A 2018 review of intermittent fasting in obese people showed that reducing calorie intake one to six days per week over at least 12 weeks was effective for reducing body weight on an average of 7 kilograms (15 lb); the results were not different from a simple calorie restricted diet, and the clinical trials reviewed were run mostly on middle-aged women from the US and the UK, limiting interpretation of the results. Intermittent fasting has not been studied in children, the elderly, or underweight people, and could be harmful in these populations.
Before you start your IF plan, it's important to talk with a professional to make sure it's right for you. Women should be especially cautious as there are some mixed opinions on whether or not certain fasting protocols are healthy for female hormone balance. In addition, if you have adrenal fatigue or gut health issues you'll want to proceed with caution. If you have a history of disordered eating, you'll probably want to avoid fasting altogether..
Also, be aware that when you exercise, your body directly burns away glycogen (a form of glucose stored in the muscles). Fasting depletes these stores, which can mean less available energy for the muscles to use when they need it. This can lead to extreme fatigue and decreased performance. The best fasting tips for athletes are to precede each workout with a low-calorie protein shake designed to deliver creatine, BCAAs, beta-alanine, and other amino acids directly to the muscles as they work them, and then to follow each workout with a whey-protein supplement to replenish depleted nutrient stores in order to speed up the recovery process. These workout supplements won’t add much to your overall calorie count, and will help ensure that your intermittent fasting schedule and your workout regimen aren’t getting in each other’s way.
Third, you've probably already fasted many times, even though you don't know it. Have you ever slept in late on the weekends and then had a late brunch? Some people do this every weekend. In situations like these, we often eat dinner the night before and then don't eat until 11am or noon or even later. There's your 16–hour fast and you didn't even think about it.
Not only that, but growth hormone is increased during fasted states (both during sleep and after a period of fasting). Combine this increased growth hormone secretion:, the decrease in insulin production (and thus increase in insulin sensitivity ), and you’re essentially priming your body for muscle growth and fat loss with intermittent fasting.