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Physiology and Benefits of Intermittent Fasting


James P.M. Odell, OMD, ND, L.Ac


Medical Disclaimer -Before embarking on any change in diet, lifestyle or activity, please seek the advice and supervision of your primary healthcare provider. This article should not be construed as medical advice, nor should it be substituted for medical advice from your healthcare provider. By continuing to read this article, you assume all responsibilities and risks for instituting lifestyle management.


Periods of voluntary abstinence from food and drink (fasting) has been practiced since earliest antiquity by peoples around the globe. Books on ethnology and religion describe a remarkable variety of fasting forms and practices.1 Many religious groups incorporate periods of fasting into their rituals, including Muslims who fast during the month of Ramadan from dawn to dusk, and Christians, Jews, Buddhist, and Hindus who traditionally fast on specified weekdays or calendar year days.


Though fasting has been practiced for millennia, only recently have studies shown its role in adaptive cellular responses that reduce oxidative damage and inflammation, optimize energy metabolism, and bolster cellular protection. Fasting extends longevity, in part, by reprogramming metabolic and stress resistance pathways. In rodents intermittent or periodic fasting protects against diabetes, cancers, heart disease, and neurodegeneration, while in humans it helps reduce obesity, hypertension, asthma, and rheumatoid arthritis. Thus, fasting has the potential to delay aging and help prevent and treat diseases while minimizing the side effects caused by chronic dietary interventions.


Types of Intermittent Fasting


A popular form of fasting is known as intermittent fasting (IF; reduced meal frequency). The fundamentals of any intermittent fasting routine revolve around a period of not eating (the fasted state) and eating (the fed state). There are different types of IF; one method of IF involves the 16/8-time frame. This method, also known as the Leangains protocol, advocates fasting every day for 14 to 16 hours and restricting the daily "eating window" to 8-10 hours. This method was popularized by Martin Berkhan who is a personal trainer and a nutritionist. Within the eating window, two, three, or more meals may be consumed. The Leangains method also incorporates exercise into the fasting period. When in a fasted state, the body is already burning fat for energy.


A number of animals and some human studies have shown that alternating fasting and eating times improves cellular health, potentially by activating an age-old adaptation to food shortage cycles called metabolic switches. Such a switch occurs when cells use up their stores of rapidly accessible, sugar-based fuel, and begin converting fat into energy in a slower metabolic process


Fasting leads to pronounced metabolic changes. The shift from carbohydrates and glucose to fatty acids and ketones as the main source of cellular fuel for the body and brain seems to be a crucial factor. It has recently been referred to as intermittent metabolic switching and glucoseto-ketone switch. The reverse step–ketone-to-glucose switch–happens upon refeeding. The glucose to ketone switch includes reduction in blood glucose, insulin and IGF-1 levels, depletion or reduction of glycogen stores, and an increase in lipolysis (the breakdown of fats and other lipids by hydrolysis to release fatty acids) and ketogenesis.


Additionally, those who want to lose weight the healthy way should exercise as well. Physical activity promotes the development of muscle mass, for which a high amount of energy is used. Regular physical activity boosts the metabolism as well as fat burning processes. Walking, jogging or strength training at the gym is all compatible with intermittent fasting. By exercising, there is an increase in the body’s rate of fat burning, hence why the Leangains methods advises working out in a fasted state for 1 hour– ideally 1 hour prior to the first meal, 3-4 days per week.


Another intermittent fasting method is the 5:2 diet that involves eating normally 5 days of the week while restricting calories to 500-600 for two days out of the week. This diet is also called ‘The Fast Diet’ and was popularized by British journalist Michael Mosley. On the fasting days, it’s recommended that women eat 500 calories and men eat 600 calories. For example, you might eat normally every day of the week except Mondays and Thursdays. For those two days, you eat two small meals (250 calories per meal for women and 300 calories for men). As critics correctly point out, there are no studies testing the 5:2 diet itself, but there are plenty of studies on the benefits of intermittent fasting. No matter what method of IF is used it is vitally important to always stay hydrated with water.


To achieve all the health benefits of IF, such as fat loss, increased metabolic rate, lower blood sugar levels, boost in the immune system and others, it is necessary to restrict consumption of caloric foods or drinks during the fasting period. Consuming non-caloric beverages will not break the fast. This is because non-caloric beverages do not cause the release of insulin, and consequently, do not interfere with fat burning and autophagy (cellular cleanup).


Non-caloric beverages include:


  • Spring water

  • Sparkling water