Fasting involves restricting the intake of caloric food / drink for a  specific length of time.  Physiologically, fasting tends to shift the body into a metabolic state known as “ketogenesis.”  As the fasting period goes on, the body’s glycogen stores will be depleted at approximately 16-36 hours after the cessation of caloric food/drink. At this point, the body begins to shift from using glucose from stored glycogen as fuel to utilizing the fatty acids from our fats stores for fuel.  This known by many researchers as the “metabolic switch.”  When the body begins to burn fatty acid ketones (ketogenesis) for fuel the body preserves muscle mass; many metabolic pathways and cellular processes are up-regulated which include autophagy (aka house cleaning) and lipolysis (fat break down); inflammation decreases and many blood bio-markers begin to normalize.

The Many Types of Intermittent Fasting

Intermittent Fasting (IF) can also be called Time-restricted eating (TRE).  In this method, there are specific times of being in the fasted state and specific times of being in the fed state.  For most who follow an IF lifestyle, the feeding window is 4-12 hours daily.  This means the fasting window is typically 12-20 hours per day where water, black coffee and non-caloric teas are consumed only.  Of this method, fasting for 16-18 hours and feeding for 6-8 hours is the most common pattern.

Just like anything we do in life, this takes practice!!  You do not just run a marathon! Right?!  Most would need to train!  You will need to train your body to be metabolically flexible where you can shift from using glucose and ketones from fatty acids for fuel readily.

The eating window can go where ever you like. I have some patients who prefer an earlier feeding period of 8am-2 pm for example and others who prefer to fast all morning and eat from 12-6.  Find the pattern that works best for you and stick with it!!  There is a wonderful app called Zero where you can track and log your fasting and eating windows.

Remember: Intermittent Fasting is NOT Caloric Restriction. 

Once you find what works for you, I suggest sticking with it and then the magic happens. Consistency over time is what creates real change. This is best off when it can become your lifestyle and not something you do here and there.

How does Intermittent Fasting Work?

Intermittent fasting is truly spectacular!  After many years of research, I have to say I am blown away by the consistent results.  Intermittent Fasting results in positive changes to hormones, blood bio-markers and cellular / energy metabolism in the body.

After you have a nice feed, food is broken down into smaller absorbable particles and absorbed in the digestive tract. Carbohydrates, are broken down into sugar or glucose which is then absorbed into the blood and sent to all of our organ systems where glucose is used as a primary source of energy.  As blood sugar rises the hormone insulin is released to regulate blood sugar levels by signaling your cells to gobble up the glucose from the blood and get it into the cells, where it provides fuel for body functions OR gets stored as Fat!  Insulin also programs the body to store excess nutrients and inhibits lipolysis or the breakdown of fat!  Insulin is not what we want floating around in high quantities if you want to lose weight and live in a healthy body!  The only way to regulate our insulin levels is to be very aware of the amount of sugar or glucose that we consume.

With an intermittent fasting schedule, the fasting period will bring  allow the body to utilize glucose in the blood.  Once blood glucose lowers, the body begins to utilize glycogen, (a storage form of glucose in liver and skeletal muscle cells ) for fuel.  After about 18 hours  of fasting (or more in some cases) insulin levels are low and lipolysis (fat breakdown) begins.  In the state of lipolysis, the body breaks down adipose tissue (fat) into free fatty acids. This is metabolic flexibility!  When there is insufficient glucose available to meet the body’s energy needs, the body will transition to a state called Ketosis where it uses fatty acids and fatty-acid derived ketone bodies (ketones) for energy.  For more of an in depth look: Liver cells are responsible for the production of ketone bodies. During ketogenesis, fatty acids are broken down in the mitochondria of cells by a process called beta-oxidation and converted to the ketones acetoacetate and beta-hydroxybutyrate.

These ketones are used by muscle cells and brain cells to generate adenosine triphosphate (ATP), the primary carrier of energy in cells.

The Metabolic States that Happen as we begin Fasting

With an intermittent fasting lifestyle your body will go through many states in your fasting period.  Many will only get to the first 2 metabolic states listed below and some will get to the first 3 metabolic states.  The 4th metabolic state will happen for those who choose to have longer fasting periods.

1. The fed state (approximately three hours post-meal) – glucose levels are high; energy comes from glycolysis (the breakdown of glucose); excess glucose is stored in the body as glycogen and excess fatty acids are stored in adipose tissue
2. The post-absorptive state or early fasting state (approximately 18 hours post-meal) – energy comes from glycogenolysis (breaking down glycogen into glucose); lipolysis begins, which increases the level of free fatty acids in the blood
3. The fasting state (approximately 36 to 48 hours post-meal) – the metabolic shift occurs and the body begins to use fatty acids and ketones as fuel
4. The starvation state (longer than 48 hours post-meal) – glucose oxidation or use is inhibited; ketone levels are increased and the body relies heavily on fatty acids for energy.  This will happen for those who choose to do longer fasting periods.

The physiological effects of fasting can also be achieved by following a ketogenic diet, which is a very high-fat, very low-carbohydrate diet. The purpose of the keto diet is to shift the body’s metabolic state into ketosis. The keto diet restricts the intake of carbohydrates to 35 grams or less per day, which minimizes the amount of glucose available for energy production. A high intake of fat provides fatty acids that are used for ketone and energy production. In my professional opinion, I do not feel that a ketogenic diet is a healthy way to go for any length of time. It does not allow the body to be “metabolically flexible” and shift easily from using glucose to fatty acids for fuel. If you are doing intermittent fasting you may choose 2-3 days a week to follow a ketogenic diet. This will help you to be metabolically flexible.

The Benefits of Intermittent Fasting

Results from human trials are now supporting some of the benefits and potential clinical applications of fasting, including its use in a number of inflammatory and cardiometabolic conditions, such as rheumatoid arthritis, asthma, cardiovascular disease, hypertension, type 2 diabetes, and weight loss. A number of mechanisms have been suggested to explain this relationship, including the effects of fasting on the the gut microbiome, circadian biology, and lifestyle behaviors.  It has been clearly noted in many studies that those who have a 6-hour feeding window experienced improvements to insulin sensitivity, oxidative stress, and blood pressure.   IF has also been shown to increase Lactobacillus Johnsonii and Lactobacillus reuteri, beneficial species of bacteria that have immunomodulatory effects. IF has also been shown to help control eating behavior, especially late at night. IF has also been shown to help patients with weight loss

Safety Considerations with Intermittent Fasting

While intermittent fasting has shown to be wildly beneficial to most there are also pre-cautions to take. IF is not for children, pregnant or breast feeding women.   While IF is generally considered to be safe, potential side effects have been reported in a small proportion of IF participants, including low energy, hunger, feeling cold, and irritability or other changes to mood and behavior.  Most of these symptoms improve with time as the body becomes more metabolically flexible.  Studies in obese adults and patients with type 2 diabetes have found intermittent fasting regimes to be safe and tolerable. However, diabetic patients should take special precautions with intermittent fasting. When there is a lack of insulin or resistance to insulin, a state called diabetic ketoacidosis (DKA) can occur. DKA is a life-threatening condition, characterized by an overproduction of ketone bodies. The symptoms include dehydration, confusion, nausea, and vomiting. The condition is treated by administering glucose and insulin, which down regulates the ketogenic pathway. DKA is more likely to occur in type 1 diabetics but also occurs in advanced type 2 diabetics.