Stages Of Fasting – What To Expect For Each Step

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By Thomas Youngerman

Reviewed by Juliana Tamayo, MS, RDN - Last Updated

Stages of Fasting – and Positive Health Outcomes

What if we told you that one simple change in your daily habits, even if only practiced two or three days each week, could potentially increase your lifespan and reduce your risk of disease, including cancer? Would that pique your interest, or would you wonder if we’re selling snake oil? Well, we’re not selling anything, and the above statement is not a hyperbole of our creation. The following is an excerpt from an article published in the New England Journal of Medicine, Effects of Intermittent Fasting on Health, Aging, and Disease:  

“Evidence is accumulating that eating in a 6-hour period and fasting for 18 hours can trigger a metabolic switch from glucose-based to ketone-based energy, with increased stress resistance, increased longevity, and a decreased incidence of diseases, including cancer and obesity.”

The eating pattern described above is referred to as the 18:6 and is a type of (IF) intermittent fasting. This article will take a quick look at fasting through the years and then delve into current intermittent fasting strategies. Our focus is the stages of intermittent fasting, what happens to your body physiologically during a fast, the positive health outcomes of these metabolic processes, and the extended benefits once you break your fast.

Intermittent Fasting

A Brief History of Fasting

Fasting has been practiced throughout the years for health and religious purposes, in rituals and initiation rites, and sometimes as an expression of protest. The father of Western medicine, Hippocrates, believed that fasting enabled the body to heal itself. In ancient times, Paracelsus, another great healer, thought fasting was the greatest remedy and referred to fasting as the “physician within.” In Ayurvedic medicine, they have long advocated fasting as a treatment.

History Of Fasting

Going back to our beginnings, when sourcing food was a constant challenge for hunters/gatherers, fasting was a natural part of life. Ghandi used fasts for social protests as did César Chávez. During the 1982 Irish hunger strike for prison reform, ten men died after fasting for extended periods. 

Today people still engage in extended fasts for religious reasons, for cleanses and detoxes. However, terms like intermittent fasting, calorie restriction, and time-restricted feeding create headlines in the mainstream media. Intermittent fasting, not long-term fasting, is our focus.

Intermittent Fasting – Gaining Credibility

A growing body of research suggests intermittent fasting, unlike many trendy diets, is becoming a more widely accepted, safe, legitimate strategy for managing weight and preventing disease. As we review the research, it’s apparent IF can help you manage weight; however, intermittent fasting is not a diet, per se; instead, it is an eating pattern focusing on when you eat and the amount of time between meals. That prescribed period when you abstain from food is the fasting stage, when metabolic processes initiate. 

Scientists, researchers, nutritionists, and M.D.s are all arriving at the same conclusion, that IF, when combined with a healthy diet and active lifestyle, can be a realistic, sustainable, and effective method for weight loss, with significant added health benefits.

Types of Fasts

One of the two most popular IF methods, which is relatively easy to sustain, is the 16:8. After reading the New England Journal of Medicine article, we may change our recommendation to an 18:6 strategy. Early studies indicated that a fast must last a minimum of twelve hours to be beneficial – some stats cited later in this article will refute that claim, shortening the required fasting period. Regardless, the 12-Hour method remains another popular, also relatively simple to maintain the pattern.

Intermittent Fasting-One Meal a Day

Other commonly practiced intermittent fasts include the Fast 5 Method, One Meal a Day, Alternate-Day fasting, and 5:2, where you eat normally for five days of the week. For the other two days, you restrict caloric intake to about one-third of your usual calories, 500 calories for women – 600 for men.

To simplify the concept, see the below chart which indicates the fasting stage of each method versus the eating window:

Intermittent Fasting Method Fasting Stage (No Caloric Intake) Feeding Window
16:8 Sixteen hours Eight hours
18:6 Eighteen hours Six hours
12:12 Twelve hours Twelve hours
Fast Five Nineteen hours Five hours
One Meal / Day Twenty-three hours One hour
Alternate Day Twenty-four hours Alternate twenty-four hours
5:2 Two days, calorie restriction Five Days

What Happens to Your Body During the Fasting Stage

We think you’ll be amazed at the incredible processes that take place during and after a fast. But first, the bad news. When you first start IF, you will be hungry, you may be irritable, and you may have difficulty concentrating. It will pass with a bit of time, and the benefits far outweigh this initial discomfort.

The typical Western diet is carbohydrate-dense, with sugars and refined grains. In your body, these foods are broken down into sugar, glucose, which your cells use for energy. Insulin is secreted to help glucose enter the body’s cells, and any excess is stored in the liver and fat cells as glycogen. Between meals, insulin levels go down, and fat cells release their stored glycogen for energy. But following the Western diet, with three square meals and snacking, you become more insulin-resistant leading to weight gain, obesity, and possibly type 2 diabetes. 

