Table 1: Baseline study population characteristics by self-reported daily caffeine intake

None
0 mg/d
(n = 190) / Low
≤200 mg/d
(n = 260) / High
>200 mg/d
(n = 175)
% / % / % / 2test
p-value
Age, years / <0.001
-- 14-19 / 52 / 38 / 15
-- 20-24 / 16 / 20 / 11
-- 25-29 / 17 / 20 / 26
-- 30-34 / 8 / 12 / 26
-- 35-40 / 7 / 9 / 21
Race/Ethnicity / 0.34
-- White/Caucasian / 71 / 77 / 79
-- Black/African American / 9 / 9 / 9
-- Asian/Pacific Islander / 7 / 4 / 5
-- Native American / 1 / 1 / 2
-- Aleutian/Eskimo / 1 / -- / --
-- Hispanic / 2 / 1 / --
-- Other/Unspecified / 11 / 8 / 5
Education ≥ high school / 75 / 75 / 91 / <0.001
Smoking / <0.001
-- Never smoked / 83 / 62 / 42
-- Former smoker / 9 / 12 / 19
-- Current smoker / 7 / 26 / 38
Dietary calcium intakea, mg/d / 0.007
-- <800 / 49 / 58 / 67
-- 800-999 / 15 / 15 / 13
-- 1,000-1,199 / 9 / 11 / 5
-- 1,200 / 27 / 16 / 14
Family history of fracture / 27 / 27 / 24 / 0.74
Current oral contraceptive useb / 17 / 20 / 16 / 0.50
Current DMPA useb / 0.13
-- Non-User / 64 / 54 / 57
-- Initiator (1 injection) / 12 / 10 / 10
-- Prevalent User (2 injections) / 24 / 35 / 33
Ever pregnant / 31 / 46 / 65 / <0.001
Mean (SD) / Mean (SD) / Mean (SD) / F-test
p-value
Caffeine, mg/d / 0 (0) / 91.2 (48.3) / 400.4 (214.2)
Age, years / 22.3 (6.3) / 23.7 (6.6) / 28.3e(6.5) / <0.001
Years since menarche / 9.8 (6.4) / 11.2 (6.6) / 15.9e(6.5) / <0.001
BMIc, kg/m2 / 24.7 (5.4) / 25.5 (5.8) / 26.0 (5.8) / 0.08
Physical activity scored / 79.8 (46.4) / 71.2 (44.9) / 86.9 (63.0) / 0.007
Dietary calcium intakea, mg/d / 946.7 (603.2) / 816.3e(534.3) / 757.4 (426.6) / <0.001
Alcohol intake, g/d / 1.7 (5.2) / 2.7 (5.7) / 5.9 (11.7) / <0.001
Hip BMC, g / 30.5 (5.4) / 30.1 (5.5) / 30.2 (4.9) / 0.65
Spine BMC, g / 59.2 (10.4) / 58.9 (11.1) / 59.1 (10.3) / 0.95
Whole Body BMC, g / 2253.3 (353.2) / 2241.8 (360.6) / 2240.0 (329.4) / 0.92
Hip BMD, g/cm2 / 0.960 (0.127) / 0.939 (0.125) / 0.932f (0.120) / 0.08
Spine BMD, g/cm2 / 1.016 (0.124) / 1.011 (0.120) / 1.028 (0.123) / 0.37
Whole Body BMD, g/cm2 / 1.092 (0.093) / 1.084 (0.090) / 1.081 (0.086) / 0.53

Abbreviations: DMPA = depot medroxyprogesterone acetate; BMI = body mass index; BMC = bone mineral content; BMD = bone mineral density

a Dietary calcium intake was derived from an annual Food Frequency Questionnaire (FFQ).

b Hormonal contraception use was reported on semi-annual health questionnaires.

c Height and weight were measured in the clinic at each visit, and the data collected were used to compute BMI.

d A weight-bearing physical activity score was derived from self-reported participation in 17 specific activities and up to four write-in activities, each assigned a value representing the degree to which the activity was weight-bearing: 0=none/low, 1=medium, or 2=high. The physical activity score was computed by summing the weight-bearing value, multiplied by the frequency of the activity for all reported activities.

e Significantly different compared to referent group (0 mg/d), p<0.05.

f Significantly lower compared to referent group (0 mg/d), p=0.036.

Table 2: Mean baseline BMC and BMD by self-reported daily caffeine intake among DMPA users and DMPA non-usersa

DMPA users (n = 263)
None
0 mg/d
(n = 69) / Low
≤200 mg/d
(n = 119) / High
>200 mg/d
(n = 75)
BMC, g / mean (95% CI) / mean (95% CI) / mean (95% CI) / Test for trend
p-value
-- Hip / 29.7 (28.5-31.0) / 29.4 (28.5-30.3) / 28.4 (27.2-29.6) / 0.16
-- Spine / 59.9 (57.2-62.7) / 57.2 (55.3-59.1) / 54.5 (51.9-57.1)b / 0.009
-- Whole Body / 2262.3 (2177.0-2347.6) / 2214.5 (2181.7-2301.2) / 2144.7 (2064.3-2225.1) / 0.06
BMD, g/cm2 / mean (95% CI) / mean (95% CI) / mean (95% CI) / Test for trend
p-value
-- Hip / 0.946 (0.918-0.974) / 0.919 (0.889-0.939) / 0.918 (0.891-0.945) / 0.21
-- Spine / 1.019 (0.989-1.049) / 0.994 (0.973-1.015) / 0.984 (0.956-1.012) / 0.12
-- Whole Body / 1.090 (1.066-1.112) / 1.080 (1.064-1.096) / 1.068 (1.047-1.090) / 0.20
DMPA non-users (n = 363)
None
0 mg/d
(n = 122) / Low
≤200 mg/d
(n = 141) / High
>200 mg/d
(n = 100)
BMC, g / mean (95% CI) / mean (95% CI) / mean (95% CI) / Test for trend
p-value
-- Hip / 31.0 (30.0-32.0) / 30.5 (29.6-31.4) / 31.2 (30.1-32.3) / 0.89
-- Spine / 59.6 (57.6-61.6) / 60.1 (58.3-62.0) / 60.6 (58.3-63.0) / 0.54
-- Whole Body / 2274.8 (2216.6-2333.0) / 2237.4 (2185.8-2289.0) / 2257.7 (2190.5-2325.0) / 0.66
BMD, g/cm2 / mean (95% CI) / mean (95% CI) / mean (95% CI) / Test for trend
p-value
-- Hip / 0.956 (0.934-0.978) / 0.949 (0.930-0.969) / 0.955 (0.930-0.980) / 0.94
-- Spine / 1.026 (1.004-1.048) / 1.021 (1.001-1.040) / 1.036 (1.011-1.061) / 0.63
-- Whole Body / 1.092 (1.076-1.108) / 1.084 (1.070-1.098) / 1.088 (1.070-1.107) / 0.74

Abbreviations: DMPA = depot medroxyprogesterone acetate; BMC = bone mineral content; BMD = bone mineral density

a Adjusted for age (continuous), race/ethnicity (indicator variables for: White/Caucasian, Black/African-American, and Other), body mass index (BMI) (continuous), physical activity (continuous), dietary calcium intake (continuous), alcohol (continuous), smoking (binary, current Y/N), family history of fracture at any point prior to or during follow-up (binary, Y/N), prior pregnancy (binary, Y/N).

b Significantly lower compared to referent group (0 mg/d), p=0.009.