It’s every dieter’s dream: Imagine being able to eat whatever you want most days of the week, limiting your intake for one or two days at a time, and still losing weight. This is what many people experience when they begin practicing intermittent fasting (IF).
Intermittent can do more than slim just your waistline. Studies show that benefits of fasting include helping stabilize blood sugar levels, reduce inflammation and keep your heart healthy.
There are a variety of approaches to intermittent fasting, whether you prefer to fast for a good chunk of hours each day or skip meals for two days each week.
Let’s look below at how intermittent fasting can serve as one way to simultaneously improve your health and achieve your weight loss goals. Consider this your intermittent fasting beginner’s guide.
What Is Intermittent Fasting?
Intermittent fasting, also known as cyclic fasting, has risen in popularity in recent years as more research emerges showing that it’s typically safe and effective.
However, intermittent fasting is hardly a new concept. It’s been used for centuries during times when food was scarce, and it even plays a central role in many major religions.
In fact, once a year, Muslims observe Ramadan, a month of fasting from dawn until sunset.
In a 2016 Cell Metabolism study, the authors discuss how fasting allow humans to rely less on our glucose (sugar) stores for energy and instead on our ketone bodies and fat stores (keto intermittent fasting). As a result, “both intermittent and periodic fasting result in benefits ranging from prevention to the enhanced treatment of diseases.”
Even fasting mimicking diets, which are not true fasting, can create beneficial changes similar to those caused by fasting.
It’s difficult to define intermittent fasting, as there’s not just one correct method for how to fast. In fact, there are many different variations that are used around the world. Each follows a different eating pattern that is often strictly adhered to in order to achieve physical or even spiritual results.
How Does Intermittent Fasting Work?
The extensive research on the concept of intermittent fasting suggests it functions in two different ways to improve various facets of health:
- First, intermittent fasting results in lowered levels of oxidative stress to cells throughout the body.
- Second, fasting improves your body’s ability to deal with stress at a cellular level. It activates cellular stress response pathways similar to very mild stressors, acting as a mild stimulant for your body’s stress response. As this occurs consistently, your body is slowly reinforced against cellular stress and is then less susceptible to cellular aging and disease development.
Common Types of Intermittent Fasting
- Alternate-Day Fasting: This entails eating only every other day. On fasting days, some eat no food at all, and others eat a very small amount, typically around 500 calories. On non-fasting calorie days, eat normally (but healthfully).
- The Warrior Diet: This diet involves eating only fruits and vegetables during the day and then eating one large meal at night.
- 16/8 Fasting (also often referred to as Time-Restricted Feeding): For this method, you fast for 16 hours every day and limit your eating to eight hours. Most often, key component of 16/8 intermittent fasting is skipping breakfast. This approach involves not eating anything after dinner and skipping breakfast the next morning.
- Eat-Stop-Eat: Practice the “Eat Stop Eat” method by picking one or two days out of the week in which you fast for 24 hours, then eat nothing from dinner one day until dinner the next day. On the other days, you should have normal calorie days.
- 5:2 Diet: For five days of the week, you eat normally. For the remaining two fast days, you should restrict your caloric intake to between 500–600 calories every day.
Related: What Is Dirty Fasting?
1. Promotes Weight Loss
A major intermittent fasting health benefit is its ability to rev up fat burning and help the pounds slide off. In fact, many people prefer intermittent fasting to traditional diets because it doesn’t require you to meticulously measure your foods and track the calories and grams consumed.
Intermittent fasting results in increased fat burning and fast weight loss by forcing your body to use fat stores as fuel. When you eat, your body uses glucose (sugar) as its primary source of energy and stores whatever is left over as glycogen in your muscles and liver.
When you don’t give your body a steady stream of glucose, it begins breaking down the glycogen to use as fuel. After the glycogen has been depleted, your body seeks out alternative sources of energy, such as fat cells, which it then breaks down to help power your body.
This is similar to the ketosis diet (or “keto diet”), in which you deprive your body of carbohydrates and force it to use stored fat for energy.
A 2020 review looked at the effects of intermittent fasting on body composition in 27 different studies and found that, on average, it slashed body weight by 1 percent to 13 percent within six months. Another study found that whole-day fasting led to similar results, with up to a 9 percent reduction in body weight.
Yet another study focused on the 16/8 method of intermittent fasting showed that it significantly reduced fat mass while retaining both muscle mass and strength. This fact is why I recommend this style of intermittent fasting the most.
All of that said, one randomized controlled trial found that calorie restriction was still needed to cause weight loss even when someone was fasting, meaning that sticking to a healthy diet is key.
2. Improves Blood Sugar
When you eat, carbohydrates are broken down into glucose (sugar) in your bloodstream. A hormone called insulin is responsible for transporting the glucose out of the bloodstream and into the cells, where it can be used as energy.
