To Fast or Not To Fast?

If you’re following conversations in the medicine and wellness worlds, you’ve probably heard about the benefits of fasting for health and longevity. You may have also heard that fasting can be dangerous or counterproductive. So which is true? 

If done properly, fasting can help speed recovery, boost immunity and cellular clean-up, decrease inflammation, and reverse conditions like diabetes and high cholesterol. As a therapy, it’s pretty accessible. (As a physician, I always look for low-cost options for my patients.)

But fasting is not for everyone. When done improperly, fasting can initiate a cascade of imbalances, leading to symptoms like poor concentration and sleep, low energy and poor performance. 

Interested in learning how and whether to fast to help your health and longevity? Read on.

Benefits of fasting

At this point, a good amount of research demonstrates the benefits of fasting. According to these studies, fasting supports: 

  1. metabolic flexibility
  2. healing 

Let’s talk about each separately. 

First is metabolic flexibility. Fasting helps support metabolic flexibility because fasting forces the body to source energy from stored fat.

Imagine that you are hungry. You’re craving a snack. What’s your next move? Maybe you go to the fridge or pantry. Maybe you go to a café or order takeout. Or maybe you dig into the chest freezer in the garage.

If you’re more likely to grab something quick and accessible (and maybe even processed), you’re acting in a way that makes a lot of evolutionary sense. As humans, we’re hard-wired to seek out foods that are immediately available and calorically dense. 

Historically, though, our ancestors didn’t have the luxury of reaching into the fridge, pantry or freezer. They would eat when they could and convert food energy to fat, stored throughout the body, and glycogen, stored in the liver and muscles. 

When our ancestors experienced food scarcity, their bodies would dig into glycogen stores. When glycogen ran out, bodies would turn to fat. In our analogy above, the glycogen stores are like going to the pantry or fridge; fat stores are the equivalent of going to the deep freezer. 

Because many of us often have easy access to calories, our bodies don’t know how to mine fat for calories. By fasting, our bodies learn how to root around in the deep freezer (aka fat stores) for energy. A body that can metabolize fat is also less likely to struggle with blood sugar regulation (and less likely to get “hangry”), weight loss, and athletic performance. 

Now let’s think about healing. 

Fasting increases the body’s healing ability because fasting increases autophagy and stem cell production. Autophagy is the body’s ability to identify and expel cells with damaged DNA. Think of autophagy as a deep cleaning of your cells. This is an important function of the body; without autophagy, damaged cells can accelerate aging and cause conditions of aging, such as cancer. After about 48 hours of fasting, autophagy increases. 

We tax our digestive systems when we eat throughout the day (and even the night). Our immune and digestive systems become more focused on processing food and don’t have the capacity for cellular clean up. Because so much immunity occurs in the gut, we can suppress immunity with frequent snacking. 

After about 48-72 hours of fasting, the body’s stem cell production increases. This allows the body to send these undifferentiated cells to damaged tissues to help them heal. This is why I recommend fasting to many patients seeking regenerative injections for musculoskeletal injuries.

All this said fasting isn’t for everyone. 

Depending on your metabolic health, fasting could lead to dangerous electrolyte imbalances. For this reason, I recommend fasting-mimicking diets for some patients, especially those who are new to fasting. Fasting-mimicking diets can help provide the benefits of metabolic stress and autophagy with fewer of the associated risks of water fasts. 

There have been studies that have linked fasting with a decrease in lean muscle mass. When I consider fasting, I make sure that I’m also thinking about protein consumption before and after. I also want to ensure I’m incorporating weight training or weight-bearing activities into my routine. 

In the long term, fasting bolsters immunity. But in the short term, fasting compromises immunity, especially on and after day three of a fast. If possible, it’s best to avoid potential exposure to contagions on days three through six. 

For people with hypothalamic-pituitary-adrenal-axis dysregulation (or HPA-axis dysregulation, sometimes called “adrenal fatigue”), inadequate carb intake can worsen symptoms. 

We also need more research to understand the impacts of fasting on all people. As with many medical studies, most participants in fasting studies are cis men. Emerging evidence shows that fasting is more likely to negatively influence sleep, muscle mass, and hormone balance in people who menstruate. Pre-menopausal women are more likely to experience the benefits of fasting during the first half of the menstrual cycle when hormones are more similar to men, who are the default fasting study subjects. 

Inadequate carbohydrate intake–through dieting, misunderstanding the needs of the body, or fasting–is common in women, particularly in women who exercise hard. I’ve worked with many women athletes whose carb-poor diets have led to hormone disruption and declining performance. Relative Energy Deficiency in Sport (RED-S) is common in female athletes. A 2019 survey of 1000 female athletes across over 40 sports, published in the British Journal of Sports Medicine, estimated the risk of low energy availability in women athletes at more than 47%. Again, I recommend menstruating women plan to both fast and train their hardest during the lowest hormone phase, or the first two weeks, of the menstrual cycle. 

How does intermittent fasting relate to all of this? 

Intermittent fasting, or time-restricted feeding, involves eating during a confined period of time. There are lots of ways to do this. The most common is eating for eight hours and fasting for sixteen within twenty-four hours. It’s important to note that no calories can be consumed during fasting; even milk in coffee or honey in tea resets the clock. 

Intermittent fasting supports metabolic health and flexibility. It can also be a great way to prepare for a longer fast. However, many of the same potential risks associated with fasting could present with intermittent fasting. 

Time-restricted feeding does not last long enough to induce significant autophagy or stem cell production. Longer fasts are needed for this.

Interested in learning more? Or get one of our fasting mimicking kits, discounted for a limited time. 

Check out Valter Luongo’s book on fasting mimicking or Stacy Sim’s book Roar for info. on women and fasting. If you’re local to Moab and Victor, come by and borrow a book from our library. 

If you are ready to try fasting, check out the recommended Prolon fasting mimicking 5-day plan and its benefits at , where you can order by choosing “Patient Registration” and save by selecting “Allison Mulcahy, MD.” A discount will be provided at purchase/checkout.

To Fast or Not To Fast?

For further learning and information, stay tuned for future posts on intermittent fasting.