5 science-backed benefits of fasting, and how to fast safely and effectively

Westend61/Getty ImagesFasting is when you do not eat foods or caloric beverages for an extended period of time.
  • The benefits of fasting can include reducing inflammation, boosting cognitive functioning, regulating blood sugar levels, improving heart health, and helping with weight loss.
  • To fast safely and effectively for health benefits, you should discuss your fasting with your doctor beforehand, especially if you have a condition like diabetes.
  • This article was medically reviewed by Samantha Cassetty, MS, RD, nutrition and wellness expert with a private practice based in New York City.
  • Visit Insider’s Health Reference library for more advice.

While many people all over the world fast for religious and cultural reasons, there are also proven health benefits of this practice, as long as you do it safely.

What is fasting?

Fasting is when you do not eat for a certain period of time. If you are fasting for health purposes, you can still drink water or black coffee.

There are many different types of fasting, says Andrew Wang, MD, PhD, a professor of immunobiology at Yale School of Medicine. The main types are:

  • Acute: when you fast for an extended amount of time every once in a while
  • Chronic: when you fast for the same amount of time every day
  • Intermittent: when you follow a fasting schedule of on-days and off-days
  • Chronic intermittent: when you follow a fasting schedule consistently

Benefits of fasting

Fasting has many mental and physical health benefits. Here are five benefits of fasting and how they work:

1. Fasting reduces inflammation

One of the main benefits of fasting is that it may reduce inflammation.

Inflammation is your body’s natural response to infection and usually disappears after damaged cells are healed. However, when your body undergoes oxidative stress — a process caused by an accumulation of free radicals — you can enter a state of chronic inflammation. That’s because these free radicals, which can come from both outside sources like pollution and bodily processes like digestion, begin attacking healthy tissues and cells.

Chronic inflammation can damage healthy cells, tissues, and organs, and is associated with many diseases, including heart attacks and strokes.

There are several ways that fasting may reduce inflammation in the body. First, when your body is in a fasted state, it is unable to convert glucose from food into energy, Wang says. Instead, your body has to rely on an alternative energy source called ketone bodies, which come from fatty acids. Your body produces fewer free radicals when burning ketones, which helps to reduce inflammation.

Fasting may also reduce inflammation by decreasing the number of monocytes — a type of inflammatory white blood cell — in the bloodstream. A small 2019 study on humans and mice found that short-term fasting for 19 hours significantly reduced monocytes in blood circulation.

2. Fasting boosts cognitive functioning

When we age, our organs are prone to chronic inflammation called “inflammaging,” Wang says. Chronic inflammation can contribute to cognitive decline and dementia potentially due to plaque buildup in the brain. Fasting helps to counter this by reducing inflammation, Wang says.

There is a growing body of research that suggests fasting can prevent the development of Alzheimer’s in animals. While the research has not yet been conducted in humans, a 2019 study found that intermittent fasting can slow cognitive decline and improve the symptoms of Alzheimer’s disease in mice.

The cognitive benefits of fasting are not restricted to the elderly. A small 2016 study tested amateur weightlifters after a 48 hour fast and found that fasting improved mental flexibility, which was defined as their ability to quickly and efficiently switch between tasks.

3. Fasting regulates blood sugar levels

Fasting may also regulate blood sugar levels. Maintaining normal blood sugar is important to protect against diseases like type 2 diabetes.

Blood sugar increases when you eat, so it naturally falls when you fast, Wang says. However, your body will prevent your blood sugar from dipping too much by making glucose itself. This keeps your blood sugar at a healthy levels, which is considered above 70 mg/dL.

A small 2019 study of men at risk for type 2 diabetes found that restricting eating to a nine-hour window, called intermittent fasting, helped improve glucose tolerance. If you have diabetes, consult a doctor before fasting, as it can increase the risk of diabetic ketoacidosis for those who use insulin to treat their diabetes.

4. Fasting improves heart health

While research on this topic is limited, Wang says it is reasonable to assume that fasting boosts heart health by reducing inflammation and protecting against diabetes — which are both risk factors for heart attacks and strokes.

A small 2012 study tested Muslims with a history of heart disease who fasted intermittently for Ramadan. After their fast ended, there was an improvement in their 10-year coronary heart disease risk score and a reduction in other heart risk factors like lipids profile, systolic blood pressure, and weight.

5. Fasting helps with weight loss

Fasting helps you lose weight by restricting the number of calories you eat. Intermittent fasting — eating your meals in an eight-hour window earlier in the day — helps with weight loss by keeping blood sugar levels lower in the evening when you are less active.

However, there is mixed evidence regarding if fasting for weight loss is more effective than a calorie-restricted diet. A 2015 review found intermittent fasting tends to lead to weight loss, with participants typically losing around seven to 11 pounds in 10 weeks. However, other studies found that intermittent fasting and calorie restriction were equally effective in helping people lose weight.

If you’re looking to lose weight, talk to your doctor about the best method for you.

How to fast safely

Fasting times can range significantly. While intermittent fasting usually involves fasting for about 16 hours a day, longer fasts can range from 24 to 72 hours.

There’s no magic amount of time you should fast for, Wang says. The best thing to do is to listen to your body and determine what type of fast works for you. If you are feeling jittery, nauseous, faint, or otherwise unwell while fasting, Wang says you should consider breaking your fast.

People who should not fast include those who are:

You should consult with your doctor before fasting if you are:

  • Under the age of 18
  • Have diabetes or blood sugar issues

The bottom line

Fasting may offer health benefits, like reduced inflammation, better heart health, and improved cognitive functioning. However, fasting is not advisable for certain people, and going too long without food can be harmful. If you are interested in fasting, it is important to listen to your body and ask your doctor if it’s safe to fast.

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