and more medical studies showcase the positives and negatives of intermittent fasting.
When done correctly and smartly, intermittent fasting can lead to weight loss, muscle gain, and promote energy, concentration, and other positive health benefits.
Below, we answer your questions to see if intermittent fasting is safe and what the health benefits are with intermittent fasting. You’ll also learn the research on how to maximize the benefits, but also when to be careful.
Intermittent fasting while pregnant and for individuals with type 1 diabetes is not recommended.
We created an ‘Intermittent Fasting How To Guide‘ to help you start with a plan, and successfully transition to intermittent fasting.
1. Does Intermittent Fasting Lead to Weight Loss If I Don’t Change My Diet?
The most commonly desired health benefit of intermittent fasting is weight loss.
A study by the University of Alabama, recently published in the journal Obesity showed that an 18:6 intermittent fasting protocol (18-hours of fasting followed by 6-hours of eating) leads to weight loss without any other dietary or physical exercise changes.
In this study, men and women with a BMI range of 25-35 (overweight and obese), spent 4-days on a 16:8 intermittent fasting plan from 8 AM to 2 PM and then 4-days eating at “typical American meal times” from 8 AM to 8 PM. Their meals, daily caloric intake, and exercise remained the same.
Through blood tests, urine samples, and a respiratory chamber, it was found that intermittent fasting reduced levels of ghrelin (the appetite hormone).
This reduction cuts the feeling of hunger, increased fullness, decreased the desire to eat, and increased metabolic flexibility (meaning that your body adapts quickly to using whatever fuel is available to it, like your stored fat and glucose).
While a small and short-term study, this is a significant first step in determining that intermittent fasting can lead to weight loss, primarily through the control of appetite.
Of course, there are many examples of intermittent fasters who do lose weight because of reducing their caloric intake, most commonly by skipping breakfast.
It is undisputed that reducing your daily calories will lead to weight loss. Note, however, that women should never eat less than 1200 calories daily while men never less than 1600 calories.
To determine if you’re maintaining or reducing your calories at a safe rate, we recommend spending a few days cataloging your meals using a free app like MyFitnessPal.
You can track your calories on our free downloadable worksheet in our article, ‘30-Day Weight Loss Challenge.‘
Scientific evidence shows that intermittent fasting can lead to weight loss without changing diet by controlling appetite.
2. Does Intermittent Fasting Increase Muscle Gain?
Let’s discuss how intermittent fasting for muscle gain works.
According to the science behind intermittent fasting, after a meal, excess glucose from the blood is stored in the liver, muscles, and fat cells. So, how does the body know to take glucose from fat cells instead of from your muscles?
The reason for this is due to human growth hormone (HGH, or also known as growth hormone, GH). Human growth hormone is vital in muscle growth, body composition, cell repair, and metabolism.
It grows muscles, increases strength, and all while reducing recovery time. For this reason, the Olympics banned it as an illegal substance and steroid in athletes in 1989.
Produced naturally by the pituitary gland, many studies have shown that fasting drastically increases the production of human growth hormone. Research cites an increase between 100%-2000% over 24 hours of fasting.
That is significant. What’s more, is HGH production occurs while fasting, while sleeping, and while exercising. Therefore, intermittent fasting can be a powerful tool to gain muscle.
A word of warning: While fasting, it is still essential to consume enough protein and exercise with strength training. Like any weight-loss diet, small amounts of lean mass (including muscle) are lost along with the fat mass.
The good news is that studies have shown that muscle mass can be maintained or grown as long as you perform proper strength and resistance training. If not, you’re at risk of losing 1-2 lbs of muscle mass over a few months.
Need help finding the best workout routine to maintain or build muscle mass during intermittent fasting? Sign up for a free fitness analysis, or try our complete Resistance Bands Workout Routine.
Studies show that intermittent fasting promotes muscle gain due to drastically increased production of human growth hormone (HGH). Proper strength training is required.
3. Does Intermittent Fasting Affect Metabolism?
As discussed in the first question about intermittent fasting and weight loss, preliminary studies show that intermittent fasting can increase metabolic flexibility (when the body adapts to using different energy sources).
This is also known as “flipping” the metabolic switch through fasting. As put by Dr. Stephen Anton from the University of Florida, Gainesville, in his 2018 study published in the journal Obesity:
“Here, we define the metabolic switch as the body’s preferential shift from utilization of glucose from glycogenolysis to fatty acids and fatty acid-derived ketones. The reason we use the word ‘preferential’ is because there is now a growing body of research to indicate ketones are the preferred fuel for both the brain and body during periods of fasting and extended exercise. Of relevance to weight management, this switch represents a shift from lipid synthesis and fat storage to mobilization of fat in the form of free fatty acids (FFAs) and fatty-acid derived ketones.”
