The etymology of the word diet shows that it comes originally from the Greek through Latin and then Old French.

Having studied Latin, Anglo-Saxon and Middle English, as well as Geography I use the word diet in its original meaning as what a person tends to eat in normal life. It wasn't until more recently (historically speaking) that it also took on the meaning of a prescribed restriction of food for medical - or other - reasons.

I certainly do not equate the word "diet" with "fad diet". There are some fad diets (prescribed or restricted eating plans) but there are also some healthy eating plans. One does not necessarily equate to the other.

diet (n.1)

c. 1200, "regular food," from Old French diete (13c.) "diet, pittance, fare," from Medieval Latin dieta "parliamentary assembly," also "a day's work; daily food allowance, food," from Latin diaeta "prescribed way of life," from Greek diaita, originally "way of life, regimen, dwelling," related to diaitasthai "lead one's life," and from diaitan, originally "separate, select" (food and drink), frequentative of *diainysthai "take apart," from dia "apart" (see dia-) + ainysthai "take," from PIE root *ai- (1) "to give, allot."

From late 14c. as "customary way of eating," also "food considered in relation to its quantity and effects," and "a course of food regulated by a physician or by medical rules," often a restriction of food or certain foods; hence to put (someone) on a diet (mid-15c.). The adjective in the sense of "slimming, having reduced calories" (Diet Coke, etc.) is attested by 1963, originally in American English.