Abstract

RESEARCH DESIGN AND METHODS

The Nurses' Health Study II (NHSII) is an ongoing prospective study of U.S. female nurses aged 25–42 years. Follow-up is conducted using biennial questionnaires from 1989 to 2005. A total of 109,172 participants remained after exclusion for diagnosis of any type of diabetes, cancer, or cardiovascular disease at baseline (2%) or for missing information on childhood body shape (2.5%), BMI at age 18 years (0.7%), or baseline BMI (0.3%). This study was approved by the institutional review board of the Partners Health Care System (Boston, MA).

RESULTS

There were 3,307 incident cases of type 2 diabetes over 16 years of follow-up, for an incidence rate of 197 cases per 100,000 person-years. Table 1 shows the characteristics for all women and by reported childhood somatotype at age 10 years categorized from 1 to ≥6. Women with somatotype 1 at age 10 years were more likely to be of minority race, to have birth weight <5.5 pounds, to have been born prematurely, and to have had a later age at menarche; whereas women with somatotype ≥6 were more likely to have birth weight >10 pounds, to actively smoke, to be nulliparous, and to have had a positive parental history of diabetes. Similar associations were found at age 5 years. Somatotypes at ages 5, 10, and 20 years were associated with each other. Of the women reporting somatotype ≥6 at age 10 years, 41% reported somatotype ≥6 at age 5 years, and 28% at age 20 years. Weight and BMI were also positively associated with somatotype; a 1–2 kg/m² increase in adolescent or adult BMI was observed per increase in somatotype category.

Somatotypes at ages 5, 10, and 20 years and BMI at age 18 years (Table 2) were all significantly and positively associated with diabetes risk (all P_trend < 0.0001). Compared with having a somatotype of 2, the RR of diabetes associated with a somatotype ≥6 at age 5 was 2.19 (95% CI 1.79–2.67), at age 10 years was 2.57 (2.20–3.01), and at age 20 years was 5.67 (4.92–6.54). The RR among women who reported a BMI at age 18 >30 kg/m² was 8.72 (7.58–10.02) compared with women with a BMI of 18–19 kg/m². Among a subgroup not missing birth weight information (n = 87,175), adjustment for childhood factors including birth weight did not substantially change these estimates.

Table 2

Childhood and adolescent size and RR (95% CI) of type 2 diabetes in adulthood in NHSII

	PY	Cases of diabetes	Age-adjusted	Model 2*	Model 3†
Age 5 somatotype
1	417,996	756	1.13 (1.02–1.25)	1.05 (0.95–1.16)	1.01 (0.90–1.13)
2	533,990	807	1.00	1.00	1.00
3	396,742	774	1.28 (1.15–1.41)	1.25 (1.14–1.38)	1.20 (1.08–1.34)
4	212,080	568	1.72 (1.54–1.91)	1.65 (1.48–1.84)	1.63 (1.45–1.84)
5	88,885	289	2.04 (1.78–2.33)	1.85 (1.62–2.12)	1.80 (1.55–2.09)
≥6	26,619	113	2.61 (2.14–3.18)	2.19 (1.79–2.67)	2.00 (1.59–2.51)
Ptrend			<0.0001	<0.0001	<0.0001
Age 10 somatotype
1	317,663	441	1.04 (0.92–1.17)	0.98 (0.87–1.11)	0.96 (0.83–1.10)
2	516,959	649	1.00	1.00	1.00
3	377,995	716	1.50 (1.35–1.67)	1.49 (1.34–1.66)	1.49 (1.32–1.67)
4	262,795	765	2.27 (2.05–2.52)	2.17 (1.95–2.41)	2.15 (1.92–2.42)
5	149,808	525	2.68 (2.39–3.01)	2.45 (2.18–2.75)	2.52 (2.21–2.86)
≥6	51,091	211	3.09 (2.64–3.61)	2.57 (2.20–3.01)	2.41 (2.02–2.88)
Ptrend			<0.0001	<0.0001	<0.0001
Age 20 somatotype
1	75,871	83	0.97 (0.77–1.23)	0.91 (0.72–1.15)	0.96 (0.73–1.25)
2	443,762	443	1.00	1.00	1.00
3	633,117	928	1.54 (1.37–1.72)	1.52 (1.36–1.70)	1.55 (1.36–1.76)
4	346,718	924	2.83 (2.53–3.17)	2.71 (2.42–3.04)	2.78 (2.44–3.16)
5	122,949	564	4.85 (4.28–5.49)	4.42 (3.90–5.01)	4.79 (4.16–5.51)
≥6	53,897	365	7.24 (6.30–8.32)	5.67 (4.92–6.54)	6.17 (5.26–7.23)
Ptrend			<0.0001	<0.0001	<0.0001
BMI at age 18
<18 kg/m2	158,206	164	1.13 (0.94–1.35)	1.06 (0.88–1.26)	1.06 (0.86–1.30)
18–<19 kg/m2	507,641	458	1.00	1.00	1.00
20–<22 kg/m2	530,083	727	1.52 (1.35–1.71)	1.49 (1.32–1.67)	1.51 (1.32–1.72)
22–<25 kg/m2	312,876	848	3.02 (2.69–3.38)	2.79 (2.49–3.13)	2.86 (2.51–3.25)
25–<30 kg/m2	127,826	721	6.32 (5.62–7.11)	5.32 (4.72–5.99)	6.10 (5.35–6.97)
≥30 kg/m2	39,682	389	11.55 (10.08–13.23)	8.72 (7.58–10.02)	9.26 (7.92–10.82)
Ptrend			<0.0001	<0.0001	<0.0001

