Everything You Need to Know About Intermittent Fasting

Medically reviewed by Natalie Butler, RD, LD on April 6, 2017Written by Jennifer Purdie

What is intermittent fasting?

“Intermittent fasting” is a term used to describe ways you can manipulate your eating patterns. It involves not eating for specific periods of time, with the intent to lose weight and improve health. Intermittent fasting can include anything from multiday fasts to skipping meals a few times a week. Many people consider it appealing because it doesn’t involve large daily calorie restriction.

Fasting is a practice that is centuries old, spanning across cultures, religions and countries. There have been claims that intermittent fasting offers other health benefits besides weight loss, such as:

  • improving the body’s sensitivity to insulin
  • lowering inflammation
  • improving the digestive system
  • reducing body fat

While some research does show short-term benefits such as these, more studies are needed to confirm these findings. Much remains to be learned about the long-term health effects of intermittent fasting.

In one study, fasting helped halt the spread of intestinal bacteria into the bloodstream. Researchers have also suggested that the positive results from fasting can help with brain function.

Types of intermittent fasting

Intermittent fasting can be accomplished in a number of ways.

The 5/2 method

In this version, you restrict your calories on two consecutive days of the week. This is the most popular intermittent fasting method.

  • Pick two days of the week, such as Monday and Tuesday. They must be consecutive days. On these two days, only eat 500-600 calories total.
  • On the other days, you can follow your regular diet. Be sure to not binge on unhealthy foods.
  • Alternative approaches that have been studied using the 2-day approach include following a complete fast on 2 non-consecutive days, or a significantly lowered calorie intake on 2 non-consecutive days.

The 16/8 method

In this version, you only eat during an eight-hour period each day.

  • Skip breakfast every day.
  • Eat only during a set 8-hour period, such as between 11 a.m. and 7 p.m. You should keep this time consistent every day. You should base these hours on when you need food for energy, such as when you’re at work and when you exercise. During these hours, you should eat mainly unprocessed, whole foods.
  • Fast for the other 16 hours. You can drink coffee and diet soda or chew sugar-free gum during this time.


In this version, you fast for 24 hours at a time.

  • Once or twice a week, you don’t eat from dinner time to dinner time the next day. You fast for 24 hours straight. You can drink calorie-free beverages during these hours.
  • You can follow your normal diet the other 5 to 6 days per week. You don’t need to count calories or restrict your diet on the non-fasting days, although you should stick to healthy foods as much as possible.

The warrior diet

This version involves eating one meal per day.

  • Fast for 20 hours per day. You can eat a few servings of raw fruit and veggies, fresh juice, and protein, if needed.
  • Eat one large meal at night. You should eat veggies first, followed by protein and then fat. If you’re still hungry, you can eat carbohydrates. You can do this only after consuming those foods in the given order.

Risk factors to consider

As with any change to your diet, you should consider the risk factors. You should also consult your doctor prior to beginning any diet program.

Potential side effects for anyone who fasts include:

  • fatigue
  • dizziness
  • low energy
  • trouble concentrating

If you decide to follow an intermittent fasting diet, you should drink lots of water. This will help with the loss of hydration you may encounter when eating less food.

The bottom line

With the heightened media attention surrounding intermittent fasting, more evidence is needed to support this type of diet. Though some studies do show that intermittent fasting results in short-term benefits like weight loss, there are few long-term studies. It’s still unclear what this type of periodic eating does to eating behaviors, body composition, metabolic rate, and overall health in the long run. It also hasn’t been confirmed that intermittent fasting has any advantages over regular daily calorie restriction.

Discuss any questions you have with your doctor, and follow their recommendations. You should consider your health history, such as:

  • any issues you’ve faced with regulating your blood sugar levels
  • the impact of fasting on your emotional and psychological state
  • your age
  • your daily activity level before starting this type of diet
  • your current weight (one short-term study reports individuals of normal weight who intermittently fasted had worsening insulin sensitivity)
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