Nutrients handling in Open Food Facts

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Revision as of 19:32, 1 June 2020 by Aleene (talk | contribs) (Added default unit)

Summary

This page describes how nutrients are stored in Open Food Facts and how they can be read or written through the API.

Why it's more complex that you might think

  • Nutrients lists are different depending on countries
  • Nutrients list are different depending on time (evolutions in the law)
    • e.g. in 2013 in Europe, the main unit for energy became kJ instead of kcal, and salt replaced sodium, with salt = 2.5 x sodium
  • Nutrients on labels may be in different units
    • e.g. sodium could be listed in mg or in g
    • US uses calories (kcal), EU uses kJ + kcal
  • Nutrients may be specified per 100g, per 100ml (which may be different than per 100g), per serving, or both
  • Nutrients may be specified for the product sold as-is, or prepared (with water, milk or something else), or both
  • Some apps, producers, databases etc. may give us values in a specific unit which is not the one that is on the label

How it works

Data input

Fields sent by the write API

Apps (+ the OFF web site edit form) send:

  • nutrition_data_per : "100g" or "serving"
  • serving_size (optional, but very useful, in particular if the nutrition facts are specified per serving)
  • for each nutrient:
    • nutrient-id (e.g. saturated-fat, energy-kcal) : value for the nutrient, for the size specified in "nutrition_data_per". This is a string ("17.8", "34,7", "< 0.1", etc).
    • nutrient-id_unit (e.g. saturated-fat_unit, energy-kcal_unit) : the unit of the value passed (e.g. "g", "mg", "kcal", "kJ")

Notes about the energy fields:

  • Energy on labels can be written in kcal, kJ or both
  • when the energy is written in both, there is no simple conversion ratio between kcal and kJ
    • This is because labelling laws in EU, US, CA etc. specify that the energy fields can (or must) be computed by multiplying and summing values for carbohydrates, proteins, fat etc.
    • That's why we now store 2 energy fields, and not just one
    • see https://esha.com/calorie-calculation-country/ for details
  • energy-kj and energy-kcal are the identifiers (nutrient-id) of the "Energy in kJ" and "Energy in kcal" fields.
  • old apps used to pass energy (as we had only one energy field instead of energy-kcal + energy-kj)
    • for those apps, the energy_unit field is used to populate the corresponding energy-kj or energy-kcal field

Fields stored and computed by the OFF backend (Product Opener)

  • nutrient-id_value : what was passed in the nutrient-id field
  • nutrient-id_unit : what was passed in the nutrient-id_unit field
  • nutrient-id : the value converted in the default unit (currently kJ for all energy fields (including energy-kcal), otherwise gram)
  • nutrient-id_[value of nutrition_data_per) is set to nutrient-id (e.g. if nutrition_data_per is "100g", the value of sugars_100g is set to the value of sugars
  • if the serving size is known, we compute the value for the other field (nutrient-id_100g or nutrient-id_serving) in default unit

Data display

The OFF web site, and apps, should use those fields to display the nutrition facts:

  • nutrient-id_100g or nutrient-id_serving (depending on the value of nutrition_data_per) : value in g or in kJ

The web site and apps must:

  • indicate if the nutrition facts are per 100g or per serving.
  • convert the value in g or in kJ to the desired unit.

Notes:

  • Don't use the nutrient-id_value and nutrient-id_unit fields
    • The nutrient-id may be empty as some apps don't pass a unit, and we use a default unit for the field
    • The value may be per 100g or per serving

Data edit

The web site and apps that allow editing the nutrition facts must populate the table with the same rules than for data display.


Possible improvements to make thing (a bit) simpler

  • In the API, rename the "nutrient-id" fields (such as energy-kj, saturated-fat) to "nutrient-id_value", so that it matches the name of the field stored.
  • Store the energy-kcal values in kcal, without converting them to kJ
    • This is probably what people intuitively expect
    • It removes unnecessary conversions from kcal to kJ and back (which is not trivial, as some people/apps may choose a different conversion factor, such as 4.2 or 4.185)
    • This would be a breaking change if some apps use the energy-kcal_100g/serving or energy-kj_100g/serving fields


Possible improvements to make thing (a bit) more complex

The current API does not allow to store values for the prepared product, but it can be done on the OFF web site.

We should extend the API to allow to write nutrition facts for the prepared product.

The fields for the prepared product are the same than for the as-sold product, except "_prepared" is appended to the nutrient-id.

e.g. saturated-fat_prepared_100g, saturated-fat_prepared_value etc.

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