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Vol. 183, No. 8 DOI: 10.1093/aje/kwv268 Advance Access publication: March 28, 2016
Original Contribution Dietary Protein Intake and Risk of Type 2 Diabetes in US Men and Women
Vasanti S. Malik*, Yanping Li, Deirdre K. Tobias, An Pan, and Frank B. Hu * Correspondence to Dr. Vasanti S. Malik, Department of Nutrition, Harvard T.H. Chan School of Public Health, 665 Huntington Avenue, Boston, MA 02115 (e-mail: [email protected]).
Initially submitted June 4, 2015; accepted for publication September 23, 2015.
Dietary proteins are important modulators of glucose metabolism. However, few longitudinal studies have evaluated the associations between intake of protein and protein type and risk of type 2 diabetes (T2D). We investigated the associations between total, animal, and vegetable protein and incident T2D in 72,992 women from the Nurses’ Health Study (1984–2008), 92,088 women from Nurses’ Health Study II (1991–2009) and 40,722 men from the Health Professionals Follow-up Study (1986–2008). During 4,146,216 person-years of follow-up, we documented 15,580 cases of T2D. In pooled multivariate models including body mass index, participants in the highest quintiles of percentage of energy derived from total protein and animal protein had 7% (95% confidence interval (CI): 1, 17) and 13% (95% CI: 6, 21) increased risks of T2D compared with those in the lowest quintiles, respectively. Percentage of energy intake from vegetable protein was associated with a moderately decreased risk of T2D (comparing extreme quintiles, hazard ratio = 0.91, 95% CI: 0.84, 0.98). Substituting 5% of energy intake from vegetable protein for animal protein was associated with a 23% (95% CI: 16, 30) reduced risk of T2D. In conclusion, higher intake of animal protein was associated with an increased risk of T2D, while higher intake of vegetable protein was associated with a modestly reduced risk. animal protein; diabetes mellitus; dietary protein; nuts; peanuts; type 2 diabetes; vegetable protein; whole grains
Abbreviations: BMI, body mass index; CI, confidence interval; EPIC, European Prospective Investigation into Cancer and Nutrition; FFQ, food frequency questionnaire; HPFS, Health Professionals Follow-up Study; HR, hazard ratio; NHS, Nurses’ Health Study; NHS II, Nurses’ Health Study II; T2D, type 2 diabetes.
investigate the associations between total protein intake and protein type (vegetable, animal) and risk of T2D in 3 large prospective cohort studies of US adults, using repeated measurements of protein intake taken over 18–24 years of follow-up. We also estimated the associations between substituting vegetable protein for animal protein, substituting protein for carbohydrates of differing quality, and substituting vegetable protein foods for animal protein and low-quality carbohydrate foods and T2D risk.
Diets high in protein have shown beneficial effects on glucose homeostasis in short-term trials (1, 2) and have thus been suggested as a potential strategy for type 2 diabetes (T2D) prevention. In contrast, findings from a few prospective cohort studies have shown positive associations between protein intake and risk of T2D that are driven by intake of animal protein (3, 4). However, because these studies used baseline intake only, they were not able to capture long-term intake patterns. Findings from epidemiologic studies evaluating food sources of protein suggest divergent associations of animal and vegetable protein with T2D risk. Intake of red and processed meat has been positively associated with risk of T2D (5, 6), while intake of plant-based sources of protein, such as nuts (7, 8) and legumes (9), has been associated with decreased risk. It is unclear whether different types of protein are differentially associated with risk of T2D and whether it is the protein per se or other components of protein-rich foods that may account for observed associations. In this study, we aimed to
METHODS Study population
Our analysis was conducted in 3 ongoing prospective cohort studies: the Nurses’ Health Study (NHS), which consists of 121,700 female registered nurses who were aged 30–55 years at baseline in 1976; Nurses’ Health Study II (NHS II), 715