Insulin doesn’t always work effectively when you have diabetes, which can result in high blood sugar levels coupled with symptoms like fatigue, thirst and frequent urination.
Some studies show that intermittent fasting benefits insulin sensitivity and blood sugar levels by keeping them well-regulated and preventing spikes and crashes. Fasting seems to be especially effective for this purpose when it lasts at least 16 hours.
In one study, participants with diabetes fasted an average of 16 hours daily for two weeks. Not only did intermittent fasting cause weight loss and a decrease in caloric intake, but it also helped significantly reduce blood sugar levels.
Another study showed that fasting decreased blood sugar by 12 percent and also lowered insulin levels by nearly 53 percent. Preventing a buildup of insulin allows it to work more efficiently and keeps your body sensitive to its effects.
3. Keeps Your Heart Healthy
Studies show that intermittent fasting helps improve cardiovascular health by lowering certain heart disease risk factors.
In one study, fasting was shown to influence several components of heart health. It increased good HDL cholesterol and decreased both bad LDL cholesterol and triglyceride levels.
One animal study in the Journal of Nutritional Biochemistry showed that IF caused an increase in levels of adiponectin, a protein involved in the metabolism of fat and sugar that may be protective against cardiovascular disease and heart attacks.
In fact, in one study, rats that fasted every other day were nearly 66 percent more likely to survive a heart attack than those on a normal diet.
4. Reduces Inflammation
Inflammation is a normal immune response to injury. Chronic inflammation, on the other hand, can lead to chronic disease.
Some research has even linked inflammation to conditions like arteriosclerosis, diabetes, obesity and cancer.
A study published in Nutrition Research followed 50 individuals observing Ramadan and showed that they had decreased levels of some inflammatory markers during Ramadan fasting.
Another study in 2015 discovered that a longer duration of nighttime fasting was associated with a decrease in markers of inflammation. In the journal Rejuvenation Research, alternate-day fasting helped reduce markers of oxidative stress.
There’s also evidence that intermittent fasting can support a healthy gut microbiome, which assists in supporting immune defenses.
While more research is needed, these studies provide promising evidence showing that IF may help reduce inflammation and fight off chronic disease.
5. Protects Your Brain
In addition to keeping your heart healthy and warding off disease, some studies have indicated that intermittent fasting protects the health of your brain.
One animal study showed that intermittent fasting helps enhance cognitive function and protect against changes in memory and learning function compared to a control group. Another animal study found that it protects the brains of mice by influencing certain proteins involved in brain aging.
Additionally, the anti-inflammatory effects of intermittent fasting may also help slow the progression of neurodegenerative disorders like Alzheimer’s disease.
Some also say that fasting promotes autophagy, or “self-eating,” which is our normal bodily process of cellular renewal, although more research on this topic is needed.
6. Decreases Leptin Resistance (and Possibly Normalizes Hunger)
Leptin, also known as the satiety hormone, is a hormone produced by the fat cells that helps signal when it’s time to stop eating. Your leptin levels drop when you’re hungry and increase when you’re full.
Because leptin is produced in the fat cells, those who are overweight or obese tend to have higher amounts of leptin circulating in the body. However, too much leptin floating around can cause leptin resistance, which makes it harder for it to effectively turn off hunger cues.
One study with 80 participants measured leptin levels during intermittent fasting and found that levels were lower at night during the fasting period. Lower levels of leptin could translate to less leptin resistance, less hunger and potentially even more weight loss.
Methods/How It Works
What are the rules for intermittent fasting?
While every type of intermittent fasting requires you to skip meals (most commonly breakfast), each type works a bit differently and has different “rules.”
What can I eat during intermittent fasting?
Unlike typical diets, with intermittent fasting there’s no need to count points or calories or plug your foods into a food diary each night.
Even if your goal is to primarily lose weight from IF, it’s still important to consume enough healthy food and calories in general. Otherwise, you risk slowing down your metabolism, which is the opposite of what you want.
Skipping meals is also not an excuse to eat junk and “empty calories,” meaning it’s important to continue to focus on consuming a nutrient-dense diet.
While practicing intermittent fasting, here are foods to emphasize when you’re not fasting and within your eating window, all of which tend to be filling plus full of nutrients:
- Vegetables: broccoli, cabbage, bell peppers, Brussels sprouts, tomatoes, asparagus, garlic, cucumber, onions, ginger, etc.
- Fruits: strawberries, oranges, lemons, blackberries, limes, raspberries, pears, apples, blueberries, etc.
