Both D-BHB and L-BHB are BHB isomers that have the same chemical formula, but some of their atoms are arranged differently. Chemically, the main difference between D-BHB and L-BHB is in the position of –OH group in the penultimate carbon atom. In the nature-identical D isomer, the OH- group of the penultimate carbon is positioned on the right side, whereas in the L isomer, the OH- group of the penultimate carbon is positioned on the left side. Note that when the human body makes BHB, it's always D-BHB. That's why supplements containing only D-BHB are more effective than those containing racemic versions of both D- and L-BHB. However, L-BHB tends to stay in the body longer and it acts as a signaling molecule. Some people argue that could have benefits beyond energy production.