Shy of some anomaly (i.e. food allergies, medical condition), weight gain or loss is based on calories in vs calories out (either intake of calories or those burnt off through exercise). A deficit or surplus of 3500 calories from your basal metabolic rate + normal daily activities will cause a difference of 1 lb. If you are focusing on body composition instead of just weight loss, then your percentage intake of macronutrients will play a role instead of just focusing on calories. There seem to be quite a few services (i.e. weight watchers) that preach that 1-2 lbs a week is a healthy way to lose weight. Personally I think 2 lbs is too aggressive (that's a deficit of 7000 calories). I tend to have my clients focus on 1 lb a week for weight loss which I want them to achieve by averaging an additional 250 calories per day burnt off by exercise and 250 by putting them in a caloric deficit. It's a much healthier way of going about it. People don't feel like they're starving and when you lose in this manner, people are actually learning how to eat properly, control portion size, and have built a habit they can hang onto for the rest of their lives. The only situation I've seen where rapid weight loss isn't damaging is if someone is taking in excessive amount of sodium. I had a lady that wanted to lose 8 lbs in 6 weeks. I told here it's doable but we'd have to hone in on her diet. When she listed out the foods she was taking in (she was Asian), it was loaded with soy sauce, tonkatsu sauce, and the like, and she was taking in an excess of 7000 mg a day (the recommended daily amount is 2300). I told her she would probably drop 4 lbs instantly if she could get down to the recommended daily amount because she was bloated and it was water weight...and she did. I've had one client lose 100 lbs but that was done over a year and a half. All I can say is be careful at how you are approaching this. You can do the math on 15 lbs in a 2 week period. Even if you had an excess amount of sodium in your diet, it's still an extreme amount of weight loss for a 2 week period. I also recommend against changing up on your diet unless this is something that is going to be life changing for you (in other words, if you're not going to continue on with it and just go back to what you were doing before you hit your weight loss goal, you'll simply gain back the weight you loss).