You might recognize this use of ketones for energy as the same goal that the ketogenic diet promises (but no, we don’t recommend the keto diet or any other fad diet).
In a nutshell, if you’ve been in a fasted state long enough to deplete the glucose and insulin in your bloodstream, a metabolic shift occurs to start burning the glucose stored in fat for energy.
The keyword is “long enough.” Depending on your diet, insulin sensitivity, and exercise level, that is somewhere between 12-36 hours.
If you maintain a poor diet while intermittent fasting, your 16:8 fast may not be long enough for you to realize a positive metabolic impact. That is why it’s crucial to have a well-balanced diet full of vegetables, beans, and legumes.
That’s also why you should experiment with extended fasting (24+ hours). We recently did a 3-day water fast and the results and experience was incredibly eye-opening. We lost a combined 16 pounds and both experienced a metabolic switch around 55 hours.
To learn how to do a 3-day water fast successfully, hear our before, during, and after story, and see our results, open the below in a new tab.
Metabolic flexibility is a health benefit of intermittent fasting, providing consistent energy levels in both men and women. A well-balanced, healthy diet is required.
4. Does Intermittent Fasting Have Anti-Aging Benefits?
I first remember hearing about the anti-aging and longevity-boosting benefits of calorie restriction during a summer internship in college. “The owner of the company,” my colleagues whispered to me whenever he was around, “lost over 100-lbs. with a 30% calorie restriction.”
Proven in rhesus macaque monkeys to increase their lifespan for three years – nearly a decade in human years – there are other links to longevity and fasting.
Along with increasing production of human growth hormone, which specializes in cell repair from free radical damage, cells made more copies of the gene SIRT3. A gene that also promotes cellular repair, while alternate-day fasting.
A similar study conducted on rodents at the Okinawa Institute of Science and Technology Graduate University in Japan found increased production of antioxidants. A substance which is also linked to our body’s responses to fasting and the oxidative stress it goes through.
While more studies are needed understand how intermittent fasting works with regards to anti-aging, the benefits of increased insulin sensitivity and weight loss are well-known benefits for a healthier, longer life.
In addition to increased insulin sensitivity and weight loss, studies suggest that higher production of the SIRT3 gene and antioxidants also help intermittent fasters fight aging.
5. Does intermittent fasting reduce the risks of cancer?
There is a small but growing body of evidence that fasting can reduce cancer risk and lessen the side-effects of chemotherapy.
A small study conducted by the University of Southern California and published in the journal Aging found that a 48-hour fast increased stress resistance to healthy, normal cells but not to cancer cells.
Since chemotherapy dose density and intensity are calculated to avoid dangerous side-effects and damage to healthy cells, fasting could enable stronger chemotherapy dosage and efficacy.
Another study, conducted on mice at the LSU School of Medicine, predicts for humans, “We hypothesize that a pretreatment of 2-3 weeks with the alternate day modified fast will improve outcomes in cancer chemotherapy, decreasing morbidity and raising cure rates.”
Of course, any fasting during cancer treatment needs to be closely monitored by medical professionals for weight loss and nutrition-related side effects.
Overall, the signs are positive for intermittent fasting reducing cancer risk as well as other diseases, like cardiovascular disease and Alzheimer’s disease.
6. Is intermittent fasting safe for both men and women?
There’s a range of studies and personal anecdotes about the safety and effectiveness of intermittent fasting. So, is intermittent fasting good for you? In a few words, the health benefits outweigh the negatives.
It is important to note that intermittent fasting affects men and women differently. The studies aren’t perfect, with many on rodents or women during Ramadan, and not a typical 16:8 intermittent fasting plan.
Studies of pregnant women over Ramadan continuously show no effect of fasting on birth weight. However, one study not to be overlooked found that female rats on calorie-restricted diets had smaller ovaries.
On top of that, women’s bodies seem to be more sensitive to calorie-restriction than men’s. If fasting for too long or with too few calories, it can affect the menstrual cycle, bone density, and the reproductive health of the woman.
These adverse side effects on reproductive health are not seen in men.
For this reason, intermittent fasting while pregnant or trying to become pregnant is not recommended.
Women may instead benefit from a wider eating window, extending the eating period from 8-hours to 10- or 12-hours.
As a woman myself, I’ve benefited from the 16:8 intermittent fasting window. It has simplified my day and created a more consistent and healthier eating routine.
The 8-hour window doesn’t seem restrictive to me, considering our story with intermittent fasting and sleep (over nine hours nightly). I’ve heard stories from lots of girlfriends as well who feel more focused and energized after fasting.
With any diet, it’s essential to discuss the implications with your doctor. In addition to women who are pregnant or trying to become pregnant, intermittent fasting is not for people who have a history of eating disorders, depression, or anxiety.
