If our civilization should someday collapse, then—with apologies to McDonald's—let this be its epitaph: “Billions and billions served.” Humanity has come a long way from its hunter-gatherer roots. Thanks to industrial-age agricultural production, global commerce and the 20th century's green revolution in farming, the world can support billions of people who once would not have found enough to eat. But goodness, look what we're feeding them.

Modern culinary extravagances include high caloric fantasies lacking even a twig's worth of nutrients, and poor nations are among their most avid consumers. Widespread obesity and malnutrition exist side by side—sometimes even within the same people. The world has become a place simultaneously of overabundance and aching starvation: the cornucopia and the empty cupboard in one.

In this special issue, Scientific American explores the relation between human health and food, which has never been more complicated and paradoxical.

After gobbling down mountains of chips, rivers of cream, stampedes of beef and poppin' fresh boxcars of baked goods, many of us fret over which best-selling diet book can salvage our health and help us see our toes again. Are we expecting too much? Nutritionist Marion Nestle says yes. In “Eating Made Simple” (beginning on page 60), she lays out why the state of nutritional science is still too incomplete to make detailed prescriptions for individual well-being. Journalist Paul Raeburn, in his contributions (“Dropping Weight ... and Keeping It Off,” on page 66, and “Can Fat Be Fit?” on page 70), also deflates expectations about how well weight-loss diets work and how many extra pounds we can pack without risk. The traditional advice in favor of exercise and moderation still applies: take the stairs, leave the cannoli.

At the physiological level, we are almost unchanged from our hunter-gatherer ancestors. We carry elaborate regulatory circuits in our heads and guts that helped us survive back when periodic famine was common and sweet, fatty desserts were not. The article from Jeffrey S. Flier and Eleftheria Maratos-Flier (“What Fuels Fat,” on page 72) and the interview with Nora D. Volkow (“This Is Your Brain on Food,” on page 84) explain what those holdovers mean for us today.

Barry M. Popkin, in “The World Is Fat” (page 88), and Per Pinstrup-Andersen and Fuzhi Cheng, in “Still Hungry” (page 96), describe the nutritional Scylla and Charybdis through which developing nations now navigate. Populations that escape famine by filling their bellies with cheap snacks and soda set themselves up for different sets of health concerns in the future. A controversial remedy for hunger might be to embrace genetically modified crops; Terri Raney and Prabhu Pingali suggest as much in their piece (“Sowing a Gene Revolution,” on page 104).

The public has become acutely aware that the food supply is increasingly vulnerable, both to terrorist actions and to accidental contamination. Check out “Is Your Food Contaminated?”—Mark Fischetti's sobering overview of the situation and of the technological fixes that might help restore a measure of security, starting on page 112. Then maybe pass on that second helping of potatoes and take a brisk walk to think it all over.