Even though intermittent fasting is so popular, there are some people and groups of people that caution against trying this diet. So that begs the question Why is Intermittent Fasting Bad?

Some people see intermittent fasting as a fad diet that people cannot maintain long-term. Some people even claim that there are bad effects of intermittent fasting. Well, depending on how this diet is done, they can be right, meaning intermittent fasting can be good or bad. 

So now you’re probably wondering why fasting is bad and there are a few reasons why.

  1. It can cause an unhealthy relationship with food, especially with people who have had an eating disorder, particularly anorexia. Fasting may also hide this type of behavior and make identifying it much more challenging.
  2. It can cause people to decrease their calories too much, which can have negative effects on overall health.
  3. It may not be healthy for individuals under certain metabolic conditions such as pregnancy or having a metabolic disorder. Studies have not been done in these populations, so we don’t know if there are any benefits to it within these groups.
  4. Some people may not adapt to the extended fasting window and may constantly feel tired, dizzy, and irritable. This will affect their overall feelings of wellness, their relationships, and their work.
  5. Most studies have been done in men. This means we don’t know if intermittent fasting is bad for women’s hormones. 
  6. We don’t fully understand if intermittent fasting is bad for muscle growth. While some studies have shown that you can maintain your muscle mass while fasting, others have shown the opposite. That said, we need more studies to better understand how to prevent muscle loss, often referred to as sarcopenia, especially in women and older people who are more prone to this (1,2). 

If you are interested in trying intermittent fasting, make sure you talk to your doctor first. Additionally, if you try it and feel sick or tired all of the time, you may want to stop because it may not be the best type of diet for you. Be aware of your body and recognize when you may be doing more harm than good. 


1. Moro T, Tinsley G, Bianco A, Marcolin G, Pacelli QF, Battaglia G, Palma A, Gentil P, Neri M, Paoli A. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med [Internet]. BioMed Central; 2016 [cited 2018 Mar 6];14:290. Available from: http://translational-medicine.biomedcentral.com/articles/10.1186/s12967-016-1044-0

2. Tinsley GM, Forsse JS, Butler NK, Paoli A, Bane AA, La Bounty PM, Morgan GB, Grandjean PW. Time-restricted feeding in young men performing resistance training: A randomized controlled trial. Eur J Sport Sci [Internet]. Routledge; 2017 [cited 2018 Mar 6];17:200–7. Available from: https://www.tandfonline.com/doi/full/10.1080/17461391.2016.1223173

Asked by: Jeffrey K.

Clinical Project Manager and Writer

With a Ph.D. in nutritional biology, Dr. Richardson is an expert in the field of nutrition, particularly intermittent fasting. Her dissertation project was an intermittent fasting study in athletes, which is where she gained substantial knowledge on the topic. She has contributed to a number of peer-reviewed articles spanning a wide array of topics and works as a freelance writer trying to make scientific knowledge accessible to the public.