how much weight will i lose if i fast for 30 days?!

I see that you are determined to do this, and I just hope that your parents notice and stop you. You could cause serious damage to your body, especially your metabolism and your heart. Fun fact: you may PERMANENTLY damage your metabolism by starving yourself. My mother did this to herself in her 20s and regrets it to this day.

I know what it is like to be a miserable, overweight teenager. It sucks. What you're describing isn't "fasting for 30 days," it's anorexia. A fast usually involves drinking a nutritional supplement beverage, and do NOT last for one month. The longest you should be on a fast is one week. An eating disorder is NOT the answer to your body issues.

What helped me immeasurably when I was 14 was joining Weight Watchers. I lost 25 pounds (I was 50 lbs overweight) and it made the rest of high school a lot easier. If you're not willing to do a program like Weight Watchers there are EASY ways you can alter your diet that will lead to weight loss, without starving yourself. Stop drinking soft drinks and juice (you're taking in a LOT of excess sugar and retaining water b/c of sodium). At each meal, leave 1/4th to 1/3rd of the food on the plate. A little cut in portions will show on the scale. Eating fruit as snacks to get you from meal to meal will cut out mindless, fatty snacks and make a world of difference. Figure out what foods trigger your eating, and don't have them in the house.

Lifting weights will help you in specific areas, but isn't going to get the cardio fat burning you are looking for. I would recommend starting running intervals. Go out walking, at a brisk pace. Run or jog for 30 seconds. Then walk again for 1-3 minutes. Then run again for 30 seconds. You only have to do it for 15-20 minutes. You'll start seeing results pretty fast. A kid in my high school dropped 50 pounds in six months this way.

Please, don't do an unhealthy fast. Once your body realizes it is starving, it will actually hold onto the fat for dear life -- it's a survival mechanism that kicks in. You will damage your health AND won't lose the weight you want.

growing up fat, Weight Watcher for 12 years, have worked with a personal trainer doing running intervals

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That's a bad idea you will become very weak and tired because your not providing vital nutrients only found in food, water has nutritional value, you will run out of nutrients and become very sick