• The best diagnostic criteria for refeeding syndrome [ Time Frame: 1 month mortality and the duration of mechanical ventilation ]
  Evaluate the best timing for intervention according to the lowest and the decline of serum phosphate
• the incidence of refeeding syndrome [ Time Frame: 3 day after treated with nutrition ]
  only according to serum phosphate standard

• duration of mechanical ventilation [ Time Frame: 30 days ]
  The duration of mechanical ventilation in General ICU of our department in the same time admitted to ICU
• survival rate [ Time Frame: 28 days ]
  including discharge from hospital and ICU
• occurrence of complications [ Time Frame: 7 days ]
  including feeding intolerance, electrolyte disturbance

Stage 1: the monitoring rate of re-feeding syndrome in our department was reviewed to evaluate the incidence of re-feeding syndrome in severe patients,which is not described in detail; Stage 2:This part is a prospective observational study，divided into two group,that is, RFS group(their serum phosphate concentration decreased to below 0.87 mmol/L within 72 h after starting nutritional support and the biological variation needed to be greater than 30% decrease from any concentration previously recorded), which is divided into three sub-group(Group 1 that a drop of >0.16 mmol/L from any previous measurement, to below 0.65 mmol/L within 72 h after starting nutritional support, Group 2 that their serum phosphate concentration decreased to below 0.87 mmol/L within 72 h after starting nutritional support and the biological variation needed to be greater than 30% decrease from any concentration previously recorded, and Group 3 that their serum phosphate concentration decreased to below 0•32 mmol/L within 72 h after starting nutritional support.), and nRFS group(non-RFS that the serum phosphate do not up to the criteria). And the RFS group will be receiving the low-calorie feeding intervention, the nRFS group will be receiving the standard feeding according to the ASPEN guideline of 2016.

Stage 3: On this basis of stage 2, it is assessed whether early low-calorie feeding could improve prognosis in high-risk patients with RFS who was defined in NICE(National institute for the health and care excellence).The part of stage 3, divided into two groups that standard calorie feeding group and low calorie feeding group, aims to assess whether early low-calorie feeding could improve prognosis in high-risk patients with RFS.

• Dietary Supplement: standard enteral nutrition
  Enteral nutrition was treated within 48 hours after ICU and the calories were 500-750kcal/d
• Dietary Supplement: restricted enteral nutrition
  Enteral nutrition was treated within 48 hours after ICU and the calories were 10-20kcal/kg/d for 3 days

• Experimental: low calorie feeding group
  Enteral nutrition was fed at 10-20kcal/kg/d for the first three days before identifying the refeeding syndrome
  Intervention: Dietary Supplement: restricted enteral nutrition
• Active Comparator: standard calorie feeding group
  Enteral nutrition was fed at 500-750kcal/d for the first three days before identifying the refeeding syndrome
  Intervention: Dietary Supplement: standard enteral nutrition
• No Intervention: RFS group
  The definition of RFS is that serum phosphate concentration decreased to below 0.87 mmol/L within 72 h after starting nutritional support and the biological variation needed to be greater than 30% decrease from any concentration previously recorded. And it is divided into three sub-group that is Group 1 that a drop of >0.16 mmol/L from any previous measurement, to below 0.65 mmol/L within 72 h after starting nutritional support, Group 2 that their serum phosphate concentration decreased to below 0.87 mmol/L within 72 h after starting nutritional support and the biological variation needed to be greater than 30% decrease from any concentration previously recorded, and Group 3 that their serum phosphate concentration decreased to below 0•32 mmol/L within 72 h after starting nutritional support.
• No Intervention: nRFS group
  It is not up to the RFS definition

• Doig GS, Simpson F, Heighes PT, Bellomo R, Chesher D, Caterson ID, Reade MC, Harrigan PW; Refeeding Syndrome Trial Investigators Group. Restricted versus continued standard caloric intake during the management of refeeding syndrome in critically ill adults: a randomised, parallel-group, multicentre, single-blind controlled trial. Lancet Respir Med. 2015 Dec;3(12):943-52. doi: 10.1016/S2213-2600(15)00418-X. Epub 2015 Nov 18.
• Rio A, Whelan K, Goff L, Reidlinger DP, Smeeton N. Occurrence of refeeding syndrome in adults started on artificial nutrition support: prospective cohort study. BMJ Open. 2013 Jan 11;3(1). pii: e002173. doi: 10.1136/bmjopen-2012-002173.

Inclusion Criteria:

• Critically ill patients at least 18 years old;
• No history of enteral nutrition or parenteral nutrition within 1 week before ICU admission or ICU admission within 1 week;
• Mechanical ventilation patients requiring enteral nutrition support for >72h

Exclusion Criteria:

• refuse to join this study;
• enteral nutrition support for less than 3 days or have enteral nutrition contraindication;
• less than 18 years old;
• artificial nutrition (enteral/parenteral nutrition) has been applied to patients before admission;
• other factors of hypophosphatemia: continuous hemodialysis, recent parathyroidectomy, or hyperphosphatemia after treatment;