Information on intermittent fasting is everywhere – magazine articles, news segments, social media posts, and sometimes even your doctor’s office – promising weight loss, ‘metabolism repair’, and improved energy. You might even hear from your family and friends that it’s helped them feel better or they lost weight. In this post, we’re going to review what the different types of fasting are, the health benefits, and if fasting is right for you.
Table of Contents
What is fasting?
Intermittent fasting, also known as just fasting, is a way of eating that focuses on when you eat, as opposed to what types of food you eat or how much food you eat. There are many variations and it may be called by many names online such as one meal a day (OMAD), fast-mimicking diet, and circadian fasting. Let’s look at the most common types of fasting and the differences between them.
Types of fasting
We’re going to focus on the three most common types of fasting out there: Alternate-day Fasting, the 5:2 diet, and Time-Restricted Fasting.
Alternate-day Fasting (ADF): This is where you alternate between a day of eating your normal diet with another day of eating little to no food or beverages (a fast).
For example,
Monday, Wednesday, and Friday: You eat your normal diet.
Tuesday, Thursday, and Saturday: You fast.
The 5:2 diet: You choose 2 days out of the week (can be two days in a row or just two days of the week that you are fasting, and eat your normal diet the remainder of the week.
For example,
Tuesday and Thursday: You fast.
Monday, Wednesday, Friday, Saturday and Sunday: You eat your normal diet.
Time-Restricted Fasting: This is by far the most popular form of fasting on social media. You only eat during a certain time period.
For example,
Fast for 16 hours
8-hour window (i.e., 10 AM – 6 PM): You eat.
Knowing is half the battle, but are these fasting diets actually good for you? Keep reading to learn how they can impact your health (good or bad).
Want to know if detox diets are a good approach instead? Check out the post on Investigating the Safety of Detox Diets.
Fasting for spiritual purposes
Fasting is also a common and important part of spiritual practice, however the purpose is not to lose weight or meet health goals. Examples include:
- Ramadan: a substantial meal after sunset, one light meal before dawn, and abstinence from food/beverages/water during all other times
- Yom Kipper: Abstinence from food/beverages/water for 25 hours
- Buddhist fasting practices: May only eat one meal per day (before noon) or practice longer 18-36 day fasts
There are some spiritual fasts that focus on the food type instead of timing. For example:
- Christian Orthodox Church fasting: 180-200 calendar days throughout the year avoiding meat, dairy products, and eggs
- Daniel fast: 21-days with removal of animal products, refined carbohydrates, preservatives, food additives, flavorings, sweeteners, alcohol, and caffeine from the diet
- Buddhism: Abstinence from meat, eggs, dairy products, garlic, onion, leeks, alcohol, and tobacco.
What are the potential health benefits of fasting?
For some patients, intermittent fasting may feel easier than a typical calorie-reduced diet. That’s because the focus is on when you eat, rather than tracking calories or thinking about what you’re eating. Also, meal planning and grocery shopping may be easier as you’re eating your normal diet.
Some studies have found that there are short-term benefits and fasting may improve biological markers in patients with prediabetes, metabolic syndrome, Type 2 diabetes mellitus, and heart disease.
Weight loss
All forms of intermittent fasting aim to limit your eating to a certain time period (called eating windows). This can result in eating less food than you would on a normal day. Any time we eat less than our daily energy needs, we are eating in a calorie deficit. This is why people are more likely to lose weight while fasting.
However, you will have the similar result with any plan that reduces daily calorie intake. Studies have shown that eating in a continuous calorie deficit reduces both BMI and waist circumference, but fasting has not always shown the same results as it can be difficult to sustain this diet. This is discussed further in the “What are the potential risks or pitfalls with fasting?” section later in this post.
If you are taking any medications for weight loss, please consult your medical provider before starting any types of fasting.
Blood sugar
Studies have found that fasting may improve morning blood sugar, A1c, and post-meal blood sugar levels in people with prediabetes and Type 2 Diabetes Mellitus.
However, the timing matters. Generally, our bodies can regulate blood sugar levels easier in the mornings. So for some people, an earlier eating window may be more beneficial. A good way to go about this is to have an eating window of 8 AM – 2 PM, instead of skipping breakfast.
