Waterfast is an interesting aspect, that you bring up. The metabolic reaction overlaps with the reaction to KD, which is unsurprising as the complete lack of dietary glucose is these diets’ common denominator. Short-term WF (as is the case for up to 3 days) seems to have some beneficial effects, such as reduction in blood glucose, insulin, and body weight. The same cannot be said for blood lipids as trial results are conflicting.

One important caveat needs to be kept in mind: about two-thirds of the weight loss achieved with longer-term WF (>3 days) is from loss of lean mass, which is generally not desirable. Refeeding post-WF will not replenish the loss in fat-free mass, unless accompanied by sufficient strength training.

But your habit of 1-3 days of WF every 6 months or so, is certainly of no such concern, and has in all likelihood a beneficial net effect.