There are a number of improvements we should make in the units structure and data. More might be added to this list.
1. Blood glucose is measured in two ways: "The international standard way of measuring blood glucose levels is in terms of a molar concentration, measured in mmol/L (millimoles per litre; or millimolar, abbreviated mM). In the United States, Germany and other countries mass concentration is measured in mg/dL (milligrams per decilitre)." - wikipedia and other sources.
For conversion we need to have a common base unit. We need to decide on the base unit we want to use, either item-per-cubic-meter vs kilogram-per-cubic-meter, and then recast the other in terms of that. To make this clean, we'll want new constants glucose_mass = 0.1801559 kilogram-per-mole (status='approximate') and mole_per_item = 6.02214076E+23, and use that to get the conversion values. Because the glucose mass is approximate, it might be best to use item-per-cubic-meter.
Remaining items were moved to the clone ticket.