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Data are RR (95% CI) unless otherwise indicated. n = 109,172.

*Model 2 adjusted for age, race, smoking status, parental history of diabetes, parity, age at first birth, and adult physical activity.

†Model 3 adjusted for age, race, parental history of diabetes, birth weight, multiple birth, prematurity, age of menarche (n = 87,175, case subjects = 2,681 in subgroup not missing birth weight information). PY, person-years.

Type 2 diabetes risk increased among women who reported increases in size whether by somatotype between ages 5 and 10 and between ages 10 and 20 or by weight gain since age 18 (Table 3). Compared with women reporting no change in somatotype, one or more unit increases in somatotype at these ages were associated with approximately twice the risk, whereas decreases in somatotype at these ages were associated with reduced risk. These associations were strengthened after adjustment for earlier somatotype. Weight gain since age 18, which is the difference between baseline weight and weight at age 18, was also significantly associated with risk of diabetes. Compared with women who had little change in weight (i.e., ±4.9 pounds), even a weight gain of 5–8 pounds doubled risks, whereas weight gain of >25 pounds increased risk by >20 times. Weight loss was associated with increased risk until after adjustment for BMI at age 18 to account for greater weight loss being associated with larger adolescent body size. In analyses stratified by BMI at age 18 years, weight loss of ≥10 pounds was significantly associated with a reduced risk of diabetes among women who were overweight (RR 0.45 [95% CI 0.22–0.91]) or obese (0.45 [0.28–0.72]) in adolescence but not among those who were lean (1.72 [0.76–3.90]).

