Is starvation mode real?

Ok, so I recently started logging my calories and I found out I only eat around 900-1000 calories per day without trying to diet. I am 5'6 and 125 pounds. I heard that I you eat too few calories, your body goes into 'starvation mode' and slows your metabolism which makes it harder to lose weight, and can even cause you to gain weight. Is this true? If so, what should I do to reverse the affects without gaining weight?
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Just a side note- I'm not trying to diet or lose weight, I just logged my calories out of curiosity. I'm never hungry though and its kind of hard to eat more without worrying I'm going to gain weight, but I don't want my metabolism to slow.
Is starvation mode real?
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