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Table 1 Characteristics of the included studies

From: The association between dietary protein intake and colorectal cancer risk: a meta-analysis

Study, year Country Study design Participants (cases) Age (years) Score quality Protein type Dietary assessment Protein intake amounts (g) RR (95% CI) Adjustment for covariates
Ghadirian P, 1997 Canada Case-control 1070 (402) 35–79 6 Total animal vegetable Quantitative food frequency questionnaire Colon
Quartile 1
Quartile 2
Quartile 3
Quartile 4
Colon 1
0.81(0.55–1.19)
0.91(0.62–1.32)
1.11(0.77–1.61)
Adjusted for gender, age, marital status, history of colon carcinoma in first-degree relatives, and total energy intake
Goldbohm RA, 1994 Netherlands Cohort 120,852 (215) 55–69 8 Animal Semiquantitative food frequency questionnaire Colon
Quintile 1
Quintile 2
Quintile 3
Quintile 4
Quintile 5
Colon 1
1.10(0.70–1.71)
1.05(0.67–1.65)
1.28(0.82–2.00)
0.90(0.57–1.42)
Adjusted for quintiles of energy and energy-adjusted nutrient intakes, sex, and dietary fiber intake
Iscovich JM, 1992 Argentina Case-control 330 (110) NA 7 Total Food-frequency questionnaire Colon
Quartile 1
Quartile 2
Quartile 3
Quartile 4
Colon 1
0.94(0.46–1.90)
0.63(0.26–1.49)
1.03(0.33–3.21)
Adjusted for fiber at 19.02 g per day, other sources of energy intake
Levi F, 2002 Switzerland Case-control 836 (286) ≤ 74 7 Total Food-frequency questionnaire Colorectal
Lowest
Middle
Highest
Colorectal 1
0.80(0.50–1.30)
1.10(0.70–1.70)
Adjusted for age, sex, education, physical activity, and residual energy
Pietinen P, 1999 Finland Cohort 27,111 (185) 50–69 8 Total
Animal
Vegetable
Food-frequency questionnaire Colorectal
83.8
96.9
105.1
115.9
Colorectal 1
1.20(0.80–1.70)
0.90(0.60–1.40)
0.90(0.50–1.50)
Adjusted for age, supplement group, smoking years, body mass index, alcohol, education, and physical activity at work, and calcium intake (except for milk protein and milk products)
Prentice RL, 2009 American Cohort 59,105 (280) 50–89 8 Total Food-frequency questionnaire Colon
Quartile 1
Quartile 2
Quartile 3
Quartile 4
Colon 1
0.97(0.70–1.34)
1.11(0.76–1.61)
0.96(0.64–1.44)
NA
Prentice RL, 2009 American Cohort 59,105 (280) 50–89 8 Total Food-frequency questionnaire Rectum
Quartile 1
Quartile 2
Quartile 3
Quartile 4
Rectum 1
1.22(0.56–2.65)
1.57(0.72–3.41)
1.08 (0.48–2.41)
NA
Slattery ML, 1994 American Case-control 623 (231) 40–79 7 Total Food-frequency questionnaire Colon
≤ 58.7
58.8–81.4
81.5–99.9
> 99.9
Colon (men) 1
2.30(1.10–4.70)
2.40(1.00–5.70)
3.70(1.30–10.50)
Adjusted for age, religion, body mass index, crude fiber, and separate logistic models were used for each variable. Calcium as continuous variables in multiple logistic regression models.
Slattery ML, 1994 American Case-control 623 (231) 40–79 7 Total Food-frequency questionnaire Colon
≤ 49.7
49.8–63.5
63.6–79.1
> 79.1
Colon (women) 1
0.90(0.40–1.90)
1.90(0.90–4.20)
2.70(0.90–7.70)
Adjusted for age, religion, body mass index, crude fiber, and separate logistic models were used for each variable. Calcium as continuous variables in multiple logistic regression models
Slattery ML, 1997 American Case-control 2389 (1099) 30–79 7 Total Food-frequency questionnaire Colon (kcal)
≤  263
264–331
332–407
408–512
> 512
Colon (men) 1
1.25(0.95–1.65)
1.35(1.01–1.81)
1.33(0.95–1.84)
1.73(1.11–2.68)
Adjusted for age, body mass index, and family history of first-degree relative with colorectal cancer, use of aspirin and/or nonsteroidal anti-inflammatory drugs, physical activity, and dietary intake of fiber, cholesterol, and calcium
Slattery ML, 1997 American Case-control 2014 (894) 30–79 7 Total Food-frequency questionnaire Colon (kcal)
≤ 209
210–259
260–316
317–399
> 399
Colon (women) 1
1.00(0.74–1.35)
1.02(0.74–1.40)
1.00(0.70–1.43)
0.91(0.56–1.48)
Adjusted for age, body mass index, and family history of first-degree relative with colorectal cancer, use of aspirin, and/or nonsteroidal anti-inflammatory drugs, physical activity, and dietary intake of fiber, cholesterol, and calcium
Sun Z, 2012 Canada Case-control 4241 (1760) 20–74 7 Total Food-frequency questionnaire Colorectal
Quintile 1
Quintile 3
Quintile 5
Colorectal 1
0.88(0.70–1.11)
0.85(0.69–1.00)
