Intermittent fasting is wildly popular due to its incredible and wide-ranging potential health benefits.
The most promising (and desired) benefit of fasting is weight loss. When you compress your feeding window or take a day off from eating, fat loss often follows.
But the benefits don’t end there. Fasting can also positively affect metabolism (lower blood sugar, lower insulin, ketone production), cellular recycling mechanisms (autophagy), the wake-sleep cycle, and even productivity.
This article is a science-based tour of these benefits. First, let’s set the stage with a definition of fasting.
What Is Fasting?
A fast is a period of temporary calorie restriction. During this period, you consume zero calories or significantly fewer calories than your metabolism demands.
The length of your fast and the level of calorie restriction depends on the type of fasting you practice. There are two main types of fasting:
- Intermittent fasting
- Extended fasting
An intermittent fast starts at 12 hours and ends at around 36 hours. Intermittent fasting can be practiced daily (12/12, 16/8, OMAD) or weekly (5:2, ADF/Alternate-Day Fasting).
The daily protocols—also called time-restricted feeding—involve eating all your calories in a compressed window. The weekly protocols involve limiting calories (0-25%) on dedicated “fasting days” during the week. Both types of protocols have been shown to have benefits.[*][*]
An extended fast is any which lasts longer than 36 hours. It shouldn’t be undertaken lightly and generally requires medical supervision.
The benefits of intermittent and extended fasting largely overlap. Both entail temporary calorie restriction, but intermittent fasting is the gentler, more sustainable method.
If you’re new to fasting, start with shorter intermittent fasts. Let’s review the benefits of this practice now.
The Benefits of Fasting
Humans evolved in a time before minimarts, Grubhub, and “fourth meal”. When food was scarce, our ancestors fasted.
During these fasts, they would burn fat and make ketones to fuel the brain. Then they would refeed and rebuild their bodies.
These days we’re constantly feeding and never fasting. If we can bring back the fasting part, we could potentially see huge benefits to our health.
#1: Weight loss
Most people fast to lose weight. It’s the primary selling point for intermittent fasting.
And it’s not an empty selling point. Multiple studies show that intermittent fasting can help both obese and non-obese people lose weight.[*][*]
What’s driving this train? To answer, we have to talk about negative energy balance.
When you’re in a negative energy balance, you’re consuming fewer calories than your body is using through resting metabolism, exercise, and daily activities. In other words, you’re eating less than you’re burning.
The connection to fasting is straightforward. When you fast, less energy is coming in than going out. That’s a formula for weight loss.[*]
When calories were held constant with one-meal-a-day intermittent fasting, researchers didn’t find a weight loss benefit vs. three meals a day in normal-weight people.[*] They did, however, see improvements in body composition (more fat loss) in the OMAD condition. That means that even when study subjects ate the exact same amount of food, they lost fat and improved body composition just by splitting those calories into one meal rather than three.
When you take a break from eating, the hormone insulin stays low. Low insulin, in turn, helps you burn body fat and make ketones.
This fat-burning state is called ketosis. Benefits of ketosis may include:
- More stable energy
- Reduced cravings
- Better appetite control
- Enhanced mental acuity
- Body recomposition (visceral fat loss)
- Increased “good” HDL cholesterol
- Lower blood sugar and insulin levels
- Lower blood pressure
- Improvements to metabolic syndrome
- ...and more
The research on ketosis for cognition is fascinating. In both animals and humans, higher levels of ketones have been linked to better mental performance in a variety of contexts.[*][*]
#3: Lower blood sugar
When a person constantly eats—especially if they’re continually consuming carbs—their blood sugar stays chronically elevated. This condition, called hyperglycemia, is the defining clinical feature of type 2 diabetes and is linked to a host of complications, including heart disease.[*][*]
Overeating has reached epidemic proportions in the Western world, and we have developed an unhealthy dependence on sugar. Are we really surprised we have a collective blood sugar problem?[*]
Fasting may be part of the solution. One randomized controlled trial found that a year of 5:2 fasting lowered average blood sugar in people with type 2 diabetes (similar to continuous calorie restriction), while other case reports have shown similar results.[*][*] Promising stuff, but more research is needed.
