What is Intermittent Fasting?

Not shockingly, eating less can reduce your risk of becoming obese. Back in my day we just called it skipping dinner. But today’s meal skippers call it “intermittent fasting,” and claim skipping meals makes them more alert and energetic, less stressed, and less prone to getting sick.

CNN asked recently, Is fasting the fountain of youth? Business Insider called it “Silicon Valley’s favorite diet.”

And science backs up a lot of the hype. Anecdotally, fasters find themselves thinking about food less, working out more, and even unintentionally limiting their caffeine intake. “I enjoyed intermittent fasting because I could still eat everything I wanted to without feeling deprived,” Jamie Friedlander wrote for Business Insider about fasting. And scientific studies have shown that intermittent fasting can help people lose fat.

Studies also show fasting is associated with health benefits beyond weight loss including:

And the best part? When you eat, not just what you eat, can make a big difference to your health. None of these benefits require fasters to eat less overall or lose any weight.

Dr. Molly Maloof, Head of Medical Science at Sano Intelligence, is a self-described “full-on fasting cult convert.”


“What I love about fasting is how good my skin looks when I fast,” Dr. Maloof said in a recent interview. She pointed to a phenomenon called autofluorescence. “You can actually measure the light output of a person’s skin,” Dr. Maloof said. “And that correlates to the level of advanced glycation end products. So the more advanced glycation end products your skin accumulates, the older your skin looks, the more wrinkles you get, because the more damage to the proteins in your skin, the collagen and elastin. What I’ve noticed is that with fasting I just see my skin start glowing after a few days. It’s pretty cool.”

“What you're doing when you’re fasting is you’re reducing entropy because you’re reducing heat exchange in your body.”  -Molly

Another benefit? “One thing intermittent fasting will teach you is that what you thought was hunger was probably thirst or boredom,” writes Jenny Sugar, PopSugar’s resident faster.  

Realizing a trend, retailers are cranking out diet books, recipe collections, devices, apps, and supplements. One store alone sells more than 1,500 intermittent-fasting-related items.

Okay, so we have some good evidence that fasting offers a ton of health benefits. But what, exactly, do people mean when they say they’re fasting? How long are they not eating? Are they really not eating anything at all, or just much less?

What is fasting?


There are at least five popular types of fasting, including alternate-day fasting, modified fasting, time-restricted fasting, and Prolon fasting.

Each has its pros and cons.

 Types of fasting.

Types of fasting.


What are the negative effects of fasting?

While the evidence is pretty strong that most people can benefit from intermittent fasting, there are some potential drawbacks to consider. During a period of fasting some people may experience the following unpleasant side-effects:

  1. Slow metabolism

    Long-term fasting can drop your metabolism. However, short-term fasting might speed it up.

  2. The shivers

    Fasting causes your blood to move from your skin to your fat stores so it can take energy from your fat to your muscles so they can use it. Less blood at your skin means you feel colder. So the less you’re eating, the more you might want to wear, especially if you work in an over- air-conditioned office.

  3. Sloth-status

    The less you’re eating, the less energetic you’re going to feel. To preserve your energy stores, your body is going to try to get you to move as little as possible. You’ll get the most benefit out of fasting, and lose the least muscle, if you ignore the instinct to sit still and keep exercising the same as before you started intermittent fasting.

  4. Dehydration

    The average person gets about 20 to 30% of their daily water from food. So if you’re not drinking a third more water than you normally would, you’ll get dehydrated. Unfortunately, dehydration often doesn’t feel like thirst. It feels like dizziness, nausea, headaches, constipation, low blood pressure and low productivity. Perhaps not surprisingly, many of the negative symptoms of a fast, including headaches, dizziness, lightheadedness, fatigue, low blood pressure, and abnormal heart rhythms, are also symptoms of dehydration. Drink water.

  5. The post-fast binge

    Mayo-clinic researchers found that after a fast dieters often crave starchy, high-calorie foods. If you give into temptation and eat a ton of sugar and refined carbohydrates once you break your fast you’ll experience a dangerous spike in blood sugar and undo a lot of the health benefits of fasting. It may also cause you some intestinal distress.

  6. Organ damage

    For some, fasting can negatively affect organ function, including the liver and kidneys. If you have known liver or kidney problems, you may want to consult a doctor before you begin fasting.

Who should avoid fasting?

Fasting isn’t recommended for people with gout, any type of diabetes, chronic kidney disease, eating disorders, heartburn, older adults, pregnant women, and children. That said, Friedlander found her GERD symptoms improved while fasting, so your mileage may vary.

An experience with the Prolon diet