Adjusted for total energy intake. Other potential confounders included age, sex, BMI, physical activity, family history of CRC, polyps, diabetes, reported colon screening procedure, cigarette smoking, alcohol drinking, education attainment, household income, marital status, regular use of NSAID, regular use of multivitamin supplements, regular use of folate supplement, regular use of calcium supplement, reported HRT (females only), province of residence, and intakes of fruits, vegetables, and red meat. Variables were included in the final model based on a ≥ 10% alternation in the parameter coefficient of interest
Tayyem RF, 2015 Jordan Case-control 417 (169) 53.8 ± 12.2 6 Total Quantitative food frequency questionnaire Colorectal
Quartile 1
Quartile 2
Quartile 3
Quartile 4
Colorectal 1
1.66(0.74–3.69)
1.74(0.78–3.90)
3.62(1.63–8.04)
Adjusted for total energy intake normality of the distributions of dietary intake variables was assessed by the Shapiro-Wilk test. Non-normally distributed variables were log transformed. Other potential confounders included age, gender, BMI, physical activity (METs/week), family history (beyond the second degree) of CRC, education attainment, household income, marital status, and tobacco use
Wakai K, 2006 Japan Case-control 3042 (507) 20–79 7 Total Food-frequency questionnaire Colon
Quartile 1
Quartile 2
Quartile 3
Quartile 4
Colon 1
0.74(0.52–1.07)
0.65(0.45–0.95)
0.74(0.51–1.07)
Adjusted for age, sex, year of first visit, season of first visit to the hospital, reason for the visit, family history of colorectal cancer, body mass index, exercise, alcohol drinking, smoking, multivitamin use, and energy intake
Wakai K, 2006 Japan Case-control 3042 (507) 20–79 7 Total Food-frequency questionnaire Rectum
Quartile 1
Quartile 2
Quartile 3
Quartile 4
Rectum 1
0.83(0.57–1.22)
0.91(0.62–1.32)
0.71(0.47–1.06)
Adjusted for age, sex, year of first visit, season of first visit to the hospital, reason for the visit, family history of colorectal cancer, body mass index, exercise, alcohol drinking, smoking, multivitamin use, and energy intake
Williams CD, 2010 American Case-control 1520 (720) 59.6 ± 10.3 8 Total Food-frequency questionnaire Colorectal
Quartile 1
Quartile 2
Quartile 3
Quartile 4
Colorectal (whites) 1
0.90(0.64–1.27)
0.86(0.57–1.30)
0.57(0.32–1.01)
Adjusted for age, sex, education, body mass index, family history, nonsteroidal anti-inflammatory drug use, physical activity, calcium, fiber, and total energy
Williams CD, 2010 American Case-control 384 (225) 58.0 ± 10.0 8 Total Food-frequency questionnaire Colorectal
Quartile 1
Quartile 2
Quartile 3
Quartile 4
Colorectal (African Americans) 1
1.05(0.51–2.16)
0.81(0.32–2.05)
0.58(0.16–2.10)
Adjusted for age, sex, education, body mass index, family history, nonsteroidal anti-inflammatory drug use, physical activity, calcium, fiber, and total energy
Yang SY, 2016 Korea Case-control 1056 (406) 30–70 8 Animal Food-frequency questionnaire Colorectal
Tertile 1
Tertile 2
Tertile 3
Colorectal (men) 1
0.71(0.47–1.07)
1.25(0.88–1.78)
Adjusted for total energy intake, waist circumference, BMI, HDL cholesterol, fasting glucose, alcohol intake, smoking status, and family history of colorectal adenoma
Yang SY, 2016 Korea Case-control 658 (151) 30–70 8 Animal Food-frequency questionnaire Colorectal
Tertile 1
Tertile 2
Tertile 3
Colorectal (women) 1
0.71(0.30–1.68)
0.87(0.52–1.46)
Adjusted for total energy intake, waist circumference, BMI, HDL cholesterol, fasting glucose, alcohol intake, smoking status, and family history of colorectal adenoma
Yang SY, 2016 Korea Case-control 1056 (406) 30–70 8 Vegetable Food-frequency questionnaire Colorectal
Tertile 1
Tertile 2
Tertile 3
Colorectal (men) 1
0.93(0.63–1.37)
0.96(0.60–1.53)
Adjusted for total energy intake, waist circumference, BMI, HDL cholesterol, fasting glucose, alcohol intake, smoking status, and family history of colorectal adenoma
Yang SY, 2016 Korea Case-control 658 (151) 30–70 8 Vegetable Food-frequency questionnaire Colorectal
Tertile 1
Tertile 2
Tertile 3
Colorectal (women) 1
0.78(0.39–1.56)
0.54(0.27–1.11)
Adjusted for total energy intake, waist circumference, BMI, HDL cholesterol, fasting glucose, alcohol intake, smoking status, and family history of colorectal adenoma
  1. Abbreviations: NA not available, BMI body mass index, CRC colorectal cancer, CI confidence interval, RR relative risk