Discussing serving sizes with kids???

I would try to address the issue with broader strokes- like try to explain nutritional needs to her overall, rather than saying "you shouldn't have more cereal" or whatever (not that you said that). Give her an overview of how and why you want a balanced diet of different foods, and explain that it's better to choose a fruit instead of more cereal because it's part of meeting all your nutritional needs.

Use the resources here:
http://www.nutrition.gov/life-stages/adolescents/tweens-and-teenshttp://www.nutrition.gov/life-stages/adolescents/tweens-and-teens

or maybe get her a book on teen nutrition so she can read it for herself.

totally this...frame the conversation around proper nutrition. My eldest boy is almost 4 and we regularly talk about nutrition and why he's getting either fruit or vegetables on his plate at every meal.

This is tipping awfully close to you projecting your own insecurities onto her. she's 5'8" and 145 pounds which is a perfectly healthy range for her to be in. Yes she had three bowls of cereal, but I remember many a day (I'll insert here that in 5th grade I was 5'6", taller than most of my teachers, all classmates, and 103 pounds) where I ate more than one bowl of cereal. As long as she is active she IS healthy. She could have eaten 3 bowls of m & m's. Take a step back, check out your own issues (ie she's eating three bowls of cereal, so she's going to get fat like me) and just let her be. Promote healthy eating, certainly, but dont project your problems onto her at 13, you could end up with a Anorexic 16 year old.

I did not get this impression at all from the OP, it sounded to me like she wants to raise nutritional awareness in a sensitive way, so she doesn't impart any insecurities on her daughter. I think you may be projecting your insecurities and/or experiences on this OP.

Children under the age of thirteen eat to their metabolismic needs. My daughter is four, and she can consume an easy three bowls of cereal every morning. She eats all day, and I was actually trying to restrict her, because I thought the sugar would be unhealthy for her, but the doctors back in TN and my current doctor in VA all say the same thing, she is growing, and she will stop eating when her body tells her she is full. She can eat two grilled cheese sandwiches, and still have a belly growling. She is four ft tall, and weights 45-50 pounds. Very tall for her age. My little cousin Blake is six months older than her, and he hardly eats anything, but he is much smaller than her. I think about 32 pounds or so, and much shorter. I would consult the doctor just in case though. I would hate for you to get the wrong info from my experience lol. Good luck!!

I wish my parents had the knowledge to have these kinds of conversations with me when I was young enough, before I developed an unhealthy relationship with food (it tastes good, so I eat lots and lots!)

My son is 8, 4'6 and 65lbs. He does tae kwon do 3 nights a week and 4 hours of gym class a week (he's the kid that goes full out and comes back sweat). He lives for the outdoors, bikes, runs, climbs, etc. He is active as anything.

I do explain to him that he needs to eat good foods to be healthy and strong, and he understands.

The benefits to a healthy, balanced diet is the best way to explain! At 13, maybe you can mention about insulin spikes from the cereal, and why veggies/fruit/protein are needed to balance things out inside the body.

Is the 3-4 serving thing new or is she a big eater (that comes with being so active)? If it's new, it could be a growth spurt!

My sons are very active 9 and 4 year olds. I try very hard to make sure they eat well-balanced meals. When they've had a bowl of cereal and are still hungry, I talk to them about what food group cereal comes from and what other groups there are to choose from and we decide together what else they are going to eat. I also limit snacks to one serving of a particular snack each day, so they don't eat all the granola bars on Monday, etc. They can have more than one snack, they just have to eat something different. I also talk to them about serving sizes, and encourage them to have one serving at a time, and then, if they truly are still hungry, come back for more. This keeps them from pouring an enormous bowl of cereal and then not being hungry enough to finish it.

Hope that helps!