Fasting for Health and Longevity: Unlocking the Power of Metabolic Rejuvenation

As a physician, I've counseled hundreds - perhaps thousands - of patients struggling with weight loss. With the advent of GLP-1 agonists, I started noticing a pattern emerge that differentiated the patients most successful - and it wasn’t simply taking the medications.

It led me to consider the role of our insulin and sugar pathways, which brought me to the role of fasting. Fasting is a practice that has been a part of human culture for millennia, with roots in various religions and spiritual traditions. But in recent years, the science behind fasting has exploded, revealing a host of benefits that go far beyond mere calorie restriction.

In this post, I'll dive into the science of fasting, exploring the most common types of fasting for health, the mechanisms by which it works, and the incredible health benefits it can provide. Whether you're looking to lose weight, improve your metabolic health, or even slow down the aging process itself, fasting might be the ancient key that provides a novel unlock for you.

In This Post

• Types of Fasting
• How Fasting Works: The Science Behind the Benefits
• Health Benefits of Fasting
• Fasting for Longevity: Can It Slow Down Aging?
• Fasting vs. Starvation: What's the Difference?
• Who Should Not Fast?
• Implementing Fasting into Your Life

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Types of Fasting

To start, we all fast! We fast each night when we sleep and yet we somehow survive 😊.

I mention this because, when most people think of fasting, they picture extended periods without food. But there are actually several different types of fasting, each with its own unique benefits and challenges.

1. Intermittent Fasting (IF): This popular approach involves alternating periods of fasting with periods of eating. Common IF protocols include:
   • 16/8: 16 hours of fasting, followed by an 8-hour eating window. This is also known as skipping breakfast, haha.
   • 5:2: 5 days of normal eating, with 2 non-consecutive days of restricted calorie intake (usually 500-600 calories).
   • Eat-Stop-Eat: 24-hour fasts once or twice per week, with normal eating on other days.
2. Time-Restricted Feeding (TRF): This concept overlaps with IF, leading sometimes to confusion. TRF simply means confining your eating to a specific window of time each day. The most common approach is the 16/8 method (16 hours of fasting, followed by an 8-hour eating window), but some people prefer shorter or longer eating windows. We often use IF and TRF somewhat interchangeably, but TRF does not include prolonged fasts beyond a single day (see below).
3. Prolonged Fasting: This involves fasting for extended periods, typically 24 hours or longer. Prolonged fasting can be challenging and may require medical supervision, particularly for those with certain health conditions, as we’ll go into later in this post.

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How Fasting Works: The Science Behind the Benefits

So how exactly does fasting work? The key lies in a process called metabolic switching. When you fast, your body shifts from using glucose (sugar) for fuel to using ketones, which are derived from stored fat.

Another way of thinking about it is that when you fast beyond 12 hours, your body starts snacking on your own fat!

This shift triggers a cascade of beneficial changes in the body, including:

1. Increased ​Autophagy​: Fasting activates a cellular "cleanup" process called autophagy, in which damaged or dysfunctional proteins and organelles are broken down and recycled.
2. Hormonal Changes: Fasting alters levels of several ​key hormones​, including insulin, growth hormone, and norepinephrine. These changes can improve insulin sensitivity, promote fat burning, and even boost brain function. Insulin is the key hormone that instructs your body to store fat - and it gets triggered on when you eat!
3. Reduced Inflammation: Chronic inflammation is a major driver of many diseases, including heart disease, diabetes, and cancer. Fasting has been shown to reduce inflammation and oxidative stress, potentially ​lowering disease risk​.

Health Benefits of Fasting

The metabolic changes induced by fasting translate into a wide range of potential health benefits, including:

1. Weight Loss: By reducing overall calorie intake and promoting fat burning, fasting can be a powerful tool for ​weight loss​ and improved body composition.
2. Improved Metabolic Health: Fasting has been shown to ​improve insulin sensitivity​, lower blood sugar levels, and reduce risk factors for metabolic diseases like type 2 diabetes.
3. Cardiovascular Health: Some studies suggest that fasting may improve heart health by reducing blood pressure, cholesterol levels, and inflammation.
4. Brain Health: Fasting has been linked to improved brain function, increased neuroplasticity, and even reduced risk of neurodegenerative diseases like Alzheimer's and Parkinson’s.

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Fasting for Longevity: Can It Slow Down Aging?

Perhaps the most exciting potential benefit of fasting is its ability to slow down the aging process itself. ​Animal studies​ have consistently shown that calorie restriction (one form of fasting) can extend ​lifespan​ and healthspan.

While the longevity effects of fasting in humans are still being studied, there's reason to believe that the benefits seen in animals may translate. Fasting has been shown to activate many of the same ​longevity pathways​ as calorie restriction, including reduced inflammation, increased autophagy, and improved metabolic health.

Fasting vs. Starvation: What's the Difference?

