Intermittent Fasting: 9 Possible Side Effects

Intermittent fasting provides numerous health benefits but also has side effects. This article covers 9 possible downsides of intermittent fasting.

Hunger and Cravings

When you reduce your calorie intake, you may experience increased hunger. According to a study involving 112 individuals, participants in an intermittent fasting group who consumed 400 or 600 calories twice weekly for a year reported higher hunger scores than those who ate a low-calorie diet with continued calorie restriction.

Digestive Issues

Intermittent fasting can result in digestive issues such as diarrhea, constipation, and nausea. Changes in your diet caused by intermittent fasting programs can also result in bloating.

Headaches and Lightheadedness

A 2020 review of 18 studies examining individuals undergoing intermittent fasting programs reported some participants experiencing mild headaches. People who commonly experience headaches also have an increased possibility of experiencing headaches during fasting than those who don’t.

Fatigue and Low Energy

Studies show that some individuals practicing different methods of intermittent fasting suffer from fatigue and low energy levels. Intermittent fasting is also linked with low blood sugar, making you feel tired and weak.

Irritability and Mood Changes

In a study involving 52 women, participants were significantly more irritable during an 18-hour fasting period than during their non-fasting period.

Sleep Disturbances

A 2020 study that observed 1,422 people who participated in fasting regimens lasting 4-21 days discovered that 15% of participants reported sleep disturbances caused by intermittent fasting.

Bad Breath

Intermittent fasting can cause bad breath due to a lack of salivary flow and an acetone increase in your breath.


You could suffer malnutrition if you fast for long periods and don’t replenish your body with enough nutrients.


During the initial days of fasting, your body releases large amounts of water and salt in your urine. You could suffer dehydration if you don’t replace the fluids and electrolytes through your urine.

Who Should Avoid Intermittent Fasting?

Healthcare professionals advise the following people to avoid intermittent fasting:

● young children and teens
● people who are pregnant or breastfeeding
● older adults who experience weakness
● people with dementia
● People with a history of traumatic brain injury or post-concussive syndrome


Although intermittent fasting is considered safe, studies show it could result in side effects such as hunger, irritability, hunger, headaches, and bad breath. Healthcare professionals also advise people who are breastfeeding or suffering from conditions like dementia to avoid intermittent fasting.

If you’re trying out intermittent fasting, consult your healthcare professional to determine if it’s safe and appropriate.