Intermittent Fasting: A Complete Guide

The How, Why, and When You Can Start Reaping Its Benefits

How to Start to Intermittent Fasting

The Spruce / Ellen Lindner

Any eating plan that doesn't restrict what or how much goes on your dinner plate is bound to gain some steam. It's no surprise that intermittent fasting (I.F.) has taken off in recent years and it's natural that skeptics are critical of its credibility given how simple it is. We'll discuss what intermittent fasting is, how it works, the science behind it, and how you can start reaping its benefits.

What is Intermittent Fasting?

The term is pretty self explanatory and easy to understand: intermittent fasting is the concept of eating some of the time and fasting the rest.

The point of intermittent fasting is manifold: proponents of this eating method claim that you can lose weight more easily than with restrictive diets, you can increase your level of human growth hormone (which in turn leads to slowing down the aging process and improves overall wellness), you're more functional physically and mentally, and your overall health will be improved. I.F. came to popularity in the early twenty-teens through "biohackers," which is a name for health and fitness advocates who seek to improve their bodies and its functional abilities through science and experimentation.

What I.F. looks like day to day has a lot of room for mobility, but the most basic version of intermittent fasting involves eating within an 8-hour window daily and fasting the other 16 hours. For example, a window of 10 AM to 6 PM or noon to 8 PM would be when you ate all your food for the day, from breakfast to dinner or "late night" snack. The remainder of the time, in these examples 6 PM to 10 AM or 8 PM to noon, you wouldn't consume anything caloric. More extreme versions of I.F. reduce the window to only 3 or so hours per day, such as eating from noon to 4 PM and fasting daily from 4 PM to noon.

An alternate version of intermittent fasting is to not restrict timing at all through 5 days of the week, then fast completely for 2 days a week. For example, this means you would eat anytime you want Monday through Wednesday, fast from Thursday morning to Friday morning, eat whenever you wanted on Friday and Saturday, and fast Sunday morning to Monday morning. Alternately, you could eat whenever you wanted Monday to Friday and fast consecutively both weekend days.

Is there scientific evidence that IF is useful? In short, yes. Fasting isn't new to humans, and it does have health benefits.

Accelerated Weight Loss

It makes perfect sense that when you eat for less time daily, you're bound to eat less calories overall. Because reducing calories is the simplest way to lose weight, intermittent fasting has been shown in studies to lead to weight loss.

In addition to facilitating weight loss through calorie reduction, intermittent fasting has also been shown to increase metabolic rate. By speeding up your metabolism, you're able to process the calories you eat more quickly and effectively.

Intermittent fasting, despite not restricting what you eat, initially leads to more weight loss than the average diet. Beyond leading to more pounds lost,I.F. results in less muscle loss than other diet, higher percentage of belly fat lost, and lowering of insulin resistance, too.

The conclusion: intermittent fasting is an effective dieting tool and studies show it leads to more weight loss than other popular diets.

Slower Aging

Fasting has been around for as long as humans. Before industrialized agriculture people didn't always have access to food. Additionally, various religions utilize fasting as part of their spiritual practices for millenia. As a wellness tool, medicine practitioners recommended fasting since the early 1900s. In recent decades it's served as a popular natural method of fighting illnesses and health problems.

While not all of natural medicine's rationales for fasting are sound, the basis of fasting as a tool for wellness actually is. The act of fasting instigates cellular repair. That process involves autophagy, which means your body's cells rid themselves of non-functional, old proteins, essentially cleaning house on a very deep level.

It also makes your body produce more human growth hormone. HGH production slows with aging, and increasing it can prove beneficial in a multitude of ways. In addition to fasting being shown to help rats live longer, human growth hormone regulates metabolism, fat loss, and muscle and bone composition. That means more human growth hormone leads to increased function in those areas. In addition, anecdotal evidence tends to lean strongly in its favor.

Personally, I did intermittent fasting from 2014 to 2017. In the first year, my human growth hormone levels, which were very low after my recovery from a half decade of chronic illness, nearly tripled. Though I haven't done I.F. in several years now, my levels continue to remain significantly higher than they were prior to my time doing it.

The conclusion: intermittent fasting induces a variety of anti-aging functions in your body.

Higher Functionality

It's a little counterintuitive to think that temporarily starving your brain, which requires copious nutrients to work well, would actually make it work better. However, science has shown that intermittent fasting does indeed lead to increased brain health by way of stimulating the growth of new nerve cells and increasing a hormone called brain-derived neurotrophic factor, which is used for memory.

Systemic inflammation can lead to pain and illness, and can be a major lifestyle disruptor. By decreasing inflammation, intermittent fasting may lead to higher overall functionality. This makes sense because we all perform our best when feeling our best.

