Introduction
Positive effects of breast-feeding have mostly been attributed to the health of the child [
1]. Other findings suggest that mothers can benefit from breast-feeding as well [
2,
3]. Although three prospective studies observed an inverse association between prolonged breast-feeding and the incidence of type 2 diabetes [
4,
5], data are still scarce, and the underlying mechanisms by which breast-feeding lowers diabetes risk remain unclear. During gestation, enormous changes occur in women’s metabolism to ensure sufficient supply to the fetus. Breast-feeding is thought to ‘reset’ these metabolic changes [
6]. Exclusive breast-feeding is accompanied by an increased energy requirement of about 2,000 kJ/day [
7], but the impact of breast-feeding on longer-term weight development post partum remains unclear [
8‐
10]. Studies also suggest higher insulin sensitivity and glucose tolerance in breast-feeding women [
7,
9]. Although these observations contribute to the hypothesis that breast-feeding reduces the risk of type 2 diabetes, systematic evaluation of different pathways is so far lacking.
Our aims were therefore threefold: first, we evaluated the association between breast-feeding and diabetes risk in a large prospective study; second, we systematically reviewed other prospective studies evaluating this hypothesis; third, we investigated possible mechanisms underlying the association between breast-feeding and maternal risk of type 2 diabetes by evaluating a large set of diabetes-related markers reflecting insulin sensitivity or lipid metabolism as well as markers of liver fat accumulation and inflammation.
Results
Table
1 shows baseline characteristics of the sub-cohort by cumulative duration of breast-feeding. Women who breast-fed longer tended to be older and were more likely to be married and better educated. Longer duration of breast-feeding was associated with less smoking and higher physical activity, but occupation and nutritional factors showed no association. Longer duration of breast-feeding was also related to a greater age at birth of the last child, a larger number of children, longer duration of breast-feeding per child, and lower use of oral contraceptive. Women who had never breast-fed had higher BMI and waist circumference levels at the baseline examination compared with women who had. The biomarkers showed no major differences across breast-feeding categories at baseline. Baseline characteristics of the sub-cohort (with and without internal cases) and all incident type 2 diabetes cases are provided in ESM Table
2.
Table 1
Baseline characteristics of parous women by duration of breast-feeding in EPIC-Potsdam
n
| 159 | 148 | 184 | 304 | 264 | |
Age at baseline, median (IQR) | 47.0 (15.0) | 46.0 (15.0) | 47.0 (16.5) | 47.0 (17.0) | 49.0 (19.0) | 0.4612 |
Marital status, % | | | | | | 0.3981 |
Unmarried | 6.92 | 5.41 | 4.35 | 2.30 | 3.41 | |
Married | 71.1 | 74.3 | 77.2 | 76.6 | 80.3 | |
Divorced | 17.6 | 16.2 | 13.6 | 16.8 | 14.4 | |
Widowed | 4.40 | 4.05 | 4.89 | 4.28 | 1.89 | |
Education, % | | | | | | 0.0148 |
No vocational training or in training | 4.40 | 2.03 | 4.89 | 3.62 | 6.82 | |
Vocational training | 41.5 | 44.6 | 37.5 | 35.2 | 31.8 | |
Technical school | 28.3 | 33.8 | 28.3 | 32.6 | 25.0 | |
Technical college, university | 25.8 | 19.6 | 29.4 | 28.6 | 36.4 | |
Occupation, % | | | | | | 0.1177 |
Sedentary occupation | 65.4 | 68.9 | 55.4 | 63.8 | 61.7 | |
Standing occupation and (heavy) manual work | 34.6 | 31.1 | 44.6 | 36.2 | 38.3 | |
Smoking, % | | | | | | 0.1801 |
Never smoker | 54.7 | 54.1 | 57.6 | 60.5 | 61.4 | |
Ex-smoker | 25.2 | 27.0 | 25.0 | 20.7 | 24.2 | |
Smoker <20 units/day | 13.8 | 14.2 | 16.9 | 15.8 | 11.7 | |
Smoker ≥20 units/day | 6.29 | 4.73 | 0.54 | 2.96 | 2.65 | |
Physical activity (h/week), median (IQR)b
| 2.00 (4.00) | 1.50 (3.50) | 1.50 (3.50) | 2.00 (3.50) | 2.50 (4.25) | 0.0970 |
