Amy Shah, MD, is double board-certified doctor who received her medical training from Cornell, Harvard and Columbia Universities. She has a thriving medical practice in the Phoenix area, where she sees more than 5,000 patients each year. In 2015, Dr. Shah was named one of the “Top 100 Women in Wellness to Watch” by MindBodyGreen and was a guest on the Dr. Oz show.

Eating all your meals in an 8-hour window (say, eating between noon and 8PM, and fasting the other 16 hours a day) causes significant weight loss without counting calories.[1]  While this type of intermittent fasting causes weight loss no matter what people eat, research shows that people who do it in a healthy manner lose twice as much weight (7% vs. 3% of their body weight) as those who fast while still eating junk. So it’s still important to follow a high-performance diet like the Bulletproof Diet while you fast.
Every study seems to support cognitive and health benefits for IF. Studies are coming out showing it may help stave off heart disease and it’s even been shown to halt or possibly reverse brain-related diseases such as Alzheimer’s. If in doubt, check out Jason Fung’s youtube videos along with a couple of youtube researchers who do wonderful analytics, an American who lives in Japan who goes by, “Things I’ve Learned” and Thomas DeLauer’s IF material. I’ve been doing IF myself for a few months now and I feel better, more energy, better sleep, and controlled weight.

While it’s not technically fasting, some doctors have reported intermittent fasting benefits by allowing such easy-to-digest food as whole fruit during the fasting window. Modifications like these can still give your digestive and metabolic system a needed rest. For example, “Fit for Life” was a popular weight loss book that suggested eating only fruit after supper and before lunch.
I believe this is a disservice to those, like me, with a history of eating disorder. It has made experimenting with IF unnecessarily stressful. Despite my worry about what might happen (reading all these baseless cautions), I went ahead and experimented. In my experience, contrary to this “expert advice”, IF has been the most profoundly effective intervention I’ve experienced for my bulemia.
Forms of intermittent fasting exist in religious practices in various groups across the world.[12] Religious fasting regimens include, but are not limited to, Vrata in Hinduism, Ramadan fasting (Islam), Yom Kippur fasting (Judaism), Orthodox Christian fasting, Fast Sunday (The Church of Jesus Christ of Latter-day Saints), and Buddhist fasting.[12] Certain religious fasting practices, like Buddhist fasting, only require abstinence from certain foods, while others, like the Jewish fast on Yom Kippur, last for a short period of time and would cause negligible effects on the body.[12]
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This is excellent, thank you! One thing I would love to see added to this research is the keto effect of just water over those 120 minutes, and THEN the keto effect of adding a tad bit of protein, such as collagen powder (I use the green tub great lakes powder; it dissolves great in coffee). I feel like that might put your test even more into an annals of science, and for us folks who don’t have the finger prick blood testers, it would be fun to know. Also, sugar free sweeteners like splenda (!!) or stevia–do they have any effect? Anyway–great read, exactly what I was looking for when I googled my question.
I just started fasting two days ago, in order to lose weight. I ended up fasting for 16 hours, and after breaking the fast I just had a salad (approx 1000 calories) and started fasting again after 4 hours. It’s been 13 hours already and I’m not hungry at all. Is it okay if I start alternate day fasting as a beginner? As i really don’t feel dizzy at all.
With methods like this, when the contact time between the water and the grounds is longer, a coarser grind is essential. If the grind is too fine, the grounds can get stuck in the filter or even get through the filter causing problems. It can make it harder to press down on the plunger and your coffee can end up with grinds in it. Worst of all, the finer grind means the coffee is left in contact with the water for too long, causing a bitter brew.
Every diet plan that has ever produced results is, at its heart, a strategy for lowering calorie intake. All-fat, no-fat, reduced-carbs, gluten-free, paleo… they may all take different approaches to weight loss, but they all have the same result. By limiting food intake, or even just limiting food options, these diets reduce our calorie consumption. And when calorie intake drops below calorie burn, that’s when we start to drop the pounds.
Diet soda doesn't contain any sugar, calories or carbohydrates, so it may seem like it's OK for fasting, but it's not that simple. Diet soda and other diet drinks are filled with artificial sweeteners, which can drastically increase sugar cravings, making fasting more challenging. Artificial sweeteners can also increase insulin resistance, which makes it harder to lose weight and increase your risk of developing diabetes.
On the flip side of that, eating keto can make your periods of fasting more manageable. For example, someone who is eating a diet higher in carbs will likely have more discomfort with intermittent fasting as the body constantly switches between glucose for fuel and ketones for fuel. By continuing to eat keto even during feeding periods, you can keep your body constantly running on ketones.
In fact, the authors of this book said that they had patients who only changed their eating habits with this twelve- to sixteen-hour “fruit” fast each day. They did not follow the diet’s other rules or count calories, and they still lost weight and got healthier. This strategy might have simply worked because the dieters replaced junk food with whole foods. In any case, people found this dietary change effective and easy to make. Traditionalists won’t call this fasting; however, it’s important to know that you may have options if you absolutely can’t abstain from food for several hours at a time.
I believe this is a disservice to those, like me, with a history of eating disorder. It has made experimenting with IF unnecessarily stressful. Despite my worry about what might happen (reading all these baseless cautions), I went ahead and experimented. In my experience, contrary to this “expert advice”, IF has been the most profoundly effective intervention I’ve experienced for my bulemia.