Am J Epidemiol. 2016;183(8):715–728
716 Malik et al.
Table 1. Age-Adjusted Characteristics of Participants According to Baseline (1984) Total Protein Intake Among 72,992 Women in the Nurses’ Health Study, 1984–2008 Quintile of Protein Intake Characteristic
1 (n = 14,598) (14.8% of Energy)a %
Mean
3 (n = 14,598) (18.0% of Energy) %
Mean
5 (n = 14,598) (21.6% of Energy) %
Mean
Demographic and lifestyle factors Age, yearsb White race/ethnicity
50.1 97.5
Body mass indexc Body mass index ≥25
24.3 31.0
Physical activity, MET-hours/week Current smoker Hypertension High cholesterol
50.1 98.1
50.7 97.6
24.8 36.1
12.8
25.7 44.6
14.2
16.1
30.1
22.6
21.8
8.0
7.6
8.8
3.1
3.2
4.2
Family history of diabetesd
26.8
28.1
31.1
Postmenopausal hormone use
22.6
25.0
26.2
Multivitamin use
33.6
Alcohol, g/day
37.0 10.2
40.8 6.8
4.2
Dietary intake Total energy, kcal/day
1,880
1,772
1,550
Carbohydrate, % of energy intake
51.3
46.4
41.8
Total fat, % of energy intake
33.0
35.1
35.1
Saturated fat, % of energy intake
11.7
12.6
12.8
Monounsaturated fat, % of energy intake
12.0
12.9
12.8
Polyunsaturated fat, % of energy intake
6.8
6.7
6.4
Trans- fat, % of energy intake
2.0
2.0
1.7
Dietary cholesterol, mg/day
229.6
283.2
344.5
Daily dietary glycemic loade
111.7
99.3
86.5
Daily dietary glycemic indexf
54.8
53.5
51.3
Total dietary fiber, g/day
15.2
16.5
17.0
4.0
4.3
4.0
252.8
287.3
326.1
1.1
1.5
Cereal fiber, g/day Magnesium, mg/day Heme iron, mg/day Potassium, mg/day Calcium, mg/day
0.76 2,570 725.2
2,908 883.4
3,207 1,040 Table continues
which consists of 116,671 female registered nurses aged 24–42 years at baseline in 1989; and the Health Professionals Follow-up Study (HPFS), which consists of 51,529 male health professionals aged 40–75 years at baseline in 1986. For each cohort, mailed questionnaires are administered biennially to collect data on lifestyle factors and health. Diet is assessed using a validated self-administered food frequency questionnaire (FFQ) every 4 years. For this analysis, we used 1984, 1991, and 1986 as the baseline years for the NHS, NHS II, and HPFS, respectively; those years respectively were then considered the year in which the FFQ was first administered in each cohort. We excluded men and women who had a diagnosis of diabetes, cardiovascular disease, or cancer at baseline, left more than 70 items on the baseline FFQ
blank, reported implausible total energy intakes, or were missing baseline data on protein intake or follow-up information on date of diabetes diagnosis. After these exclusions, a total of 72,992 participants from the NHS, 92,088 participants from NHS II, and 40,722 participants from the HPFS remained. All 3 studies were approved by the institutional review boards of Brigham and Women’s Hospital and the Harvard T.H. Chan School of Public Health (Boston, Massachusetts). Assessment of dietary protein
Diet was assessed using a 131-item FFQ, administered at baseline and every 4 years. Each FFQ asked participants how Am J Epidemiol. 2016;183(8):715–728
Dietary Protein and Risk of Type 2 Diabetes 717
Table 1. Continued Quintile of Protein Intake Characteristic
1 (n = 14,598) (14.8% of Energy)a %
Mean
3 (n = 14,598) (18.0% of Energy) %
Mean
5 (n = 14,598) (21.6% of Energy) %
Mean
Vitamin C, mg/day
301.2
326.0
389.8
Vitamin E, mg/day
71.1
83.2
104.3
Whole grains, servings/day
1.0
1.2
1.2
Fruit, servings/day
1.3
1.4
1.4
Vegetables, servings/day
2.4
2.8
3.2
Red meat, servings/day
0.62
0.88
0.93
Processed meat, servings/day
0.35
0.32
0.23
Fish, servings/day
0.10
0.15
0.28
Chicken, servings/day
0.20
0.29
0.46
Eggs, servings/day
0.30
0.36
0.38
Dairy foods, servings/day
1.7
2.0
2.1
Legumes, servings/day
0.35
0.40
0.42
Nuts, servings/day
0.31
0.33
0.25
Peanuts, servings/day
0.12
0.12
0.09
Peanut butter, servings/day
0.20
0.21
0.16
Potatoes, servings/day
0.43
0.41
0.29
Coffee, servings/day
1.9
1.8
1.6
Sugar-sweetened beverages, servings/day
0.69
0.23
0.10
Abbreviations: GI, glycemic index; MET, metabolic equivalent of task. a Median percentage of energy intake in the quintile. b Not adjusted for age. c Weight (kg)/height (m)2. d First-degree relatives. e Average glycemic load was calculated by multiplying the amount of carbohydrates in the diet by the average glycemic index. For 1 serving of a food, a glycemic load of ≥20 is considered high, 11–19 is considered medium, and ≤10 is considered low. Among these 72,992 women from the Nurses’ Health Study, the mean glycemic load was 102.5 (range, 0–213). f Average dietary glycemic index (GI) was calculated by summing the products of 1) the carbohydrate content of each food item per serving, 2) the average daily number of servings of that food, and 3) the food’s GI value (derived from available databases and publications) and dividing by total daily carbohydrate content. Foods with a GI value of ≤55 are considered to have a low GI, foods with a value of 56–69 are considered to have a medium GI, and foods with a value of ≥70 are considered to have a high GI. Among these 72,992 women from the Nurses’ Health Study, the mean GI was 52.8 (range, 0.01–70.9).
