Intermittent Fasting 101: Unlock Your Metabolic Health
Fasting. The most popular dieting approach based on extending the non-eating window. It’s meant to put our bodies in a fasted state, known to offer numerous benefits on metabolic health, energy increase, longevity, cognition, and weight loss.
From explaining how fasting works on a cellular level to its most potent benefits and different types of fasting, we cover it all. This is your Fasting 101 spot. You’ll finally learn what breaks a fast, and whether keto is superior to fasting. Enjoy!
16/8 Intermittent Fasting
Intermittent fasting (IF) is an eating pattern alternating between feeding and fasting periods. It essentially shortens the eating window, allowing for a longer fasting phase. Fasting emphasizes when, rather than what you eat.
There are numerous fasting protocols, amongst which the 16:8 fasting is the most popular. It respects a 16-hour fasting window with an 8-hour eating window in which all meals are consumed. Extending the fast this way evidently supports metabolic function and hits the reset button, particularly in those who constantly overconsume calories.
Intermittent means occurring at intervals, being interrupted. Fasting means not eating. Intermittent fasting is, therefore, fasting that occurs at intervals, regardless of the duration. When talking about IF, we mostly mean daily fasting, not long fasts, so the 16:8 is a great example. In science, the term time-restricted eating (TRF) is preferred.
Why is 16 hours the magic number for fasting?
It is not. 16-hour food abstinence is just long enough for the majority to experience the metabolic shift that promotes the benefits of fasting. That greatly depends on glucose metabolism, metabolic flexibility, eating history, genotype, and digestive enzymes. It is not a one size fits all, but rather a general guideline thought of as relatively safe, sustainable, and practical for most.
What can you drink during intermittent fasting?
During fasting, one can drink zero-calorie beverages. Mineral or regular water, herbal teas, and black coffee. No additives, no sugars, no creamers are allowed. And no, zero-calorie sodas filled with aspartame still break a fast, as most chewing gums do.
Is it better to fast 12 or 16 hours?
There is no right or wrong answer. Adapting the fasting window to fit one’s needs is what matters. 16 hours is the gold standard in the literature, as a sufficiently long fasting window to promote most of the benefits. For some, reducing this window may be a better option to reduce the intensity and risk of running into hormonal imbalances, nutrient deficiencies, and hypoglycemia. More is not always better.
How Does Intermittent Fasting Work?
The body is in a constant flux of growth and breakdown. For growth, like muscle or fat gain to occur, the body needs greater amount of calories. For breakdown like fat-loss or detox to occur, the body requires fewer calories.
During feeding windows, we ingest calories which increase insulin levels, stimulating growth. Overconsumption of food, particularly carbohydrates, puts a strain on our metabolic system. Insulin is chronically elevated as a result, slowly developing metabolic dysfunction which results in fatigue, making us hungrier.
During fasting we don’t ingest calories, in particular glucose, so insulin levels lower. The body can tap into fat stores, mobilize and use fats for energy. Evidently, ketone levels increase, promoting fat-loss and energy. The primary reason fasting works for many is resetting metabolic function, addressing the cause of weight gain, a loop of fatigue one overeats to compensate for.
Beyond, fasting stimulates other catabolic reactions in the body related to anti-aging and longevity. It helps the body clear itself of toxins, reset hormonal levels, enrich gut microbiota, and many others.
What happens physiologically when you fast?
During fasting, glucose and insulin levels drop. The body is directed to tap into fats for energy, thus increasing ketone levels. This metabolic shift makes the energy creation machinery slightly more efficient and less strenuous on the body, and multiple organs.
Longer fasts (>16-24 hours) tend to stimulate autophagy, in which the body starts the intracellular cleanup processes, recycling damaged cells into new, functional ones.
What is the mechanism of the fasting diet?