Lastly, the act of cooking and eating fewer meals per day means that time is freed up for other life activities and less time stressing over meal prep.

The conclusion: intermittent fasting helps your brain, reduces inflammation, and can simplify your lifestyle.

Carbonated water with lemon
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How to Get Started With Intermittent Fasting

First, it's important to understand there will likely be some trial and error involved. While the preliminary step to practicing intermittent fasting is to choose your daily eating window, that doesn't mean your choice will work best for your body. After all, your brain may have different ideas than your stomach about when it wants food! So, choose your daily window (or your fasting days if you'd prefer to begin with the more difficult five on, two off method), but be flexible. Start with an 8-hour window and don't try to move to a shorter window until you have successfully eaten only within this window for long enough that it feels natural and effortless.

Next, you'll want some noncaloric liquids on hand for the period of day before and possibly after your eating window. That's because the average person eats in a much larger window daily within a standard 8-hour work day. Acceptable liquids include water, black coffee, tea, or any noncaloric water-based beverage such as a flavored sparkling water.

The final step to getting started with I.F. is simply to only eat within the designated chosen window. In order to avoid missing days because your window ends up being too short, you should stick to the same window daily until you're well into the process. Otherwise, you'll have a lot of math to do and can end up wasting a day by not fasting for long enough to have the desired impact.

It's important to realize that as you begin this journey, you'll probably experience hunger in the first few days or weeks because you're changing a lifelong eating pattern. Moreover, you may feel weaker or tired as you adjust to I.F., but that should change quickly once your body has acclimated.

Is Intermittent Fasting Safe For Everyone?

In short, no. I.F. is not a match for all.

If you have a history of disordered eating, I.F. is probably not a good fit as focusing on when you do and don't eat may be triggering and not an emotionally positive experience. If an eating disorder is a part of your past, consult with your doctor, therapist, or other treatment provider before considering it further.

Because I.F. naturally leads to some amount of weight loss, it's not a good idea if you are already at an ideal weight or if you are underweight.

I.F. will serve you best when you are not pregnant, breastfeeding, or trying to conceive because you do not want to restrict calories during any of these important times of life or anything to possibly lead to your child not receiving sufficient nutrients. While people of all genders can do I.F., there are some potential issues for female-bodies people, such as a decrease in menstruation, to exercise caution around.

Finally, if you have any major medical condition at all, you'll want to discuss I.F. with your doctor before beginning. Whether that condition is diabetes, cancer, low blood pressure, etc., err on the side of caution and make sure to discuss it thoroughly first with your provider.

Should I Try It?

Intermittent Fasting is generally considered a safe and effective tool for both weight loss and overall improved health. I.F. is far less restrictive than other diets in that it doesn't at all limit what you can eat, only when you can. If you'd like to try I.F., all you have to do is choose a window of time for eating. Then, you're ready to start!

Article Sources
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  1. Welton S, Minty R, O’Driscoll T, et al. Intermittent fasting and weight loss. Can Fam Physician. 2020;66(2):117-125

  2. Mansell PI, Fellows IW, Macdonald IA. Enhanced thermogenic response to epinephrine after 48-h starvation in humans. Am J Physiol. 1990;258(1 Pt 2):R87-93

  3. Barnosky AR, Hoddy KK, Unterman TG, Varady KA. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings, Translational Research. 2014;164(4):302-311

  4. Varady KA. Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?, Obes Rev. 2011;12(7):e593-601

  5. Alirezaei M, Kemball CC, Flynn CT, Wood MR, Whitton JL, Kiosses WB. Short-term fasting induces profound neuronal autophagy, Autophagy. 2010;6(6):702-710

  6. Hartman ML, Veldhuis JD, Johnson ML, et al. Augmented growth hormone (Gh) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men, J Clin Endocrinol Metab. 1992;74(4):757-765

  7. Goodrick CL, Ingram DK, Reynolds MA, Freeman JR, Cider NL. Effects of intermittent feeding upon growth and life span in rats, GER. 1982;28(4):233-241

  8. Lee J, Duan W, Long JM, Ingram DK, Mattson MP. Dietary restriction increases the number of newly generated neural cells, and induces BDNF expression, in the dentate gyrus of rats, J Mol Neurosci. 2000;15(2):99-108

  9. Miranda M, Morici JF, Zanoni MB, Bekinschtein P. Brain-derived neurotrophic factor: a key molecule for memory in the healthy and the pathological brain, Front Cell Neurosci. 2019;13

  10. Aksungar FB, Topkaya AE, Akyildiz M. Interleukin-6, C-reactive protein and biochemical parameters during prolonged intermittent fasting, Ann Nutr Metab. 2007;51(1):88-95