Table 3

Changes in size and RR (95% CI) of type 2 diabetes in NHSII

	PY	Cases of diabetes	Age-adjusted	Model 2*	Model 3†
Age 5–10 years
Decrease	19,572	29	0.93 (0.65–1.35)	0.95 (0.65–1.37)	0.63 (0.43–0.91)
No change	1,235,621	1,943	1.00	1.00	1.00
+1	310,086	950	2.00 (1.85–2.16)	1.86 (1.72–2.01)	1.89 (1.75–2.05)
≥2	111,033	385	2.16 (1.93–2.41)	1.95 (1.75–2.18)	2.19 (1.95–2.45)
Age 10–20 years
Decrease	109,956	196	1.04 (0.89–1.21)	1.00 (0.86–1.17)	0.53 (0.45–0.62)
No change	758,071	1,294	1.00	1.00	1.00
+1	621,736	1,274	1.27 (1.18–1.37)	1.22 (1.12–1.31)	1.86 (1.71–2.02)
≥2	186,550	543	1.89 (1.70–2.08)	1.72 (1.55–1.90)	2.87 (2.58–3.20)
Weight change since age 18
Loss ≥10 pounds	31,322	61	5.44 (4.11–7.20)	4.48 (3.38–5.94)	1.26 (0.93–1.71)
Loss 5–<10 pounds	58,170	41	2.02 (1.45–2.82)	1.84 (1.32–2.56)	1.17 (0.84–1.63)
No change (±5 pounds)	714,230	249	1.00 (referent)	1.00 (referent)	1.00 (referent)
Gain 5–<8 pounds	226,283	169	2.05 (1.69–2.49)	1.99 (1.64–2.42)	2.10 (1.73–2.55)
Gain 8–<11 pounds	182,828	219	3.24 (2.70–3.88)	3.05 (2.54–3.66)	3.10 (2.58–3.72)
Gain 11–<15 pounds	177,159	437	6.54 (5.60–7.65)	5.99 (5.12–7.00)	5.78 (4.94–6.77)
Gain 15–<20 pounds	114,008	477	10.92 (9.36–12.74)	9.67 (8.28–11.30)	8.74 (7.48–10.20)
Gain 20–<25 pounds	76,539	464	15.72 (13.46–18.36)	13.42 (11.48–15.70)	11.11 (9.49–13.00)
Gain ≥25 pounds	95,774	1,190	32.28 (28.11–37.06)	27.13 (23.57–31.23)	20.41 (17.70–23.52)

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Data are RR (95% CI) unless otherwise indicated. n = 109,172.

*Model 2 adjusted for age, race, smoking status, parental history of diabetes, parity, age at first birth, and adult physical activity.

†Model 2 variables with additional adjustment for earlier size, i.e., somatotype at age 5 and 10 years and BMI at age 18 years. PY, person-years.

The prevalences of overweight by somatotype ≥5 at ages 5, 10, and 20 years were 7, 12, and 11%, respectively. Most of the cases (83%) in the cohort occurred among women who were overweight (BMI ≥25 kg/m²) at baseline with an incidence rate of 562 per 100,000 person-years. Among these women, the prevalences of overweight by somatotype ≥5 were 12, 21, and 23%, respectively.

To evaluate the cumulative effect of being overweight across the life course, women were jointly categorized as being overweight using somatotype at age 10, BMI at age 18 years, and BMI at baseline (with mean age of 34 years) (Fig. 1). Women who were overweight only as an adult (BMI >25 kg/m²) had an adjusted RR of 8.23 (95% CI 7.41–9.15) compared with women who were never overweight at any age. The adjusted RR increased to 15.10 (13.21–17.26) for women who were also overweight at age 10 (somatotype ≥5) and age 18 (BMI >25 kg/m²). However, women who were overweight at age 10 but came off the trajectory and became lean in adulthood (i.e., not being overweight in adulthood) did not have a significantly increased risk of diabetes (1.02, 0.74–1.40).

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Figure 1

Adjusted RRs (95% CI) of type 2 diabetes by somatotype ≥5 at age 10, BMI ≥25 kg/m² at age 18 years, and BMI ≥25 kg/m² at baseline (average age of 34 years), after adjustment for age, race, smoking status, parental history of diabetes, parity, age at first birth, and physical activity. Number of cases of type 2 diabetes for each category: 503 for none; 50 for 10 years; 21 for 18 years; 10 for 10 and 18 years; 1,469 for 34 years; 260 for 10 and 34 years; 664 for 18 and 34 years; and 441 for all ages.

CONCLUSIONS

Among 109,172 women followed for >16 years, somatotypes at ages 5 and 10 years were positively associated with the risk of incident type 2 diabetes after adjustment for major risk factors. However, women who were overweight at age 10 but were lean in adulthood did not have an increased risk of diabetes associated with childhood overweight, underscoring the importance of continued efforts to control adiposity among overweight children.