often, on average, they consumed standard portions of foods and beverages, using 9 possible responses ranging from “never or less than once per month” to “6 or more times per day.” Nutrient and energy intakes were calculated by multiplying the frequency of consumption of each unit of food and beverage by nutrient and energy contents and summing across all items. Values were obtained using the US Department of Agriculture food composition database (10). Intakes of total, animal, and vegetable protein were calculated for each participant and expressed as a percentage of total energy by multiplying the grams of protein consumed per day by the number of kilocalories in 1 gram of protein (4 kcal/g) and then dividing by total caloric intake (11). The reproducibility and validity of these FFQs have been described in detail elsewhere (12– 16). In a subsample of NHS participants, the coefficients for correlations between the FFQ and multiple dietary records Am J Epidemiol. 2016;183(8):715–728
were 0.50 for protein, 0.57 for fat, and 0.64 for carbohydrate (14, 16). Similar values were reported in a subsample of HPFS participants (13). Assessment of T2D
Participants who reported a diagnosis of T2D on the biennial questionnaire were mailed a supplementary questionnaire about symptoms and treatment. In accordance with the National Diabetes Data Group criteria (17), a case of T2D was confirmed if at least 1 of the following was reported on the supplementary questionnaire: 1) 1 or more classic symptoms of T2D (excessive thirst, polyuria, weight loss, hunger) and fasting plasma glucose concentration of ≥11.1 mmol/L; 2) ≥2 elevated plasma glucose measurements taken on different occasions (fasting concentrations of ≥7.8 mmol/L, random plasma
718 Malik et al.
Table 2. Age-Adjusted Characteristics of Participants According to Baseline (1991) Total Protein Intake Among 92,088 Women in Nurses’ Health Study II, 1991–2009 Quintile of Protein Intake Characteristic
1 (n = 18,417) (15.3% of Energy)a %
Mean
3 (n = 18,466) (18.8% of Energy) %
Mean
5 (n = 18,417) (22.6% of Energy) %
Mean
Demographic and lifestyle factors Age, yearsb White race/ethnicity
35.9 95.6
Body mass indexc Body mass index ≥25
31.0
Hypertension
36.4 96.1
23.8
Physical activity, MET-hours/week Current smoker
36.0 96.8 24.5 36.1
25.5 44.6
20.7
20.3
22.4
14.6
11.8
11.7
5.8
5.9
7.3
High cholesterol
13.7
13.8
16.4
Family history of diabetesd
32.4
34.3
36.5
4.3
4.4
5.0
Oral contraceptive use
12.0
10.4
10.1
Multivitamin use
42.7
43.9
Postmenopausal hormone use
Alcohol, g/day
44.5
4.0
3.1
2.3
Dietary intake Total energy, kcal/day
1,901
1,815
1,626
Carbohydrate, % of energy intake
55.8
49.4
44.6
Total fat, % of energy intake
30.4
32.1
31.5
Saturated fat, % of energy intake
10.7
11.4
11.2
Monounsaturated fat, % of energy intake
11.7
12.2
11.7
Polyunsaturated fat, % of energy intake
5.6
5.7
5.6
Trans- fat, % of energy intake
1.8
1.7
1.4
Dietary cholesterol, mg/day
187.1
241.7
297.6
Daily dietary glycemic loade
139.1
120.3
106.4
Daily dietary glycemic indexf
55.3
53.9
52.4
Total dietary fiber, g/day
17.7
18.3
18.6
5.6
5.7
5.4
285.2
314.9
345.2
Cereal fiber, g/day Magnesium, mg/day Heme iron, mg/day
0.75
1.1
1.5 Table continues
glucose concentrations of ≥11.1 mmol/L, and/or concentrations of ≥11.1 mmol/L after ≥2 hours shown by an oral glucose tolerance test) in the absence of symptoms; or 3) treatment with hypoglycemic medication (insulin or oral hypoglycemic agent). For cases identified after 1998, we applied the American Diabetes Association criteria (18), in which the threshold for diagnosis of diabetes changed from a fasting plasma glucose concentration of 7.8 mmol/L to a concentration of 7.0 mmol/L. The validity of the supplementary questionnaire has been documented through medical record review (19, 20). Covariates
The biennial follow-up questionnaires collected updated information on lifestyle factors and medical history, including
age, body weight, smoking status, physical activity, medication use, and history of chronic diseases. Family history of diabetes in first-degree relatives was assessed in 1982 and 1988 in the NHS; in 1989, 1997, 2001, and 2005 in NHS II; and in 1987 in the HPFS. Information on dietary factors was obtained from the FFQs. Statistical analysis
