After ~8-14 hours fasting the body uses the last of the glycogen reserves (stored glucose). This time frame varies based on the individual. Once glycogen is depleted, the body shifts to a state of gluconeogenesis. Fat is used to make glucose and the body spares protein. Within 24 hours, autophagy sets in, which is a natural detox process the body undergoes of removing damaged cells and regenerating new healthy cells. This process helps to reduce risk for cancer and promotes longevity. Within 48 hours fasting, and often within just 24 hrs fasting, the hunger hormone ghrelin circulates at high levels which promotes the secretion of the human growth hormone (HGH). High circulating levels of HGH alert the body to preserve lean body mass and utilize stored fat for energy. HGH is also associated with enhancing exercise performance, recovering from injury, and promoting cellular repair. One study conducted at the Intermountain Medical Heart Center Institute showed HGH increased an average of 1300% for women and 2000% for men during a 24 hr fast. During the fasting period, insulin levels are very low which helps to reduce inflammation. Conversely, in the fed state, cells are in growth mode and genes related to stress resistance, fat burning, and damage repair are turned off.
Moderate side effects one may experience during a fast include nausea, headache, insomnia, back pain, fatigue, dyspepsia. A chart review of 2539 charts assessing for adverse events during a medically supervised, water only fasting at True North Healthcare between 2006-2011 published in BMC Complementary and Alternative Medicine concluded adverse effects experienced during a water only fast are “tolerable and mild to moderate in nature.” Of this population, there were two serious adverse effect reported. One being dehydration in a 73 yo M after 3 days water fasting and the other, hyponatremia in a 70 yo M after day 9 water only fast.
The mindset going into a fast also plays a role. For example, there are psycho-physiological differences between starvation and fasting as noted in the 2013 Expert Panel Update of the 2002 Consensus Guidelines for Fasting Therapy. Starvation is normally an unchosen, mentally stressful, continual condition. Fasting on the other hand is intentional, limited in length, and ideally practiced by individuals in an appropriate nutritional state. Starvation is often accompanied by feelings of angst, suffering, and sometimes trauma. This unsteady psychological condition increases the body’s use of micronutrients and protein. However, during a voluntary fast, serotonin and brain-derived neurotrophic factor (BDNF) availability are increased which promotes a more euphoric state. Due to the limited duration, the body adapts to make use of the body’s energy reserves in order to meet the body’s requirements for macro and micro-nutrients. The body adapts with energy-sparing mechanisms and switches from glucose utilization to fat utilization and spares protein if enough fat is available for energy. Cells switch to a protective mode and aging pathways are deactivated.
Stay tuned for some testimonials from a few of us who have been incorporating a fast periodically and how to incorporate it into your lifestyle.
Lauren has been a registered dietitian for 6 years and helps others develop practical and enjoyable lifelong nutrition habits to improve health, well-being, energy levels, confidence, and digestion. She focuses largely on a whole food, plant-based nutrition approach to health.
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