Limited studies have investigated the long-term consequences of childhood overweight and the risk of type 2 diabetes in adulthood, with inferences hindered by small numbers of case subjects and/or lack of differentiation of diabetes type (2–8). In addition, some studies included case subjects with young age at onset (<30 years old) (3,5,7). One study using data based on records (185 cases) showed that the cumulative incidence of diabetes was doubled (from 4.2 to 8.4%) among women in the largest BMI group at age 11 years (>17.4 kg/m²) compared with those in the smallest BMI group (≤15.3 kg/m²) (4). We found a similar increase in risk using recalled somatotypes. In contrast, a study of older French women using recalled body shape, found an association between childhood thinness and diabetes risk (8). However, this birth cohort vastly differed from the NHSII as demonstrated by >60% of their case subjects having reported extreme leanness (somatotype 1) at age 8 years, possibly due to their nutritional status having been affected by World War II (8). Thus, our findings among a more recent birth cohort may be more relevant.

In addition to absolute size, longitudinal changes through different ages are also relevant to the risk of diabetes (17,18). Under normal development, infants lose weight after 6 months of age and continue doing so until ~5 years of age when their adiposity rebounds (18). Although we were unable to assess age at adiposity rebound, earlier rebound has been associated with increased risk of type 2 diabetes (17), and there is some suggestion that age at rebound matters more than the absolute size of the child at any point in time because it corresponds to weight gain. Our findings of increased risk associated with increases in somatotype and weight gain support the importance of change in size in addition to absolute size. These findings offer support for continued weight reduction efforts across the life span.

Moreover, in analyses of the cumulative effect of overweight through childhood, adolescence, and adulthood, the risk for type 2 diabetes was greatest among women reporting overweight by all three measures, even though the difference was marginal when compared with adolescent overweight. However, women who became lean in adulthood did not have an increased risk of diabetes associated with childhood overweight. Similar observations have been made from other studies of youth overweight and type 2 diabetes or related traits, with findings becoming nonsignificant after accounting for adult BMI (6,19,20). Tracking of metabolic risk factors (e.g., HDL and triglycerides) has also been observed after 21 years of follow-up from childhood (21), supporting the fact that cumulative overweight is linked with prolonged exposure to metabolic irregularities, putting children on the path to β-cell dysfunction earlier. It should be noted that although there is tracking of overweight to obesity from childhood to adulthood, the trajectory is not fixed from youth (18,22). It has been shown that ~10–30% of overweight children do not go on to be overweight as adults with conflicting evidence as to whether there is greater tracking in girls than in boys (22). Conversely, the majority of adults who are overweight were not in childhood (22) as confirmed here with only 12–20% of the overweight women reporting childhood overweight.

Our study had some limitations. Although experts have recommended BMI for the measurement of childhood size (18,23), we used recalled somatotype. Misclassification of childhood size is inevitable. However, two previous studies have shown that the accuracy of recall between childhood somatotypes and recorded childhood BMI did not differ by adult BMI (10,11). The misclassification of childhood size is thus more likely to be nondifferential, which cannot explain the observed positive association between childhood size and type 2 diabetes risk. In addition, our findings were in agreement with those of other studies that used childhood measures of weight and height (4,20). The generalizability of our findings may be limited to white women who comprised >95% of our study population. More research is needed in minority populations who have higher rates of overweight and diabetes (1,24). Last, we cannot rule out the possibility of residual confounding by unmeasured confounders because of the observational nature of the study.

As for strengths, the NHSII included a large number of women (>3,000 case subjects) and adjusted for many risk factors. Follow-up for previous studies required recontacting participants in adulthood with low to moderate success rates (20–60%) (5,7,19,20), whereas we evaluated somatotypes at baseline and >90% of the women remained for follow-up.

In summary, our findings demonstrate that the importance of childhood overweight stems largely from adult overweight. Women who do not continue to be overweight in adulthood do not have increased risks. It remains important then to promote lifestyle changes from youth so that the adverse trajectory could be avoided. Multiple interventions to address childhood overweight have been suggested (23), but these remain to be fully tested.

Acknowledgments

Footnotes

References