[jokamoto]

Two possible ways:

Being underweight is dangerous, just as being overweight is dangerous. Instead of heart disease and joint damage, you end up with osteoporosis, anemia, etc.

Losing weight without paying attention to nutrient intake and balance can have similar effects, even if you'd be at or above healthy weight losing 50lbs.

[Tharkun]

Erhm, yeah, good luck losing that amount of weight easily and ending up underweight...

[chrisdbaldwin]

Stop eating carbs and see what happens. It's surprisingly easy to end up underweight when sugar isn't present.