Weight loss is the decrease of your body's mass over time. The bulk of this mass loss comes from your fat stores. Weight loss can also come from loss muscle and bone mass.
How the process of losing weight works?
You lose weight by eating fewer calories than your maintenance level. By consuming less calories than your maintenance your body cannot sustain it's weight and will cause weight loss until it reaches a new maintenance level.
How to lose weight?
Consume fewer calories than your current maintenance, your body will lose weight and adjust to this calorie level. When your body has adjusted it will stop losing weight, in this case you would need to eat fewer calories below this maintenance to continue losing weight.
Having lost weight, is it sustainable?
Yes, provided you keep eating the same amount of calories daily that it took you to reach your goal weight.
Risks of losing too much weight
Being under weight can be as dangerous as being over weight. This is because the body is malnourished and therefore weakened. The risks include fragile bones, weakened immune system, fertility issues, hair loss and anaemia among many others.