The mechanism of fasting involves a dynamic shift in the body’s energy and metabolic systems to adapt to the absence of food. This process engages a couple of potential mechanisms like:
- energy conservation and fat utilization
- lowering insulin and blood glucose levels
- ameliorating inflammation and oxidative stress
- stimulating autophagy, Sirtuin-1, and cellular repair
- increasing gut microbiota diversity
- improving lipid profile (LDL, HDL, TG)
- improving neuropeptide signalling (PYY, CHK, GLP-1)
- improving the ratio of leptin/adiponectin
5 Intermittent Fasting Benefits
Due to the change of fuels used, fasting can stimulate numerous positive adaptations in the body. Mobilization of fats is the primary mechanism behind the effects of fasting, in regards to improving metabolic function, heart health and increasing energy.
Fat Loss
Fats stored in adipose tissue serve as a reservoir of energy. Beyond, they play a role in nutrient absorption, hormone production, inflammation, and structural support. Overeating food, regardless of macronutrient, is stored as excess adipose tissue. Whilst fats are essential for functioning, high % of fat disrupts health, energy, and metabolic function. Fasting increases fat loss and energy by mobilizing fats.
A substantial body of evidence strongly suggests that fasting is associated with weight loss and fat loss. (1) (2) (3) (4) Fasting has proven effective in decreasing visceral fat, waist circumference, and improving central fat distribution. (5) (6)
Calorie restriction appears to be one of the main drivers of fat loss. When compared to CR only, fasting was not superior for weight loss acutely. (7) (8) However, it did improve other metabolic markers which may aid in better weight management later. (6) Fasting is not that different from calorie restriction, physiologically. In most people, similar benefits occur due to increased fat mobilization, for which fasting is superior.
Fasting puts many in an unintentional calorie deficit, a practical way to achieve weight loss, that’s evidently easier to adhere to.
A vast array of hormones continuously signal energy balance and hunger to the brain, which regulates appetite.
- Ghrelin and Neuropeptide Y stimulate hunger
- Leptin, Insulin, Cholecystokinin (CKK), GLP-1, and Peptide YY signal satiety
Obesity or metabolic dysfunction disrupts this signalling, tipping the scale towards being hungrier and more tired. Our evolutionary way of dealing with the problem is eating more and moving less, which has proven inefficient.
Fasting, just as calorie restriction, acutely increases ghrelin, the hunger hormone, although to a lesser degree. It also decreases satiety hormones like GLP-1, PYY, and CKK which makes us hungrier. (9) (10) However, long-term adaptations to fasting (and weight loss) have been shown to improve neuropeptide signalling and modify appetite-regulating hormones to reestablish energy homeostasis in favour of better body composition. (11) (12)
Metabolic Function
Metabolic function is about converting nutrients to energy. It greatly depends on body composition, nutrient intake, total calories, and meal timing. Losing weight, particularly in overweight people, can strongly improve metabolic function. Fasting is one of the strategies used to do it.
Overeating, particularly carbohydrates, will lead to elevation in insulin and weight gain. (13) (14) Over time, this disrupts metabolic function, leading to poor glycemic control, fatigue, and more weight gain.
Fasting, by reducing the amount or time-window of glucose ingestion, hits the metabolic switch, where the body starts burning fat for fuel whilst insulin is low.
The data shows a strong positive effect of fasting on metabolic function. In most studies, fasting was proven effective in improving metabolic markers, particularly in overweight, T2D, or obese population. (15) (16) (17)
- Insulin
- Glucose
- Hemoglobin A1c
- Lipid Profile
- Leptin
- CRP
The primary mechanism behind its metabolic benefit is mobilization of fat, the same mechanism that occurs in CR or keto diet.
During fasting, ketone levels increase while glucose levels drop. (6) Beyond weight loss, research shows substantial improvements in glycemic control, lipid profile, and insulin sensitivity. (2) (3) (18)
When glucose availability is low, insulin isn’t secreted as much. This gives the pancreas a break, resetting its function. Early time-restricted feeding was shown to improve β cell function, the pancreatic cells that secrete insulin. (19)
Adiponectin, a hormone related to metabolic health that controls blood sugar levels and insulin secretion, can also be increased via fasting. (20)
The cumulative effect of fasting on all these markers: glucose, insulin, cytokines, leptin, adiponectin, can have a substantial benefit in the long run. It’s a spiral that supports fat loss, energy, and longevity.
