I think the issue here is that the BMI-for-age reference extends from birth to 228 months (i.e. 19 years). For people older tham 19 years you can use fixed thresholds to classify underweight-for-height. A number of classification schemes are available. Here is an example:

    Normal       >= 18.5 kg/m^2
    Grade I      10.0 - 18.4 kg/m^2
    Grade II     16.0 - 16.9 kg/m^2
    Grade IIi    <= 15.9 kg/m^2

These are adapted from:

    Ferro-Luzzi A, Sette S, Franklin M, James WPT. (1992) A simplified 
    approach to assessing adult chronic energy deficiency. European
    Journal of Clinical Nutrition 46:173-186

and are presented in this RNIS guideline.

These thresholds are probably not suitable for older (i.e. > 50 years) people.

There are issues with BMI being strongly affected by body-shape. My preference would be to use MUAC but the choice of indicator(s) should be informed by the application.

19 years old are still adolescents and following the standard of WHO and CDC, if they are not more than 19 years old BMI-for-age percentile should be used for them..but if they are more than 19 years old, then normal adult BMI measurement should be used for them. Thank you

Since they have become women already and no longer girls, child-bearing has changed their body structure and so adult measurement should be used for them at this stage since they are now women.

Dear Mija, 

This is a really good question as its really confusing which references cover this age group! 

WHO BMI reference goes up to 19.0 years (228 months), so anyone that is 19 years exactly can use the reference, but anyone that is 19 years and one month should use the adult BMI single cut-off. 18 years and 11 months can use the WHO children's reference. 

The IOTF also have a very useful BMI reference, but it goes up to just before 19 years (227 months), so again, here, you would need to switch the the adult cut-off when they turn 19.

I dont know the exact cut-off for the CDC reference but I wouldn't use that for international measurements anyway, since that is only USA children. 

I hope that helps a little 

Thanks Mark, Natasha and others. It remains very confusing as I cannot find one clear reference from a normative ageny that states use 'normal' BMI for 19 years and over.

WHO Europe says

https://www.euro.who.int/en/health-topics/disease-prevention/nutrition/a-healthy-lifestyle/body-mass-index-bmi

For adults over 20 years old, BMI falls into one of the following categories. etc

Another older doc from WHO insinuates also that adults are 20 years and over: page 11

https://apps.who.int/iris/bitstream/handle/10665/41720/WHO_TRS_731.pdf?sequence=1&isAllowed=y

when they state 10-19 years they included 19 years. Yet, we have no BMI for age for 19 years, only 19 years and 0 months.

(TRS 854 refers to the reference TRS 731)

CDC says, from 20 years (included) and over (but let''s forget this one for now)

FANTA says 'normal' BMI for 18 years and over

https://www.fantaproject.org/sites/default/files/resources/MODULE-5-FANTA-Anthropometry-Guide-May2018.pdf

https://www.fantaproject.org/tools/anthropometry-guide

And yes Natasha, the IOTF is there also and refers to BMI 18+ years but is created in the context of measuring obesity.

So to my understanding, to date, we do not have a clear reference defending to use 'normal' BMI for 19 years and older. What am I missing? 

Mija

Dear Mija,

You are correct that there is no clear guidance. The BMI cut-off values presented by the IOTF that range from the ages of 2-19 years were estimated to identify three degrees of thiness, as observed in adults. the BMI ranges used are described below.

 The IOTF values, form the ages of 16 to 18 years are presented below.

As you can see from the table above, from the ages of 16 years, the IOTF cut-off values differ very little, and there is no change from the age of 18 years, so extending the BMI assessment at 19 years to below 20 years seems a reasonable option. The change of criteria from IOTF to adult BMI is unavoidable at 19 years or at 20 years.

The same is true for using the 2007 WHO growth references. See the table below for the cut-off values for 7 centiles. Again little variations from the ages of 16 years, and you can just extend the values from 19 years until before 20 years.

I hope the above is useful.

With kind regards,

Carlos

Dear Carlos, that is very helpful indeed. Thanks to you and all the other colleagues who took the time and energy to respond. Very much apprecaited. Mija