Intermittent fasting (IF) is currently one of the world's most popular health and fitness trends.
People are using it to lose weight, improve health and simplify their healthy lifestyle.
Many studies show that it can have powerful effects on your body and brain, and may even help you live longer (, , ).
This is the ultimate beginner's guide to intermittent fasting.
Intermittent fasting (IF) is a term for an eating pattern that cycles between periods of fasting and eating.
It does not say anything about which foods you should eat, but rather when you should eat them.
In this respect, it is not a "diet" in the conventional sense. It is more accurately described as an "eating pattern."
Common intermittent fasting methods involve daily 16 hour fasts, or fasting for 24 hours, twice per week.
Humans have actually been fasting throughout evolution. Sometimes it was done because food was not available, and it has also been a part of major religions, including Islam, Christianity and Buddhism.
When you think about it, our hunter-gatherer ancestors didn't have supermarkets, refrigerators or food available year-round.
Sometimes we couldn't find anything to eat, and our bodies evolved to be able to function without food for extended periods of time.
If anything, fasting from time to time is more "natural" than constantly eating 3-4 (or more) meals per day.
For a more detailed explanation of what intermittent fasting is, read this article: What is Intermittent Fasting?
Bottom Line: Intermittent fasting (IF) is a term for an eating pattern that cycles between periods of fasting and eating. It is currently very popular in the health and fitness community.
Intermittent fasting has been very popular for many years and several different methods have been used.
All of them involve splitting the day or week into "eating periods" and "fasting periods." During the fasting periods, you eat either very little or nothing at all.
These are the most popular methods:
- The 16/8 Method: Also called the protocol, it involves skipping breakfast and restricting your daily eating period to 8 hours, for example from 1 pm to 9 pm. Then you "fast" for 16 hours in between.
- Eat-Stop-Eat: This involves fasting for 24 hours, once or twice a week, for example by not eating from dinner one day until dinner the next day.
- The 5:2 Diet: On two non-consecutive days of the week, only eat 500-600 calories. Eat normally the other 5 days. More details here.
By making you eat fewer calories, all of these methods should make you lose weight as long as you don't compensate by eating much more during the eating periods.
I've personally found the 16/8 method to be the simplest, most sustainable and easiest to stick to. It is also the most popular.
There is more detailed information on the different protocols here: 6 Intermittent Fasting Methods.
Bottom Line: There are several different ways to do intermittent fasting. All of them split the day or week into "eating periods" and "fasting periods."
When you fast, several things happen in your body on the cellular and molecular level.
For example, your body changes hormone levels to make stored body fat more accessible.
Your cells also initiate important repair processes, and change the expression of genes.
Here are some changes that occur in your body when you fast:
- Human Growth Hormone (HGH): The levels of growth hormone skyrocket, increasing as much as 5-fold. This has benefits for fat loss and muscle gain, to name a few (, , , ).
- Insulin: Insulin sensitivity improves and levels of insulin drop dramatically. Lower insulin levels make stored body fat more accessible ().
- Cellular repair: When fasted, your cells initiate cellular repair processes. This includes autophagy, where cells digest and remove old and dysfunctional proteins that build up inside cells (, )
- Gene expression: There are changes in the function of genes related to longevity and protection against disease (, ).
Bottom Line: When you fast, human growth hormone levels go up and insulin levels go down. Your body’s cells also change the expression of genes and initiate important cellular repair processes.
Weight loss is the most common reason that people try intermittent fasting ().
By making you eat fewer meals, intermittent fasting can lead to an automatic reduction in calorie intake.
Additionally, intermittent fasting changes hormone levels to facilitate weight loss.
In addition to lower insulin and increased growth hormone levels, it increases release of the fat burning hormone norepinephrine (noradrenaline).
Because of these changes in hormones, short-term fasting may actually increase your metabolic rate by 3.6-14% (, ).
By helping you eat less (fewer calories in) and helping you burn more (more calories out), intermittent fasting causes weight loss by changing both sides of the calorie equation.
Studies show that intermittent fasting can be a very powerful weight loss tool. In a review study from 2014, it was shown to cause weight loss of 3-8% over periods of 3-24 weeks ().
That is actually a very large amount compared to most weight loss studies.
According to this study, people also lost 4-7% of their waist circumference (). This indicates that they lost significant amounts of the harmful belly fat that builds up around the organs and causes disease.
There is also one study showing that intermittent fasting causes less muscle loss than the more standard method of continuous calorie restriction ().
However, keep in mind that the main reason this works, is that it helps you eat fewer calories overall. If you binge and eat massive amounts during the eating periods, then you may not lose any weight at all.
Read this article for more information on IF and Weight Loss.
Bottom Line: Intermittent fasting may boost metabolism slightly, while helping you eat fewer calories. It is a very effective way to lose weight and belly fat.
Many studies have been done on intermittent fasting, in both animals and humans.
These studies have shown that it can have powerful benefits for weight control and the health of your body and brain. It may even help you live longer.
Here are the main health benefits of intermittent fasting:
- Weight loss: As mentioned above, intermittent fasting can help you lose weight and belly fat, without having to consciously restrict calories (, ).
- Insulin resistance: Intermittent fasting can reduce insulin resistance, lowering blood sugar by 3-6% and fasting insulin levels by 20-31% (). This should protect against type 2 diabetes.
- Inflammation: Some studies show reductions in markers of inflammation, a key driver of many chronic diseases (, , ).
- Heart health: Intermittent fasting may reduce LDL cholesterol, blood triglycerides, inflammatory markers, blood sugar and insulin resistance. These are all risk factors for heart disease (, , ).
