What Are the Potential Side Effects and Disadvantages of Intermittent Fasting?

Intermittent fasting is an eating regimen in which you restrict the timing, and sometimes even the quantity, of meals. The potential side effects and disadvantages of intermittent fasting include malnutrition, dehydration, hunger, and more.

Intermittent fasting has become quite popular as a dietary trend recently. It has been associated with many well-documented health benefits like weight loss, decreased blood pressure, and decreased blood sugar levels, but its disadvantages aren’t as well-known. 

Read on to find out everything you need to know about intermittent fasting side effects, types, and more.

Intermittent fasting is an eating regimen in which you restrict the timing, and sometimes even the quantity, of meals. The main goal of intermittent fasting is to prompt your body to enter a state called ketosis. In this state, the body burns stored fat for fuel instead of using the dietary carbs that we usually eat.

The longer you fast, the longer your body maintains its ketosis state, and the more fat you can burn, eventually resulting in weight loss.

The three most popular types of intermittent fasting regimens include:

• Alternate day fasting: During this type of fast, you’re allowed to eat normally on one day, but the next day, you must fast completely or eat a small, calorie-restricted meal (e.g., less than 500 calories).
• Daily time-restricted fasting: During this type, you fast for fixed periods during the day (e.g., for the 16:8 fast, you fast for 16 hours every day and eat all meals during a fixed 8-hour window).
• Periodic fasting: During this type, you fast on specific days of the week. For example, you can eat normally during the week from Monday to Friday and then fast on weekends (Saturday to Sunday).

Here are the potential side effects of intermittent fasting:

Malnutrition

Prolonged fasts or poorly planned calorie-restriction fasts can lead to malnutrition. If you overeat to compensate for the nutrient deficit after a fast, you will end up damaging your body. 

Moreover, extreme calorie restriction is also not sustainable in the long term, and you become more likely to break your fast. So, plan your fasting regimen well to balance healthy, nutritious meals with fasting periods and avoid malnutrition.

Dehydration

During the initial phase of fasting, your body expels large quantities of water and salt through your urine. This process is called natriuresis of fasting or natural diuresis. If you don’t immediately replace the water and salt (fluid and electrolytes) lost during this period, you could become dehydrated.

Additionally, during fasting periods, since you’re not eating, you may forget to drink water, promoting dehydration. Make sure to stay hydrated throughout your fasting periods, and monitor the color of your pee (urine) regularly. A pale yellow color is normal, whereas a dark color may indicate dehydration.

Cravings and hunger

Increased hunger is a common phenomenon that you may experience during the initial phase of fasting, as your body is not yet used to the decreased calorie intake.

These symptoms usually clear up as your body adapts to fasting regularly and for long periods, though.

Bad breath

During fasting, your body uses stored fats as fuel. A byproduct of this process is acetone, which collects in your blood and may be discernible in your breath during the fasting period.

Because you’re not eating, there’s also less salivary flow in your mouth. This could result in increased acetone build-up in your breath, resulting in bad breath (halitosis).

Additionally, the large quantities of water lost during the early days of fasting could make you dehydrated, giving you a dry mouth and worsening your bad breath.

Digestion problems

Sudden dietary changes such as prolonged fasting can negatively impact your digestive system, resulting in symptoms like diarrhea, constipation, nausea, and bloating.

The dehydration that occurs during intermittent fasting can also worsen constipation. Staying hydrated and eating nutrient-rich foods is critical to preventing these symptoms and maintaining your gastric health.

Headaches and dizziness

Headaches commonly occur during fasting, especially in the early days. These headaches typically affect the front of the brain and are usually of mild to moderate intensity.

If you have a tendency to develop headaches anyway, then you’re even more likely to get a headache while fasting. Since you’re not eating, your low blood sugar levels (and caffeine withdrawal if you’re a regular coffee drinker who abstains while fasting) may trigger these headaches.

Moodiness and irritability

When you’re eating fewer carbs, the resulting low blood sugar levels can make you feel irritated, stressed, and anxious. They can also decrease your concentration.

However, once you overcome this moodiness and irritability, you’re likely to feel pride, satisfaction, and a great sense of achievement after completing your fast.

Fatigue

The low blood sugar levels during fasting can make you feel tired or weak. If you’re also experiencing sleep disturbances during the night due to fasting, this may make you feel very tired throughout the day.

These low energy levels may last only through the initial phase of fasting, though. Once your body goes into ketosis and you begin to burn fat for fuel, you may feel energized again. At this stage, regular fasting is unlikely to make you feel fatigued anymore.

Sleep difficulties

One of the most common side effects of intermittent fasting is a decline in sleep quality, resulting in insomnia and disturbed sleep.

In a study of more than a thousand people who were fasting for 4 to 21 days, more than 15% experienced sleep disturbances: one of the most frequently reported side effects.

The fatigue and dehydration caused by the loss of water and electrolytes during the early fast days may have been partially responsible for these sleep disturbances.

Although intermittent fasting has many beneficial effects, certain populations are at a higher risk of experiencing harmful side effects. These include:

• Pregnant or breastfeeding women
• Children or teenagers
• Older adults with health conditions like chronic fatigue
• People with various health conditions like eating disorders, nutritional deficiencies, dementia, traumatic brain injuries, or concussions
• People who are taking medications that aren’t suitable for fasting

If you experience intermittent fasting side effects like headaches, dizziness, nausea, irritability, unbearable hunger, or fatigue for prolonged periods without any resolution, consider stopping the fast and contact your healthcare provider immediately for medical guidance.

Fasts that make you miserable can be counterproductive. Instead, it may be preferable to focus on eating nutritious meals, sleeping well, engaging in regular physical activity, and managing your stress levels to recover your health. Only fast when you feel physically and mentally prepared.