We calculated person-time for each participant from the date of return of the baseline questionnaire to the date of diagnosis of T2D, death, loss to follow-up, or the end of the follow-up period, whichever occurred first. Cox proportional hazards regression was used to model the association between protein intake and risk of T2D. Protein intake was expressed as a Am J Epidemiol. 2016;183(8):715–728
Dietary Protein and Risk of Type 2 Diabetes 719
Table 2. Continued Quintile of Protein Intake Characteristic
1 (n = 18,417) (15.3% of Energy)a %
Potassium, mg/day
Mean
2,649
3 (n = 18,466) (18.8% of Energy) %
Mean
5 (n = 18,417) (22.6% of Energy) %
Mean
2,944
3,189
1,028
1,145
Calcium, mg/day
857.7
Vitamin C, mg/day
264.5
246.5
268.3
Vitamin E, mg/day
47.1
41.2
48.6
Whole grains, servings/day
1.3
1.4
1.3
Fruit, servings/day
1.2
1.2
1.2
Vegetables, servings/day
3.0
3.3
3.6
Red meat, servings/day
0.57
0.77
0.75
Processed meat, servings/day
0.25
0.24
0.17
Fish, servings/day
0.18
0.27
0.42
Chicken, servings/day
0.39
0.66
1.1
Eggs, servings/day
0.17
0.19
0.17
Dairy foods, servings/day
2.2
2.5
2.4
Legumes, servings/day
0.35
0.38
0.40
Nuts, servings/day
0.29
0.26
0.18
Peanuts, servings/day
0.05
0.04
0.02
Peanut butter, servings/day
0.20
0.20
0.14
Potatoes, servings/day
0.39
0.39
0.31
Coffee, servings/day
1.5
1.6
1.5
Sugar-sweetened beverages, servings/day
1.1
0.36
0.13
Abbreviations: GI, glycemic index; MET, metabolic equivalent. a Median percentage of energy intake in the quintile. b Not adjusted for age. c Weight (kg)/height (m)2. d First-degree relatives. e Average glycemic load was calculated by multiplying the amount of carbohydrates in the diet by the average glycemic index. For 1 serving of a food, a glycemic load of ≥20 is considered high, 11–19 is considered medium, and ≤10 is considered low. Among these 92,088 women from Nurses’ Health Study II, the mean glycemic load was 119.0 (range, 47.8–222.4). f Average dietary glycemic index (GI) was calculated by summing the products of 1) the carbohydrate content of each food item per serving, 2) the average daily number of servings of that food, and 3) the food’s GI value (derived from available databases and publications) and dividing by total daily carbohydrate content. Foods with a GI value of ≤55 are considered to have a low GI, foods with a value of 56–69 are considered to have a medium GI, and foods with a value of ≥70 are considered to have a high GI. Among these 92,088 women from Nurses’ Health Study II, the mean GI was 52.9 (range, 37.0–64.8).
percentage of total energy using the nutrient density method (16) and categorized into quintiles. Regression models included age in years as the time scale, stratified by calendar time in 2-year intervals. Multivariate models adjusted for race/ethnicity (white or nonwhite), family history of diabetes (yes/no), and various lifestyle factors, including smoking status (never smoker, past smoker, or current smoker of 1–14, 15–24, or ≥25 cigarettes/day), alcohol intake (0, 0.1–4.9, 5.0–14.9, or ≥15 g/day in women; 0, 0.1–4.9, 5.0–29.9, or ≥30 g/day in men), physical activity (3.0, 3.0–8.9, 9.0– 17.9, 18.0–26.9, or ≥27.0 metabolic equivalent-hours/ week), total energy intake (kcal/day; quintiles), and, for women, menopausal status, postmenopausal hormone use (NHS and NHS II; premenopausal, postmenopausal with no history Am J Epidemiol. 2016;183(8):715–728
of hormone replacement, or postmenopausal with current hormone replacement), and oral contraceptive use (NHS II only; never user, past user, or current user). Results were also adjusted for percentages of energy derived from trans- fat, saturated fat, monounsaturated fat, and polyunsaturated fat; dietary fiber; dietary cholesterol; and glycemic index, and included mutual adjustment for percentages of energy derived from animal protein and vegetable protein (quintiles). Since body weight may partly mediate the association between protein intake and risk of T2D (21), we subsequently adjusted for body mass index (BMI), defined as weight (kg)/ height (m)2 (
Vol. 183, No. 8 DOI: 10.1093/aje/kwv268 Advance Access publication: March 28, 2016
Original Contribution Dietary Protein Intake and Risk of Type 2 Diabetes in US Men and Women
Vasanti S. Malik*, Yanping Li, Deirdre K. Tobias, An Pan, and Frank B. Hu * Correspondence to Dr. Vasanti S. Malik, Department of Nutrition, Harvard T.H. Chan School of Public Health, 665 Huntington Avenue, Boston, MA 02115 (e-mail: [email protected]).
