Of course, we could play that game for hours. Will insurance cover these life-extending technologies? Will we provide them to convicted criminals who've been given life sentences? What do you do about a brutal dictator who is never going to die of old age? The book could have used more discussion, or at least acknowledgement, of ethical considerations.

There's certainly plenty to digest, though. Cloning replacement organs, correcting genetic defects, turning cells into living computers, and supplementing, or even entirely replacing, our blood cells with more efficient nanobots are just a few of the 21st century developments the authors foresee. New technologies should be able to sidestep many, if not all, of the religious and ethical questions that swirl around issues like stem cell research. The book's Bridge Two and Bridge Three sidebars describe fascinating areas of research, from drugs that will allow us to eat whatever we want and still maintain optimal weight, to nanobot-based neural implants that will vastly amplify our thinking and communicating abilities. (Telepathy, anyone?)

For the present, the book provides several useful services. It explains the basis for a great many popular health claims. Why is it healthier to consume fewer carbohydrates? Why are some fats good and others bad? What about those supplements that reduce free radicals in the bloodstream, and how do free radicals actually harm you? If you're patient, you'll find explanations in this book for many of the health claims and fads you may have been wondering about. You can also satisfy some of your curiosity about what is actually happening in the body when conditions like cancer, diabetes and heart disease occur.

The authors provide their own nutritional guidelines, with convincing explanations of how and why they differ from common wisdom or government specifications. There's a food pyramid that departs markedly from even the most recent government version, along with fairly simple formulas for how much of what nutrients an individual who wants to lose weight or improve health should consume. They also blunt a few saws, like the common modern assumption that any and all exposure to the sun is bad for you, and challenge the medical establishment's faith in some common practices like coronary bypass surgery and angioplasty.

With respect to lifestyle, the authors' advice includes a lot of common sense: choose healthy (and whenever possible organic) food, exercise, drink plenty of water, avoid sugar and addictive drugs, avoid stress, get enough sleep, quit smoking and so on.

Thus after a breathtaking, futuristic beginning, it turns out that the heart of the book is, as promised, Bridge One material covering what you can do today to achieve and maintain optimum health.

The book enters some unfamiliar territory in the area of nutritional supplementation. Here, its central ideas aren't all proven facts. They are intriguing, however, and seem sensible given the scientific evidence presented.