Heart Health
Cardiovascular health can be partly measured by looking at blood pressure, lipid profile, and inflammation. Most of the favourable effects of CR or IF on weight loss concurrently reduce cardiometabolic risk, as lipid profile and blood pressure improves.
Both strategies, fasting and calorie restriction have shown improvements in cardiometabolic status. (21) (6) Restricting energy seems to reduce inflammation and improve adiponectin levels. (22) The reduction in visceral fat is likely a major factor for improving other metrics related to heart health, like increased blood pressure.
Essentially, fasting stimulates lipolysis, mobilizing fat for energy. This increases ketone levels which reduces reliance on glucose.
Generally, fasting is associated with improvements in lipid profile. It can assist in reducing total cholesterol (TC), LDL cholesterol, and triglyceride levels (TG). (23) (21) (24) (25) By regulating the levels of apolipoprotein A4, it may aid in decreasing the oxidation of LDL, the bad cholesterol. (6)
In obese women, time-restricted feeding of 16:8 hours promoted a 12% decrease in the cardiovascular risk. (26)
The data shows a strong association between:
Anti-Aging
Aging is a slow process of degradation or decline in function. An inevitable process congruent with DNA shortening, oxidative stress, and lower recoverability. In the quest for a longer life, many scientists look for strategies that improve mitochondrial function and stimulate autophagy.
Calorie restriction or fasting is one of the most potent strategies to stimulate autophagy. It’s effective in upregulation autophagy markers and stimulating Sirtuin 1, an important enzyme related to longevity. (29) (30) (31)
Autophagy is a cellular repair mechanism. A catabolic, self-eating process where dysfunctional cellular components are being recycled. This energy is used as a building block that helps replace misfolded proteins and junk cells. Essentially, an internal detox. (32)
Longer fasts are required to stimulate autophagy, but the timing is still largely dependent on genetic factors and calories, particularly protein. A study which used 5-day prolonged fasting method showed improved gut microbiota diversity correlated with SIRT1 gene expression, related to longevity. (33)
Healthy aging is a balance between growth and breakdown. Catabolic processes stimulated by fasting can be particularly effective for those who tend to spend longer periods in anabolic phases of elevated insulin. It’s a natural way the body cleanses itself from the inside out.
Brain Function
Brain health is tightly related to cognition. The better the neural structure of the brain, the better it functions and slower it ages. Fasting is one of many strategies proposed to improve brain health and slow down neurodegeneration.
In a fed state there is less blood available for the brain, as it’s used for digestion, and blood sugar rises. This is why after a heavy meal, particularly carb-rich meal, we tend to feel sleepy and tired. Acutely this tends to diminish cognitive performance, alertness, and mental clarity.
Ketones present an alternative source of fuel the brain can run more efficiently on. Increasing ketone levels, which is achieved via fasting, acutely improves mental clarity. Fasting has also been shown to increase Orexin-A levels, an alertness-promoting hormone. (34)
Generally, fasting isn’t strongly associated to improved cognitive function in healthy humans. However, not overfeeding allows more energy to be used for cognitive tasks, rather than digestion.
Fasting stimulates certain pathways that help the brain cleanse. It’s evident that fasting increases ketone levels, thus helping to ameliorate oxidative stress and inflammation in the brain.
Most neurodegenerative diseases develop or weaken due to poor glucose metabolism in the brain. Enhancing this energy-creation machinery, and reducing the strain on the brain is proposed to be one of the pathways to slower brain aging. (35) (36)
Animal studies show a potential for fasting or calorie restriction to (37)
- reduce β-amyloid accumulation
- enhance synaptic adaptations
- promote vascular health
Such pathways can play a strong role in brain protection and reduce cognitive decline that comes with age. (38)
BDNF or brain-derived neurotrophic factor is a key player in neural development, which supports brain function. Higher levels are associated with healthier, better performing brains.
