All food databases store nutrient concentrations per 100 G weight of the food. For foods that are typically used by volume measures one must also know the density of the food, ie. how many G are in 100 mL in order to be able to figure out the weight of the food actually consumed. Food databases usually include conversion factors which allow the calculation of density. For instance if a food factor is given for a cup and we assume a cup to be 250 mL then we can calculate the density. Density of a food also depends on how it is served. A cupped of diced vs a cup of crushed pineapple. Each one of these would have a different density.
In scientific studies with multiple coders of food data it is important to ensure consistency. When a subject mentions pasta as a consumed food, which, of multiple pasta codes must the coder use? Is more information required? A user manual which documents the decisions made during coding is essential. Coder training in the use of the manual is also essential.
The more accurate and consistent is the data coding and entry the more valuable will be the resulting data and the more likely will the study be able to show nutrient effects.