Initially submitted June 4, 2015; accepted for publication September 23, 2015.
Dietary proteins are important modulators of glucose metabolism. However, few longitudinal studies have evaluated the associations between intake of protein and protein type and risk of type 2 diabetes (T2D). We investigated the associations between total, animal, and vegetable protein and incident T2D in 72,992 women from the Nurses’ Health Study (1984–2008), 92,088 women from Nurses’ Health Study II (1991–2009) and 40,722 men from the Health Professionals Follow-up Study (1986–2008). During 4,146,216 person-years of follow-up, we documented 15,580 cases of T2D. In pooled multivariate models including body mass index, participants in the highest quintiles of percentage of energy derived from total protein and animal protein had 7% (95% confidence interval (CI): 1, 17) and 13% (95% CI: 6, 21) increased risks of T2D compared with those in the lowest quintiles, respectively. Percentage of energy intake from vegetable protein was associated with a moderately decreased risk of T2D (comparing extreme quintiles, hazard ratio = 0.91, 95% CI: 0.84, 0.98). Substituting 5% of energy intake from vegetable protein for animal protein was associated with a 23% (95% CI: 16, 30) reduced risk of T2D. In conclusion, higher intake of animal protein was associated with an increased risk of T2D, while higher intake of vegetable protein was associated with a modestly reduced risk. animal protein; diabetes mellitus; dietary protein; nuts; peanuts; type 2 diabetes; vegetable protein; whole grains
Abbreviations: BMI, body mass index; CI, confidence interval; EPIC, European Prospective Investigation into Cancer and Nutrition; FFQ, food frequency questionnaire; HPFS, Health Professionals Follow-up Study; HR, hazard ratio; NHS, Nurses’ Health Study; NHS II, Nurses’ Health Study II; T2D, type 2 diabetes.
investigate the associations between total protein intake and protein type (vegetable, animal) and risk of T2D in 3 large prospective cohort studies of US adults, using repeated measurements of protein intake taken over 18–24 years of follow-up. We also estimated the associations between substituting vegetable protein for animal protein, substituting protein for carbohydrates of differing quality, and substituting vegetable protein foods for animal protein and low-quality carbohydrate foods and T2D risk.
Diets high in protein have shown beneficial effects on glucose homeostasis in short-term trials (1, 2) and have thus been suggested as a potential strategy for type 2 diabetes (T2D) prevention. In contrast, findings from a few prospective cohort studies have shown positive associations between protein intake and risk of T2D that are driven by intake of animal protein (3, 4). However, because these studies used baseline intake only, they were not able to capture long-term intake patterns. Findings from epidemiologic studies evaluating food sources of protein suggest divergent associations of animal and vegetable protein with T2D risk. Intake of red and processed meat has been positively associated with risk of T2D (5, 6), while intake of plant-based sources of protein, such as nuts (7, 8) and legumes (9), has been associated with decreased risk. It is unclear whether different types of protein are differentially associated with risk of T2D and whether it is the protein per se or other components of protein-rich foods that may account for observed associations. In this study, we aimed to
METHODS Study population
Our analysis was conducted in 3 ongoing prospective cohort studies: the Nurses’ Health Study (NHS), which consists of 121,700 female registered nurses who were aged 30–55 years at baseline in 1976; Nurses’ Health Study II (NHS II), 715
