TR
I am a big fan of the eat less and move more diet. And ultimately if you are in positive calories you gain weight and negative calories you loose weight.
However there is some tinkering around the edges to do with the calories you burn. Metabolic rate can be altered. Eating chillis and currys increases metabolic rate. Exercising increases burn rate during the exercise and also afterwards to repair, replenish and grow muscles. And yep muscle maintenance burns more all the time.
Extremely low calorie intake can induce a starvation mode in some, reducing long term metabolic rate. Although how the 5:2 diet affects that versus long term negative calorie intake it would be difficult to comment.
Many overweight people get so over a long period. 100kcal extra per day puts on a potential 1lb of fat every 35 days. Over a year that is 10lbs. Over 2-3 years that is 2 stone of weight. For 100kcal/.day which is