In “The Science of Yoga,” William J. Broad brings something unusual to his subject: an open mind. Broad, the book’s biographical note informs us, has practiced yoga since 1970. For nearly that long he has also been a science reporter for The New York Times, writing books like “Teller’s War: The Top-Secret Story Behind the Star Wars Deception” along the way. But Broad brings neither the boosterism of a yoga devotee nor the leeriness of a professional skeptic to his project — just curiosity, energy and a commitment to follow where his investigations lead. That route turns out to be a long and meandering one, ending up at an ambiguous, or at least ambivalent, conclusion. Though “The Science of Yoga” lacks the clarity of a book that sets out to define and defend a preconceived position, what it does offer is an intellectually honest exploration that is true to yoga’s own winding path.

Broad’s objective is simple enough: to evaluate in scientific terms the claims made for yoga. But this turns out to be more complicated than it seems. For one thing, there are the sheer number and variety of those claims: yoga, it is said, can prevent heart disease, reverse aging, eliminate pain, and bestow serenity and peace. Broad patiently and exhaustively examines the evidence for each of these assertions, revealing surprises along the way. Yes, yoga can reduce anxiety and improve mood. No, it won’t help the overweight shed pounds. Yes, it may actually slow the body’s biological clock. Broad doesn’t just discuss the results of the scientific literature; he weighs the relative prestige of the journal in which the studies were published and scrutinizes each experiment’s design and methodology. This is more information than some readers may want, but Broad leaves no doubt that he’s done his homework.

This dogged pursuit of the truth about yoga enables Broad to excavate its remarkable history. He combs through decades of studies, talks to hundreds of scientists and practitioners and roams the world in search of the real deal on yoga. Locating its origins in India thousands of years ago, he recounts his visits to “historians, archives, literary societies and more, traveling by bus, subway, bicycle rickshaw and train (open doors, looking out over villages and smoky morning fires).” In Calcutta, he visits a library so obscure and little-used that dust covers the books and cobwebs hang, horror-movie style, from the ceiling. What he finds in these records bears little resemblance to the yoga we know today as the quintessential activity of a clean-living, upper-middle-class American lifestyle. The yogis of old, Broad notes, “were often vagabonds who engaged in ritual sex or showmen who contorted their bodies to win alms — even while dedicating their lives to high spirituality.” They read palms, interpreted dreams and sold charms; they promoted yoga as the way to sexual ecstasy (“yoga,” Broad tells us, means “union,” and not just the spiritual kind).