By Dr. Kristine Sarna, MD
Fasting has been practiced for centuries across many faiths and cultures around the world. Traditionally, it has been valued for its role in self-reflection, spiritual growth, and personal discipline. What many people may not realize is that fasting can also offer powerful benefits for physical health when done thoughtfully and safely.
When we eat regularly throughout the day, our bodies are constantly focused on digestion and nutrient processing. During periods of fasting, however, the body is able to shift its attention elsewhere — specifically toward internal repair and regeneration. This process is sometimes described as the body “cleaning house.”
What Happens in the Body During a Fast?
When food intake is restricted, the body begins clearing out damaged or dysfunctional cells (often referred to as “zombie cells”), removing cellular debris, and supporting the regeneration of healthier new cells. Toxins are more efficiently eliminated, inflammation may decrease, and many people report noticeable improvements in energy levels and mental clarity.
These cellular repair processes are a major reason fasting has gained attention in the medical and wellness communities.
How Long Should You Fast?
One of the most common questions I hear is: How long do I need to fast to see benefits, and is it safe?
Research suggests that longer fasts — particularly a 72-hour (three-day) water fast — may produce the most significant cellular benefits, including enhanced detoxification and stimulation of stem cell regeneration. However, this type of prolonged fast is not appropriate for everyone. A 72-hour water fast can increase the risk of hypoglycemia, dehydration, and electrolyte imbalances, especially in individuals with underlying medical conditions.
Because of these risks, any extended fast should only be done under medical supervision, and after discussing your individual health history with your provider.
The Benefits of Shorter Fasts
Fortunately, you don’t need to fast for three days to experience meaningful health benefits.
A 24-hour water fast, done occasionally, can be both effective and safe for many people. Even a single day of fasting allows the body to initiate repair processes and metabolic reset.
Another highly beneficial and more sustainable approach is intermittent fasting, also known as timed eating. In this method, food intake is limited to an 8-hour window each day, followed by a 16-hour water fast. This routine can be practiced daily and has been shown to improve metabolic health, insulin sensitivity, and overall energy levels.
Some individuals choose to incorporate fasting weekly, monthly, or quarterly, depending on their lifestyle and health goals. The key takeaway is that benefits accumulate with each fasting cycle, regardless of the specific schedule you choose.
Exercise During Fasting
Because energy production naturally decreases during fasting, aggressive or high-intensity exercise is not recommended. However, gentle movement — such as walking — is encouraged. Light activity supports circulation, lymphatic flow, and toxin clearance without placing undue stress on the body.
A Final Word on Safety
Fasting can be a powerful tool for improving health, but it is not one-size-fits-all. Before starting any fasting regimen — especially prolonged or water-only fasts — it is essential to speak with your medical provider about your personal risks, medications, and health conditions.
When approached with intention and proper guidance, fasting can be a meaningful way to support cellular health, mental clarity, and overall well-being.
— Dr. Kristine Sarna, MD