The data on BDNF in humans is mixed, with one meta-analysis showing varying results. In some studies fasting greatly increases, in others it decreases or is insignificant. (39) Studies in animals consistently report upregulation of BDNF and improved cognition via fasting. (40)
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Fasting Methods
Real fasting is abstaining from calories for an extended period. Some fasting methods add the element of calorie restriction. While juice fasting, keto diet, or alternate day fasting might not be fasting technically, due to the similarity in the way they work, the benefits are similar.
Hereby we’ll cover the four main fasting methods:
16/8 Intermittent fasting (IF)
So-called time-restricted eating, 16/8 fasting occurs daily. It involves fasting for 16 hours and eating during an 8-hour feeding window. An example, 12 PM to 8 PM is eating, the rest is fasting. It is one of the most practical, socially convenient, relatively safe method. It can be further adapted to less intense 14/10 or more intense 18/6 cycle.
5:2 Fasting
5:2 fasting involves eating normally for five days a week while restricting calories to <25% on the two fasting days. It’s been shown effective for weight management, but due to the higher calorie (and glucose) deficit, it is slightly less balanced in terms of glycemic control.
Alternate Day Fasting (ADF)
This approach alternates between normal eating and fasting days. During fasting days, little (<25%) to no calories are consumed. It’s more intense, leading to greater calorie deficit and higher risk of hypoglycemia. Very challenging to maintain.
Eat-Stop-Eat
Involves a full 24-hour fast once or twice a week. For instance, one might fast from dinner one day to dinner the next. It can help reduce calorie intake while promoting autophagy and metabolic benefits.
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Intermittent Fasting for Beginners
Entering fasting can be quite challenging for a beginner. Beyond proper hydration, we’ll list a couple of helpful tips that can make one’s fasting journey easier, safer, and more sustainable.
Fasting Protocol
It’s important to choose a method that suits one’s lifestyle and goals. For beginners, the 16/8 method (16-hour fast, 8-hour eating window) is a great place to begin. This protocol aligns well with most daily routines and is relatively easy to sustain.
Adaptation Phase
Transitioning to intermittent fasting can take time. Early on, one might feel hunger or low energy as the body adjusts to burning fat for fuel. While most people adapt within a week or two, it is partly dependent on metabolic function, glycemic control, and diet.
Fasting App
Apps like Zero or Fastient can help one track the fasting schedule, monitor progress, and stay motivated. These tools often provide educational resources and reminders to keep us on track.
Hydration and Electrolytes
Fasting naturally reduces food intake, which can lead to lower electrolyte levels. Sustaining hydration is crucial for maintaining optimal function. Beyond drinking enough water, it’s smart to consider adding zero-calorie, zero-sugar electrolytes, such as sodium, potassium, and magnesium, to support hydration and prevent fatigue or muscle cramps. Best bet is high-quality, pure, mineral-rich salt.
Last Meal Plan
Most overestimate their fasting by overextending the feeding window by ~30 minutes, not accounting for the small cookie or a glass of wine after dinner. It’s essential to finish eating within the planned timeframe, as the body still is in a fed state a little after the last meal.
Zero-Calorie Beverages
While fasting, one can drink only water, herbal tea, or black coffee. Be cautious about added ingredients like cream or sugar, as even small amounts break your fast by triggering insulin responses.
Glucose Tracking
A Continuous Glucose Monitor (CGM) can provide valuable insights into how one’s body responds to fasting and eating. Stable glucose levels during eating windows can enhance energy, reduce cravings, and support metabolic health.
Healthy Fats
Healthy fats like avocados, nuts, seeds, and olive oil provide sustained energy, making it easier to extend fasting periods without feeling overly hungry. They also help stabilize blood sugar levels, supporting metabolic flexibility. Being reliant on glucose during extended fasting is not optimal, so combining healthy fats, complex carbs, and protein is crucial to in the hunger games.
