Can you lose ten pounds in one week by having 800 to 900 calories a day?

Diet and exercise together would allow you to meet that goal, but make sure that you take some vitamins and if possible check in with your doctor, just in case you have a condition that might be best to monitor.

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It is NEVER a good idea to go below 1200 calories for anyone. In your case, I'm assuming your a growing teenager who, as a bare minimum, needs at least 1500 calories to function properly, and that's not even including the energy you need to have for normal things such as walking or even typing on the keyboard. If you go below 1200 cals your body will go into something called starvation mode, in which your body stops using energy for the less important things such as your hormonal system, which will result in you losing your period and most likely the chance of you ever being able to conceive permanently because of the massive stress your body is under. You also have the risk of developping full-blown anorexia, which in most cases with lead to death. Anorexia is a TERRIBLE disease, believe me, I've had it before, and I'm in recovery now. No matter how much you want to be thinner, its not worth it. Have anorexia - for me - was absolute Hell. Anyway when I had it, I got down to the lowest weight possible which for me was 88 pounds, which might to someone with anorexia is the best thing possible, but I looked horrible, my ribs protruded, my eyes looked sunken in, my arms were, literally, stick-like, and to top it off, my lack of food made me irratatted constantly and I often fainted and woke up hours later with no idea what had happened. To cut a long story short, I spent 3 months in a hospital recovering, just to be able to go home, where I couldn't even walk around, and am now still recovering. In the beginning before the whole eating disorder thing, I had been at healthy weight of 110 pounds at 5"2. Yes, the exact same as you, and I too thought I had to lose weight, but now looking at myself now compared to before, I honestly looked better at the normal healthy weight of 110 pounds, then now at 94. My skin was creamy, when now its a dull grey color and dry, my eyes shone and now their sunken in, and my hair, once a thick shining golden blond, now looks like a floor mop, thinning and dull. Losing weight is never the answer, unless your very obese, and your health is in danger. In your case, by losing the weight you'll put your health in danger, when in reality your at the ideal weight for your height.