Am J Epidemiol. 2016;183(8):715–728
716 Malik et al.
Table 1. Age-Adjusted Characteristics of Participants According to Baseline (1984) Total Protein Intake Among 72,992 Women in the Nurses’ Health Study, 1984–2008 Quintile of Protein Intake Characteristic
1 (n = 14,598) (14.8% of Energy)a %
Mean
3 (n = 14,598) (18.0% of Energy) %
Mean
5 (n = 14,598) (21.6% of Energy) %
Mean
Demographic and lifestyle factors Age, yearsb White race/ethnicity
50.1 97.5
Body mass indexc Body mass index ≥25
24.3 31.0
Physical activity, MET-hours/week Current smoker Hypertension High cholesterol
50.1 98.1
50.7 97.6
24.8 36.1
12.8
25.7 44.6
14.2
16.1
30.1
22.6
21.8
8.0
7.6
8.8
3.1
3.2
4.2
Family history of diabetesd
26.8
28.1
31.1
Postmenopausal hormone use
22.6
25.0
26.2
Multivitamin use
33.6
Alcohol, g/day
37.0 10.2
40.8 6.8
4.2
Dietary intake Total energy, kcal/day
1,880
1,772
1,550
Carbohydrate, % of energy intake
51.3
46.4
41.8
Total fat, % of energy intake
33.0
35.1
35.1
Saturated fat, % of energy intake
11.7
12.6
12.8
Monounsaturated fat, % of energy intake
12.0
12.9
12.8
Polyunsaturated fat, % of energy intake
6.8
6.7
6.4
Trans- fat, % of energy intake
2.0
2.0
1.7
Dietary cholesterol, mg/day
229.6
283.2
344.5
Daily dietary glycemic loade
111.7
99.3
86.5
Daily dietary glycemic indexf
54.8
53.5
51.3
Total dietary fiber, g/day
15.2
16.5
17.0
4.0
4.3
4.0
252.8
287.3
326.1
1.1
1.5
Cereal fiber, g/day Magnesium, mg/day Heme iron, mg/day Potassium, mg/day Calcium, mg/day
0.76 2,570 725.2
2,908 883.4
3,207 1,040 Table continues
which consists of 116,671 female registered nurses aged 24–42 years at baseline in 1989; and the Health Professionals Follow-up Study (HPFS), which consists of 51,529 male health professionals aged 40–75 years at baseline in 1986. For each cohort, mailed questionnaires are administered biennially to collect data on lifestyle factors and health. Diet is assessed using a validated self-administered food frequency questionnaire (FFQ) every 4 years. For this analysis, we used 1984, 1991, and 1986 as the baseline years for the NHS, NHS II, and HPFS, respectively; those years respectively were then considered the year in which the FFQ was first administered in each cohort. We excluded men and women who had a diagnosis of diabetes, cardiovascular disease, or cancer at baseline, left more than 70 items on the baseline FFQ
blank, reported implausible total energy intakes, or were missing baseline data on protein intake or follow-up information on date of diabetes diagnosis. After these exclusions, a total of 72,992 participants from the NHS, 92,088 participants from NHS II, and 40,722 participants from the HPFS remained. All 3 studies were approved by the institutional review boards of Brigham and Women’s Hospital and the Harvard T.H. Chan School of Public Health (Boston, Massachusetts). Assessment of dietary protein
Diet was assessed using a 131-item FFQ, administered at baseline and every 4 years. Each FFQ asked participants how Am J Epidemiol. 2016;183(8):715–728
Dietary Protein and Risk of Type 2 Diabetes 717
Table 1. Continued Quintile of Protein Intake Characteristic
1 (n = 14,598) (14.8% of Energy)a %
Mean
3 (n = 14,598) (18.0% of Energy) %
Mean
5 (n = 14,598) (21.6% of Energy) %
Mean
Vitamin C, mg/day
301.2
326.0
389.8
Vitamin E, mg/day
71.1
83.2
104.3
Whole grains, servings/day
1.0
1.2
1.2
Fruit, servings/day
1.3
1.4
1.4
Vegetables, servings/day
2.4
2.8
3.2
Red meat, servings/day
0.62
0.88
0.93
Processed meat, servings/day
0.35
0.32
0.23
Fish, servings/day
0.10
0.15
0.28
Chicken, servings/day
0.20
0.29
0.46
Eggs, servings/day
0.30
0.36
0.38
Dairy foods, servings/day
1.7
2.0
2.1
Legumes, servings/day
0.35
0.40
0.42
Nuts, servings/day
0.31
0.33
0.25
Peanuts, servings/day
0.12
0.12
0.09
Peanut butter, servings/day
0.20
0.21
0.16
Potatoes, servings/day
0.43
0.41
0.29
Coffee, servings/day
1.9
1.8
1.6
Sugar-sweetened beverages, servings/day
0.69
0.23
0.10
Abbreviations: GI, glycemic index; MET, metabolic equivalent of task. a Median percentage of energy intake in the quintile. b Not adjusted for age. c Weight (kg)/height (m)2. d First-degree relatives. e Average glycemic load was calculated by multiplying the amount of carbohydrates in the diet by the average glycemic index. For 1 serving of a food, a glycemic load of ≥20 is considered high, 11–19 is considered medium, and ≤10 is considered low. Among these 72,992 women from the Nurses’ Health Study, the mean glycemic load was 102.5 (range, 0–213). f Average dietary glycemic index (GI) was calculated by summing the products of 1) the carbohydrate content of each food item per serving, 2) the average daily number of servings of that food, and 3) the food’s GI value (derived from available databases and publications) and dividing by total daily carbohydrate content. Foods with a GI value of ≤55 are considered to have a low GI, foods with a value of 56–69 are considered to have a medium GI, and foods with a value of ≥70 are considered to have a high GI. Among these 72,992 women from the Nurses’ Health Study, the mean GI was 52.8 (range, 0.01–70.9).
