During the first four hours after ingestion, the body metabolizes carbohydrates, fat, and protein into glucose, fatty acids, and amino acids. This is an anabolic, or growth, period that entails energy production and storage. (1)
As glucose enters the bloodstream, the pancreas secretes insulin to help transport glucose inside cells for immediate use. Anything not used immediately will find its way to the liver for short-term storage as glycogen. Long-term storage takes the form of adipose tissue throughout the body. (1)
During this first phase of digestion, one will typically feel full and satisfied because the body has readily accessible energy stores. This full feeling stems from the hormone Leptin, the levels of which will rise after a meal. Leptin will give way to the hormone Ghrelin as glucose depletes and the body becomes hungry again. (2)
As fasting persists, blood glucose and insulin levels steadily drop, and the body turns to the liver and skeletal system’s supply of glycogen for energy. (3) During these hours, there is a flood of Glucagon into the system via the pancreas. Glucagon is a catabolic hormone that initiates the breakdown of glycogen to maintain blood glucose levels. (3) As these stores deplete, the body will look elsewhere for more energy.
Sometimes, one can experience any of the following mild physical effects of not eating during these hours: (4)
- Fast heart rate
- Problems with coordination
- Blurred vision and/or speech
On the other hand, clinicians have noted many positive effects from fasting, such as: (12,13)
- Increase vigilance
- Improved mood, for example in the setting of depression
- Increased sense of tranquility
- Increased brain availability of serotonin, endogenous opioids, and endocannabinoids
With both glucose and glycogen depleted, the body begins to digest triglycerides which starts the metabolic state of ketosis. The free fatty acids and glycerol in these triglycerides undergo a conversion in the liver into ketone bodies and glucose, both of which the body uses as fuel. (5)
Ketogenic diets prioritize this time window because ketones replace glucose as the primary source of energy. In other words, the body is burning adipose tissue for fuel. The benefits of ketosis go far beyond weight management as well. Studies have shown improvements, with the aid of ketosis, in all of the following: (6,12)
- Bipolar disorder
- Major depressive disorder
- Mild cognitive impairment associated with Alzheimer disease
- Rheumatic diseases
- Chronic pain syndromes
Studies show that alternate-day fasting trials of 3 to 12 weeks in duration appear to be most effective at reducing body weight, body fat, total cholesterol, and triglycerides. This is with scheduled periods of eating in the absence of a diagnosed eating disorder (ED). For those who struggle with ED, Within Health recommends consulting with our team before embarking on that journey to ensure it is the best path for our members. (14)
Any prolonged fasting of this kind should be done with the aid of a medical care professional.
The body will continue in ketosis and one may notice an uptick in both mental clarity and energy. Ketones have been shown to enhance antioxidants and decrease free radicals. (7)
The more severe effects of not eating typically begin after 72 hours. (8) As the body continues the hunt for glucose, the next source stems from the digestion of proteins found in vital muscle tissue.
With an adequate amount of water, the human body can continue to survive anywhere from three weeks to 70 days by digesting proteins in the muscular system. (8,9) Survival is the body’s main concern at this phase, so it will divert energy to critical organ operation.
Deprivation of nutrients at this stage is largely reversible depending on the situation presented. If someone has gone many weeks without eating, to suddenly reintroduce food again could trigger refeeding syndrome. Treatment for this requires inpatient monitored feeding to correct any sudden electrolyte shifts. (10)
Under the proper care of a medical professional, the potential physical and mental benefits from fasting can be positive, when done in mindful ways. Intentionally restricting food for days at a time, when it causes significant emotional distress, is a sign that fasting should be avoided for the time being and likely is not something to be tried again.