Whole potatoes are processed more slowly by the body than processed potatoes because of the fiber. This reduces the sugar spike from the carbs (same thing in whole oranges vs orange juice).
Processed food, in general, should be minimized. To the degree that things are processed, the processing should be minimized.
So, cooking is one processing step. Adding butter is another processing step. Adding salt is another. Mashing the potato is another. Freezing and later thawing is another. Etc. Think of the healthiness of food in that way.
The adulteration and degradation process can be thought of in terms of original materials, additions, reductions, contamination, etc. Home cooking from your own garden has a lot to recommend it, even if it is not 'convenient'.
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