Consumption of garlic and risk of colorectal cancer: An updated meta-analysis of prospective studies

Table 1 Main characteristics of included studies

Ref.	Study	Country	Follow-up		Number of participants, age (yr)			Dietary	Study	Adjustment for confounders
			Start	End	Male	Female	Total	assessment	quality
Meng et al[7] (2013)	The Nurses’ Health Study (NHS)	United States	1984	June 2008	NA	76208 (1339 cases), 30-55 yr	76208 (1339 cases)	Validated FFQ	8	Age, BMI, smoking, history of colorectal cancer, history of endoscopy screening, regular aspirin use, physical activity, postmenopausal hormone use, beef, pork and lamb as a main dish, consumption of processed meat, alcohol consumption, energy-adjusted total calcium intake, total folate, total vitamin D intake, total energy intake
Meng et al[7] (2013)	The Health Professionals Follow-up Study (HPFS)	United States	1986	Jan 2008	45592 (1029 cases), 40-75 yr	NA	45592 (1029 cases)	Validated FFQ	8	Age, BMI, smoking, history of colorectal cancer, history of endoscopy screening, regular aspirin use, physical activity, postmenopausal hormone use, beef, pork and lamb as a main dish, consumption of processed meat, alcohol consumption, energy-adjusted total calcium intake, total folate, total vitamin D intake, total energy intake
McCullough et al[8] (2012)	The American Cancer Society Cancer Prevention Study (CPS) II Nutrition Cohort	United States	1999	June 2007	42824 (579 cases)	56876 (551 cases)	99700 (1130 cases)	Validated FFQ	9	Age, energy intake, gender, history of endoscopy, BMI, physical activity, smoking, alcohol consumption, NSAID use, PMH use, total calcium intake, fruits, vegetable and red/processed meat intake
Satia et al[9] (2009)	The VITamins And Lifestyle (VITAL) study	United States	Oct 2000-Dec 2002	Dec 2006	36516 (220 cases), 50-76 yr	39568 (208 cases), 50-76 yr	76084 (428 cases)	Validated FFQ	9	Age, gender, education, physical activity, fruit and vegetable consumption, BMI, NSAID use, sigmoidoscopy, history of arthritis
Sellers et al[23] (1998)	The Iowa Women's Health Study	United States	1986	Dec 1995	NA	35216 (241 cases), 55-69 yr	35216 (241 cases)	Validated FFQ	9	Age, total energy intake, history of colorectal polyps
Dorant et al[22] (1996)	The Netherlands Cohort Study	Netherlands	Sep 1986	Dec 1989	1525 (243 cases), 55-69 yr	1598 (200 cases), 55-69 yr	3223 (443 cases)	Validated FFQ	9	Age, gender, family history of intestinal cancer, education, smoking, previous history of chronic intestinal disease or cholecystectomy, vitamin C and beta-carotene intake

FFQ: Food frequency questionnaire; NSAID: Non-steroidal anti-inflammatory drugs.

Table 2 Subgroup analyses of garlic consumption and risk of colorectal cancer

Subgroup	RC garlic intake					Garlic supplement intake
	No. of studies1	Relative risk(95%CI)	Q-test	I2(%)	Egger’s test	No. of studies	Relative risk(95%CI)	Q-test	I2(%)	Egger’s test
Colorectal cancer	4	1.07 (0.95-1.19)	0.66	0.0	0.50	5	1.12 (0.96-1.31)	0.11	46.9	0.61
Male	2	1.18 (0.99-1.41)	0.28	15.7	NA	3	1.24 (0.96-1.59)	0.31	13.6	0.75
Female	3	1.04 (0.80-1.34)	0.07	62.1	0.55	3	0.85 (0.64-1.11)	0.39	0.0	0.95
Colon cancer	4	1.07 (0.94-1.21)	0.63	0.0	0.23	3	1.01 (0.77-1.32)	0.15	47.3	0.58
Male	2	1.18 (0.98-1.43)	0.61	0.0	NA	2	1.30 (0.91-1.85)	0.39	0.0	NA
Female	3	1.01 (0.86-1.19)	0.10	56.5	0.49	2	0.73 (0.50-1.09)	0.85	0.0	NA
Rectal cancer	3	1.02 (0.90-1.17)	0.93	0.0	0.59	3	1.17 (0.74-1.83)	0.41	0.0	0.31
Male	2	1.13 (0.86-1.49)	0.26	22.5	NA	2	1.59 (0.94-2.69)	0.85	0.0	NA
Female	2	0.97 (0.67-1.40)	0.46	0.0	NA	2	0.69 (0.32-1.48)	1.00	0.0	NA
Proximal colon cancer	3	1.06 (0.88-1.28)	0.53	0.0	0.04	2	0.78 (0.51-1.21)	0.77	0.0	NA
Distal colon cancer	3	1.12 (0.87-1.45)	0.55	0.0	0.89	2	1.15 (0.43-3.12)	0.04	75.4	NA

¹Data from NHS and HPFS (both conducted by Meng et al[2]. in 2013) were counted as different studies. RC: Raw and cooked.