BMR+Harris-benedict equation: case of negative output

Question

I was using the BMR and Harris benedict equations. I got a negative result when i used the following information just by curiosity:

• Age: 80
• Height :1feet 0 inches (ie 12 inches)
• weight :22 pounds
• Gender: male
• Activity level-->little or no exercise : BMR x 1.2

Then using the formulas at the mentionned links:

Men: BMR = 66 + ( 6.23 x weight in pounds ) + ( 12.7 x height in inches ) - ( 6.8 x age in year )
  ==> BMR = -188.54 
  ==> Energy needs = BMR * activity level = -188.54 * 1.2 = -226.248

I know the information are probably not common (80 years old, 1feet 0 inch, 22 pounds), but what does it mean when you get a negative BMR and a negative Energy need?

I did not try all possible cases where u might have a negative value (For negative, it is in general required: left side < right side where right side is ( 6.8 x age in year ) ) but if let's say a nutritionist is using this formula on a client and gets a negative result what do they do?

Answer 1

It means the equation isn't perfect, but just an estimation. You happen to have found one case where it fails quite a bit. Of course, it doesn't help that you didn't describe an actual human being (the shortest verified person is 54.6cm, that is 21.5 inches). I don't feel like doing the path, but I'm sure the formula provide reasonable estimations for reasonable input parameters.

Answer 2

If you take a look at A Biometric Study of Human Basal Metabolism, where Harris and Benedict introduced their formula, you will see one important sentence:

These equations have been tabulated for values of weight from 25.0 to 124.9kgm. [sic], for stature from 151cm to 200cm., and for age from 21 to 70 years, so that the most probable basal metabolism of an unknown subject may be easily determined.

So, this formula is not supposed to give accurate results for

• children
• people with unusual heights (±25cm from average)
• obese people

Answer 3

Indeed. Despite it's widespread use, the formula is clearly not extensive. Try running the relevant calculations against the equations for male/female using a 2 year old weighing 27 lbs and 36 inches in height and you'll find female have higher BMR!

If you consider the physical difference between male and female at toddler age is pretty minimal the I consider the equation should be used with caution.

female: BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years).

male: BMR = 66 + (6.23 x weight in pounds) + (12.7 x height in inches) - (6.8 x age in year).