often, on average, they consumed standard portions of foods and beverages, using 9 possible responses ranging from “never or less than once per month” to “6 or more times per day.” Nutrient and energy intakes were calculated by multiplying the frequency of consumption of each unit of food and beverage by nutrient and energy contents and summing across all items. Values were obtained using the US Department of Agriculture food composition database (10). Intakes of total, animal, and vegetable protein were calculated for each participant and expressed as a percentage of total energy by multiplying the grams of protein consumed per day by the number of kilocalories in 1 gram of protein (4 kcal/g) and then dividing by total caloric intake (11). The reproducibility and validity of these FFQs have been described in detail elsewhere (12– 16). In a subsample of NHS participants, the coefficients for correlations between the FFQ and multiple dietary records Am J Epidemiol. 2016;183(8):715–728
were 0.50 for protein, 0.57 for fat, and 0.64 for carbohydrate (14, 16). Similar values were reported in a subsample of HPFS participants (13). Assessment of T2D
Participants who reported a diagnosis of T2D on the biennial questionnaire were mailed a supplementary questionnaire about symptoms and treatment. In accordance with the National Diabetes Data Group criteria (17), a case of T2D was confirmed if at least 1 of the following was reported on the supplementary questionnaire: 1) 1 or more classic symptoms of T2D (excessive thirst, polyuria, weight loss, hunger) and fasting plasma glucose concentration of ≥11.1 mmol/L; 2) ≥2 elevated plasma glucose measurements taken on different occasions (fasting concentrations of ≥7.8 mmol/L, random plasma
718 Malik et al.
Table 2. Age-Adjusted Characteristics of Participants According to Baseline (1991) Total Protein Intake Among 92,088 Women in Nurses’ Health Study II, 1991–2009 Quintile of Protein Intake Characteristic
1 (n = 18,417) (15.3% of Energy)a %
Mean
3 (n = 18,466) (18.8% of Energy) %
Mean
5 (n = 18,417) (22.6% of Energy) %
Mean
Demographic and lifestyle factors Age, yearsb White race/ethnicity
35.9 95.6
Body mass indexc Body mass index ≥25
31.0
Hypertension
36.4 96.1
23.8
Physical activity, MET-hours/week Current smoker
36.0 96.8 24.5 36.1
25.5 44.6
20.7
20.3
22.4
14.6
11.8
11.7
5.8
5.9
7.3
High cholesterol
13.7
13.8
16.4
Family history of diabetesd
32.4
34.3
36.5
4.3
4.4
5.0
Oral contraceptive use
12.0
10.4
10.1
Multivitamin use
42.7
43.9
Postmenopausal hormone use
Alcohol, g/day
44.5
4.0
3.1
2.3
Dietary intake Total energy, kcal/day
1,901
1,815
1,626
Carbohydrate, % of energy intake
55.8
49.4
44.6
Total fat, % of energy intake
30.4
32.1
31.5
Saturated fat, % of energy intake
10.7
11.4
11.2
Monounsaturated fat, % of energy intake
11.7
12.2
11.7
Polyunsaturated fat, % of energy intake
5.6
5.7
5.6
Trans- fat, % of energy intake
1.8
1.7
1.4
Dietary cholesterol, mg/day
187.1
241.7
297.6
Daily dietary glycemic loade
139.1
120.3
106.4
Daily dietary glycemic indexf
55.3
53.9
52.4
Total dietary fiber, g/day
17.7
18.3
18.6
5.6
5.7
5.4
285.2
314.9
345.2
Cereal fiber, g/day Magnesium, mg/day Heme iron, mg/day
0.75
1.1
1.5 Table continues
glucose concentrations of ≥11.1 mmol/L, and/or concentrations of ≥11.1 mmol/L after ≥2 hours shown by an oral glucose tolerance test) in the absence of symptoms; or 3) treatment with hypoglycemic medication (insulin or oral hypoglycemic agent). For cases identified after 1998, we applied the American Diabetes Association criteria (18), in which the threshold for diagnosis of diabetes changed from a fasting plasma glucose concentration of 7.8 mmol/L to a concentration of 7.0 mmol/L. The validity of the supplementary questionnaire has been documented through medical record review (19, 20). Covariates
The biennial follow-up questionnaires collected updated information on lifestyle factors and medical history, including
age, body weight, smoking status, physical activity, medication use, and history of chronic diseases. Family history of diabetes in first-degree relatives was assessed in 1982 and 1988 in the NHS; in 1989, 1997, 2001, and 2005 in NHS II; and in 1987 in the HPFS. Information on dietary factors was obtained from the FFQs. Statistical analysis