Finally, while intermittent fasting can reduce the likelihood of developing diabetes, if you already have diabetes (mainly type 1 diabetes), it is not advised to try intermittent fasting.
Intermittent fasting is safe for both men and women, although the effects differ.
Do not intermittent fast while pregnant.
Intermittent fasting for diabetes is recommended for prevention, but it is not if you already have diabetes.
How to maximize the benefits of intermittent fasting?
Based on the science-backed health benefits above, we know that intermittent fasting is healthy. However, the advantages that your body receives from intermittent fasting (i.e. weight loss and muscle gain) rely heavily on your eating habits.
Weight loss can occur with merely eating within a smaller time frame, but we also see others use it as an excuse to overeat and binge. Muscle mass can be increased, but only with the right diet and training.
Therefore, remember these three rules while intermittent fasting to increase the health benefits.
1. Optimize your food
Continue to eat a healthy, well-balanced diet full of vegetables, legumes, and beans, which are the healthiest foods to eat.
While you may still cut calories when compared to a typical eating day, become familiar with your daily calorie goals, and never consume less than 1200 calories for women and 1600 calories for men.
We share our typical intermittent fasting eating plan with a printable meal calendar below.
2. Strength train
While intermittent fasting, strength training is essential, or you could lose muscle mass. Our recommendation is taking advantage of the benefits of using resistance bands.
We like to workout in the morning, toward the end of our fasting window, because studies show that glucose can be converted straight to muscles post-workout. Intermittent fasting and morning workouts have synergistic benefits.
Open the below article in a new tab to learn how to maximize the benefits of working out in a fasted state.
3. Follow a plan
Like our ‘Intermittent Fasting How-To Guide‘ to work your way up to a 16:8 intermittent fasting plan. Optimize your new eating schedule and experiment with longer or shorter fasting windows.
For people with a poor diet, it will take longer for the glucose to deplete in your bloodstream, and therefore could require a fasting window of up to 36 hours for you to reap the full benefits of intermittent fasting.
If you’re already intermittent fasting or considering it, remember these three things:
1. Your diet affects how many hours you need to be fasting to reap the benefits (a poor diet might be up to 36-hours)
2. If you’re not strength and resistance training, some of your weight loss = muscle loss
3. Many people binge during their window, while many under-eat. Both are bad
4. Incorporate 3-day fasts into your plan.
I have questions. What do I do?
Please reference our fasting page or ask away in the comments below. We’ll get back to you as soon as possible and add your recommendations to this article accordingly.
Intermittent Fasting Meal Plan (Printable PDF)
Download and instantly access the intermittent fasting meal plans, including a printable meal planner to create daily menus.
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Such a well written and accessible article!
I have also been IF for years. I always do at least 12 each night, but I think 14.5 is my sweet spot 🙂
Thank you, Anne, we appreciate your comment! It’s so important to find what works for you. Finding where your sweet spot is so valuable and takes good intuition 😉 I’ve been thinking of experimenting more with a shorter fasting window from 16-hours, especially now that I’ve read research showing it can be more beneficial for women. I’ll keep this post updated with our new findings as we learn more!
Hi Alex, great article! Are there optimal fasting windows depending on a person’s exercise times? I train between 6:30 – 9pm and I’ve tried IF where I’ve had my last meal ~ 5pm. After class, sometimes I’m not hungry and other times I’m starving. I don’t usually like to eat late at night but sometimes I wonder if I should replenish my body with nutrients after training. Thanks for doing all this research!
Thanks so much for your question, Michelle! It’s a common one too: When to work out within the eating or fasting windows?
During my research, I read many anecdotes about people successfully pairing intermittent fasting with cycling calories and cycling carbs. This means that on days that they’re working out, they frontload some of their week’s calories and carbs, and then eat lighter on their off-days.
For someone like you who works out daily and as part of a profession, it gets a bit harder. Traditional research shows that eating right after a workout is a good way for sugars to help build muscle directly without getting stored as fat. I’m not sure how flexible your eating window can be, and if you can experiment with pushing it back slightly, it could help.
Also, there is research saying that women benefit optimally with a 16:8 window, which extends the eating window by 2-hours if you were following the typical 18:6 plan. With this, perhaps you can sneak a snack or banana in during your training.
Along with this, if you already have a healthy diet, you don’t need to fast as long as others to reap the same benefits. Someone who eats a lot of junk needs more time for their sugar levels to clear out of their bloodstream than someone who already eats healthily.
I hope this helps, but it’s very case-by-case. For Ryan, it took him about a month to get control of his hanger and eating habits under IF. So, whatever you decide to experiment with, give it some time and collect the data 🙂