From the article "The Sirtfood Diet encourages participants to cut down their calorie intake to 1000kcals for the first three days of ‘phase one’ — a week-long period where you drink lots of green juice and exercise daily. Your calorie intake is then increased to 1500kcals for the remainder of phase one. "
My understanding over the years is you require 1500cals/ day to maintain body function such as breathing, brain function etc... The following link is a little different so maybe my figure was for sleeping 24hrs/ day.
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The amount of food we consume daily has a significant impact on bodily function. Most adults need a minimum of 2000 calories to sustain metabolism, muscle activity, and brain function. "
https://www.news-medical.net/health/How-Many-Calories-Should-You-Eat-Per-Day.aspx
I feel a little uncomfortable that a pro-1k-1.5k diet is reviewed (not overly negative).... It could lead some people to very poor choices inadvertently.