Intermittent Fasting 101



August 2, 2021


Intermittent Fasting 101

by Mona Jauhar RDN, LD


First things first. Intermittent fasting is not a diet, it is a pattern of eating. It doesn’t necessarily change what you eat but when you eat. Also known as intermittent energy restriction, it is a generic term used for various meal timing schedules that cycle between periods of restricting calories and not restricting calories. A growing body of research suggests that the timing of the fast is key, and can make IF a more realistic, sustainable, and effective approach for weight loss, as well as for diabetes prevention.

Common intermittent fasting methods involve daily 16-hour fasts or fasting for 24 hours, twice per week. Is it unnatural? Not more unnatural than eating every 3-4 hours actually. Fasting has been part of many religions and cultures for centuries and it has been established that humans can easily go without food and water for many hours. There are a number of variations of fasting, but some of the more common approaches are defined here. Ask your healthcare practitioner to determine if and what type of fasting routine is appropriate for you.

Time Restricted Feeding (TRF) describes a shortened window of time when a person consumes calories from food and beverages. Time-restricted eating is a type of diet that focuses on the timing of eating. Instead of limiting the types of food or number of calories that people consume, this diet restricts the amount of time they can spend eating. This is also called “prolonged nightly fasting” and usually extends a person’s typical overnight fast.

Alternate Day Fasting (ADF) also called fasting intervals, describes a cycle of fasting on one day and eating on the next day. This way you only need to restrict what you eat half of the time. On fasting days, you’re allowed to drink as many calorie-free beverages as you like. Some individuals may only consume water and electrolytes on these fasting days.

Modified Fasting describes a type of fast in which a person restricts calories by 20-30%, or reduces calories to 600 per day for a specified number of days per week. This is more of a choose-your-own adventure. You might do the time-restricted fasting (fast for 16 hours, eat for eight, for instance) every other day or once or twice a week. This type of fast is also called intermittent energy restriction.

Fasting Mimicking Diet (FMD) typically lasts about five days — you’ll keep your carb, protein, and calorie intake low and your fat intake high. Calories are kept at around 40% of normal intake. This keeps you nourished with nutrients and electrolytes. It also puts less stress on your body than normal fasting while still giving you the same benefits.


Benefits of intermittent fasting

Intermittent Fasting has turned many firmly held health notions on their head. Eat small, frequent meals throughout the day? Not really. Never skip breakfast? No such thing. Fasting is an extreme measure? Depends on how you do it. Studies show that intermittent fasting and time restricted eating can improve insulin sensitivity, reduce obesity, and reduce inflammatory cytokines. They also improve the efficiency of our mitochondria and brain health. Let’s examine these claims:  

  1. Reducing calories isn’t the primary objective of IF, it is the good stuff that happens during the digestive process in the hours between meals. Fasting gives the GI tract a much needed break and helps the body conserve energy for other important functions.
  2. Long fasting leads to autophagy, or the body’s process of cleaning out damaged cells; this appears to begin around hour 22 of a fast. It is in times of starvation that autophagy keeps the body going by breaking down cellular material and reusing it for necessary processes. It is believed to be helpful in treating neurodegenerative diseases, such as Parkinson’s and Alzheimer’s and some cancers.
  3. If enhances hormone function to facilitate weight loss. Lower insulin levels, higher growth hormone levels and increased amounts of norepinephrine (noradrenaline) all increase the breakdown of body fat and facilitate its use for energy.
  4. Fasting also boosts mitochondria, the furnaces in our cells that burn fuel for energy. Lackluster mitochondria may contribute to conditions like metabolic syndrome, cardiovascular disease, cancer, and neurodegenerative disorders.
  5. Intermittent fasting has been shown to have major benefits for insulin resistance and lead to an impressive reduction in blood sugar levels and hence in chances of developing type 2 Diabetes.

Some important dos and don’t s

  1. Be hydrated: Drink plenty of filtered water on fasting days and if required take electrolytes or other supplements.
  2. Light exercise: Ditch the gym, go for walks or try some breathing and yoga on fast days. Be aware of your movement as you may feel dizzy or lightheaded, especially when first starting a fast.
  3. Eat right: Prioritize whole, unprocessed, nutrient-dense foods, especially as you are limiting your intake on fasting days.
  4. Fasting isn’t for everyone: Some individuals with diabetes, metabolic syndrome, or recurring hypoglycemia may be “metabolically inflexible.” It is recommended that these individuals work with medical experts to firstly improve their metabolic flexibility so they may be able to fast eventually, if needed. Fasting is not recommended for frail individuals, those who are pregnant or breastfeeding, individuals with eating disorders or disordered eating behaviors, those with an underweight BMI, insulin-dependent diabetics, those with heart arrhythmias, or low blood pressure.


  1. Patterson R, et al. Intermittent Fasting and Human Metabolic Health. J Acad Nutr Diet. 2015;115(8):1203–1212. doi:10.1016/j.jand.2015.02.018
  2. Wei M, et al. Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease. Sci Transl Med. 2017;9(377): eaai8700. doi: 10.1126/scitranslmed. aai8700
  3. Mattson M, Longo V, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46–58. doi:10.1016/j.arr.2016.10.005
  4. de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541–2551. doi:10.1056/NEJMra1905136
  5. Trepanowski J, Kroeger C, Barnosky A, et al. Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: a randomized clinical trial. JAMA Intern Med. 2017;177(7):930–938. doi:10.1001/jamainternmed.2017.0936


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