- Meat: grass-fed beef, lamb, venison, wild game
- Fish: wild-caught salmon, tuna, mackerel, anchovies, sardines
- Poultry: organic chicken, turkey, goose, duck
- Cage-free eggs
- Nuts: almonds, cashews, pecans, pistachios, macadamia nuts, walnuts, Brazil nuts
- Seeds: hemp seeds, pumpkin seeds, sunflower seeds, chia seeds, flaxseeds
- Legumes: black beans, kidney beans, pinto beans, lima beans, chickpeas, lentils
- Whole grains: quinoa, barley, buckwheat, millet, brown rice
- Healthy fats: olive oil, coconut oil, MCT oil, grass-fed butter, ghee, avocado oil
- Dairy products: goat milk, kefir, goat cheese, probiotic yogurt, raw milk
- Condiments: hummus, guacamole, apple cider vinegar, mustard, salsa, balsamic vinegar, liquid aminos
- Herbs and spices: basil, oregano, rosemary, turmeric, cinnamon, paprika, cumin, black pepper, etc.
- Natural sweeteners: stevia, raw honey, maple syrup, dates, monk fruit
- Beverages: water, tea, kombucha, bone broth
How to Intermittent Fast Safely
As described above, there are many types of intermittent fasting with different options that can fit any schedule or lifestyle. It’s best to experiment and find the one that works best based on your individual needs.
For beginners, the easiest starting point is the intermittent fasting 16/8 method, a form of time-restricted eating. This typically just involves skipping your evening snack after dinner and skipping breakfast the next morning as well.
If you don’t eat anything between 8 p.m. and 12 p.m. the next day, for instance, you’ve already fasted for 16 hours.
If 16 hours of fasting feels too daunting at first, begin with a shorter fast lasting about 13 to 14 hours. Increase how long you fast as your body gets accustomed to it.
While the 16/8 method has been shown to be generally safe for most people, longer fasts ranging from 14 to 72 hours may not be safe for everyone. It also takes your body time to adjust to the effects of fasting, so don’t push yourself too hard at first.
How much weight can you safely lose in a month with intermittent fasting?
It’s reasonable to expect to lose about 0.5 to two pounds per week (or up to about eight pounds per month). The bigger your body mass when you start fasting, the likelier it is that you’ll lose weight more quickly.
Keep in mind that intermittent fasting should be viewed as a change in lifestyle rather than a diet. Be sure to focus on filling your diet with healthy whole foods during the days that you do eat to squeeze as many nutrients as possible into your day.
If you’re physically active, intermittent fasting and working out are OK. While you can exercise during fast days, don’t push yourself too hard, and remember to drink plenty of water.
If you’re fasting for longer than 72 hours, however, it’s advisable to limit physical activity.
Additionally, always listen to your body. If you feel weakness or fatigue when you go an entire day with no food, try increasing your intake a bit, and have a light meal or snack.
Alternatively, try out one of the other methods of IF to find what works for you.
Risks and Side Effects
Why might intermittent fasting be bad for some people? It’s not intended for people with eating disorders, women who are pregnant or those who already have low body weight.
When taken too far, it can potentially cause side effects such as:
- weakness, brain fog
- increased hunger
- trouble sleeping (especially if you’re fasting at night)
If you suffer from low blood sugar, for example, going without eating all day may lead to dangerous side effects due to a drop in blood sugar causing symptoms like shakiness, heart palpitations and fatigue. If you have diabetes, it’s best to work with your doctor to determine if intermittent fasting is right for you.
If you have a history of eating disorders, this is also not a good tool for you as it may encourage unhealthy behaviors and trigger symptoms. If you are a child or teenager and still growing, intermittent fasting is not recommended either.
Among those who have gallstone disease, fasting may actually increase the risk of gallbladder problems and should be avoided.
Finally, studies show that fasting may alter levels of your thyroid hormones. If you suffer from any thyroid issues, you may want to reconsider intermittent fasting to avoid alterations in these important hormones.
Those who are sick may also want to reconsider fasting as it can deprive your body of the steady stream of nutrients that it needs to heal and get better.
Generally speaking, yes, it’s safe, but women tend to be more sensitive to the effects of fasting compared to men. Certain women may encounter hormone issues if they fast for days on end — they may benefit from doing intermittent fasting only a few days a week rather than every day, for example.
Those who are pregnant and breastfeeding should also avoid fasting and focus instead on a nutritious diet rich in vitamins and minerals.
- Intermittent fasting is one method to increase fat-burning and lose weight while supporting metabolic and cognitive health.
- Other health benefits include help regulating blood sugar, protecting your brain, keeping your heart healthy and reducing inflammation.
- There are many ways to fast, with variations that can fit into any lifestyle. A popular method is skipping breakfast and fasting for about 16 hours per day, including overnight.
- This dietary tool is typically safe but not a good fit for everyone, especially those suffering from certain health conditions, such as diabetes, gallstones, an eating disorder or a thyroid disorder.
- To avoid side effects, start slowly, and be sure to eat a nutrient-rich diet in order to prevent any deficiencies or issues like fatigue.