If only we could allow insulin levels to go down far enough, long enough to burn off fat. If we could limit the eating window to twelve hours or less, we would give the body time to properly digest its food and make stored body fat more accessible as an energy source. We could flip the metabolic switch from glucose to fatty acid-derived ketones as the primary fuel, burning fat stores. 

Just how long does it take to begin burning fat rather than glucose? Researchers tell us that in the fed state, blood levels of ketone bodies are low, yet they rise within eight to twelve hours after beginning a fast and escalate as the fast continues. Perhaps more importantly, fasting also triggers other cellular functions that improve metabolism, lower blood sugar, reduce inflammation, and initiate autophagy, a process of cleaning out toxins and damaged cells.  

Your fasting stage needs to be an absolute minimum of eight hours. We recommend the 12:12, 16:8 (18:6) or longer. The above research indicates, the longer the fasting stage, the greater the benefits.

Fasting, it seems, does more than just help us enter ketosis to burn calories and lose weight. Fasting puts the body under mild stress, like resistance training, as muscles adapt and grow stronger, so our cells adapt to fasting by enhancing their ability to cope. When we practice an intermittent fasting regimen, the periodic flipping of the metabolic switch during the fasting stage elicits systemic and cellular responses that carry over into the fed state, enhancing mental and physical performance and disease resistance. 

Quoting the New England Journal article once more:

“The research reviewed here shows that most if not all organ systems respond to intermittent fasting in ways that enable the organism to tolerate or overcome the challenge and then restore homeostasis. Repeated exposure to fasting periods results in lasting adaptive responses that confer resistance to subsequent challenges. Cells respond to intermittent fasting by engaging in a coordinated adaptive stress response that leads to increased expression of antioxidant defenses, DNA repair, protein quality control, mitochondrial biogenesis and autophagy, and down-regulation of inflammation.”

Not only is the length of the fasting stage crucial, but the frequency and the repeated exposure, are also critical.  

The article cites intermittent fasting regimens as essential for improved function and resistance to a broad range of damaging health outcomes and diseases – benefits untapped in those who overeat or are sedentary.

To recap, during the fasting stage, the body switches from glucose to ketones as the primary fuel source. The longer the fast, the more ketones produced in the bloodstream. Other metabolic processes occur during the fast such as increased defense against free radical damage, DNA repair, autophagy, and reduced inflammation. A repeated regimen of IF prepares the body to respond to future stressors. Many of the benefits observed during the fasting stage will last once you break your fast, particularly if you do so with the “right” type of breakfast meal. 

Clinical Studies of Fasting

With its potential for significant beneficial health outcomes, beyond weight loss, intermittent fasting has been the subject of numerous clinical studies by researchers. Our bonus section will discuss many of the diseases IF is thought to impact, including longevity, cancer, cardiovascular disease, neurodegenerative disorders, asthma, arthritis, and MS. The earliest scientific research attributed many of the positive effects of fasting to limiting free radical damage and weight loss. More recent studies show that the health benefits of IF are not simply reducing oxidative stress. Fasting activates cellular processes that improve glucose regulation, initiate autophagy, enhance stress resistance, and suppress inflammation. The ketone bodies produced during IF not only serve as a primary fuel source but also signal molecular reactions with cell and organ function effects.  

Contrary to what detractors of IF believe, studies show that physical function is also improved with intermittent fasting. Mice fed on an alternate-day fasting regimen have more endurance than those with unlimited access to food – balance and coordination are also improved in animals on a time-restricted feeding regimen. Of particular note, a two-month study of young men who fasted for sixteen hours a day and resistance-trained proved that they could lose body fat while maintaining their muscle mass. Fasting has benefits beyond the physical, including improved mental function. In several clinical trials with the elderly, fasting and caloric restriction improved memory, executive function, and cognition. 

Intermittent Fasting-Weight Loss

Clinical Studies of CR (Caloric Restriction)

Intermittent fasting is known to “mimic” the effects of caloric restriction. Clinical research of CR includes two landmark, twenty-year studies of monkeys, one by the University of Wisconsin, the other by the National Institute on Aging. Each study attempted to establish a link between longevity and caloric restriction. The Wisconsin study showed a positive effect on health and survival, whereas the NIH study showed no significant impact on life span. However, there was a reduction in age-related conditions, such as cancer, heart disease, and diabetes.

If a reduction in age-related disease is not sufficient motivation for intermittent fasting and CR, perhaps the photo below will provide additional inspiration. The monkey on the left followed a CR regimen while the one on the right was fed a normal diet at regular intervals. 

Intermittent Fasting twenty-year studies of monkeys

Research now proves that intermittent fasting not only “mimics” the effects of calorie restriction but seems to confer additional health benefits, to a greater extent, than can be attributed to CR alone.

 Why Don’t More People Fast?