#4: Autophagy (Anti-Aging)
Fasting activates a cellular cleanup program called autophagy. When a cell undergoes autophagy, it digests old and damaged parts and replaces them with fresh ones.
Higher levels of autophagy have an anti-aging effect in animals, which may be true in humans too.[*] When autophagy is impaired, the brain, heart, kidneys, and other organs suffer.[*][*]
As a general rule, longer fasts have the potential to activate more autophagy than shorter fasts. But since autophagy is so hard to measure, nobody knows the optimal protocol.
#5: Brain health
Entering ketosis during a fast isn’t just about burning body fat. It’s also about making ketones that fuel your brain.
Normally, your brain runs entirely on glucose. But in a state of ketosis, ketones shoulder much of that burden.
Ketosis appears to have cognitive benefits in both humans and animals. For instance, in older people, elevating ketone levels led to better performance in a variety of mental tasks.[*]
Ketosis may also be therapeutic for Alzheimer’s disease. Research on Alzheimer’s suggests that the aging brain loses its ability to use glucose as fuel, leading to cognitive impairment.[*]
But the aging brain doesn’t seem to lose its ability to use ketones. Stay tuned for more research on this front.
#6: Heart health
Any intervention that helps with obesity (a major heart disease risk factor) has the potential to benefit the heart.[*] Fasting included.
But it’s not just weight loss. Intermittent fasting has also been shown to improve other markers of heart health.
For example, one review found that alternate-day fasting (ADF) reduced low-density lipoprotein (LDL) cholesterol and triglycerides in some of the most rigorous studies—both positive movements for heart health.[*] And another found that intermittent fasting improved multiple cardiovascular risk markers. [*]
Still, we don’t have data on fasting for preventing specific hard outcomes like a heart attack or stroke.
#7: Anti-inflammatory effects
Another benefit of having ketones in your blood? Anti-inflammatory effects.
Specifically, the ketone body beta-hydroxybutyrate has been shown to suppress an inflammatory complex called the NLRP3 inflammasome.[*] It sounds like a droid from Star Wars, but it’s actually central to the damaging, unnecessary immune response known as chronic inflammation.
Fasting may reduce the risk of high blood sugar (hyperglycemia)—a pro-inflammatory state. When you eat less (and less frequently), you have the potential to reduce blood sugar levels.
The signs that intermittent fasting may help with cancer are promising.
First, consider the Warburg Effect—the tendency of cancer cells to suck up glucose.[*] Keeping blood glucose low with fasting may starve cancer of its favorite fuel.
A similar principle applies to insulin. Insulin is a growth hormone, and high insulin levels have been linked to increased rates of cancer.[*] Fasting can help to keep insulin low.
Getting to specific data, researchers have found that fasting increases the effect of chemotherapy in mice.[*] And in women with HER-2 negative breast cancer, a 24-fast was shown to reduce toxic side effects of chemotherapy.[*]
#9: Circadian rhythm enhancement
The circadian rhythm is your 24-hour wake-sleep cycle. Its proper functioning affects every area of human health, including sleep quality, fat metabolism, and energy levels throughout the day.
By not snacking overnight—the most basic form of time-restricted feeding— it is possible to enhance your circadian rhythm.[*] You give your body a chance to wind down and promote restful sleep. You tune up your fat-burning capacity. You skip the needless calories—all good things.
#10: More productive time
Time-restricted feeding also frees up your schedule. When you don’t have to worry about meal prep, eating, and cleanup, your calendar opens up nicely. Furthermore, ketosis provides a steady energy source for your brain in the form of ketones without having to deal with rapid blood sugar fluctuations, which can enhance sustained concentration.
Skipping breakfast can be a valuable productivity hack. You conquer the hard stuff on an empty stomach, and then you treat yourself with a rewarding lunch.
In a sense, your brain is like a trained seal. If you toss it a fish when it does something good, it will soon associate the behavior with the reward.
Knowing the reward is coming will motivate the behavior. You’ll not only get more work done, but you’ll enjoy it more.
