Fasting- The Good, The Bad, and The Science

lifestyle medicine Nov 16, 2022
Fasting has been a tool used by many for health reasons as well as spiritual reasons for decades. As a coach, I have come across many patients who love using tools such as intermittent fasting to reach their goals. However, it’s hard for me to not reason an eyebrow when I hear of people using fasting as a way to counteract poor daily dietary choices. Just as many try to use exercise to “burn off“ their diet choices that day, I have seen some use fasting as a way to counteract a diet rich in calorically-dense junk foods. Not only does this sabotage weight loss goals, but this also has negative health consequences. This approach will lead to decreased intake of micronutrients, and phytonutrients, and nourishing our bodies with foods required to function at our best. Intermittent fasting can be a great way to improve blood sugars, boost immune function, enhance cognitive function, improve digestion, and stabilize blood sugars when done properly. However, fasting is definitely not for everyone, and some may benefit more from specific types of fasting than others.
There are many factors that must be considered before attempting to do any fast. Let's dive in!
The number one factor to consider before attempting to fast is quality sleep. Sleep is essential for restoring our body, promoting healing, bringing balance to hormones, regulating the nervous system, and maintaining a healthy digestive system. Sleep is essential when considering whether or not one should fast because fasting can be an additional stressor on the body. Yes, fasting can be a positive form of stress on the body, but when one is undergoing physical stress already from poor sleep, fasting may have counterproductive results. When our bodies undergo significant chronic stress, physically or mentally, our resources are depleted and even stressors that were once positive may become negative stressors. Fasting does promote cellular cleanup, known as autophagy, especially during the night. But if you are not getting enough sleep or do not have quality sleep, it is advised that you optimize your sleep pattern prior to including fasting in your routine. If resources are depleted from lack of sleep, your immune system and metabolism will not be working at their best, so fasting may even sabotage weight loss goals and have counterproductive effects on blood sugars.
Speaking of additional stress on the body, another major factor we must look into before starting a fast is exercise. This is another form of stress that is typically positive for the body, but if we are over-exercising or not also allowing time to recover and missing out on quality sleep, we may need to cut back on our exercise or opt for lighter movement such as tai chi, walking, etc. It is advised that you do not engage in strenuous or intense exercise on days when you are fasting, especially if you are doing a prolonged fast. People who enjoy intermittent fasting regularly may need to increase protein on the days when they choose to exercise. It is important therefore to speak with a coach, or your provider team/dietitian to create a plan that works for you.
It may also be useful to know the different types of fasting and how they differ from each other. Intermittent fasting is eating within a certain time-frame only, and allowing a certain number of hours between dinner and breakfast. Typically, people will try to fast for about 16 hours and eat within an eight-hour window. This can be especially useful if you are following a circadian rhythm diet and trying to avoid eating past dark. The downside of this however is that this might not always work for people's schedules or their family's routines. Intermittent fasting may benefit those who are struggling with chronic digestive issues such as acid reflux, bloating, and constipation/diarrhea, as it gives the digestive tract a long break between dinner and breakfast the following day. It is important to review the factors listed previously before deciding if intermittent fasting will help you achieve your health outcomes.
There are a plethora of fascinating benefits fasting can achieve for the body, such as improving insulin resistance through blood sugar regulation, boosting immune function, as well improving markers associated with metabolic disease(s). Fasting for a longer period of time allows enhanced cellular cleanup known as autophagy. Autophagy is a process that allows cells that might have mitochondria damage or DNA damage to literally engulf themselves as a protective mechanism. This is so imperative for protecting our immune system and preventing of chronic inflammatory diseases, such as cancer.
In order to achieve enhanced autophagy AND stem cell regeneration, a longer fast is required. Stem cell regeneration allows the rejuvenation of damaged tissue, as well as the regeneration of new, healthy cells. This means better immune function and adequate blood cell production. In order to stem cell regeneration to occur, at least four days of fasting must be achieved. While it is possible to fast from all food for this long of a period of time, it is extremely challenging and may not be recommended. Therefore, Prolon Fasting Mimicking Diet has allowed many to complete a 5-day fast without having to avoid all food. This may be a great option for those who would like to drastically improve metabolic and inflammatory markers, such as Insulin growth like factor, CRP, and even cholesterol levels, blood sugars, and blood pressure.
Who should potentially not fast?
As listed previously, those with sleep issues or inadequate quality sleep should ideally focus on restoring a healthy sleep routine before engaging in regular fasting. It is also useful to note that those who are struggling with hormone imbalances, low thyroid function, chronic stress, or adrenal/cortisol issues may exacerbate their symptoms by fasting. As mentioned before, fasting can be an additional stressor to the body. It is advised that balance is restored to the body as well as improved stress management in order to avoid causing more harm than good to the endocrine system. Those who have an underweight BMI or struggle to maintain a healthy weight should also speak to their provider before thinking about fasting. It is worth mentioning that fasting may potentially trigger those who have experienced a poor relationship with food. We always encourage checking in with yourself and your relationship with food when making any major changes to your diet, and adjust as needed to ensure that you are nurturing your body, mind, and spirit through lifestyle changes.
As always, we highly recommend speaking with your provider team to create a plan for you if you are interested in incorporating fasting.
Monda, Vincenzo, et al. “Exercise Modifies the Gut Microbiota with Positive Health Effects.” Oxidative Medicine and Cellular Longevity, U.S. National Library of Medicine, 2017,
Wpadmin. “Welcome.” Valter Longo, Wpadmin Https://, 18 Jan. 2022,
Cienfuegos, Sofia, et al. “Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials.” Nutrients, U.S. National Library of Medicine, 3 June 2022,
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