1.5 stones is about 21 lbs (I had to look it up). That's a lot of weight, but not completely impossible. The danger that comes from extreme calorie drops has to do with making sure your body has enough of the right stuff to keep itself happy.
- You will typically lose a lot more weight when you first start off, and then the progress will begin to taper.
- To answer whether it is healthy really depends on what you did to get there.
Typically, one week isn't going to make or break you from a health standpoint. However, you can't sustain that pace for too long.
Make sure your diet includes the following bare minimums:
- 0.5g protein per pound lean body mass (estimated is fine); 0.8g protein per pound if exercise is a big component
- 0.35g fat per pound lean body mass
- Fruits and veggies for micronutrients
Without those bare minimums, your body runs the risk of going into metabolic shutdown. Practically, that means no more fat loss, and you run the risk of burning your muscles and organs for energy. Those goals are easy to accomplish when you are only trying to lose 1-2 lbs a week.
Some of that initial weight loss was water weight, some was fat, but some could also be lean mass and that's where the danger lies. Again, 1 week isn't going to cause major lasting damage, but prolonged periods like that can cause other problems.
My suggestion is this:
- Celebrate the 21 lbs you lost!
- Adjust the diet so you aren't losing quite as fast. Many dieters skimp on protein, and protein is what helps protect lean mass.
- Make sure you are getting all your essential nutrients. Take supplements if necessary to meet them.
- Make sure the diet is something you can handle long term.
The last bullet point is really important. Once your body gets used to eating a certain way, and you suddenly increase the amount you eat, you are likely to regain the weight you lost. However, when you make small adjustments to lose what you need to lose, you only have to keep eating at the new amounts to maintain your new weight.