Expert: Florence M Rollwagen - 8/14/2008

Question
What percentage of our total body is comprised of non-human material (bacteria, parasites, etc.)? Just curious. And thank you for volunteering your knowledge. It's a wonderful act of kindness not unappreciated.

Answer
Hi Loren:  Thanks for your question and kind comments.

First, in a healthy person, there are no parasites or bacteria outside the intestines.  The intestines contain food breakdown products, water and bacteria.

To answer your question, we need to make some estimates and calculations.

Human body composition is about 45-70% water, depending on fat content.  So let’s assume 50% for ease of calculation.

Humans produce about a half pound of feces/day

So a 150 lb person would produce 0.5 lbs of feces which are about 80% water. So that’s 0.5 lbs X 0.8, or 0.4 lbs.  Feces are about 60% bacteria, so 0.4 X 0.6 is 0.24 lbs dry weight.

Doing the calculation, a 150 lb person actually “weighs” 75 dry lbs.  The bacteria weigh 0.24 lbs, so that comes to [0.24/75] or 0.0032, or 0.032%  

As noted, we made some estimates, such as percent water in the body, the weight of feces and the percent of bacterial content.  But I think we are in the ball park of 0.02 to 0.05% weight.

I hope this answer has helped you.  Please write back if you have more questions.

FM Rollwagen, PhD