Dr. Richard Prayson from the Cleveland Clinic along with a family member and a colleague from anatomic pathology recently asked and answered this question. Their results are in a recent publication in the Annals of Diagnostic Pathology.
It turns out in their study of eight different brands of hamburgers looked at are actually comprised of little meat (12% on average), more than 1/2 their weight is water and unexpected tissue types found in some hamburgers were as diverse as bone, cartilage & plant material. On a positive note, no brain tissue was identified in their study although parasites were found in 2 burgers.
"Americans consume about 5 billion hamburgers a year. It is presumed that most
hamburgers arecomposed primarily of meat. The purpose of this study is to assess
the content of 8 fast food hamburger brands using histologic methods. Eight
different brands of hamburgers were evaluated for water content by weight and
microscopically for recognizable tissue types. Glial fibrillary acidic protein (GFAP)
staining was used to evaluate for brain tissue. Water content by weight ranged from
37.7% to 62.4% (mean, 49%). Meat content in the hamburgers ranged from 2.1% to
14.8% (median, 12.1%). The cost per gram of hamburger ranged from $0.02 to $0.16
(median, $0.03) and did not correlate with meat content. Electron microscopy showed
relatively preserved skeletal muscle. A variety of tissue types besides skeletal muscle
were observed including connective tissue (n = 8), blood vessels (n = 8), peripheral
nerve (n = 8), adipose tissue (n = 7), plant material (n = 4), cartilage (n = 3),
and bone (n = 2). In 2 hamburgers, intracellular parasites (Sarcocystis) were
identified. The GFAP immunostaining was not observed in any of the hamburgers.
Lipid content on oil-red-O staining was graded as 1+ (moderate) in 6 burgers and
2+ (marked) in 2 burgers. Fast food hamburgers are comprised of little meat
(median, 12.1%). Approximately half of their weight is made up of water. Unexpected
tissue types found in some hamburgers included bone, cartilage, and plant material;
no brain tissue was present. Sarcocystis parasites were discovered in 2 hamburgers."