Is Intermittent Fasting Healthy?

There is a lot of buzz around intermittent fasting. Some people swear by it, while others think it's just another diet trend. So, is intermittent fasting healthy? The answer is: it depends.

Illustration of a person's hand holding a fork near a clock that reads 1:15.

Intermittent fasting can be a great way to lose weight and improve your health, but it needs to be individualized to your body.

This article will discuss intermittent fasting basics, the benefits, how to do it safely, and how to make adjustments to fit your lifestyle.

What is intermittent fasting? 

Intermittent fasting (IF) is an eating pattern with set periods of eating and fasting. Fasting means you completely abstain from food or drinks with calories. In the absence of nutrients, physiological adaptations kick in that can benefit your health (more on this below)<sup>1</sup>. 

IF can include a calorie deficit, but it doesn't have to, which is why many people like it. It's more about when you eat than how much (although food quality matters too), making it less restrictive than a typical diet. 

What happens in the body during fasting?

Weight loss is the most popular reason for experimenting with IF. A set cutoff to stop eating helps some people reduce late-night or mindless snacking, and it could also naturally reduce how much you eat since the window is shorter.

But IF could support weight loss and other health benefits because of how your body responds to going longer without food. As mentioned briefly above, IF works with your body's natural physiological adaptations that happen when you aren't eating.

When your body isn't digesting and absorbing nutrients, it switches to reparative and clean-up processes. Glucose and insulin levels drop, and fatty acids become an energy source promoting weight loss and other health benefits<sup>2</sup>. 

We will dive into the specific health benefits linked to IF later in the article, but here are some of the foundational changes that happen during fasting.

Fasting promotes autophagy

Autophagy is a process where the body cleans out old, damaged cells. It's a self-protective tool that reduces inflammation and oxidative stress to support cellular health<sup>3, 4</sup>. 

The metabolic pathway that turns on autophagy can only happen in the absence of nutrients (especially glucose) and when insulin levels are low, so fasting is a way to turn it on.

Fasting reduces inflammation and supports healthy immune cells

Inflammation is a known contributor to chronic disease risk. In combination with autophagy, IF appears to reduce inflammatory markers like homocysteine, interleukin 6, and C-reactive protein (CRP), all associated with cardiovascular disease<sup>5</sup>. These markers are all signals that turn on inflammation in your body, so IF could interrupt the process to lower the inflammatory burden.

What are the different intermittent fasting plans?

IF can be adapted to fit your lifestyle. Here are the different intermittent fasting plans:

The 16:8, 18:6, or 12:12 methods

All of these are variations known as time-restricted eating (TRE). They include daily periods of fasting (16-hours, 18-hours, or 12-hours) alternated with set eating windows (6-hours, 8-hours, or 12-hours).

16:8 method—16:8 is the most popular type of IF because it only means pushing breakfast a few hours later than usual for many people. For example, if you follow 16:8, you'd stop eating dinner by 8 pm and have your first meal of the day at 12pm. 

18:6 method—The 18:6 method means you'd extend your fast even more, so it's not a beginner method and is overly restrictive for many people.

12:12 method—Alternatively, the 12:12 is a gentle, natural way of following IF for people who want the benefits but can't or don't want to follow a longer fast. It still provides a solid fasting window for reparative processes, but it is easier to implement. Here you'd stop eating at 8pm and have breakfast the next day at 8am, which is a pretty natural pattern for most.

Alternate day fasting and eat-stop-eat

These two patterns are some of the more restrictive forms of IF and are not appropriate for everyone. 

  • Alternate day fasting typically means fasting for 24 hours and then eating normally the following day, repeating the pattern consistently. 
  • Eat-stop-eat is similar in that it includes a full 24 hour fast, but you'd only do it once or twice a week and not on consecutive days. The other five days, you'd eat without any restriction. 

For example, someone following eat-stop-eat would fast on Monday and Thursday for the entire day but eat the rest of the week as usual.

The 5:2 Fast

The 5:2 fast is similar to eat-stop-eat, but it allows small amounts of food  (500 to 600 calories) during the two days of fasting. This is often used in research studies, but again is very restrictive and not for fasting beginners.

One Meal A Day (OMAD)

Just as it sounds, with OMAD, you eat one meal a day (or within a short window). While some may lose weight using this pattern, in most cases, it's just too stressful on the body and is a setup for disordered eating.

Types of Intermittent Fasting Infographic: a visual summary of the details of intermittent fasting plans featured in the content above.

Is it a good idea to intermittent fast every day?

As with anything in the world of nutrition, what works for one person may not work for you. Bio-individuality means that we will all respond differently, so it's essential to understand how your body responds to fasting as you decide how to proceed.

Some people do fine with daily longer fasts, while others see better results with a gentler fast only a few days a week. Overly stressing the body can negatively impact your health, so if you notice your energy levels drop or some of the benefits you saw at the beginning start to plateau, it's time to take another look.

<p class="pro-tip">Key takeaway: Fasting can be modified to fit your lifestyle and should not add excess stress. If you feel starving, low energy, or aren't seeing the health benefits you were looking for, it may not be the best fit for you.</p>

What can I eat or drink while intermittent fasting?

While fasting, you can have:

  • Plain water
  • Sparkling flavored water
  • Unsweetened tea
  • Black coffee

Since metabolic adaptations happen when your body goes without nutrients, fasting means you should avoid eating or drinking anything with calories. These beverages also help you stay hydrated even when you aren't eating.

What are intermittent fasting health benefits?

There is a lot of research on the benefits of intermittent fasting, so much so that it seems like there's a new headline daily. Here are some of the top benefits of IF.