Traditional wisdom is calories in versus calories out for weight loss. A healthy diet and exercise are the established, time-tested methods for weight loss. So why are so many people overweight? Why are obesity and type II diabetes so prevalent in the U.S.? Because most people lack the discipline to maintain this type of weight loss regimen for the long term and fall off the wagon, regaining any weight loss. 

Intermittent fasting is flexible, easy, and free – unlike other diet programs and weight loss supplements. Additionally, there are now many apps and tools out there to help with maintaining and tracking your fasting, like DoFasting, that can make it even easier. However, the Western diet is so ingrained in our culture that simply changing your eating pattern is rarely considered by most and seldom prescribed by physicians today.

Conclusion: Stages of Fasting

We’ve mentioned reduced oxidative stress (limiting damage from free radicals) and suppressing chronic inflammation several times in this article. In studies on aging and mortality, these two conditions alone contribute significantly to the onset of numerous age-related diseases, such as atherosclerosis, cancer, inflammatory joint conditions, asthma, diabetes, and kidney diseases. 

Weight reduction is nice to have; health and longevity are even more essential. 

We highly recommend you consider intermittent fasting, even if only for one to two days per week. Limit the hours of the day when you eat. Start with a simple 12:12 and add an hour per day to your fasting stage until you reach sixteen to eighteen hours in a fasted stated. 



This bonus section describes the physiological responses during the fasting stage and how they affect certain conditions. This section is paraphrased and includes direct quotes from the New England Journal of Medicine article on the Effects of Intermittent Fasting on Health, Aging, and Disease.  

Intermittent Fasting-Life Extension

Life Extension

“Reducing food availability over a lifetime (caloric restriction) has remarkable effects on aging and the life span in animals.” It was originally thought that the health benefits of caloric restriction resulted from a reduction in the production of damaging free radicals. “Subsequently, hundreds of studies in animals and scores of clinical studies of controlled intermittent fasting regimens have been conducted in which metabolic switching from liver-derived glucose to adipose cell-derived ketones occurs daily or several days each week. Although the magnitude of the effect of intermittent fasting on lifespan extension is variable (influenced by sex, diet, and genetic factors), studies in mice and nonhuman primates show consistent effects of caloric restriction on the health span.”

Cardiovascular Health

“Intermittent fasting improves multiple indicators of cardiovascular health in animals and humans, including blood pressure; resting heart rate; levels of high-density and low-density lipoprotein (HDL and LDL) cholesterol, triglycerides, glucose, and insulin; and insulin resistance. In addition, intermittent fasting reduces markers of systemic (chronic) inflammation and oxidative stress that are associated with atherosclerosis.” 

“Improvements in cardiovascular health indicators typically become evident within two to four weeks after the start of alternate-day fasting and then dissipate over a period of several weeks after resumption of a normal diet.”


Most early research relative to IF or CR on cancer was on laboratory animals. Since then, “numerous studies in animals have shown that daily caloric restriction or alternate-day fasting reduces the occurrence of tumors during normal aging in rodents and suppresses the growth of many types of induced tumors – while increasing their sensitivity to chemotherapy and irradiation. IF is thought to impair energy metabolism in cancer cells, inhibiting their growth and rendering them susceptible to clinical treatments.”

In human clinical trials, “several case studies involving patients with glioblastoma suggest that intermittent fasting can suppress tumor growth and extend survival.” 

Neurodegenerative Disorders

To paraphrase, there is data that suggests excessive energy intake (calories) during midlife increases the risks of stroke, Alzheimer’s disease, and Parkinson’s disease. “There is strong preclinical evidence that alternate-day fasting can delay the onset and progression of the disease processes in animal models of Alzheimer’s disease and Parkinson’s disease.”

Asthma and Arthritis

Once again, the reduction of oxidative stress and inflammation, attributable to processes initiated during intermittent fasting, “reduces the symptoms of asthma and airway resistance in obese patients.”

“Intermittent fasting would also be expected to be beneficial in rheumatoid arthritis, and indeed, there is evidence supporting its use in patients with arthritis.”

Multiple Sclerosis

MS is an autoimmune disorder adversely affected by chronic inflammation. “Two recent pilot studies showed that patients with multiple sclerosis who adhere to intermittent fasting regimens had reduced symptoms in as short a period as two months.”

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Thomas Youngerman

Thomas Youngerman is an entrepreneur and author in the health and wellness space with extensive experience in the supplement industry. He has owned multiple sports nutrition stores and served as the Category Manager and Director of Business Development for a regional chain of nutrition stores. Thomas created a successful line of men’s healthy aging supplements that was distributed in GNC, The Vitamin Shoppe, and Kroger, and later sold to a West Coast corporation. Thomas was previously a certified nutrition coach. He has a strong understanding of nutrition, supplement formulations, DSHEA, cGMP, and FDA regulations.