Many recipe sites that already use schema.org markup have tags or categories for restricted diets such as Gluten Free or Low Fat but no way to markup those attributes formally. I would like to propose a closed set of common dietary restrictions that could be used to tag such recipes.

Here are some examples of where these kinds of concepts are used on popular, existing recipe sites.

• All Recipes has a section for Special Diets that includes Dairy Free, Low Sodium, etc.
• Food Network has sections for Gluten Free, Low Sodium and others.
• Food.com has a faceted search category called Diet & Nutrition (seen under the Advanced Search drop-down with queries such as chicken recipes), which includes categories such as Diabetic, Kosher, Vegetarian and others.

In addition to these large, general purpose recipe sites, there are smaller boutique sites that address specific dietary needs.

• joyofkosher.com focuses on Kosher recipes.
• myhalalkitchen.com specializes in Halal recipes.
• vegkitchen.com showcases vegetarian recipes.

While there are a many different types (and sub-types) of diets, providing a small list of common ones should cover most of the prevalent use cases and simplify the task of the recipe publisher. The list I propose (in a forthcoming pull request) was inspired by the meal types offered by major airlines. While there is no official standard as to the specific list of dietary names, there is a large degree of overlap among the world's major international airlines. The list was adapted to suitability to the recipe domain and use of those dietary types in major recipe sites.