Volume 6, Issue 47 October 2009
Old Bones Hold the Secret to Strong Skeletons
Sabrina Agarwal travels the world studying ancient skeletons for clues to preventing osteoporosis, often bringing samples back to study in the Skeletal Biology Lab in the Department of Anthropology. Image credit: courtesy Sabrina Agarwal
Among seniors, the brittle bones of osteoporosis are now nearly as ubiquitous as gray hair and bifocals. American women in particular now face a 1 in 2 chance of fracturing their bones after age 50. Those fracture rates are among the highest in the world despite all of the milk, cheese and calcium supplements we ingest.
But fragile bones, argues Berkeley professor of anthropology Sabrina Agarwal, are a relatively recent phenomenon. "Modern women shouldn't think of osteoporosis as something that's inevitable, that's just part of being a woman growing old and a consequence of menopause," she says.
A biological anthropologist, Agarwal studies skeletons from earlier human populations as well as modern primates to understand how bones age. What she finds is that fragility fractures, such as those of the hip and wrist so characteristic of seniors with osteoporosis, were almost unheard of in earlier times.
"Populations in the past had very different lifestyles and did very different things. That makes them ideal for seeing what bone loss, maintenance and growth look like under different circumstances," Agarwal says. Pinpointing those differences, she hopes, may lead to better advice on how to prevent and treat osteoporosis today.
Human bone undergoes changes in strength and structure with age. This is due both to changes in bone density but also internal structure. From top: Fourth lumbar vertebrae from a juvenile; adult; and an older adult from the Neolithic community of Çatalhöyük, Turkey. Image credit: Courtesy Sabrina Agarwal, in Agarwal and Glencross (in press), "Bone Loss and Fragility Through the Lifecycle: A Paleopathological Perspective." In: Moffat, T, and Prowse, T (eds). Biosocial Perspectives on Human Diet and Nutrition. Berghahn Press (Oxford, New York)
One of the populations Agarwal has studied is from the medieval town of Wharram-Percy, in northern England. These rural peasant women, Agarwal has found, lose bone very differently than their modern industrialized counterparts. Instead of losing a dramatic amount of bone immediately after menopause, Wharram-Percy's women lost it gradually during their reproductive years. Further, those women who attained the age of 50 or older had little or no fragility-related fractures and possessed just as much bone as the village men. This is dramatically different from patterns of bone loss and osteoporosis seen in modern Western women.
By comparison, the medieval urbanites Agarwal has studied fall somewhere in between. Like modern Americans, women, buried in the Royal Mint cemetery near the Tower of London, lost the most bone after menopause, but still suffered few fragility-related fractures in old age.
Agarwal points to several factors that probably helped keep medieval women's skeletons strong throughout life. One is hard physical labor. Wharram-Percy women toiled in the fields for many hours a day as well as maintained a household in a time before running water and electricity. Many studies have shown that bones respond to biomechanical stress by becoming stronger.
Another factor is childbearing. In medieval times, women were generally either pregnant or nursing during the decades between 20 and 45. Most had four to five children over their lifetimes, and breast fed them for several years rather than just a few months. Medieval women also reached menarche several years later and menopause several years earlier than women today. As a result, they would have experienced about 40 periods throughout their lives compared to the current average of 210.
"It's a huge hormonal difference," Agarwal says. "Hormones affect breast cancer, the function of the ovaries, and heart risk. Yet no one has investigated the effect of this dramatically different hormonal milieu on bone."
The differences between urban and rural populations, she says, might be a combination of sunshine and female hormones. Rural women were more likely to work outdoors in the fields, where they could absorb more of the sunlight required to make the Vitamin D needed for strong bones. And urban women might have nursed their babies for slightly shorter periods of time.
The skeletons of a child and adult buried in the medieval-era churchyard at Wharram Percy, England. Image credit: Sabrina Agarwal, Mays (1998) The Archaeology of Human Bones, Routledge, London
Bone health, Agarwal concludes, is "very dependent on what environment you're living in and what lifestyle you're following."
Quality as well as quantity appears to affect bone strength. While modern medicine is primarily concerned with bone quantity, particularly bone mineral density, the organization and material properties of bone may influence bone fragility even more. The bones of the hip and spine have interiors that resemble a porous honeycomb. Such trabecular bone is a primary source of the bone mineral lost during osteopenia and its more advanced state, osteoporosis. How the body remodels the internal rod and stick structure that remains, Agarwal suspects, affects its structural soundness.
"If it's reorganized poorly, you can have a higher risk of fracture. As you absorb bone in your spine, you might lose more of the horizontal pieces, but the vertical ones might stay put to prevent collapse. If what you have left is reorganized or thickens in a manner that withstands biomechanical force; you potentially recover in a different way."
Agarwal is analyzing the bones of captive macaque monkeys with known life histories to gain insights into what drives bone reconfiguration. She relies on animals because bone scans that can be easily and safely conducted on humans do not reveal the interior details of trabecular bone.
The bottom line, says Agarwal, is to find better ways to prevent osteoporosis today. "I'm not saying that women should have ten babies and breast feed each for four years. But if we can identify the factors that are most important to maintain bone from an evolutionary and biocultural approach, we can experimentally figure out what to develop to address these problems."