Explain diffence between d-glucose and d-fructose?

Slight differences. Glucose, fructose and galactose are all three dietary monosaccharides. The d (or dextro rotary) isomer of each is the one found naturally in nature. There chemical structures are similar, but slightly different. Both have 4 calories/gram ingested. There is disagreement on whether they exert different effects after ingestion. Both are "empty" calores with no nutrition.