- Cancer: Animal studies suggest that intermittent fasting may help prevent cancer (, , , ).
- Brain health: Intermittent fasting increases a brain hormone called BDNF, and may aid the growth of new nerve cells (, , ). It may also protect against Alzheimer's disease ().
- Anti-aging: Intermittent fasting can extend lifespan in rats. Studies showed that fasted rats live as much as 36-83% longer (, ).
Keep in mind that the research is still in its early stages. Many of the studies were small, short in duration or conducted in animals. Many questions have yet to be answered in higher quality human studies ().
More evidence-based details here: 10 Benefits of Intermittent Fasting.
Bottom Line: Intermittent fasting can have many benefits for your body and brain. It can cause weight loss, and may protect against type 2 diabetes, heart disease and cancer. It may also help you live longer.
Eating healthy is simple, but it can be incredibly hard to stick to.
One of the main obstacles is all the work required to plan for and cook healthy meals.
If you do intermittent fasting, this gets easier because you don't need to plan, cook or clean up after as many meals as before.
Intermittent fasting is actually very popular among the "life hacking" crowd because it improves your health while simplifying your life at the same time.
Bottom Line: One of the major benefits of intermittent fasting is that it makes healthy eating simpler. There are fewer meals that you need to prepare, cook and clean up after.
Intermittent fasting is certainly not for everyone.
If you are underweight, or have a history of eating disorders, then you should not do intermittent fasting without consulting with a health professional first.
In these cases, it can be downright harmful.
Should Women Fast?
There is that intermittent fasting may not be as beneficial for women, as it is for men.
For example, one study showed that it improved insulin sensitivity in men, but worsened blood sugar control in women ().
Although there are no human studies on this, studies in rats have shown that intermittent fasting can make female rats emaciated, masculinized, infertile and cause them to miss cycles (, ).
There are plenty of anecdotal reports from women who became amenorrheic (their menstrual period stopped) when they started doing IF, then went back to normal when they stopped doing it.
For these reasons, women should definitely with intermittent fasting. Ease into it, and if you have any problems like amenorrhea then stop doing it immediately.
If you have problems with fertility and/or are trying to conceive, then consider holding off on intermittent fasting for now. Intermittent fasting is probably a bad idea when pregnant or breastfeeding.
Bottom Line: People who are underweight or have a history of eating disorders should not fast. There is also some evidence that intermittent fasting may be harmful for some women.
Hunger is the main side effect of intermittent fasting.
You may also feel weak and that your brain isn't performing as well as you're used to.
This may only be temporary, as it can take some time for your body to adapt to the new meal schedule.
If you have a medical condition, then you should consult with your doctor before trying intermittent fasting.
This is particularly important if you:
- Have diabetes.
- Have problems with blood sugar regulation.
- Have low blood pressure.
- Take medications.
- Are underweight.
- Have a history of eating disorders.
- Are a female who is trying to conceive.
- Are a female with a history of amenorrhea.
- Are pregnant or breastfeeding.
Bottom Line: The most common side effect of intermittent fasting is hunger. People with certain medical conditions should not fast without consulting with a doctor first.
Here are answers to the most common questions about intermittent fasting.
1. Can I drink liquids during the fast?
Coffee can be particularly beneficial during a fast, because it can blunt hunger.
2. Isn't it unhealthy to skip breakfast?
No. The problem is that most stereotypical breakfast skippers have unhealthy lifestyles. If you make sure to eat healthy food for the rest of the day then it is fine.
3. Can I take supplements while fasting?
Yes. However, keep in mind that some supplements (like fat-soluble vitamins) may work better when taken with meals.
4. Can I work out while fasted?
Yes, fasted workouts are fine. Some people recommend taking (BCAAs) before a fasted workout.
5. Will fasting cause muscle loss?
All weight loss methods can cause muscle loss, that is why it is important to lift weights and keep protein intake high. One study shows that intermittent fasting causes less muscle loss than regular calorie restriction ().
6. Will fasting slow down my metabolism?
No. Studies show that short-term fasts actually boost metabolism (, ). However, longer fasts (3 days or more) can suppress metabolism ().
7. Should kids fast?
That's probably a bad idea.
Chances are that you've already done many "intermittent fasts" in your life.
If you've ever eaten dinner, then slept late and not eaten until lunch the next day, then you've probably already done a 16+ hour fast.
Many people actually instinctively eat this way. They simply don't feel hungry in the morning.
I personally find that the 16/8 method is the simplest and most sustainable way to do intermittent fasting. I recommend that you try that one first.
If you find that it is easy and you feel good during the fast, then you can try moving on to more advanced fasts like 24-hour fasts 1-2 times per week (Eat-Stop-Eat) or only eating 500-600 calories 1-2 days per week (the 5:2 diet).
Another approach is to simply fast whenever it is convenient. As in, skip meals from time to time when you're not hungry or don't have time to cook.
There is no need to follow a structured intermittent fasting plan to derive at least some of the benefits.
I recommend that you experiment with the different approaches and find something that you enjoy and fits your schedule.
Bottom Line: It is recommended to start with the 16/8 method, then perhaps later move on to longer fasts. It is important to experiment and find something that works for you.
Intermittent fasting is not something that anyone needs to do.
If you don't like the idea of fasting, then you can safely ignore all of this. Just continue to do what works for you.
At the end of the day, there is no one-size-fits-all solution in nutrition. The best diet for you is the one you can stick to in the long run.
Intermittent fasting is great for some people, not others. The only way to find out which group you belong to is to try it out.
If you feel good when fasting and find it to be a sustainable way of eating, then it can be a very powerful tool to lose weight and improve health.