According to its fervent proponents, humans were not made to be eating constantly; rather, we were designed to eat for short periods of time followed by hours (and sometimes days) of fasting. Much like vegetarianism, intermittent fasting is practiced for two main reasons: health and religion/spirituality. There are no food or caloric restrictions, but rather a restriction on WHEN to eat. The most popular methods include the 16/8 (8 hours of eating followed by 16 hours of fasting), the “Eat-Stop-Eat” method (two 24-hour fasts per week on non-consecutive days); and the 5/2 method (only 500 calories for 2 non-consecutive days, and 5 days of normal eating per week). While it's generally considered a healthy approach to eating when done moderately, some studies suggest extreme fasting may lead to hair loss and anxiety. 

There are many ways to do intermittent fasting — ranging from fasting for a number of hours each day up to an entire 24-hour fasting period one or two times a week. “If you're trying to kick a habit like eating late into the night, then stopping eating earlier in the evening and fasting overnight could be beneficial for you,” says Hultin. “There are many types of intermittent fasting, so ensuring you pick one that works for you and your lifestyle is important.”
"Your body begins to work differently. This study proves that small amounts of nourishment throughout the day are better than the same amount of food concentrated in three big sittings. If we feed the body at regular intervals we send a signal to the body that it doesn't have to store calories. Conversely, when we skip meals we send just the opposite signal for the body to store calories, creating a negative effect on the metabolism." — Dr. Wayne Scott Andersen, co-founder and Medical Director at Take Shape For Life
"These diets are so restrictive that of course you're going to lose weight fast because you're not eating enough calories to sustain basic activities of your body, let alone any exercise. That's nothing that any person can sustain for the long term," Hogan said. "The weight's going to come back if you do lose any weight, and then it's going to be harder to lose weight in the future."
With this eating style, you’re looking at a lot of menu planning and preparation. A review published in August 2017 in Nutrients suggests the diet could lead to weight loss, but the Academy of Nutrition and Dietetics warns the plan could also cause certain nutrient deficiencies, such as in calcium and vitamin D. (3,4) And, therefore, according to an article published in the January–February 2016 issue of the Royal Australian College of General Practitioners, anyone at risk for osteoporosis should avoid it. (5)
Consequently, researchers have widely discredited the hCG diet, which involves using hCG injections, pellets, sprays, or drops, and consuming  as few as 500 calories daily. The diet is problematic not only because there’s a lack of research on hCG supplements, but also because the calorie requirement is dangerously low, potentially leading to nutrient deficiencies, fatigue, hormone imbalances, blood clots, and other issues. Thus, most experts agree the hCG diet is not safe for anyone, the Mayo Clinic notes. (35)
Forget old low-carb diet plans that focused on processed protein bars and shakes. This year, the keto diet got high marks for low carb. Keto, short for "ketogenic," is all about training the body to burn fat for fuel. How? By eating fat—and lots of it. Most keto diets recommend getting at least 70 percent of your daily calories from fat and the rest from protein. The goal is to eat as few carbohydrates as possible. Proponents say it helps them drop weight fast with little or no hunger in addition to perks like more energy and mental clarity. (Interested? Here's everything you need to know about the keto diet.)
Right now, this is the trendiest of all diet theories. It prioritizes the consumption of foods high in protein and fat while limiting the number of carbs per day to less than 20%. This means abstaining from grains, starchy vegetables, and fruits, and filling up on protein and fat. The most famous of all low-carb plans is the Keto Diet, an approach based on the metabolic process of ketosis, which happens when you restrict your carb intake to less than 0.7 grams per day and your body starts using fat, and not carbs, as its main fuel. 
If you’ve been eating fast food for years, get real about your approach: You’re probably not going to stick to an organic, gluten-free, paleo overhaul for very long. "You want to change as little as possible to create calorie deficit," says Dr. Seltzer, who insists the best way to support sustainable weight loss is to incorporate small changes into existing habits. So instead of giving up your daily BLT bagels in favor of an egg-white wrap, try ordering your sandwich on a lighter English muffin. Or say you eat a snack bar every afternoon: Swap your 300-calorie bar for a 150-calorie alternative. "Your brain will feel the same way about it, so you won’t feel deprived," he says. 
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