Make sure you still eat enough. Intermittent fasting does help you naturally eat less during the day, but be sure you’re still eating nutritious ketogenic foods to avoid any deficiencies or metabolic issues. Use a website or app to calculate ideal caloric intake and your ketogenic macros for each day, then track them to make sure you’re getting sufficient nutrition.


Eating all your meals in an 8-hour window (say, eating between noon and 8PM, and fasting the other 16 hours a day) causes significant weight loss without counting calories.[1]  While this type of intermittent fasting causes weight loss no matter what people eat, research shows that people who do it in a healthy manner lose twice as much weight (7% vs. 3% of their body weight) as those who fast while still eating junk. So it’s still important to follow a high-performance diet like the Bulletproof Diet while you fast.
Processed foods are usually packed with sugar additives and other undesirable ingredients that improve the taste but negatively affect your health. Even if "sugar" isn't listed on the label of ingredients, chances are it contains a substitute in some form. Whether it's sucrose, dextrose, high-fructose corn syrup or something similar, these ingredients can have the same (or worse) effects as sugar.
This is a new area, but the research that has come out since this article is also positive, and promising. One example: In this June 2018 study of 23 people with obesity, 12 weeks of 8-hour time-restricted feeding resulted a 2.6% decrease in body weight and a 7 point decrease in systolic blood pressure, which was significant when compared to controls: https://www.ncbi.nlm.nih.gov/pubmed/29951594

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Hi Thea, That’s wonderful that IF has worked for you. Diets, and particularly fasting, can be very triggering for others with a history of an eating disorder. People who have been in remission can relapse. For more about what concerns and problems others have had, there is alot of information out there, and for starters I recommend this thorough article from Psychology Today: https://www.psychologytoday.com/us/blog/hunger-artist/201411/the-fast-diet-fast-route-disordered-eating
Intermittent fasting may work amazingly well for some people, and terribly for others. Most importantly, if you do decide to give intermittent fasting a try, be sure to listen to your body’s feedback. Easing into intermittent fasting by starting with shorter fasting windows can help with initial symptoms of hunger and discomfort. But if it becomes too uncomfortable, be honest with yourself, accept it, and move on.

Black coffee won't break your fast or take you out of ketosis. In fact, according to a report published in the Journal of Basic and Clinical Physiology and Pharmacology in 2017, caffeine may even increase your metabolism, which will help promote weight loss. Black coffee also helps curb your appetite, which can make getting through your morning fast much more manageable.
Cons: Going 24 hours without any calories may be too difficult for some — especially at first. Many people struggle with going extended periods of time with no food, citing annoying symptoms including headaches, fatigue, or feeling cranky or anxious (though these side effects can dimish over time). The long fasting period can also make it more tempting to binge after a fast. This can be easily fixed… but it takes a lot of self-control, which some people lack.

Two ground-breaking studies have recently been published on the effects of intermittent fasting on males. One group of researchers studied the effects that 16 hours of intermittent fasting had on males that lift weights. They found that muscle mass stayed the same, fat mass decreased significantly, and the males who fasted for 16 hours a day burned more fat for fuel compared to the control group that only fasted for 12 hours.
Intermittent fasting, commonly referred to as IF, has become all the rage lately, and it doesn't look like the trend is going away anytime soon. The practice of intermittent fasting, or time-restricted feeding, is when a person limits the period of time in which they consume calories down to a limited number of hours. So for example, I practice IF with an eight-hour eating window, which means that I will only eat between the hours of 12 p.m. and 8 p.m.
If you have an addictive relationship with food and you struggle with portion control, track your calorie intake in your meals to make sure you’re not overeating. If you skip breakfast, you might be so hungry from this that you OVEREAT for lunch and this can lead to weight gain. Again, the important thing here is that with intermittent fasting you’re eating fewer calories than normal because you’re skipping a meal every day.
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