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as a path to ultimate well-being, their descriptions are typically vague. Science nails the issue.
    For example, recent studies indicate that yoga releases natural substances in the brain that act as strong antidepressants, suggesting great promise for the enhancement of personal health. Globally, depression cripples more than one hundred million people. Every year, its hopelessness results in nearly a million suicides.
    Amy Weintraub, a major figure in this book, recounts how yoga saved her life by cutting through clouds of despondency.
    But if some findings uplift, others contradict the onslaught of bold claims and proffered cures.
    Take body weight—a topic of enormous sensitivity for anyone trying to look good. For decades, teachers of yoga have hailed the discipline as a great way to shed pounds. But it turns out that yoga works so well at reducing the body’s metabolic rate that—all things being equal—people who take up the practice will burn fewer calories, prompting them to gain weight and deposit new layers of fat. And for better or worse, scientists have found that the individuals most skilled at lowering their metabolisms are women. Of course, other aspects of yoga do fight pounds successfully. The discipline builds body awareness and its calming influence can help reduce stress eating. Most yoga teachers are lithe, not lumpy. But when yoga succeeds at weight control, the scientific evidence suggests that it does so in spite of—not because of—its basic impact on the human metabolism.
    That’s one of yoga’s dirty little secrets. It turns out there are plenty of others, some quite significant.
    Yoga has produced waves of injuries. Take strokes, which arise when clogged vessels divert blood from the brain. Doctors have found that certain poses can result in brain damage that turns practitioners into cripples with drooping eyelids and unresponsive limbs.
    Darker still, some authorities warn of madness. As Carl Jung put it, advanced yoga can “let loose a flood of sufferings of which no sane person ever dreamed.” Many yoga books cite Jung approvingly but always seem to miss that quote. Even so, it represented his considered opinion after two decades of study and reflection.
    Overall, the risks and benefits turned out to be far greater than anything I ever imagined. Yoga can kill and maim—or save your life and make you feel like a god. That’s quite a range. In comparison, it makes most other sports and exercises seem like child’s play.
    My research has prompted me to change my own routine. I have deemphasized or dropped certain poses, added others, and in general now handle yoga with much greater care. I hope you benefit, too.
    I see this book as similar to informed consent—the information that the subjects of medical experiments and novel treatments are given to make sure they understand the stakes, pro and con.
    To me, the benefits unquestionably outweigh the risks. The discipline on balance does more good than harm. Still, yoga makes sense only if done intelligently so as to limit the degree of personal danger. I’m convinced that even modest precautions will avert waves of pain, remorse, grief, and disability.
    The heroes of this book are the hundreds of scientists and physicians who toiled inconspicuously over the decades to uncover the truth despite the obstcenter1es of scarce funding and institutional apathy. Their early inquiries not only began the process of illuminating yoga but, as it turns out, produced a remarkable side effect. They helped transform the nature of the discipline.
    Yoga at the start was an obscure cult steeped in magic and eroticism. At the end, it fixated on health and fitness.
    To my surprise, it turned out that science played an important role in the modernization. As investigators began to show how the ostensible wonders of yoga had natural explanations, the discipline worked hard to reinvent itself. A new generation of gurus downplayed the rapturous and the