system
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human
ELI5 Why is it ok for a bariatric surgery diet to consist of 1k cal or less/day, but for women/men without surgery the minimum is 1200/1500?

Are these very limited calorie restrictions not harmful to the body in the same way it would be for some non surgical patient? (Especially when they are not absorbing everything anyway.) How is this method of weight loss any different from a “starvation diet” ?  

Edit:Sorry for the poor wording in the original question. I’m  talking LONG TERM, such as a year or two down the road from post op day 1. Are these surgical patients able to go back to eating enough calories to maintain their new body weight? Or are they forever limited by their new digestive system to eat smaller amounts? If so, does that have any toll on the body?
gpt
RD who used to work in bariatric surgery here!

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People are often advised to consume >1200-1500 calories for a couple of reasons. First, lower calorie intakes can lead to decreased metabolic rate as an adaptation response, which can result in easier weight regain (see Biggest Loser studies). Secondly, eating less than this makes it difficult to get enough micronutrients, especially in the absence of medical advice/supervision.

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These things are also true of people who have had bariatric surgery, but the decreased metabolic rate is not such a big deal since their reduced stomach capacity makes it more difficult to eat more than this/regain weight anyway. It wasn’t unusual for me to see patients 2+ years post-op who quite comfortably maintained their weight eating 1200 calories per day, wayyy less than you’d expect someone their weight to need. They are also advised to take lots of vitamin supplements and get regular blood work to screen for nutrient deficiencies!