Yoga enthusiasts link the practice to a long list of health benefits, including greater flexibility and range of motion, stronger muscles, better posture and balance, reduced emotional and physical stress, and increased self-awareness and self-esteem.
But definitively proving these benefits is challenging, requiring years of costly research. A pharmaceutical company is unlikely to fund a study that doesn't involve a drug, and in any event, the research requires a large group of volunteers tracked over a very long time.
The subjects must provide health measurements at the outset, learn the proper poses, continue to do them regularly for years and be regularly evaluated.
No one knows these challenges better than Dr. Loren M. Fishman, a physiatrist at Columbia University who specializes in rehabilitative medicine. For years, he has been gathering evidence on yoga and bone health, hoping to determine whether yoga might be an effective therapy for osteoporosis.
The idea is not widely accepted in the medical community, but then, researchers know comparatively little about complementary medicine in general. So in 2005, Dr. Fishman began a small pilot study of yoga moves that turned up some encouraging results. Eleven practitioners had increased bone density in their spine and hips, he reported in 2009, compared with seven controls who did not practice yoga.
Knowing that more than 700,000 spinal fractures and more than 300,000 hip fractures occur annually in the United States, Dr. Fishman hoped that similar findings from a much larger study might convince doctors that this low-cost and less dangerous alternative to bone-loss drugs is worth pursuing.
Those medications can produce adverse side effects like gastrointestinal distress and fractures of the femur. Indeed, a recent study published in Clinical Interventions in Aging found that among 126,188 women found to have osteoporosis, all of whom had Medicare Part D drug coverage, only 28 percent started bone drug therapy within a year of diagnosis.
Many of those who avoided drugs were trying to avoid gastrointestinal problems.
On the other hand, yoga's "side effects," Dr. Fishman and colleagues wrote recently, "include better posture, improved balance' enhanced coordination, greater range of motion, higher strength, reduced levels of anxiety and better gait.
Weight-bearing activity is often recommended to patients with bone loss, and Dr. Fishman argues that certain yoga positions fit the bill.
"Yoga puts more pressure on bone than gravity does," he said in an interview. "By opposing one group of muscles against another, it stimulates osteocytes, the bone-making cells."
Most experts argue that it's difficult, perhaps impossible, for adults to gain significant bone mass. Undeterred, Dr. Fishman invested a chunk of his own money and with three collaborators - Yi-Hsueh Lu of The Rockefeller University, Bernard Rosner of Brigham and Women's Hospital, and Dr. Gregory Chang of New York University - solicited volunteers worldwide via the Internet for a follow-up to his small pilot study.
Of the 741 people who joined his experiment from 2005 to 2015, 227 (202 of them women) followed through with doing the 12 assigned yoga poses daily or at least every other day. The average age of the 227 participants upon joining the study was 68, and 83 percent had osteoporosis or its precursor, osteopenia.
The 12 poses, by their English names, were tree, triangle, warrior II, side-angle, twisted triangle, locust, bridge, supine hand-to-foot I, supine hand-to-foot II, straight-legged twist, bent-knee twist and corpse pose. Each pose was held for 30 seconds. The daily regimen, once learned, took 12 minutes to complete.
The researchers collected data at the start of the study on the participants'bone density measurements, blood and urine chemistry and X-rays of their spines and hips. They were each given a DVD of the 12 yoga poses used in the pilot study and an online program in which to record what they did and how often.
A decade after the start of the study, bone density measurements were again taken and emailed to the researchers; many participants also had repeat X-rays done. The findings, as reported last month in Topics of Geriatric Rehabilitation, showed improved bone density in the spine and femur of the 227 participants who were moderately or fully compliant with the assigned yoga exercises.