Hydration While Fasting
Since fasting has a diuretic effect, it’s essential to replace fluids regularly. This helps prevent dehydration symptoms like fatigue, headaches, and weakness.
What can you drink? As long as the beverage contains minimal calories, it shouldn’t meaningfully interfere with your fast.
Healthy options include:
- Tea (herbal, green, or black)
- Lemon water
- Beverages sweetened with stevia, monk fruit, or erythritol
- Bone broth
Keto Beef Bone Broth is an excellent option because it provides electrolytes like sodium, calcium, and phosphorus. Along with water, fasting makes you excrete electrolytes at higher rates[*]—so it’s crucial to replace them.
An important point to note, drinking plain water and neglecting electrolytes will only exacerbate an electrolyte deficiency, provoking a series of unpleasant symptoms.[*]
What To Eat During Your Feeding Window
When you practice any form of fasting, proper nutrition is essential. You have a limited window in which to cram protein, fatty acids, vitamins, minerals, and polyphenols. You need to make that window count.
Whenever possible, select foods based on nutrient density. For example, vegetables are more nutrient-dense than grains, meat and fish are more nutrient-dense than protein powder, and real eggs are more nutrient-dense than chocolate eggs. (Um, thanks Captain Obvious!).
Your calories during feeding windows will come from protein, fat, and carbs—the three main macronutrients. Of the three, focus on getting enough protein first to help maintain muscle, synthesize hormones, and support a vast array of biochemical reactions.
Because protein kickstarts the growth and repair process, it’s wise to break your fast with a high-protein meal. Meat, fish, and eggs are all excellent sources, or opt for tofu or tempeh for a plant-based protein boost.
For fasts 24 hours or longer, make your first meal back a high-protein snack of 200-300 calories. (Eat a bigger meal about an hour later). This helps put the brakes on muscle loss and stimulates satiety hormones, so you don’t subsequently overeat.[*]
One final point is to consider eating a Keto diet to support your fasting regimen. Eating Keto helps you fat-adapt. This makes it easier to access body fat during a fast and simultaneously helps to reduce hunger hormones.[*]
The point is that Keto and fasting work together like a Swiss bobsled team: efficiently.
What to Expect With Fasting
If you’re new to fasting, it’s important to set realistic expectations. The benefits won’t occur overnight, at least not in full force.
The key is to find a sustainable fasting protocol that you can stick with and that is suitable for your individual health needs. Start with 12 or 13 hours of time-restricted feeding and work your way up as your comfort level and schedule permit.
Better still, ease yourself through the process using a diet tracking app like Carb Manager to help build your custom intermittent fasting program. This is a great way to manage all your essential health and dietary metrics while tracking your long term fasting goals and progress.
Just remember, shorter fasts can be just as beneficial as longer fasts. It all depends on your goals and unique physiology. Find what works for you, and enjoy the journey.
Please note that as with any significant diet or lifestyle change, we recommend working alongside a registered health professional, especially if you are currently on any prescribed medications or undergoing any medical treatments. Fasting is not appropriate for everyone and should be avoided by those with a history of eating disorders, pregnant and breastfeeding women, and anyone under 18.
Connie 3 days ago
Apparently Carb Manger doesn't answer anyone's questions. Hate that.
MyCarbManager@5656 16 days ago
Is cream in coffee considered “breaking the fast”?
RousingMacadamia952275 2 days ago
yes...it gives a slight raise to insulin...considered food
BlithesomeKale640443 3 days ago
I would like to know this, too.
kattykathey 6 days ago
I would like to know also although Inhave a feeling the answer is going to be yes.
datrices18b12 a month ago
Check out Spark on Amazon! Perfect suppression and energy without jitters
Kristine 2 months ago
I had a baby 15months ago and I have been struggling to get back into a keto diet. Even fasting has been a struggle I always want to eat. When I try to get back to a keto diet I feel ill. I don't understand it worked so well before I was pregnant. Any advice?
FantasticArugula756275 2 months ago
Look up dr mindy pelz, she takes hormones into the equation and they are responsible for a lot.
