A very common problem that I see when people start their efforts on weight loss and workouts, is cutting calories too low, and they end up under-eating, and not consuming enough calories. Sometimes this is intentional and sometimes people are doing it by accident.

This can be just as detrimental to weight loss, as over-eating. What seems to happen is that when we are concerned and conscious of what we are eating, and really focusing on not taking in any bad foods, we over compensate and many times cut too much, or pick too many foods that are low in calories. 

When are calories drop too low, you can actually see very good weight loss, for a short time. This happens obviously just for the sheer reason that we stopped eating so much. But after a little while, our body starts to catch on. It realizes you aren't giving it enough nourishment, so it decides that it needs to hold on to that extra fat, as energy reserves. For all it knows, a famine is coming. 

So basically, two very bad things can begin to happen. For one, your metabolism will start to slow, this is bad obviously because now we just aren't burning as many calories, which means its going to be even harder to lose weight.

Second, and possibly worse, your body is going to start to break down muscle tissue for energy. It has already realized you aren't giving it enough food. And since fat is more difficult to break down compared to muscle and carbohydrates, your body now turns itself against its own muscle, and begins to break that down for energy. That compounds our slow metabolism even more, because muscles are pretty much the generators for resting metabolic rate. 

Even worse, when you go back to eating a normal amount of calories, you have less muscle and a slower metabolism to keep things going. So if you gain any of the weight back, you are now more unhealthy than you started because you have less muscle mass than when you began, and if you end up at the same beginning weight, you are now the same weight with a higher fat percentage. 

As a general rule of thumb, women should usually start around 1100-1200 calories and make adjustments as needed, men about 1400-1500. It's a good starting point and you will reduce the risk of running your body into starvation mode.