1. Fasting can help weight loss without calorie restriction

Some people lose weight because they eat less by limiting their eating window, but IF may support weight loss for other reasons<sup>6</sup>. A study comparing weight loss interventions found that women following IF lost as much weight as those following a calorie-restricted diet<sup>7</sup>. 

Many studies echo these results and conclude that IF is as effective, if not more, than calorie restriction for weight loss<sup>8</sup>. Since the body utilizes fat for fuel during fasting, IF may also be a way to target and turn on fat-burning to promote weight loss<sup>3</sup>.

These studies are examples of why IF is so popular for weight loss, as it may feel less restrictive and easier to follow longer-term.

2. Intermittent fasting could lower diabetes risk

IF is also used to support blood sugar and insulin levels. Research suggests that IF helps people with diabetes improve blood sugar levels without following the typical low-calorie weight loss advice<sup>9</sup>.

In human and animal studies, IF appears as effective (if not more) than caloric restriction to improve insulin sensitivity or how efficiently your cells can move sugar out of the blood<sup>9, 10</sup>. Further, IF can improve blood sugar and glycemic response without any other changes or dietary restrictions<sup>11</sup>.

The timing of the fast may also come into play, as one study found that an IF pattern where the first meal was earlier in the day improved beta-cell responsiveness (the cells in the pancreas that make insulin), insulin sensitivity, and markers of oxidative stress compared to a group eating later during a 12-hour window<sup>12</sup>.

3. Intermittent fasting may support healthy aging

Fasting could support longevity and healthy aging. More research is needed, but animal studies show that fasting can increase life expectancy<sup>2</sup>. The effect on longevity is likely related to autophagy, and Autophagy cleans out damaged cells to reduce oxidative damage,  a potential root cause in the aging process<sup>13</sup>.

4. Intermittent fasting could protect your brain

Once again, the power of autophagy extends to brain cells by removing aging and damaged cells that could decrease the risk of neurological diseases<sup>14</sup>. Inflammation (aka inflammaging) is also closely associated with the aging process. Fasting is also linked to improved cognitive function, including memory and neurodegeneration<sup>15</sup>.

5. Intermittent fasting is heart-healthy

IF may also offer cardiovascular protection with or without weight loss by lowering cholesterol, blood pressure, and triglycerides<sup>11, 16</sup>. And as mentioned earlier, the inflammatory markers associated with increased risk of cardiovascular diseases are positively affected by intermittent fasting<sup>3</sup>.

Is intermittent fasting safe for everyone?

Fasting can be safe for nearly everyone because you can adapt it to your lifestyle. It's possible to take any diet alteration. Extended fasts are slippery for disordered eating habits, especially if followed very rigidly.

The good news is that you don't have to be overly restrictive, as simply fasting for 12-hours overnight still provides benefits. Most people do well within the 12- and 16-hour fasting window, but it's important to listen to your body even then.

Signs you've gone a bit too long include fatigue or hunger to the point of dizziness or nausea or simply not feeling well. The goal of IF is to help you feel good, so if it's not working for you, don't force it.

There are several contraindications to fasting, including<sup>2</sup>:

  • Pregnancy or breastfeeding
  • History of eating disorder
  • Underweight
  • Reactive hypoglycemia

If you are taking any medications, especially those for diabetes that affect your blood sugar, it's essential to check with your doctor before fasting. Children or older adults should not participate in extended fasts as well.

Intermittent fasting and women's hormones

It's always crucial to note that women may respond differently to fasting than men because of hormones. Women's bodies and hormones are susceptible to perceived stress from nutrient scarcity, so restriction of any kind, from calories to prolonged fasting, can throw off hormone balance and adversely impact a woman's menstrual cycle<sup>17, 18</sup>.

Does this mean all women shouldn't fast? No, many women fast and see a lot of benefits. But depending on how your body responds, you may need to adjust the length of your fast or how many days you do it. Always pay attention to the signs your body gives you, and if you notice any changes to your cycle, it's time to make adjustments or stop fasting altogether.

Getting started with intermittent fasting

If you're brand new to IF, here are some helpful tips to get you started:

  • Plan ahead and think about what you'll eat to break your fast, as well as the rest of the day. If you're hungry, you are more likely to make an impulsive choice, even if it's not the healthiest option. Make sure that the first meal has fiber, fat, and protein to balance your blood sugar and energy response.
  • Don't restrict yourself during your eating window. Make sure you eat enough food during your eating window. Over restriction means you'll be more hungry while fasting, plus it adds unnecessary stress to your body.
  • Choose nutrient-dense foods. Food quality still matters because it provides essential vitamins, minerals, and phytochemicals. Avoid processed and packaged foods and reach for whole foods like vegetables, fruits, legumes, nuts, and seeds instead.

Above all, start slow and listen to your body. Diving into an extended fast is not a great idea if you're new to fasting, but an overnight fast is a great place to start. You may find that it fits perfectly with your lifestyle, or you can try to extend it a bit if it works for you.

See How Your Body Responds to IF with Signos 

Signos makes it easy to see exactly how your body responds to intermittent fasting. You can learn more about how using real-time feedback from a continuous glucose monitor (CGM) makes it that much easier to find an eating and fasting pattern to match your lifestyle here.

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About the Author

Caitlin Beale Headshot
Caitlin Beale is a registered dietitian and nutrition writer with a master’s degree in nutrition. She has a background in acute care, integrative wellness, and clinical nutrition.
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Please note: The Signos team is committed to sharing insightful and actionable health articles that are backed by scientific research, supported by expert reviews, and vetted by experienced health editors. The Signos blog is not intended to diagnose, treat, cure or prevent any disease. If you have or suspect you have a medical problem, promptly contact your professional healthcare provider. Read more about our editorial process and content philosophy here.

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