Creating a food with multiple serving sizes will likely led to an error.
When creating a food with multiple serving sizes the following can happen:
User adds 30g serving size.
Then it ads 100g of serving size. Given that 100g will have less rounding errors than 30g, user will likely entry values for 100g and not for 30g.
So in "Nutrition per serving size" listbox user will select 100g. Then the user starts entering values , display serving size is changed silently from 100g to 30g
The user thinks he is saving data for serving size 100g but it is entrying data 30g.