Jeff Lichtman: I was going to mention that, but I see you got to it first… =) Carbohydrate stereochemistry is my very least favorite part of chemical nomenclature. I am also of the opinion that Emil Fischer’s vast contribution to organic chemistry is entirely negated by his insane and confusing method of drawing molecules.
Anyway…D-tagatose is a ketohexose (6 carbons, with a C=O group at the second carbon, as opposed to aldohexoses, which are aldehydes with -HC=O on the first carbon). D-fructose is also a ketohexose. The difference between them is the configuration of the -OH group at the fourth carbon. D-Fructose is sweeter than glucose, and is the sweetest common sugar; it’s possible that D-tagatose is also sweet.
All sugars, if I remember correctly, eventually must be changed into D-fructose to undergo glycolysis (carbohydrate metabolism). Humans do not have any enzymes capable of changing D-tagatose into D-fructose, so they cannot metabolize it. Apparently, some bacteria, including E. coli and some other common bacteria, do have enzymes for metabolizing D-tagatose as part of their pathway for digesting lactose. Thus, I would imagine that D-tagatose would be digested in humans by the intestinal flora, probably with no nutritive value to the host.
Strangely, it appears that ‘left-handed’ (-)-glucose/L-glucose/l-glucose really is as sweet as (+)-glucose/D-glucose/d-glucose. Assumedly, the same thing is true of the ‘left-handed’ form of fructose (which would rotate polarized light to the right, but would be L-fructose…carbohydrate stereochemistry is profoundly confusing like this!). But it couldn’t be recognized by the enzymes which digest sugars in your body (except possibly by some bacteria), and therefore would have no nutritive value.
I say strangely because there are many examples of two enantiomers having very different tastes and smells. The classic example is carvone; (+)-carvone smells like caraway seeds, (-)-carvone smells like spearmint. Another example is aspartame itself; aspartame is made from two L-amino acids and is much sweeter than sugar. The enantiomer of aspartame, made with D-amino acids, is extremely bitter.
I found a good introduction to carbohydrate nomenclature here. (There’s a mnemonic for the names of the aldohexoses, including glucose: All altruists gladly make gum in gallon tanks. It was first suggested by the inventor of napalm.)
There’s a discussion of this topic from another site here.