Relating to diet or the regulation of food intake, especially for health or medical purposes; an older or alternate form of 'dietary.'
From 'dietic' + the suffix '-al', creating an alternative adjective form that was more common in older English texts from the 16th-19th centuries.
You'll rarely see 'dietical' used today because 'dietary' replaced it, which is a common pattern in English where multiple word forms compete and one eventually wins out—in this case, the shorter form won.
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