Child v adult BMI calculations

[RenegadeKeeblerElf]

I've just had the results through from my daughter's year 6 (11 years old before anyone gets confused!) height/weight check. Now I'm not in denial, I know she is carrying a little extra fat around her middle, and she definitely could eat more healthily, but I'm not worried about her. She's going through a growth spurt and it will even out. But what I found interesting is that I put her measurements into both the adult and child BMI calculators on the NHS website (she is adult sized and having regular periods so biologically basically an adult) and although she is overweight for a child, for an adult her BMI is 23. I wonder if the child calculator just can't deal with children who go through puberty early, maybe it should include a question about that to help with accuracy? Has anyone else found the same?

The child calculator goes to 18 so would take puberty into account. I would not use the adult one, she is not an adult. I would think that the fact healthy bmi has a range, not a target, would mean it would already take into account normal variations in body shape, timing of puberty and so on. If she went through puberty unusually early could you put in her date of birth to make her a couple of years older and see what that says?

Fat around the middle is a sign that she is likely overweight though.

Teenagers grow up and then build muscle, their BMI increases gradually into young adulthood.

You may find it helpful to look at a chart eg www.rcpch.ac.uk/sites/default/files/2018-03/boys_and_girls_bmi_chart.pdf

On this you can see that even at 18 the normal BMI range for girls is a little lower than adults. A BMI of 23 is still on the "overweight" line for a 13 yr old which may give you a more useful perspective allowing for her pubertal stage.

Make the most of her growth spurt now, while it lasts, but try to help her eat more healthily and keep active so that she can become a slim and strong teenager.

Bmi calculators are a bit rubbish imo. I'm no expert though. Its just a guide. If she's active, eats well ish and similar to her peers she is probably fine. Its easy to tell children who are overweight when they are with their peers. She will probably grow loads over the next few years and her body will change. As long as you are modelling a healthy lifestyle I'm sure its fine. If the nurse was that concerned surly they would have said so.

Its easy to tell children who are overweight when they are with their peers.
I disagree on this point. It's about 40% of children overweight or obese in year 6 so they would be unlikely to stand out except maybe in a very affluent cohort.

This is interesting. I know my eldest (12) is overweight according to the calculator so have been actively trying to change this for his sake but I just added his height and weight in as an adult and his BMI is 21.5 so lower end of healthy… How odd.

Absolutely. Although I do know she is a little overweight, she is very similar to her peers, and definitely not the most overweight. I just found it interesting that the exact same height and weight on an adult would be perfectly healthy and wondered what the difference was.

It's also very difficult to get her to eat more healthily without making her worry about weight/body image, especially when her older sister has a much faster metabolism and will likely always be slim regardless of what she eats!

You've basically answered your own question OP.
You say she's going through a growth spurt but then she is adult sized and having regular periods so biologically basically an adult, but those things must be mutually exclusive mustn't they? She isn't biologically an adult - she is still growing. Even though she is well into puberty, she hasn't yet reached full physical maturity and the normal body composition of a woman. Growth plates must still be open if she is growing so she won't have a fully mature skeleton and muscles, nor would I imagine, the normal fat distribution of an adult woman That will all be developing and she is obviously in a transitional stage but she isn't yet an adult so you can't apply the adult standards yet.
The data that the growth charts are based on will have come from a very large number of children who, at that age must be at very varied stages of puberty so that will be accounted for within the range of normal. I would think it's not unreasonable to think that a child who is further on in puberty than their peers might be towards the higher end of the normal BMI range for their age, but not that they should be measured on the adult range.

But she will keep growing in her teens so if that is her adult BMI now that will make her an overweight adult? I developed quickly and had early periods but my breasts etc were still bigger by the time I was an actual adult. Or have I misunderstood?

I'm not trying to say she isn't overweight, I do recognise that she is. I just find it interesting that the same height and weight for a child versus an adult gives a different result, in an intellectual/theoretical way.

And she probably won't grow much more, at least not in height. She is very similar to me and I grew early and then stopped - I'm the same height now that I was when I was 12, and she is almost as tall as me. She will probably also be like me and struggle with her weight throughout her life so I'm trying not to make her self conscious too early.

Height isn't everything. I stopped growing when I was 11- but my body (and brain!) kept developing and changing.

You need to go with what's age appropriate.

It's simply because children's and adult's bodies are a different shape and composition until they are fully developed.
I'm very small, and people are always saying to me "oh you're lucky, you can buy kids' clothes and shoes and save the VAT" but of course I can't. I have a small body, but it is a small woman's body, not a child's. I have hips, a waist and breasts so children's clothes intended for my height are completely the wrong shape. If I bought a pair of boots from the children's department I probably wouldn't be able to zip them over my calves because I am both more muscular and have more subcutaneous fat than child would have. A child with the same size feet as me should weigh significantly less than me, so were I to buy say a pair of children's running shoes they would at best wear out quickly and at worst be dangerously unsupportive.
The same height and weight give you the same BMI number whatever the age obviously, but the interpretation of that result is age dependent. I have a healthy BMI but a typical 11 year old who was my height and weight would be significantly overweight. An adult body has a fully developed skeleton, more musculature, different fat deposition etc so is supposed to be heavier for any given height than a child's. Obviously it is a spectrum and some older children will be closer to becoming adults than others and there is a range of normal but full maturation isn't achieved at the onset of puberty, it takes years. Children aren't small adults - that's the opening line in almost every introduction to paediatrics for medical students - they are very different, anatomically, physiologically and psychologically which is why there are different ranges for normal in just about everything, for different ages.

Were you an overweight child as well? Studies have shown that overweight and obese children are taller than their peers in early childhood, but stop growing at a younger age. Is your daughter physically active? If not, I’d see if there is an activity that she can do that she enjoys, and one that she can carry though into adulthood. Not to lose weight, but it will make her stronger and healthier, and could well have a positive impact on her mental health and make her less self conscious.

Others have said better than I could about how children aren’t just shorter adults. Paediatric medicine doesn’t age out at puberty, because they still aren’t adults.

she is adult sized and having regular periods so biologically basically an adult

She is 11. She is not 'basically biologically and adult'