Two substances in raw eggs have been shown to block nutrient availability.
Conalbumin is a protein that can bind together with iron and block its availability.
Avidin is a second egg protein that can bind together with biotin (a B-vitamin) and
make it unavailable.
The cooking of eggs helps denature both of these proteins, and can increase the
availability of both iron and biotin from eggs. Of course, another reason for
cooking eggs involves health safety. The U.S. Centers for Disease Control and
Prevention estimate that 1 of every 20,000 eggs may be contaminated with the
bacterium Salmonella, which is actually passed from the infected hen to the egg
before the shell is formed.
Avidin was irreversibly denatured at temperatures higher than 70 deg C (158 deg F), but that the complex was stable to 100 deg C (212 deg F).
ref: Pritchard, A. B., McCormick, D. B., and Wright, L. D. 1966. Optical rotatory dispersion studies of the heat denaturation of avidin and avidin-biotin complex. Biochem. Biophysc. Res. Commun. 25, 524-528.
Residual avidin activity in cooked egg white was demonstrated using an improved dye binding assay. Mean residual avidin activity in fried, poached and boiled (2 min) egg white was 33%, 71% and 40%, respectively, of the activity of raw egg white.
ref: Durance, T. D. 1991. Residual avidin activity in cooked egg white assayed with improved sensitivity. J. Food Sci. 56, 707-709, 729.
For ileostomy patients:
The true ileal digestibility of cooked and raw egg protein amounted to 90.9% plus or minus 0.8% and
51.3% plus or minus 9.8%, respectively. Five ileostomy patients were studied.
(An ileostomy involves bringing the ileum (the last portion of the small intestine) to
the abdominal surface. When waste matter reaches the ileum it is liquid, so an appliance
is needed to collect it.) These people do not digest the way normal people do.
Evenepoel, P., B. Geypens, A. Luypaerts, M. Hiele, Y. Ghoos, and P. Rutgeerts.
“Digestibility of cooked and raw egg protein, assessed by stable isotope techniques.”
J. Nutr. 128: 1716-1722, 1998
For normal people:
5.73% and 35.10% (P < 0.005) of cooked and raw test meal, respectively, escaped
digestion and absorption in the small intestine.
Pieter Evenepoel, Dirk Claus, Benny Geypens, Martin Hiele, Karen Geboes, Paul Rutgeerts,
and Yvo Ghoos “Amount and fate of egg protein escaping assimilation in the small
intestine of humans” Am J Physiol. 1999 Nov;277(5 Pt 1):G935-43.