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Intermittent Fasting

What is Intermittent Fasting?

Intermittent Fasting or IF is not a diet.

Simply put, intermittent fasting is a dieting strategy or eating pattern which times all your daily food intake into a specific number of contiguous hours to produce an extended fasting phase which can go up to 24 hours. For example, one might choose to time all their meals within an 8 hour window everyday. For the remaining 16 hours, they would have only water, black coffee or tea (items with no calorific value). Hence, this would result in an intermittent fast of 16 hours every day.

Intermittent Fasting does not specify calorie count or macro split. Hence it is not a diet per se. It is a time based strategy people use for various health and convenience benefits.

To some extent, everyone uses intermittent fasting because we generally tend to sleep for 7-8 hours and it results in a 8-9 hour fast. The most popular variants of Intermittent fasting seem to be:

  • 16:8 fast (Fasting window of 16 hours including sleep time, followed by an eating window of 8 hours per day)
  • Eat-stop-eat or 5:2 fast (Whole day spent fasting twice a week)

Science behind Fasting

After several hours pass without caloric food intake, our body leaves the fed state and enters a fasted state. Whether we are fasting or in fed state, our body needs to maintain blood glucose levels in order to function. In humans, depending upon their level of physical activity, 12 to 24 hours of fasting typically results in a 20% or greater decrease in serum glucose[1] and depletion of the hepatic glycogen(glycogen stored in liver), accompanied by a switch to a metabolic mode in which non-hepatic glucose, fat-derived ketone bodies and free fatty acids are used as energy sources[2]. Different sets of hormones assist our metabolic processes that either cause breakdown of any tissue (fat, muscle, bone etc) or growth. These processes are called catabolism and anabolism. While both can occur simultaneously at different tissues, one set of hormones hinders the other set like a seesaw. Anabolic hormone like Insulin helps in both fat storage and muscle growth. Insulin increases on intake of any food and Glucagon(which can trigger fat and muscle breakdown) decreases. Glucagon increases and Insulin decreases during fasting which causes breakdown of fat to provides the energy(ATP) we need during intermittent fasting. This fueling mechanism undergoes further changes if the fasting is continued more than 2-3 days but that is not relevant to intermittent fasting where the fasting window is usually upto to 24 hours. So we do lose fat while fasting because we use fat stores as energy when food intake is stopped and glycogen is getting depleted. However, our body is smart enough to store any excess energy as fat and replenish fat stores during our eating window. So there is no real contest to the fact that energy equations hold good no matter which dieting strategy we use.

Intermittent fasting is therefore a method used to prolong fasted state where catabolic process of fat breakdown for energy happens along with other hormonal changes that might be beneficial for health. However, our body is an extremely smart complex system with lots of counterbalances and personalizations. One solution does not fit all.

Who is it for?

While anyone could use this strategy, most people use it for the following reasons

  • They are on a caloric deficit diet (trying to lose fat) but they find bigger meals more satisfying than smaller frequent meals
  • They find certain times inconvenient to eat. Eg. Some people do not feel like eating in the morning or while they are at work.
  • They would believe there will be hormonal benefits of this practice for their fitness goal (fat loss, blood sugar regulation). These claims however, need to be vetted to see whether they are applicable for us or not.

The fasting and eating window should be selected according to the lifestyle one follows so that it is convenient and sustainable. The factors to consider for this are:

  • Training intensity and timing
  • Routine social activities and personal preferences

Who should avoid it?

Some people have a tendency to binge after a fasting window or find it inconvenient to consume the required amounts of food within a short window of time. This might result in deviating from their desired goals. Such people should avoid IF simply because of these reasons.

The following sets of people should avoid it without proper medical consultation and monitoring:

  • Pregnant women
  • Breastfeeding women
  • Underweight person
  • Child
  • Person with any medical condition currently under treatment

Things to know while using IF

  1. Adequate water intake is crucial for our well-being.
  2. Respect your bodily cues: Sleep quality, immunity, energy levels and hormonal changes can happen due to this dieting strategy. Hence it can affect our health and quality of life. These changes may not be generic for all. So, respect your unique bodily cues.
  3. Exercise and fasting: Intense exercise during the fasting window or exercise immediately after a large meal in the eating window might be troublesome. It is important to change and regulate the timings to your overall benefit.
  4. Nutrient dense diet is still required: Intermittent fasting does not condone wolfing down empty calories during the eating window. As always, your body cannot perform at its best without the fulfillment of micronutrients(vitamins, minerals) and essential macronutrients(fats, proteins).
  5. Managing hunger pangs: Our body secretes hunger hormones according to the timing we are used to. Hence, it is natural to feel hunger pangs when we increase our fasting window. Zero calorie beverages like water, coffee or tea (*without any milk/sugar/cream etc) can help us tide over these hunger pangs. Coffee has proven[3] useful for suppressing appetite.

Note: Though there are many wonderful claimed benefits of Intermittent Fasting; care must be taken before believing popular websites. Studies are often conducted on a group of people that share some attributes like gender, age group, body composition and medical conditions. Sometimes studies are conducted on animals. Hence before believing such claims, we need to find out whether these conclusions are true and whether they apply to us at all. We attempt to address these concerns in part 2 of this article “Intermittent Fasting: Vetting popular claims.


[1] “Fasting: Molecular Mechanisms and Clinical Applications – NCBI – NIH.” 16 Jan. 2014, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3946160/.

[2] “How does the body adapt to starvation? (video) | Khan Academy.” 15 Jun. 2014, https://www.khanacademy.org/test-prep/mcat/biomolecules/fat-and-protein-metabolism/v/how-does-the-body-adapt-to-starvation.

[3] “Effect of different amounts of coffee on dietary … – Wiley Online Library.” 29 Nov. 2012, https://onlinelibrary.wiley.com/doi/full/10.1002/oby.20190.

Intermittent Fasting

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