It’s important to keep in mind the symptoms of hypoglycemia (low blood sugar):
If you have a blood sugar monitor, it is recommended to check as soon as you start feeling these symptoms to ensure that your blood sugar levels are not too low (at or below 70 mg/dL). To treat low blood sugar, follow the 15-15 rule from the American Diabetes Association here.
If you use insulin or take any diabetes medication, please consult your medical provider before starting any types of fasting.
Heart health
Fasting has been shown to improve heart health, including lowering blood pressure and reducing the risk for developing Type 2 Diabetes Mellitus (a major risk factor for heart disease). One study found that a time-restricted fast reduced blood pressure by an average of 8.1 mmHg (systolic) and 5.1 mmHg (diastolic). This means if you typically have a high blood pressure of 145/90, you may have a blood pressure of 137/85 after a year of following a time-restricted fast (about a 5% decrease). To really understand the impact of fasting on blood pressure in humans, more studies are needed.
Research is mixed on whether fasting impacts other important biological markers of heart health, such as LDL cholesterol levels.
If you are on any medications for heart health, blood pressure, cholesterol, excess fluid, blood clots, etc. (such as anticoagulants, anti-platelet agents, ace-inhibitors, angiotensin II receptor blockers, beta-blockers, calcium channel blockers, cholesterol lowering, diuretics, and/or vasodilators), please consult your medical provider before starting any types of fasting.
What are the potential pitfalls of fasting?
Intermittent fasting can lead to poor overall nutrition. This is because you aren’t eating enough during your eating window to meet your protein, vitamins, minerals, or other nutrient needs.
Imagine those busy days where you have to skip lunch to meet work deadlines or pick up your kiddo early from daycare. This can cut into your eating window. Or you might feel overstuffed all day because you were so hungry following the fast that you ate too much food right away and didn’t eat anything else the rest of the day.
In these cases, fasting can be difficult to maintain. Adherence to a plan is key for any weight loss journey. Any lack of consistency with fasting may lead to less weight loss or weight gain.
What are the potential risks of fasting?
Dehydration is also a big risk with fasting. Some of the foods we consume throughout the day help with our hydration needs. For example, fruits and vegetables contain between 70-90% water and cooked salmon filets about 65% water.
There can also be side effects of fasting due to long periods of not eating. Side effects include dizziness, nausea, insomnia, increased risk of falling, headaches, weakness, constipation, abdominal pain, and excessive hunger.
If you have or are at risk for developing diabetes, fasting can increase the risk of severe side effects such as low blood sugar.
If you have or are at risk for cardiovascular disease/heart disease, fasting can increase the risk of severe complications, like heart attack or stroke. Time-restricted fasting in particular has been associated with a higher risk of cardiovascular mortality if you already have heart disease.
Research on the long-term impacts of intermittent fasting, especially in humans with these conditions, is preliminary and extremely limited.
Should I fast?
Intermittent fasting can sound appealing as it seems simple: “I just won’t eat overnight.” However, what seems like an easy fix is rarely the solution.
We want to create a lifestyle change that is something that you can sustain long-term. Before implementing any change, ask yourself:
- Will this cause me to miss celebrating important events? “Can I eat the cake at my child’s birthday party?”
- Will this prevent going out with friends? “Should I skip this meal and just drink water?”
- Will this add stress to my life? “Will I get hangry and snap at my family?”
If you answered yes to any of the above questions, this change may not work long-term for your lifestyle.
If you’re looking for something sustainable that meets your health goals, reach out to Carrie. Schedule a free 30-minute video call to learn more.
Bottom Line
For some people, developing a shorter eating window may be a helpful tool in reducing portions. If that’s the case for you, make that eating window full of nutritious food that is satisfying and supports all your health goals. Be sure to talk to your healthcare team so they can help you decide if fasting is the right approach for you.
Written by Krystal Welp, dietetic intern
Reviewed by Carrie Weitzel, registered dietitian
Krystal Welp is a dietetic intern with Metropolitan State University of Denver. Her interest in nutrition stems from her own personal health challenges but has evolved into a broader commitment to providing evidence-based, culturally sensitive support to our community.