In the chapters between, he tries to parse fact from fiction, true health benefits from hype: Can yoga cure depression? Cause weight loss? Improve sex? Help arthritis? Diabetes? Rotator cuff injuries? Can it bolster creativity? Cause strokes?
It's about time that such an analysis was done, says Broad, himself a longtime yoga aficionado, given the flourishing, unregulated yoga industry, with growing legions of toddlers doing downward-facing dog and moms sweating in steamy Bikram yoga studios.
The two halves of my own brain approached this book with contradictory feelings. I've practiced yoga for nearly three decades, after discovering early on that it reduced my writer's shoulder aches by making me mindful that I was scrunching up my muscles. I love my weekly class with one of the deans of yoga in this region, Hari Zandler, who has studied with great gurus in India. I credit the strength of my back, my good balance, and my flexibility to his teachings.
I didn't want to read a book that undermined my convictions. And I worried it would take some of the rosy afterglow out of the experience.
On the other hand, as former medical editor of The Inquirer, I should be open-minded to what science has found, another part of my brain argued.
Dutifully, I read on. As feared, there's some bad news.
You could hurt yourself. While cases of injury are very few compared with sports such as biking or running, federal statistics on emergency room visits document growing numbers of sprains, strains, and joint dislocations caused by yoga. Most concerning, in a very small number of instances people have suffered strokes in poses that cause neck compression, disrupting blood supply to the brain. These are akin to the hair-washing neck injuries dubbed "beauty parlor syndrome" a few years ago.
Many yoga teachers, Broad warns, have yet to address the possibility of harm. Others are eliminating the more risky positions. Also, he cautions, training for instructors varies widely, with no national standards.
Another negative: Yoga does not, in general, increase your metabolism or aerobic fitness enough to help your heart or to lose weight, despite what some ads promise. It tends to do the opposite, slowing the metabolism.
But there's good news on the cardiac front, for different reasons.
Yoga, which reduces stress, can bring down blood pressure and cut cardiovascular risk, a 2009 study at the University of Pennsylvania found. And researchers at the University of Virginia who in 2005 reviewed 70 studies found yoga promising as a "safe and cost-effective" way to improve heart health.
A study at Duke found that yogis showed no improvement in oxygen transport or aerobic conditioning, but they experienced "enhanced sleep, energy, health, endurance and flexibility," Broad writes. They also felt better about their sex lives and social lives. They were more self-confident and in better moods.
Recent brain imaging studies in Boston have found that even yoga beginners show a boost in the anxiety-reducing chemical GABA. This possibly explains why yoga helps people feel better about themselves, and some claim it alleviates depression.
Other studies have found that yoga improves balance in elderly women, likely reducing their risk of falling and breaking bones. It slows degeneration of the spine, possibly because spinal flexing keeps disks bathed in nutrients. And there's evidence that it helps fight inflammation, benefiting those with rheumatoid arthritis.
As for yogis living longer? An enzyme called telomerase, linked to the lifespan of cells, appears to get a boost from a combination of a Dean Ornish low-fat diet and daily yoga.
With so much science, history, and unpronounceable names of famous yogis, this is not a titillating book by any means, but there is a chapter on sex and kundalini yoga. Does fast breathing increase desire? Can kundalini yoga create its own form of orgasm? These remain largely unanswered, as does yoga's effect on creativity, though a small Penn study showed increased blood flow on the right side of the brain.
What's needed now? With the science of yoga still in its infancy, Broad pleads for a central depository for yoga research and an expansion of serious, well-funded studies, "given that yoga's demonstrated skills at disease prevention might result in savings of billions of dollars in traditional health-care costs."
In the meantime, I'll be doing my weekly upward dog and downward dog, sun salutations, and triangle poses, ending with a relaxing savasana, my mind empty and my body grateful.
Dorothy Brown, a retired Inquirer editor, blogs at www.unretiring.blogspot.com.