This topic is a hot button issue for me.
My DD has a BMI that is far below 'the chart'. I'm taking, like 1.5 or 2" below it. We had to look around the page to find the plot point, and he thought her weight was entered incorrectly at first. She is 11.
Anyway, her doctor just had a fairly in-depth discussion with DD about the importance of eating enough calories, with a huge reminder that she's a kid. He focused on the fact that she trains a TON, far more than most kids, and that she needs to not worry so much about what she's eating. Occasional fast food is ok, treats are ok. In moderation, as long as she's also eating enough good proteins, fruits, and vegetables. Basically, nachos as a frequent dinner option = bad, but as a snack sometimes is ok.
He was fairly concerned that she's setting herself up for an eating disorder due to things she's read and heard over the years... No not just the gym, gym/health class, television, and friends, too. He has even encouraged drinking calories during practice (by whatever means). He suggested a 50:50 water:Gatorade blend. Water is important, but so is consuming enough calories.
Plus, Gatorade tastes good, so she's more likely to drink more fluids.
My DD has often struggled to eat enough. When she was in 2nd grade, her teacher insisted snacks must come "from nature". The former bio/chem major in me argued that everything is "from nature". Fact is, my kid had high-calorie, less-healthy snacks because (1) it was a nut-free classroom, (2) the snack couldn't be refrigerated, (3) she eats a very light breakfast... Milk and fruit, usually, and (4) she has anxiety issues and rarely ate much lunch in the cafeteria because it was so overwhelming. Teacher and I argued over it the entire school year, until I finally went to the office just to get her to stop snack-shaming my kid.
So, blanket statements upset me, every kid is different.
Anyway, DD has gained a pound or so since the appt, and has grown 1". Whether that's just genetics or the increase in calories, I'll never know.
Now, if I could get her younger brother to stop avoiding the healthy stuff and to eat fewer darn pretzels, life would be great.