Hey Milca just copied this post from a similar question I answered a bit back... pretty much covers it all..
When it comes to loose skin a lot of different things factor in, skin health is a big part of it a lot of people neglect hydration which is the most important thing when it comes to your skins elasticity, nutrition is also key a lot foods like chicken breast and other fatty protein sources contain a lot of collagen which helps improve elasticity but your skin is the last place to receive nutrients so your gonna need a good amount of veggies in your diet and cut out as much processed and junk food. Also when it comes to weight loss a lot of people jump the gun and focus only on the number on the scale. Measure your BF% and take progress pics. Incorporating weight lifting opposed to cardio is a better route for the more extreme weight loss (50+lbs). When dropping a lot of weight only doing cardio a lot of what your loosing is actually muscle and you'll end with a soft flabby physique.
So to sum it all up stay hydrated, eat healthy, lift weights, and most importantly take your time with it fast transformations rarely end well, stick to around 6-8lbs a month your skin is a living organ and will adapt to your body's changes but it takes time, there's no point in rushing it just to be disappointed with the end result.
