Fasting isn't a weight loss technique, really. Your weight and leanness is determined by your genetics and by your diet and how it affects your metabolism. Fasting can be useful for spiritual and detoxification, but obviously one doesn't need to detox a whole lot. The human body is built for intermittent fasting -- our history was one of periods of lots of food followed by periods of not so much -- so it's the way all animals live when not in artificial civilizations, but if you look at photos of people who still live that way you won't necessarily find leanness. As for building muscle, fasting isn't related to that -- most bodybuilders eat a ton and natural humans don't have great muscle definition -- strength doesn't come from the size of the muscle but the trained or instinctive use of motion with muscle. Defined muscles are just a modern cosmetic thing.
According to Mark Sissons, author of lifestyle and diet book "The Primal Blueprint," intermittent fasting can promote rapid weight loss and is aligned with natural hunter-gatherer behaviors, simulating when food supplies were periodically scare.
By strategically skipping meals you reduce your overall energy intake and, providing you avoid overeating before or after your fast, you will consume significantly less calories. By consuming fewer calories than you need, your body will burn body fat for fuel and consequently you will lose weight. IF requires no special preparation, specific foods, supplements or cooking ability which makes IF convenient for anyone who want a simple, no-frills approach to weight loss.
According to "The Miracle of Fasting: Proven Throughout History for Physical, Mental, Spiritual Rejuvenation" by Patricia and Paul Bragg, intermittent fasting as associated with a number of benefits and the effects of IF have been studied both in animals and humans. The authors say IF can lead to lowered blood glucose levels, reduced blood pressure, increased sensitivity to the action of insulin, increased fat metabolism, digestive system detoxification and increased fat metabolism.
Intermittent fasts can vary in length from a few hours, for example missing a meal, to abstaining from food for 24 to 48 hours. The length of your fast depends on your will-power, previous fasting experience and daily physical activity levels. If you are new to fasting or very active, fasts should be limited to eight to12 hours initially to ensure you suffer as few ill effects as possible. So-called daylight fasts are ideal for fasting novices. A daylight fast means that you have an early breakfast and a late dinner and consume only water in the hours in between. A more advanced fast might involve eating only on alternate days. Alternate Day fasting, ADF for short, is much more demanding than the daylight fast.
Eating less than normal can trigger severe hunger in some people while others find that they tire easily, suffer head aches, nausea and have trouble concentrating. Because of hunger, fasters may find that they eat more than usual both before and after a planned fast which essentially eliminates the benefit of the fast. See your doctor to confirm your medical suitability to fasting before trying any form of fast.
Although short fasts are seen as safe and can be effective for weight management, longer fasts can have an adverse effect on your short and long term health. Rapid weight loss can be easily regained during the post-fast period if you eat too much. Fasts do not address your day to day eating habits and fail to teach correct food selection or portion control. Fasting can be socially exclusive -- especially within a family setting where meals are eaten together. Longer fasts may cause muscle atrophy -- a condition where muscles are broken down for energy -- which may result in a lowered metabolism.
Not saying Gym is wrong, because I really don't know, I just know his sources have no scientific double blind evidence to prove that it works. I used to manage health food stores, and had customers who fasted one day a week, or by other schedules. They certainly didn't alter their regular body shape. Why not just give it a try and see what happens and report back to us?
I will say, though, that daylight fasting doesn't sound like a good idea, at least to me. That leaves you eating when you're least active. This is something Muslims do for a month during the holiday Ramadan, but obviously not for health reasons. They get up before dawn, stuff themselves, then work all day and eat after dark, when they stuff themselves again. It would seem for health purposes this would be the wrong time to fast -- it would be better just logically to fast in darkness, when most people (though not me) are least active and need the food the least.
Most Muslims will tell you they gain weight when they fast. I don't know what your routine is but you need to be eating a few hours before you excercise so unless u exercise during the night day time fasting won't be suitable anyway. If your trying to build lean muscle low weights and repetition is the only way. If you want to see progress faster then you will need to increase your calorie intake as your metabolism adapts