Not knowing your frame size I chose a medium frame size and using you height and current weight I calculated your ideal weight range. For your height and weight at a medium frame you would fall between 138 to 149 pounds. Your current weight of 145 pounds puts you just under the top end of that weight range.
To lose 20 pounds would make you underweight which is more dangerous than being slightly overweight, which you are definitely not. Even if you consider yourself small framed you are just 6 pounds overweight, really not that much to be concerned about. A 10 pound weight loss would put you in the middle of the weight range. If you are large framed you are 1 pound under weight. I will provide a link to the weight calculator I used.
I suggest before you start a diet you check with your doctor to see if you truly need to lose weight. Also remember muscle weighs more than fat so if you’re athletic you will weigh more than a friend of similar size as you.
Another thing to consider is that while we as a whole, that is world society, is very concerned about obesity. We are also concerned with people trying to look like film stars. There is such a thing as being too thin and being under weight causes a whole host of problems medically left unchecked, corrected can have a lifelong and life shortening effect.
xXxPuNki-PiXiExXx answered Tuesday October 11 2011, 5:28 am: I agree with the previous answer, but it really depends on the person as well as their diet and exercise plan. In about a year I lost just over 20 pounds because I did it really slowly. This is because I went on and off diets and exercise plans constantly, and my lifestyle changed in this period of time (I changed schools and had to walk from the train station to school every day which meant more exercise). I wasn't exactly too serious about losing weight, I just wanted to get healthier and so I was okay with losing it slowly. I also had trouble keeping my weight down and occasionally my weight would rise back up a few kilos.
I know one girl who's lost about 50 pounds in a year or so but she was a lot more determined than I was, and stuck to her diet and exercise plan. And it's not so much about how long it will take, but making yourself healthier now to benefit yourself in the long run.
I also know people who can lose weight really quickly simply because they enjoy exercise and it's something they can do really easily. But I do know that these people also gain weight back quite quickly if they go back to their normal diet and lack of exercise after they've lost weight.
Pook answered Tuesday October 11 2011, 2:45 am: To lose weight at a realistic, healthy pace, you're looking at a 1-2 pound loss per week. 1lb a week is equivalent to a 500 calorie deficit per day, where
Caloric deficit = Base Metabolic Rate + calories burned from exercise - calories consumed
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