Fasting 101: Benefits, Risks and When to Do It

Medically Reviewed By William C. Lloyd III, MD, FACS
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You may have recently heard of intermittent fasting, but it isn’t a brand-new diet plan. People have been doing it for centuries. Cavemen who weren’t always able to find food had to fast for long periods of time, and there have always been groups who fast for religious or spiritual reasons. Today, human bodies are still equipped to handle periods of fasting and often function better after a break from processing food.

Most of the fasting people do now is intermittent fasting, where you consume no food or very little during certain time periods while maintaining enough nutrients in your system. These breaks allow your body to burn fat instead of glucose and give your cells time to repair themselves. An intermittent fasting routine can help improve your health, but it’s important to talk with your doctor first and be aware of possible risks.

Benefits of Fasting

Is fasting healthy? There have been many studies with animals, as well as human clinical trials, that show the health benefits of fasting. Here’s how it works: When your body doesn’t have any glucose from food to fuel it, it makes adjustments to your hormones to access body fat for energy. During fasting periods, your body also uses this resting time to make new cells, repair old ones, and remove cells that aren’t functioning well anymore. Imagine how great you feel after a nap. That’s how your body feels inside after fasting, and you’ll see positive changes on the outside too.

Some benefits of intermittent fasting include:

  • Weight loss: Because fasting causes your body to burn fat instead of glucose, you’ll likely lose some weight. Intermittent fasting can also help you reduce calorie consumption, since you won’t be eating as often.
  • Better sleep: If your natural sleep cycle is off, fasting may get your circadian rhythm back in balance as it helps your cells rest, repair and reset.
  • Lower risk of chronic illness: Studies show intermittent fasting lowers blood pressure, reduces inflammation, and helps maintain stable blood sugar levels. This can prevent or improve conditions like heart disease, diabetes, and arthritis.
  • Protection against neurological diseases: Fasting increases the brain hormone BDNF, which stimulates the production of new brain cells and strengthens existing cells. This helps protect against neurological problems, such as Alzheimer’s disease.

Risks to Consider When Fasting

Though the benefits of fasting may be appealing, it’s not for everyone. One study reported intermittent fasting may cause negative changes in mood and energy levels, increase obsessive thoughts about food, and lead to overeating during non-fasting periods. Fasting is also not recommended for people who take medications that require them to eat regularly.

If any of these conditions/situations apply to you, avoid fasting or only fast with guidance from your doctor:

  • Pregnant or breastfeeding
  • Eating disorder
  • Actively growing, such as in adolescence
  • Taking medicine that requires food

It can take a few weeks for your body to adjust to fasting. If you experience prolonged side effects like dizziness, trouble concentrating, headaches, irritability, or any unpleasant mental or physical effects, fasting may not be right for your body.

Ready to Try Fasting?

Before you start a fasting program, it’s smart to talk with your doctor about the best approach and when to fast. It’s important to find the right fasting plan for your body and lifestyle.

Common fasting routines include:

  • Fast every other day.
  • Eat normally five days a week and fast two non-consecutive days.
  • Time-restricted daily fast where you eat normally during an 8-hour window each day and fast for 16 hours. For example, skip breakfast and eat lunch and dinner between noon and 8 p.m.

When you’re starting a time-restricted schedule, you can ease into it with a 12-hour fast and 12-hour eating window, adding an hour onto your fasting window every few days until you reach 16 hours of fasting. You may even want to try a few full-day fasts each week to see how it feels.

Remember, nutrition is very important while fasting. You’ll get the biggest benefits when you eat healthy meals during your non-fasting time. Think fruits, vegetables, protein, and limited amounts of refined carbohydrates. During your fasting days or hours, drink water, tea, coffee or clear broth to stay hydrated.

Intermittent fasting can have a variety of health benefits—if it’s right for you. Talk with your doctor about how to approach fasting safely and to discuss any health risks that may prevent you from incorporating fasting into your diet. Together, you can find the right eating plan to maintain a healthy weight and lower your risk of chronic conditions.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Aug 17
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
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