Potential Side Effects of Fasting and How to Avoid Them

While fasting can offer a wide range of health benefits, some people may experience side effects, depending on how caloric restriction is implemented, the duration of fasting, and other factors related to nutrition and individual health.

Discover potential side effects of fasting and their common causes, and explore practical strategies to maintain a safe and effective fasting routine.

What is fasting?

Fasting is a mealtime approach that involves prolonged periods without food rather than continuous eating throughout the day, often extending the time between meals and reducing the frequency of eating. 

Several fasting approaches exist, including intermittent fasting, one-meal-a-day (OMAD) fasts, alternate-day fasting, and prolonged fasts such as 24- or 48-hour fasts. 

These time-restricted eating plans vary in structure and duration but are based on the same core principle of designated fasting periods followed by defined eating windows.

Health benefits associated with fasting include improved insulin sensitivity, which supports better blood sugar control and overall metabolic and heart health.

A study published in Cell Metabolism found that limiting food intake to a consistent daily eating window improved insulin sensitivity and cardiometabolic markers, without the need for intentional calorie restriction.

Weight loss is another commonly reported outcome of fasting, as structured meal timing supports fat utilization and hormonal balance linked to improved weight management. 

A review in the Journal of Diabetes Research reported that fasting may support weight loss through improvements in metabolic markers associated with obesity.

When practiced appropriately, fasting can support metabolic health and body weight regulation, whereas inconsistent or poorly planned approaches may lead to unwanted effects.

Watch the video below to learn about the benefits and side effects of fasting.

8 potential side effects of fasting

Fasting can impact hydration status, nutrient availability, and energy balance. This can lead to side effects in some individuals, particularly when fasting is prolonged or introduced without adequate preparation.

Here are eight potential side effects of fasting.

1. Fatigue and mental sluggishness

Frequent fasting, combined with inadequate nutrient intake, can contribute to low energy levels and mental fatigue, especially during prolonged fasting periods.

The body requires specific vitamins and minerals, particularly electrolytes and B vitamins, to maintain optimal metabolic energy-making processes.

Lack of these nutrients can lead to low energy production linked to common fasting side effects such as weakness, brain fog, or difficulty concentrating.

A study published in the Journal of International Medical Research summarizes, “B complex vitamins are essential for mitochondrial function, and a lack of just one of these vitamins may compromise an entire sequence of biochemical reactions necessary for transforming food into physiological energy.”

2. Headaches

Headaches are commonly reported during fasting, especially when fluid or electrolyte intake is insufficient. Reduced sodium intake and fasting-related changes in hydration status may contribute to pressure-like or tension headaches.

In some cases, caffeine withdrawal can also play a role, particularly for individuals who regularly consume caffeinated beverages. 

Fasting headaches are often temporary and typically improve as hydration and electrolyte balance stabilize.

3. Mood changes and irritability

Some individuals experience irritability, mood disturbances, or difficulty concentrating when fasting. 

These effects are often reported during the early adaptation phases to caloric restriction and may be related to shifts in blood sugar regulation or changes in stress hormone activity.

Mood-related symptoms typically subside as eating patterns become more consistent and the body adapts to longer periods without food.

4. Dizziness or lightheadedness

Lightheadedness or mild dizziness may occur when fasting is started abruptly or when meals are skipped without adequate hydration. 

Temporary changes in blood pressure regulation, often related to fluid or electrolyte shifts, may contribute to these symptoms.

These sensations tend to lessen as the body adapts to changes in fluid balance and meal timing.

5. Digestive discomfort or nausea

Digestive symptoms, including nausea or stomach discomfort, may occur after longer fasting periods or when food is reintroduced too quickly after periods of caloric restriction.

Fasting-related digestive problems are often associated with temporary changes in stomach acid production, bile flow, and digestive enzyme activity, and typically improve when easy-to-digest meals are consumed to break a fast.

6. Sleep disturbances 

Changes in meal timing, hydration status, or electrolyte balance may affect sleep quality, especially in those new to fasting. 

While some individuals report difficulty falling asleep or fragmented sleep during early adaptation, fasting is generally associated with better long-term sleep quality and more balanced circadian control. 

7. Hair loss

Hair thinning may occur when fasting is combined with inadequate intake of protein and nutrients needed to support healthy hair growth. Shedding of hair isn’t caused by fasting itself but may develop when nutritional needs aren’t met during eating periods.

Ensuring adequate nutrient intake during eating windows helps support normal hair growth and overall tissue maintenance.

8. Change in body composition 

Unfavorable changes in body composition, such as loss of lean mass, may occur when fasting is paired with insufficient protein intake or prolonged low-calorie eating patterns.

Maintaining an adequate intake of high-quality protein and prioritizing nutrient-dense meals during eating periods supports the preservation of lean muscle mass while fasting.

Tips to avoid fasting side effects

Not everyone experiences side effects while fasting, as proper preparation, gradual adaptation, and a nutrient-dense eating plan significantly reduce the risk of adverse reactions.

Here are five tips to avoid fasting side effects.

1. Follow a consistent fasting plan

Maintaining a consistent fasting schedule may help the body adapt more effectively to changes in meal timing.

This consistency supports metabolic adjustment and reduces the likelihood of developing fatigue, irritability, or digestive discomfort during fasting.

2. Extend fasting periods gradually

Starting with shorter fasting windows allows the body to adapt more comfortably. Gradually extending fasting periods may help reduce fatigue, headaches, and dizziness during early adaptation.

Gradually extending fasting periods helps the body slowly adjust energy metabolism and hormone levels, making the transition to longer fasts smoother and more sustainable.

3. Maintain hydration and electrolyte balance

Adequate fluid intake supports cardiovascular function and blood pressure regulation, and maintaining electrolyte balance is essential for nerve signaling, muscle function, and overall stability.

Prioritizing adequate hydration and electrolyte replacement can help minimize headaches, dizziness, and fatigue during fasting by supporting normal circulation and blood pressure regulation.

4. Prioritize nutritious whole foods 

Meals consumed during eating windows should provide sufficient protein, healthy fats, and essential micronutrients, and pairing fasting with a poor diet increases the likelihood of side effects.

“The purpose of eating isn’t just to get fuel, it’s also to replenish nutrients,” explains Dr. Berg. “During fasting, nutrient intake is limited, and if your meals don’t supply the nutrients your body needs, you may develop deficiencies and associated health issues.”

A nutrient-dense, low-carbohydrate diet such as Healthy Keto® complements fasting plans by prioritizing nutrient-rich foods while promoting fat burning and ketosis. 

5. Break fasts gradually

Reintroducing food slowly after extended periods of fasting supports digestive health and reduces gastrointestinal strain.

Starting with easily digestible options, such as bone broth, soups, or cooked vegetables, allows the digestive system to resume activity more comfortably before progressing to larger or more complex meals.

Who shouldn’t fast?

Fasting isn’t recommended for pregnant or breastfeeding individuals, older adults experiencing significant muscle loss, or those with a history of eating disorders.

Certain health conditions may increase the risk of adverse effects during fasting. Individuals with diabetes who use medications that affect blood sugar, those with recurrent hypoglycemic symptoms, or people at risk of malnutrition should consult a healthcare professional before starting any fasting regimen.

FAQ

Sources

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC5990470/
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC10005873/
3. https://journals.sagepub.com/doi/abs/10.1177/147323000703500301