I'm wondering about how L-sugars taste. They are obviously not digestible by humans, but what do they taste like? If they are sweet, too, why isn't sucrose with the unnatural chirality used as a zero-calorie table sugar substitute?
Take a look at tagatose. It is a 6-carbon sugar (a ketohexose like fructose) but is inefficiently metabolized and has about 30% the caloric content of sucrose. It is on the FDA GRAS (generally recognized as safe) list. Unlike common artificial sweeteners, you can cook with it like sugar. It has been industrially produced as a sugar alternative, but it is not clear if it commercially available to consumers at this time.