Intermittent fasting, often shortened to “IF”, is a huge trend in the health and wellness world. Proponents of intermittent fasting say it’s a great way to lose weight, boost your metabolism, and raise your energy levels – but is that really true?
IF involves eating for a certain period of time and then not eating at all. There are a number of different methods of intermittent fasting. Although it’s associated with some health benefits, it might also pose some risks for certain people.
Here’s what you need to know before trying intermittent fasting.
What is intermittent fasting?
Intermittent fasting is an eating pattern that involves fasting for a set amount of time. The goal of intermittent fasting isn’t to prescribe what you should eat, but rather when you should eat. Sometimes, intermittent fasting is paired with a diet or eating style – such as paleo or keto – for added benefits.
During the fasting periods, you eat nothing (or you eat extremely little). Typically, you’re encouraged to drink water during this period. Some people also drink black coffee or tea during the fasting period.
For the most part, intermittent fasting works for weight loss because you’ll generally eat less throughout the day. By decreasing your overall caloric intake, you will probably lose weight.
However, intermittent fasting doesn’t work for everyone – some people find it difficult to stick to. Plus, there are some risks and side effects you’ll need to be aware of before you try it for yourself.
Intermittent fasting methods
There are a few different approaches to intermittent fasting. During the eating period – or “feast” period – you can eat whatever you’d like (within reason). Don’t go overboard, and try to eat a variety of healthy, nutritious foods, including protein sources, vegetables, nuts, and legumes.
During the fasting period, drink plenty of water. Black coffee and tea are also allowed during fasting. You can take your dietary supplements during the fasting period, but many supplements are best taken with a meal.
The 16/8 method
One of the most common intermittent fasting methods is the 16/8 method. With this pattern, you’ll fast for 16 hours (including the time you spend sleeping) and eat within the remaining eight hours of the day. This is also called the Leangains protocol.
The 20/4 method
20/4 is a slightly more difficult version of the 16/8 method. As the name suggests, you fast for 20 hours and eat within four hours. Because four hours is a very short period of time, you’ll usually only fit in a meal and a snack.
The 5:2 method
This involves eating normally for five days a week and then fasting for two days a week. On those two days, you’ll eat very little – typically around 500-600 calories per day.
Alternate day fasting
Alternate day fasting means that you’d fast every other day. There are a few different approaches to alternate day fasting – some people fast fully for 24 hours at a time, while others fast for a shorter period of time.
Eat Stop Eat
This method, popularized by fitness expert Brad Pilon, involves eating normally and then fasting for 24 hours. Typically, you’d fast once or twice a week.
How intermittent fasting affects the body
When you stop eating for an extended amount of time, your body adjusts to accommodate the lack of calories. As a result of fasting, your hormones and cells will be affected.
First, your body’s levels of human growth hormone (HGH) can increase dramatically during a fast. Higher levels of HGH could promote fat loss and muscle gain.
When we fast, our level of insulin drops and our insulin sensitivity improves. This also makes it easier for your body to use stored fat, thus helping with fat loss.
Because of the changes to your HGH and insulin levels, fasting can actually increase your metabolic rate. This means your body will burn even more calories.
As a result, intermittent fasting can be a good way to lose weight: one study found that participants lost 3 to 8% of their weight over 3 to 24 weeks. The participants also lost 4 to 7% of their waist circumference over 6 to 24 weeks.
It’s not just about weight loss. Some evidence shows that fasting can also affect gene expression, possibly leading to protection against disease and longevity.
Health benefits of intermittent fasting
There are many alleged health benefits of intermittent fasting.
The main reason why people do intermittent fasting is because of weight loss. So far, most of the research suggests that intermittent fasting does help with weight loss – as long as you stick to healthy options during your eating period. It’s important not to binge eat.
One of the reasons why intermittent fasting can help you lose fat is because of how it affects your HGH and insulin levels. Another reason is that you’re probably taking in fewer calories overall.
Last, because you only have to worry about cooking one or two meals a day (depending on the method you choose), you’re more likely to make a better choice about the meal. It can be difficult and time-consuming to cook three healthy meals a day, and many time-strapped people choose a high-calorie takeout or snack instead. But if you only need to cook once a day, you’ll have the time and energy to create a balanced, nutrient-rich meal that you’ll truly enjoy.
As mentioned before, intermittent fasting can increase insulin sensitivity. This could be hugely helpful for those with insulin resistance.
Insulin is a hormone that “tells” your body’s cells to pick up sugar from your blood and use it. When your cells become less sensitive to insulin, which means your blood sugar levels stay high you have insulin resistance. The pancreas then produces more insulin, which increases your insulin levels.
Insulin resistance can lead to diabetes, so it’s important to watch your insulin levels. You might benefit from regular fasting insulin tests, as these tests can detect whether you’re trending towards pre-diabetes.
Some evidence suggests that fasting could reduce inflammation. Chronic inflammation can cause a variety of diseases. Your body’s inflammatory response can damage your cells and tissues, leading to a range of health problems.
It’s important to note that none of these studies can guarantee an outcome for you. Many of the alleged benefits of intermittent fasting should be studied further.
Can I exercise while fasting?
Yes! In fact, many fitness enthusiasts swear by doing fasted workouts. However, it’s not a good idea to push yourself into training extra-hard when you’ve just started with intermittent fasting. Rather ease your body into it.
