What is undereating and the effects on the body?

There has been research in "caloric restriction" in lab mice, and the effects were positive in terms of a longer lifespan. The conclusion was that, while food is good, it is also a killer in the long term. Putting fewer calories into your body over the course of your life will give you a longer life span. The downside is you'll probably look older than your age. I'm not an expert on the subject so I would suggest you google "caloric restriction studies". Note that caloric restriction is not the same as "dieting", so don't get them confused. Dieting is a temporary reduction of caloric intake (which really isn't good for a human body), while "caloric restriction" is developing a lifelong eating plan, of something like 1000 calories/day or less.