Understanding the potential adverse effects of intermittent fasting is limited by an inadequate number of rigorous clinical trials. One 2015 review of preliminary clinical studies found that short-term intermittent fasting may produce minor adverse effects, such as continuous feelings of weakness and hunger, headaches, fainting, or dehydration.[29] Long-term, periodic fasting may cause eating disorders or malnutrition, with increased susceptibility to infectious diseases.[29]
What if we told you that the answer to losing weight, improving body composition, and feeling better isn’t about dieting, but instead skipping meals every once in a while? For some, intermittent fasting, or going a longer period of time — usually between 14 and 36 hours — with few to no calories, can be a lot easier than you may think. And the benefits might be worth it. If you think about it, all of us “fast” every single day — we just call it sleeping. Intermittent fasting just means extending that fasting period, and being a bit more conscious of your eating schedule overall. But is it right for you? And which method is best?
As a lifestyle-leaning research doctor, I needed to understand the science. The Obesity Code seemed the most evidence-based summary resource, and I loved it. Fung successfully combines plenty of research, his clinical experience, and sensible nutrition advice, and also addresses the socioeconomic forces conspiring to make us fat. He is very clear that we should eat more fruits and veggies, fiber, healthy protein, and fats, and avoid sugar, refined grains, processed foods, and for God’s sake, stop snacking. Check, check, check, I agree. The only part that was still questionable in my mind was the intermittent fasting part.
Other studies have found that fasting was as effective as chemotherapeutic agents in delaying progression of different tumors and increased the effectiveness of chemotherapeutic drugs against melanoma, glioma, and breast cancer cells. Although this research may not apply to your life, it does suggest that intermittent fasting can help support your body in times of toxic stress.
No cravings. Fat doesn’t spike your blood sugar levels. In fact, a keto diet is so effective at stabilizing blood sugar that it got type II diabetics off their medication entirely, according to a recent study.[15] If you pair a keto diet with fasting, your blood sugar will stay stable and low (but not too low) all day. Say goodbye to the cravings, fatigue, and mood swings that make high-carb fasting so difficult.
The thing is that if you want to slim down and get rid of excess pounds, then the fasted state is really the best way to do it. If you keep taking food into your body, insulin levels will remain high, and you will keep burning glucose for fuel instead of burning fat. Of course, to remain in the fasted state, you need to not be in the fed state, and that can be a problem. Starving yourself all of the time isn’t enjoyable, and it’s not healthy. To be blunt about it, your body needs nutrients, it just doesn’t need them all day, every day.
Crescendo fasting only requires you to fast a few days a week instead of every day. My experience is that women get a lot more benefit from doing it this way without accidentally throwing their hormones into frenzy. This is a more gentle approach that helps the body more easily adapt to fasting. And if women do it right, it can be an amazing way to shave off body fat, improve inflammatory markers and gain energy. (7)

It has totally regulated my appetite and normalised my relationship with food. My obsessive thoughts have completely subsided, my black and white thinking around food has gone, and I no longer binge! This is amazing. For the first time in my adult life I feel like I know what it is like to have a normal relatinoship with food. I eat when I eat, a range of healthy whole foods and occasional less healthy foods. In normal amounts. In manageable amounts. And when my meal is over, I stop! Normal for others, a seeming impossibility for me (and, I’m guessing, others with eating disorders).

Autophagy is spring cleaning for your cells. It’s Latin for “self-eating,” which is spot-on: when autophagy turns on, your cells sift through their internal parts, get rid of anything that’s damaged or old, and install shiny new versions.[8] Autophagy is like a tune-up for your car: afterward everything runs more smoothly. It reduces inflammation and even boosts longevity. Intermittent fasting triggers, to quote researchers, “profound” autophagy, especially in your brain.[9]
The problem is that we tend to spend so much time in the fed state, fueling our bodies exclusively with glucose, that we become addicted. Once the flow of glucose stops and our insulin levels drop, we start getting hunger pains. This is one of the reasons that overweight individuals — people with more than adequate energy stores in their fat cells — often have such a difficult time cutting back on food intake. They’re not starving, but they’re not getting the glucose their bodies are used to. 

It doesn't matter when you start your 8–hour eating period. You can start at 8am and stop at 4pm. Or you start at 2pm and stop at 10pm. Do whatever works for you. I tend to find that eating around 1pm and 8pm works well because those times allow me to eat lunch and dinner with friends and family. Breakfast is typically a meal that I eat on my own, so skipping it isn't a big deal.
It can level up your brain, including positively counteracting conditions like Parkinson’s, Alzheimer’s, and dementia. As explained here in this TEDx talk by Mark Mattson, Professor at Johns Hopkins University and Chief of the Laboratory of Neurosciences at the National Institute on Aging fasting is grounded in serious research and more studies are coming out showing the benefits:
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