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PURPOSE: Whereas, the impact of morbid obesity on colorectal resection (CR) outcomes has been studied, there is limited data concerning CR outcomes in the underweight population (BMI <18.5). This study's goal was to assess the underweight population (UN) that comes to CR and CR-related morbidity.
METHODS: The ACS NSQIP database was queried from 2005-2010 for the CR codes. Patients (pts) who, preoperatively (preop), were ventilator dependent, ASA 5, hypotensive, had SIRS, sepsis, and emergent surgery were excluded. Pts were divided into 3 BMI groups: UN, BMI<18.5; Normal/Obese (NO), BMI ≥18.5 to ≤40; and morbidly obese (MO), BMI >40. Demographic parameters were assessed as well as surgical indications, comorbidities, preop laboratory data, and complications (including surgical site infections (SSI), transfusions, reoperation, etc). The statistical methods used were two sample for population proportions for categorical variables and Wilcoxon rank sum tests for continuous variables.
RESULTS: A total of 73,516 CR pts were identified; the breakdown as per BMI was UN, 2,180, 3%; NO, 67,732, 92%; MO, 3,604, 4.9%. In the UN group there is a significantly higher proportion of colitis/enteritis pts (UN 20.4%, NO 9%, MO 5.2%) and obstruction/volvulus pts (UN 8.3%, NO 3.3%, MO 2.5%) as well as a much lower proportion of diverticulitis cases (UN 8.4%, NO 21.1%, MO 23.4%). The percent of UN cancer pts (44%) was modestly but significantly lower than the NO (47%) or MO (47.6%) groups yet the UN group had more disseminated cases. More UN pts reported weight loss (21.6%) than in the NO (4.7%) or MO (1.7%) groups. Also, more UN pts (13.6%) were on steroids (vs NO, 6.1%; MO, 4.5%, p<0.05). Finally, the UN group had a significantly lower mean albumin level and hematocrit. Significantly fewer UN CR's were done using laparoscopic (LAP) methods than in the other groups (UN 34%, NO 45%, MO 39%). There were also more total colectomies and Hartman's procedures in UN group (p<0.05). There were significantly more complications in the underweight group (UN)(20.4%) than in the NO group (15.6%), yet, there was no difference between the UN and MO groups (20.1%). The UN group's rate of transfusions, sepsis, and reoperations were higher than noted in the NO group. Of note, there were significantly fewer superficial surgical site wound infections in the UN vs the other 2 groups (UN 5.6%, NO 8.1%, MO 16%).
CONCLUSIONS: The complication rate was notably higher in the UN group which may be related to the higher incidence of colitis and obstruction cases and the greater percentage of pts with weight loss and steroid use all of which are associated with high complication rates. Diverticulitis is rare in UN pts. For unclear reasons laparoscopic methods were used less often in the UN group.

		UN (n=2180)	NO (n=67732)	MO (n=3604)	UN vs. NO P value	UN vs. MO P value
Gender	Male/ Female	700/ 1480	33712/ 34020	1334/ 2270	<.0001	0.00016
Age	Median(range)	62(16-90)	63(19-90)	58(16-90)	0.0034	<.0001
Indication
	Malignant Neoplasm	960	31805	1716	0.0071	0.0083
	Benign Neoplasm	169	8494	497	<.0001	<.0001
	Diverticular Disease	183	14283	843	<.0001	<.0001
	Colitis/enteritis	444	6092	187	<.0001	<.0001
	Obstruction/voluvulus	180	2208	90	<.0001	<.0001
	Rectal Prolapse	68	861	10	<.0001	<.0001
	Perforation/ hemorrhage	13	254	21	n.s.	n.s.
	Other Benign Disease	163	3735	240
Comorbidities
	Current smoker	630	11950	569	<.0001	<.0001
	Steroid intake	297	4134	161	<.0001	<.0001
	Weight loss in last 6 mo	470	3206	63	<.0001	<.0001
	Disseminated cancer	124	2548	93	n.s.	<.0001
Preoperative data
	Hematocrit (Mean±SD)	35.8 ±5.3	38.3 ±5.4	38.1±5.3	<.0001	n.s.
	Albumin (Mean±SD)	3.5 ±0.8	3.8 ±0.6	3.7 ±0.6	<.0001	<.0001

UN; underweight group (BMI <18.5), NO; Normal/Obese(BMI ≥18.5 to ≤40), MO; morbidly obese(BMI >40), n.s.; not significant

		UN (n=2180)	NO (n=67732)	MO (n=3604)	UN vs. NO P value	UN vs. MO P value
Surgical Procedure
	Laparoscopic Surgery	742	30682	1409	<0.0001	<0.0001
	Abdominoperinial Resection	2	118	7	n.s.	n.s.
	Coloproctectomy	231	5891	242	0.0020	<0.0001
	Hartmann Procedure	95	1877	106	<0.0001	0.0044
	Partial colectomy	1197	44214	2472	n.s.	n.s.
	Partial colectomy with removal of terminal ileum	436	11217	595	<0.0001	0.0008
	Total colectomy	219	4415	182	<0.0001	<0.0001
Complications
	Major complication	445	10539	723	<.0001	n.s.
	Re-operation	156	3521	227	<.0001	n.s.
	Superficial SSI	121	5464	578	<.0001	<.0001
	Bleeding/Transfusions	81	1545	95	<.0001	0.0203
	Sepsis	116	2639	195	0.0008	n.s.
	Septic Shock	45	1026	81	0.0394	n.s.

UN; underweight group (BMI <18.5), NO; Normal/Obese(BMI ≥18.5 to ≤40), MO; morbidly obese(BMI >40), n.s.; not significant, SSI; surgical site infections