Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked （A）, （B）, （C） and （D）. You should decide on the best choice and mark the corresponding letter on Answer Sheet 2 with a single line through the centre.
Questions 52 to 56 are based on the following passage.
With advancing age, our bodies experience an inevitable loss of bone. Two major studies in elderly populations now underscore the importance of dietary protein in this structural erosion, which can lead to osteoporosis （骨质疏松证）. The reports come to sharply different conclusions, however, about the effect of animal protein on the rate of bone loss.
In one study, diets rich in animal protein correlated with greater bone loss and fracture risk. In the other, animal protein appeared to protect bone.
Such contradictions, which are not very satisfying, often emerge from observational studies where researchers record the natural habits and health of people, notes Robert P. Heaney of Creighton University Medical Center in Omaha, Neb. "That's not a criticism of such studies," he says, "just a reminder of the uncontrollable variables common in populations."
However, he notes, the new studies are important because they're in the vanguard （前锋,先锋） of efforts to understand protein's impact on age-related bone loss. This condition underlies a growing U.S. epidemic of hip fractures. Treating them costs more than $10 billion annually.
The first of the new analyses computed bone loss over 7 years in some 750 elderly women, all taking part in a larger osteoporosis" study that's been running since 1986 at four U.S. sites.
Women eating the most animal protein-roughly four times their vegetable protein intake lost 0.8 percent of the bone in their hips annually, notes endocrinologist Deborah E. Sellmeyer, director of the University of California, San Francisco-Mount Zion Osteoporosis Center. This is four times the rate of bone loss in participants who ate about equal parts animal and vegetable protein.
The risk of hip fractures in women eating the highest proportion of animal protein was 3.1 times that in women regularly downing equal quantities of animal and vegetable protein, the team reports in the January American Journal of Clinical Nutrition.
The findings make sense, Sellmeyer argues, because digestion of animal protein releases large amounts of acid that the kidneys must excrete.