One common misconception about fasting is that it's the same as starvation. While both involve periods without food, there are some key differences between the two:

1. Intentionality: Fasting is a voluntary, controlled practice that involves deliberately abstaining from food for a set period of time. Starvation, on the other hand, is an involuntary state of severe food deprivation, often due to external circumstances like poverty, famine, or eating disorders.
2. Duration: Fasting is typically practiced for shorter periods, ranging from a few hours to a few days. Starvation, however, can last for much longer periods, leading to severe nutrient deficiencies and health problems.
3. Physiological Response: During a fast, the body switches from using glucose to using fatty acids (broken down fats) for fuel, which can have various health benefits (as discussed earlier). In starvation, however, the body's nutrient reserves become severely depleted, leading to muscle wasting, organ damage, and other serious health issues.
4. Mental and Emotional Impact: Fasting is often practiced for spiritual, health, or personal reasons, and can be a positive, empowering experience. Starvation, on the other hand, is a traumatic experience that can have severe psychological and emotional consequences.

The takeaway is that fasting is voluntary with a pro-health intention, while starvation is not. You should only fast if your body has enough nutrients stored up (usually in the form of fat or adipose cells). If you are malnourished, it is not be a good idea to fast (ask your doctor if you are not sure!).

Who Should Not Fast?

While fasting can be a beneficial practice for many people, it's not appropriate for everyone. Here are some groups who should avoid fasting or only fast under close medical supervision:

1. Pregnant and Breastfeeding Women: Fasting during pregnancy and breastfeeding is complicated and potentially harmful to both the mother and the baby. These periods require increased nutrient intake to support growth and development, and fasting can lead to nutrient deficiencies. There is limited research in this space given the difficulty of setting up a study with pregnant or breastfeeding women, and unfortunately there is ​no consensus​ among physicians about when fasting, religious or otherwise, is acceptable in such populations.
2. Children and Adolescents: Children and teenagers are still growing and developing, and they have increased nutrient needs. Fasting can negatively impact their growth, development, and ​overall health​.
3. People with Eating Disorders: Fasting can be ​triggering​ for those with a history of eating disorders like anorexia nervosa or bulimia. It can exacerbate disordered eating patterns and lead to relapses. I can't emphasize enough how people who have ongoing struggles with eating disorder should consult their clinician or avoid fasting practices completely. That being said, limited ​research​ has shown short-term fasting to be safe among young adults without an eating disorder.
4. People with Severe Health Conditions: As a catch-all, it’s imperative that people with severe medical conditions (​diabetes​ that is uncontrolled or on medications such as insulin, chronic kidney or ​liver disease​, ​severe heart disease​, ​adrenal insufficiency​, or general ​fragility​) consult their physicians whenever considering a major health practice like fasting. That being said, there are surprising conditions that might benefit from structured fasting practices (cancer, for example).
5. People Taking Certain Medications: Some medications need to be taken with food to work properly or to avoid side effects. Fasting can interfere with the effectiveness and safety of these medications. It's crucial to talk to your doctor before fasting if you're taking any prescription medications.
6. Some Athletes: Fasting can lead to decreased energy levels, impaired performance, and slower recovery in certain athletes and highly active individuals. These groups may need to modify their fasting practices or avoid them altogether to maintain optimal performance and health.

Fasting is not for everyone. It's a powerful intervention that can have profound effects on the body, both positive and negative. If you have any underlying health conditions or concerns, always talk to your healthcare provider first!

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Implementing Fasting into Your Life

Fasting can seem intimidating, but it doesn’t have to be. I have some great lifehacks that helped me and my patients get started that you can try out.

1. Start Slow: If you're new to fasting, begin with a shorter fasting window (e.g., 12/12) and gradually work your way up to longer periods. I personally have found success ramping up a few hours at a time (8 hours fasting, then 12 hours, then 14 hours, and then finally my typical 16-18 hours).
2. Stay Hydrated: During your fasting periods, be sure to drink plenty of water, tea, or other non-caloric beverages to stay hydrated. On prolonged (over 24 hours) fasts, I prefer adding electrolytes (without sugar or other additives) to my water to keep up my blood pressure and energy. Simple Himalayan salt or just table salt can work, or you can get fancy for prolonged fasts with ​fasting salts​ (which include sodium, potassium, and magnesium).
3. Focus on Nutrient Density: When you do eat, make sure you're consuming nutrient-dense foods like vegetables, fruits, whole grains, lean proteins, and healthy fats. This is the key factor that will prevent you from becoming malnourished or feeling off when practicing fasting!
4. Listen to Your Body: Fasting isn't for everyone, and it's important to pay attention to how your body responds. If you feel unwell or experience negative side effects, stop fasting and consult with a healthcare professional. It's important to understand, before you start fasting, what symptoms are expected and what are potentially alarming.

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The Bottom Line

Fasting is an incredible tool for improving health, enhancing metabolic function, and potentially even slowing down the aging process. I have found it to be a useful lifehack for longevity and fat loss in myself and my patients. By understanding the science behind fasting and implementing it carefully into your life, you can unlock a host of benefits that go far beyond simple weight management.

Cheers to your health!

Hillary Lin, MD

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