On the first few days of intermittent fasting, you might prefer light exercise like yoga, a walk, or a short swim. Once you’re used to your new eating pattern, you can embark on something a little more intense.
If you have a personal trainer, it’s a good idea to tell them that you’re doing intermittent fasting. Because intermittent fasting is associated with fat loss and muscle gain, many fitness enthusiasts do fasted strength training exercises as well as cardio workouts.
You might benefit from taking branched-chain amino acids (BCAAs) before fasted exercise. BCAA supplements can be found at your local health store and are said to improve muscle growth, reduce muscle soreness, and reduce post-workout fatigue.
Should women do intermittent fasting?
Some studies have suggested that intermittent fasting is more beneficial for men than it is for women. For example, in one study, blood sugar control worsened in women after three weeks of intermittent fasting, although the male subjects were not affected in the same way. It’s not totally clear why this is the case.
But we do know women are generally more sensitive to caloric restrictions. A lack of calories can affect certain female hormones – luteinizing hormone (LH) and follicle stimulating hormone (FSH). This disruption can cause irregular periods, fertility issues, and issues with bone health.
That type of response is probably evolutionary: if you don’t have access to sufficient food, becoming pregnant can be dangerous for both you and the baby. As such, our bodies evolved to become less fertile when we’re taking in fewer calories.
If you’re currently trying to conceive, it’s not a good idea to start intermittent fasting as it can impact your hormone levels. And of course, you shouldn’t try intermittent fasting if you’re pregnant or breastfeeding.
This doesn’t mean that women can’t do intermittent fasting, but women are generally advised to do a less intense method of intermittent fasting. It could entail fasting for fewer hours, or for fewer days each week.
Who should be cautious of intermittent fasting?
You should avoid intermittent fasting if you’re pregnant or breastfeeding (or trying to get pregnant). It is not recommended that children fast, either. You should also avoid it if you have a history of eating disorders or disordered eating.
While it’s always important to talk with your doctor before you try a new eating regime, it’s especially important to speak to your doctor about intermittent fasting if you:
- Take prescription medication
- Have diabetes or prediabetes
- Have low blood pressure
- Have a gastrointestinal condition
When talking to your doctor about intermittent fasting, you should outline:
- Why you want to try intermittent fasting (your goal)
- Your medical history, including mental health history
- The medications you currently take (including over-the-counter products and supplements)
- Your exercise regimen
- Your current diet and eating habits
This information will help them assess whether intermittent fasting will be safe and effective for you.
Overall, intermittent fasting is safe for most people – as long as you eat a decent amount of nutrient-dense, healthy food during your feasting period!
Safety and side effects of intermittent fasting
The most obvious side effect of intermittent fasting is hunger. You might also experience low energy levels at first. Many people also struggle to concentrate for the first few days of intermittent fasting.
However, after a few days, your body should start getting used to it. You might even notice an increase in energy and focus, especially if you tended to overeat before.
If you feel too ill to continue with intermittent fasting, it’s a good idea to stop. You could go for a less intense method and fast for fewer hours at a time. Listen to your body – if it doesn’t work for you, that’s okay! Speak to your doctor about an alternative.
To be safe, you should ease into intermittent fasting: start with shorter fasting periods and increase your fasting time if you’d like. Once again, it’s important to speak with your doctor before trying intermittent fasting.
If you’re trying to improve your overall health and wellness, you’ll benefit from personalized health coaching – which is just one of the many included benefits and services we offer Knew Health Members. In addition to health coaching, we offer free and discounted lab tests, discounts on supplements, and free fitness resources.
Ahmed, A. et al. 2018. Impact of intermittent fasting on human health: an extended review of metabolic cascades. https://www.tandfonline.com/doi/full/10.1080/10942912.2018.1560312
Barnosky, A. R. et al. 2014. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. https://www.sciencedirect.com/science/article/abs/pii/S193152441400200X
Blackman. M. et al. 2002. Growth hormone and sex steroid administration in healthy aged women and men: a randomized controlled trial. https://pubmed.ncbi.nlm.nih.gov/12425705/
Heilbronn, L. et al. 2005. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. https://pubmed.ncbi.nlm.nih.gov/15640462/
Heilbronn, L. et al. 2005. Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting. https://pubmed.ncbi.nlm.nih.gov/15833943/
Martin, A. et al. 2006. Caloric restriction and intermittent fasting: Two potential diets for successful brain aging. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2622429/
Takayuki, T. et al. 2019. Diverse metabolic reactions activated during 58-hr fasting are revealed by non-targeted metabolomic analysis of human blood. https://www.nature.com/articles/s41598-018-36674-9
Zhu, Y. et al. 2013. Metabolic regulation of Sirtuins upon fasting and the implication for cancer. https://pubmed.ncbi.nlm.nih.gov/24048020/
Disclaimer: This information is being provided to you for educational and informational purposes only. It is being provided to educate you about how to take care of your body and as a self-help tool for your own use so that you can reach your own health goals. It is not intended to treat or cure any specific illness and is not to replace the guidance provided by your own medical practitioner. This information is to be used at your own risk based on your own judgment. If you suspect you have a medical problem, we urge you to take appropriate action by seeking medical attention.