Health-conscious consumers read nutritional labels, but it's nearly impossible to get the nutrients we need with diet alone. To get the USDA-recommended daily quota of vitamin D, for example, you need to eat 15 eggs or 26 sardines; of iron, 414 almonds or 15 cups of broccoli. So we rely on nutritional supplements--vitamins and minerals, probiotics and enzymes--but the variety of pills, products, and elixirs on the market today is overwhelming. And,...