Have you gone through the ASPEN guidelines:

http://www.nutritioncare.org/Guidelines_and_Clinical_Resources/A_S_P_E_N__s_Parenteral_Nutrition_Resources/

Since many years, we know that severe malnourished patients undergoing surgery have an increased rate of complications. This review (Evans DC et al. Nutr Clin Pract. 2014; 29:10-21) remembers us that preoperative optimization of metabolic state with nutrition support can improve outcomes and reduce complications in severe malnourished patients undergoing surgery.

Perioperative parenteral nutrition should improve nutritional parameters, sometimes not reflected in weight, must be expect changes in albumin, pre albumin, ferritin, lymphocytes, retinol bound protein for example. Like another answer before, ASPEN and ESPEN guidelines can be useful.