We calculated person-time for each participant from the date of return of the baseline questionnaire to the date of diagnosis of T2D, death, loss to follow-up, or the end of the follow-up period, whichever occurred first. Cox proportional hazards regression was used to model the association between protein intake and risk of T2D. Protein intake was expressed as a Am J Epidemiol. 2016;183(8):715–728
Dietary Protein and Risk of Type 2 Diabetes 719
Table 2. Continued Quintile of Protein Intake Characteristic
1 (n = 18,417) (15.3% of Energy)a %
Potassium, mg/day
Mean
2,649
3 (n = 18,466) (18.8% of Energy) %
Mean
5 (n = 18,417) (22.6% of Energy) %
Mean
2,944
3,189
1,028
1,145
Calcium, mg/day
857.7
Vitamin C, mg/day
264.5
246.5
268.3
Vitamin E, mg/day
47.1
41.2
48.6
Whole grains, servings/day
1.3
1.4
1.3
Fruit, servings/day
1.2
1.2
1.2
Vegetables, servings/day
3.0
3.3
3.6
Red meat, servings/day
0.57
0.77
0.75
Processed meat, servings/day
0.25
0.24
0.17
Fish, servings/day
0.18
0.27
0.42
Chicken, servings/day
0.39
0.66
1.1
Eggs, servings/day
0.17
0.19
0.17
Dairy foods, servings/day
2.2
2.5
2.4
Legumes, servings/day
0.35
0.38
0.40
Nuts, servings/day
0.29
0.26
0.18
Peanuts, servings/day
0.05
0.04
0.02
Peanut butter, servings/day
0.20
0.20
0.14
Potatoes, servings/day
0.39
0.39
0.31
Coffee, servings/day
1.5
1.6
1.5
Sugar-sweetened beverages, servings/day
1.1
0.36
0.13
Abbreviations: GI, glycemic index; MET, metabolic equivalent. a Median percentage of energy intake in the quintile. b Not adjusted for age. c Weight (kg)/height (m)2. d First-degree relatives. e Average glycemic load was calculated by multiplying the amount of carbohydrates in the diet by the average glycemic index. For 1 serving of a food, a glycemic load of ≥20 is considered high, 11–19 is considered medium, and ≤10 is considered low. Among these 92,088 women from Nurses’ Health Study II, the mean glycemic load was 119.0 (range, 47.8–222.4). f Average dietary glycemic index (GI) was calculated by summing the products of 1) the carbohydrate content of each food item per serving, 2) the average daily number of servings of that food, and 3) the food’s GI value (derived from available databases and publications) and dividing by total daily carbohydrate content. Foods with a GI value of ≤55 are considered to have a low GI, foods with a value of 56–69 are considered to have a medium GI, and foods with a value of ≥70 are considered to have a high GI. Among these 92,088 women from Nurses’ Health Study II, the mean GI was 52.9 (range, 37.0–64.8).
percentage of total energy using the nutrient density method (16) and categorized into quintiles. Regression models included age in years as the time scale, stratified by calendar time in 2-year intervals. Multivariate models adjusted for race/ethnicity (white or nonwhite), family history of diabetes (yes/no), and various lifestyle factors, including smoking status (never smoker, past smoker, or current smoker of 1–14, 15–24, or ≥25 cigarettes/day), alcohol intake (0, 0.1–4.9, 5.0–14.9, or ≥15 g/day in women; 0, 0.1–4.9, 5.0–29.9, or ≥30 g/day in men), physical activity (3.0, 3.0–8.9, 9.0– 17.9, 18.0–26.9, or ≥27.0 metabolic equivalent-hours/ week), total energy intake (kcal/day; quintiles), and, for women, menopausal status, postmenopausal hormone use (NHS and NHS II; premenopausal, postmenopausal with no history Am J Epidemiol. 2016;183(8):715–728
of hormone replacement, or postmenopausal with current hormone replacement), and oral contraceptive use (NHS II only; never user, past user, or current user). Results were also adjusted for percentages of energy derived from trans- fat, saturated fat, monounsaturated fat, and polyunsaturated fat; dietary fiber; dietary cholesterol; and glycemic index, and included mutual adjustment for percentages of energy derived from animal protein and vegetable protein (quintiles). Since body weight may partly mediate the association between protein intake and risk of T2D (21), we subsequently adjusted for body mass index (BMI), defined as weight (kg)/ height (m)2 (
