My 16 year old skips breakfast, brings her lunch bag home full of everything I packed her & refuses to eat dinner. She drank a smoothie for lunch yesterday. She drinks plenty of water and sports drinks, but she runs cross country, she's in athletics and she's a dancer. I'm concerned after all that daily activity she's not properly nourishing herself. I've never dealt with anything like this before. I'm not sure what approach to take. Should I up her vitamin intake or should I take her to her doctor? How urgent a matter is this or am I just being an over concerned mom?

I would take he to the doctor and have an exam and have her doctor talk to her about the importance of a proper diet.
My daughter also ran cross country and other activities and she ate like a horse. They are constantly burning calories.

There are a lot of reasons a 16 year old girl could be refusing to eat. Regardless, a visit to the doctor would do well. Teenage girls are fairly prone to eating disorders, my girlfriends sister had developed one during her early years in high school. It's best to determine what it is sooner rather than later.

Are there sports medicine clinics near you? I'd take her to one, for a good stern lecture on what her body is going to do without food - basically burn her own muscle to provide calories, when stores of glycogen run out, which is soon. It can be a nutritionist to lecture her.
Ask her if anyone in her dance class, teacher or classmate, told her she has to lose weight. Often that is the culprit, more than sports, where eating is considered very important.