ScarlettRoseCat 2 months ago
Hey Kristine. Post Partum mama too. If by feeling ill, you are referring to the "Keto flu" the answer is to increase your sodium intake. There is something about decreasing carbs that releases a lot of sodium so you need to replenish it to not feel headaches etc. On my physician monitored program they recommend 5g of sodium either via salt (1/4 teaspoon =1) or bullion cubes (to make a broth).
AmazingMacadamia618403 3 months ago
Is anyone doing 18/6 fast ? ,I have been doing this fast ; past week with great success in general .The 18/6 appear to suppress long covid symtom ( burning stinging pain nerves in legs ) - past 2 yrs .My daughter has same symtom since contracting covid .
queenvaleen 3 months ago
Out of curiosity, if I am “fasting” from 7:00pm - 10:00am next day, can my break fast begin with a premier protein 30G shake or does it need to be solid food? Just started Keto and I am still learning do׳s and don’ts
Bay 12 10 days ago
Go with eggs and ratio keto yogurt better in the stomach and none of all that processed goo. Read the label on that protein shake and google those ones you don’t know and decided if it is still a real meal supplement
Kristenmccoy0526 2 months ago
I regularly break my fast around 11AM with a protein shake. Just make sure you cram in the nutrient dense whole foods throughout the day.
UpbeatAvocado262400 2 months ago
Better to break with whole foods because it is nutrient dense. However, a protein shake is next best thing. Just keep an eye for the ingredients of the shake.
AmazingCauliflower899364 3 months ago
Yes the premier protein will break the fast. Water coffee tea is about all with fasting
RousingRadish912259 3 months ago
Anyone doing OMAD? I just started...im curious what ur one meal is calorie wise....i try to stick around 900 but still not loosing...i am in ketosis though
AwesomeRadish815167 2 months ago
I have been eating OMAD since February it’s fantastic, you need to make sure your eating enough especially protein as it keeps our bones and muscles healthy, also muscle replaces fat and weighs more . A friend who was successful with losing her weight told me to STOP going on the scale as it was consuming my mood and success, I haven’t weighed myself in 2months and I have dropped 2pant size without exercise as it’s winter I don’t want to go out lol . But my mindset is in a good place and I feel so strong and energetic and I am looking great ! Best feeling when someone says wow you have lost weight. So stay focused hide the scale drink lots of water and electrolytes big help and eat lots of protein for your meal , I love pork steaks so yummy lots of fat , I have it with asparagus, broccoli, zucchini whatever low carb veggies . And tell yourself I got this ! 🙌💃😉
BlithesomeAvocado490869 2 months ago
Hi, give it time.. Make sure you hitting your macros and is there an exercise routine to help speed it up
MarvellousArugula417497 3 months ago
Are there any good fat loss supplements that don't break your fast? I know supplements are not essential... but it would be helpful in the beginning to deal with hunger and cravings. Thank you!
FabulousKetone741163 3 months ago
Look into a high quality probiotic with lean bacteria. They can boost fat loss, reduce cravings and wont break your fast. Just make sure it contains the research-backed ingredients. You can find a very good one at: www.LeanProbiotic.info - Many people overlook gut microbiome health but it's super important. Hope this helps.
GorgeousRadish317224 3 months ago
Do antacids break your fast
Kittycatmom a month ago
I dont THINK so. You can still take your medications.
kDog487 3 months ago
Humans did not “evolve” we are created by God. I wish I could log the time that ate. I normally wait until later in the day to log my food.
rn a month ago
I agree. I like to pre log when I meal plan for the day
TeriseD a month ago
Stop dreaming. There is no "god", no invisible man, no supreme being or magical power in the sky. Of course we evolved from lower life forms. Everything on this planet evolved from atoms and particles scattered all throughout the universe.
UpbeatKale399074 3 months ago
If you go to the three dots to the right by the meal your logging you can change the time. It’s the meal time option and you can put in the time you ate. As for “evolving” they mean from starting as hunter/gatherers to what we are today. We had to work got our food. Now we don’t have to do that. Supermarket shopping, food delivery etc. are our ways of finding food, therefore not expending energy working for food sources.