Cons: Even though there is flexibility in when you eat, Leangains has pretty specific guidelines for what to eat, especially in relation to when you’re working out. The strict nutrition plan and scheduling meals perfectly around workouts can make the program a bit tougher to adhere to. (You can learn more about the specifics — as well as when to time these meals — directly from Leangains here and here.)
So what’s the first step in getting started? Each method has its own guidelines for how long to fast and what to eat during the “feeding” phase. Below, you’ll find the five most popular methods and the basics of how they work. Keep in mind, intermittent fasting isn’t for everyone. Those with health conditions of any kind should check with their doctor before changing up their usual routine. Note that personal goals and lifestyle are key factors to consider when choosing a fasting method.

The fasting phase of The Warrior Diet is really more about “undereating.” During the 20-hour fast, you can eat a few servings of raw fruit or veggies, fresh juice, and a few servings of protein, if desired. This is supposed to maximize the Sympathetic Nervous System’s “fight or flight” response, which is intended to promote alertness, boost energy, and stimulate fat burning.
Of course, fasting — regardless of the method — isn’t for everyone. If you have any medical conditions or special dietary requirements, it’s smart to consult a doctor before giving intermittent fasting a shot. Anyone who tries it should also plan to be highly self-aware while fasting. If it’s not agreeing with you, or if you need to eat a little something to hold you over, that’s just fine. It takes our bodies time to adjust, and some require more than others. Keep in mind that hormones can make it harder for women to follow a fasting plan than for men. “Be cautious at first, and start slowly [with a shorter fast],” Shanks recommends. If it doesn’t make you feel better, try something different, or accept the fact that maybe fasting isn’t for you.
There are MUCH bigger fish to fry with regards to getting healthy than a few calories here and there during a fast. 80% adherence that you stick with for a year is better than 100% adherence that you abandon after a month because it was too restrictive. If you’re trying to get to a minimum bodyfat percentage, you’ll need to be more strict – until then, however, do what allows you to stay compliant!
How It Works: Not completely satisfied with the IF diets listed above? This method takes the best parts of Eat Stop Eat, The Warrior Diet and Leangains, and combines it all into one plan. You also get one cheat day each week (yay!) — followed by a 36-hour fast (which may be not-so-yay for some). After that, the remainder of the seven-day cycle is split up between the different fasting protocols.
In Prime Women’s recently launched PLATE weight management program, Dr. Kathryn Waldrep recommends eating within a nine hour window and choosing that time frame based on your body’s circadian rhythms. Early risers might eat between 9:00 am and 6:00 pm. Night owls would eat their first meal at noon and finish their last meal at 9:00 in the evening. As more and more research has been done around IF and circadian rhythms, there seems to be more and more evidence on the soundness of this approach to eating for weight management.
^ Jump up to: a b c Harris, L; Hamilton, S; Azevedo, LB; Olajide, J; De Brún, C; Waller, G; Whittaker, V; Sharp, T; Lean, M; Hankey, C; Ells, L (February 2018). "Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis". JBI Database of Systematic Reviews and Implementation Reports. 16 (2): 507–547. doi:10.11124/JBISRIR-2016-003248. PMID 29419624.

When you eat a meal, your body spends a few hours processing that food, burning what it can from what you just consumed.  Because it has all of this readily-available, easy to burn energy (thanks to the food you ate), your body will choose to use that as energy rather than the fat you have stored.  This is especially true if you just consumed carbohydrates/sugar, as your body prefers to burn sugar as energy before any other source.
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