Does Animal Protein Play a Role?
The cause of osteoporosis is unknown. In exploring possible links between
osteoporosis and what we eat, some researchers have developed a hypothesis
and a model that point to sulfur-containing animal proteins as a culprit
in the bone disease.
The theory has commanded the attention of nutrition researchers, including
scientists at the ARS Western Human Nutrition Research Center in Davis,
California. They have teamed up with university colleagues to crack
some of the secrets of osteoporosis. In a novel study, they recruited
women who eat both animal and plant foodsthe omnivore regimen
typical of most Americansand women who only eat plant-derived
foods. What better way to monitor the possible effect of sulfur-containing
proteins than by comparing the bone health of vegan volunteers, who
don't eat animal proteins, with that of omnivore volunteers, who do?
ARS physiologist Marta D. Van Loan of the Western Human Nutrition Research
Center collaborated in the investigation with Anita M. Oberbauer of
the University of California, Davis, and with Lydia-Anne Stawasz, formerly
at the Davis campus and now at the University of California, Irvine.
Forty-eight healthy, nonsmoking women, aged 18 to 40, volunteered for
the 10-month study. At three intervals during the experiment, the women
submitted records of the types and amounts of foods they had eaten during
the previous 3 days. The records gave the researchers an indication
of the amount of protein each volunteer had eaten.
The volunteers gave blood and urine specimens at each lab visit. The
samples were analyzed for any of several standard indicators of bone
health. These included bone formation, as indicated by the amount of
a chemical called osteocalcin; and bone resorption, or the amount of
calcium removed from bone and reabsorbed into the bloodstream, as measured
by another biochemical, N-teleopeptide. Other measures included renal
net acid excretion and urinary calciumboth indicators of how much
calcium was excreted from the body.
The model that other scientists developed predicts that to maintain
the correct balance of calcium in the blood, or homeostasis, renal net
acid excretion and urinary calcium increase as intake of sulfur-containing
animal proteins increase.
But preliminary results suggest that osteoporosis may in fact be more
complicated than the model predicts. The Davis scientists applied a
statistical proceduremultivariate regression analysisto
determine the relative impact of each of the variables, or factors,
they examined. As expected, they found that the vegan volunteers ate
less protein than the omnivore volunteers. Also, as predicted by the
model, renal net acid excretion and urinary calcium were higher in the
volunteers who ate more protein (the omnivore women) than in those who
ate less (the vegan participants).
Less Bone Formed
But two findings were unexpected. First, bone resorptionin which
calcium is taken away from bones via the bloodstreamwas the same
for omnivore women as for vegan women.
"The current model predicts increased bone resorption for people
who consume large amounts of animal protein, so it was somewhat surprising
that bone resorption was the same for both groups of our volunteers,"
Van Loan notes.
Second, bone formation was significantly less in omnivore women than
in vegan women. This happened even though the omnivore women had a higher
calcium intake than did the vegan volunteers. (The volunteers did not
differ in their intake of other nutrients that affect bone health, such
Using the model as a basis, "one would not have predicted a significantly
greater amount of bone formation for vegan volunteers than for omnivore
volunteers," Van Loan adds.
The implication for people who eat high amounts of animal protein may
be important: Specifically, over time, the net effect of a lower amount
of bone formation would likely be a decrease in bone density. Explains
Van Loan, "If you have less bone formation, the result is the same
as if you had an increase in bone resorption. So, even though bone resorption
was the same in both groups of volunteers, the lower amount of bone
formation in the omnivore women could lead to a decrease in their bone
The findings, if borne out in larger studies, may lead to a modified
model. What's more, the investigation may lead to other useful lines
of inquiry for other studies.By Marcia
Wood, Agricultural Research Service Information Staff.
This research is part of Human Nutrition, an ARS National Program
(#107) described on the World Wide Web at www.nps.ars.usda.gov.
Marta D. Van Loan is
with the USDA-ARS Western Human
Nutrition Research Center, One Shields Ave., Davis, CA 95616; phone
(530) 752-4160, fax (530) 752-5271.