I havn't dropped 100 pounds yet but I am down 70. I am not having issues with loose skin. As long as you lose weight at a slow, healthy pace, you shouldn't have a problem. It seems like forever, but I am so much healthier now.
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Most people that you see with loose skin are from dropping weight too fast, like that TV show where they lose 30 pounds a week, or they have had the surgery to lose weight. That is not to say everyone is like that. Skin looses elasticity as we age, i can't say that you won't have loose skin. Sometimes I think I see loose skin on me but I am not going to let that stop me. If I am 145-150 and I don't like what I look like, because of skin, that is when I will think about it because the weight will shorten life. Skin just hangs there.
Don't let your fears be an excuse. Deal with one issue at a time. If you are into what-ifs too much, you won't accomplish any thing. Your biggest issue is that you are too heavy for your bones and muscles to carry you around. You heart has to work extra hard to make sure all the tissue has an ample supply of blood. That will shorten your life. The quality of life when you are overweight isn't great either. You may have great friend and family and say 'wait a minute, quality of life is fine', it really isn't. It sucks to not fit into places, like certain chairs or playground equipment to play with your kids. Been there, done that have the double extra large t-shirt to show it. It stinks when you overhear someone say something about your weight, or how fat people shouldn't wear things like that and you know they are talking about you.
As you loose weight, and get stronger, you will feel better about yourself. When you feel good about yourself, flaws have less meaning. Someone can say I don't like you hair that style and instead of saying, 'what is wrong with it?', you'll say, 'I like it' and it really will be all that matters.
You will make new friends, some that are into being healthy and will encourage you to be the same. One of new friends is a teacher at karate school, and she runs me ragged and in return, I run her ragged and the next day we will both complain of aches and pains, but we earned them pains and do it all over again, gladly.