NICE Guidance CG32: Nutrition Support In Adults updated 2017 from original in 2006 (online version). Not specific to critical care, so you don't need to read all of it, but an essential mention in answering a question about nutrition support.
It defines which patients are malnourished and those at risk of malnutrition in which nutrition support should be considered. It defines those at risk of refeeding syndrome and how to manage this. It also provides guidance on when to consider TPN.
Malnourished (any of the following)
At risk of malnutrition
Eaten little or nothing for more than 5 days and/or are likely to eat little or nothing for 5 days or longer
A poor absorptive capacity and/or high nutrient losses and/or increased nutritional needs from causes such as catabolism.