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Intermittent Fasting: Choosing the right fasting window

Deciding when and how to fast isn't an easy choice once you're getting started. Here is some tips on how to choose the right fasting window.
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Written by
Dr. Sara Diana Garduno Diaz
PhD & Senior Nutrition Consultant

You’ve nailed step one of intermittent fasting: deciding to do it. 


Step two might be a bit more challenging: deciding how to do IF the right way to ensure it works best for your health and wellness goals. 


You may be slightly overwhelmed with this step, what, with all 43 million results coming up on your search… What information do you trust?


Well, that’s why you’ve come to us. Let us be your guide to start your IF journey by helping you to decide on the best fasting window that’s suited to your needs.


Decided on your fasting window


The first thing you need to do is to think about your day. When do you think it would be the most suitable time to eat and when would you think it’s the most suitable time to fast?


This may seem like a silly question because most of us are working 9-5 jobs and we sleep at night. But for those of you who work across different time zones, or for those of you who work shifts, your fasting and eating windows will look a lot different. 


There are a number of ways to do IF. If you know what to expect upfront, and you’ve made a plan to fit the eating and fasting time into your schedule, you’re far more likely to be able to implement and maintain the key strategies you need to ensure success with IF. 



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Here are the most common types of IF:


1. 16:8


The 16:8 window is the most popular choice of IF as those who implement this type of fasting say it is sustainable, fairly easy to do, and can be a convenient way to lose weight and promote overall health*. The premise is that as long as you’re fasting for 16 hours in the day, you can eat whatever you like the remaining 8 hours of the day. 


Typically, the suggestion is to eat within the hours of 10:00 a.m. and 6:00 p.m., fasting then in the evening, overnight and into the morning. Of course, adjusting these times to suit your schedule is ‘allowed’, as long as you’re sticking to the 16 hours of consecutive fasting. 

2. 14:10 fasting


This option requires you to only fast for 14 hours every day and is similar to the previous one. You may lose some benefits of fasting such as autophagy, but we’ll talk about this in another article :)



3. 12:12 Fasting, also known as overnight fasting.


If a 14 hour fast seems too long, or you enjoy the occasional late dinner or earlier breakfast, an overnight fast is another option. 


Instead of 14 hours, this fast is only 12 hours long. Research shows that it only takes 12 hours for your body to use all of its glucose reserves, which helps to regulate insulin levels*; one of the aims of fasting. While the results will be milder than you’d see with a longer fast, the overnight fast is generally easy to accomplish. 


4. 5:2 


Another popular option is to eat without any restriction 5 days a week and then eat only 500 calories the other two days of the week. This approach works for most people as it provides freedom to choose the 2 fasting days each week, which can be scheduled around events*. 


5. OMAD (One meal a day)


You’ll only eat one meal a day when you’re doing whole-day fasting. This is a really tough approach, and again, may lead to hormonal changes in women. Many people try to eat only once a day, but full up on coffee or diet drinks to keep hunger at bay, which can also have negative effects. 


6. Alternate Day Fasting


Alternate day fasting involves eating every other day, and has shown to produce great results for weight loss. However, a downfall is that hunger levels don’t appear to subside much over time, which means that alternate day fasting is unlikely to be a long term strategy*.


7. Prolonged fasting


Prolonged fasts can involve not eating for 24 hours and above. In fact, some people even participate in 21 day fasts! Prolonged fasts are great for pushing the body into a deeper state of repair and have great benefits such as improving blood sugar control11. It’s notable that a doctor should be consulted before performing any prolonged fasts, to ensure it’s fitted for each person.


There you have it! Decide on the fasting and eating window that suits you, and give it a go. Of course, you can also adjust your windows as you progress in your IF journey!

References:

  1. Collier R. Intermittent fasting: the science of going without. CMAJ Canadian Medical Association. 2013;185:E363–4.
  2. Stockman, MC., Thomas, D., Burke, J. et al. Intermittent Fasting: Is the Wait Worth the Weight?. Curr Obes Rep 7, 172–185 (2018).
  3. Moro T, Tinsley G, Bianco A, Marcolin G, Pacelli QF, Battaglia G, Palma A, Gentil P, Neri M, Paoli A. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290.
  4. Ruderman NBAT, Cahill GF. Gluconeogenesis and its disorders in man. In Gluconeogenesis: Its Regulation in Mammalian. New York: Wiley; 1976:515-532.
  5. Schneider JE. Energy balance and reproduction. Physiol Behav. 2004 Apr;81(2):289-317.
  6. Solianik R, Sujeta A. Two-day fasting evokes stress, but does not affect mood, brain activity, cognitive, psychomotor, and motor performance in overweight women. Behav Brain Res. 2018 Feb 15;338:166–72.
  7. Van Pelt RE, Gavin KM, Kohrt WM. Regulation of Body Composition and Bioenergetics by Estrogens. Endocrinol Metab Clin North Am. 2015 Sep;44(3):663–76
  8. Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metab. 2018;27(6):1212-1221.e3.
  9. Hirsh SP, Pons M, Joyal SV, Swick AG. Avoiding holiday seasonal weight gain with nutrient-supported intermittent energy restriction: a pilot study. J Nutr Sci 2019;8:e11.
  10. Klempel et al. Dietary and physical activity adaptations to alternate day modified fasting: implications for optimal weight loss. Nutrition Journal 2010, 9:35
  11. Effects of prolonged fasting and sustained lipolysis on insulin secretion and insulin sensitivity in normal subjects
  12. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism






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