Coffee consumption (ml/day), median (IQR) | 300 (450) | 450 (300) | 300 (300) | 300 (300) | 300 (450) | 0.0846 |
Red meat intake (g/day), median (IQR) | 27.4 (27.8) | 30.3 (23.2) | 33.7 (24.3) | 31.3 (26.1) | 31.6 (25.2) | 0.6453 |
Whole-grain bread intake (g/day), median (IQR) | 30.3 (62.4) | 25.7 (61.6) | 32.3 (62.1) | 29.8 (65.2) | 36.9 (62.0) | 0.2935 |
Alcohol (g/day), median (IQR) | 5.07 (8.66) | 5.36 (10.6) | 5.25 (7.82) | 5.19 (7.87) | 5.02 (8.67) | 0.9801 |
Age at birth of first child, median (IQR) | 23.0 (5.00) | 23.0 (4.50) | 23.0 (4.00) | 23.0 (4.00) | 23.0 (5.00) | 0.0813 |
Age at birth of last child, mean (SD) | 26.6 (5.03) | 25.9 (4.12) | 26.2 (4.14) | 26.9 (4.24) | 28.2 (4.14) | <0.0001 |
Number of children, median (IQR) | 1 (1) | 2 (1) | 2 (1) | 2 (1) | 2 (1) | <0.0001 |
Breast-feeding duration per child in months, median (IQR) | | 0.28 (0.38) | 0.81 (0.50) | 1.81 (0.97) | 4.71 (3.08) | <0.0001 |
Use of oral contraceptives, % | | | | | | <0.0001 |
No use | 18.9 | 12.2 | 10.9 | 15.1 | 17.1 | |
≤5 years | 13.8 | 14.2 | 16.9 | 15.8 | 28.4 | |
>5 and ≤10 years | 12.0 | 16.9 | 18.5 | 17.1 | 20.1 | |
>10 years | 55.4 | 56.8 | 53.8 | 52.0 | 34.5 | |
BMI at age of 25 years (kg/m2), median (IQR) | 22.3 (3.28) | 21.6 (3.06) | 22.1 (2.97) | 21.4 (3.04) | 21.6 (3.40) | 0.0032 |
BMI at baseline (kg/m2), median (IQR) | 25.8 (6.56) | 24.2 (4.98) | 24.9 (5.67) | 24.3 (5.25) | 24.6 (5.80) | 0.0056 |
Waist circumference (cm), median (IQR) | 80.0 (17.0) | 78.0 (14.5) | 78.3 (16.0) | 77.5 (13.8) | 79.0 (16.0) | 0.0168 |
Triacylglycerols (mmol/l), median (IQR) | 1.20 (0.92) | 1.02 (0.59) | 1.03 (0.70) | 1.09 (0.68) | 1.01 (0.62) | 0.0060 |
HDL-cholesterol (mmol/l), mean (SD) | 1.52 (0.37) | 1.51 (0.39) | 1.52 (0.37) | 1.57 (0.39) | 1.57 (0.37) | 0.2561 |
LDL-cholesterol (mmol/l), mean (SD) | 3.00 (0.96) | 2.90 (0.79) | 2.95 (0.87) | 3.06 (0.85) | 3.11 (0.95) | 0.2215 |
CRP (nmol/l), median (IQR) | 9.90 (25.4) | 7.71 (26.0) | 9.90 (28.8) | 7.71 (18.8) | 6.67 (14.4) | 0.0697 |
Fetuin-A (μg/ml), mean (SD) | 268 (62.4) | 268 (64.9) | 274 (60.4) | 267 (62.4) | 265 (64.6) | 0.6200 |
GGT (μkat/l), median (IQR) | 0.23 (0.23) | 0.23 (0.20) | 0.21 (0.19) | 0.21 (0.17) | 0.19 (0.20) | 0.2433 |
Adiponectin (μg/ml), median (IQR) | 8.52 (5.81) | 9.29 (5.20) | 9.42 (5.77) | 8.93 (5.52) | 9.43 (4.78) | 0.1943 |
We used multivariate linear regression models to evaluate the covariate-adjusted associations between different biomarkers and cumulative duration of breast-feeding (ESM Table
3). No single biomarker was strongly associated with breast-feeding duration. However, we observed an inverse relation between breast-feeding duration and triacylglycerols, which became non-significant after adjustment for anthropometry. In contrast, HDL-cholesterol and adiponectin were positively associated.
Table
2 presents associations between duration of breast-feeding and risk of type 2 diabetes. In age-adjusted models, mothers who had ever breast-fed had a lower risk of type 2 diabetes than mothers who had never breast-fed (HR 0.62; 95% CI 0.43, 0.88). This inverse association was also observable when different durations of breast-feeding were evaluated, with women who had breast-fed for ≥6 months having the lowest risk compared with women who had never breast-fed (HR 0.46; 95% CI 0.29, 0.73). Adjustment for lifestyle and other reproductive factors strengthened these associations (HR 0.31 [95% CI 0.18, 0.55] for women who had breast-fed for ≥6 months compared with those who had never breast-fed).
Table 2
HRs (95% CI) for type 2 diabetes by duration of breast-feeding, EPIC-Potsdam study
n cases | 49 | 177 | 49 | 31 | 38 | 66 | 42 | 226 |
Age-adjusted HR (95% CI) | 1 (Ref.) | 0.62 (0.43, 0.88) | 1 (Ref.) | 0.76 (0.46, 1.27) | 0.65 (0.40, 1.04) | 0.67 (0.44, 1.03) | 0.46 (0.29, 0.73) | 0.84 (0.68, 1.04) |
Adjusted HR (95% CI) model I | 1 | 0.66 (0.45, 0.97) | 1 | 0.87 (0.51, 1.51) | 0.68 (0.40, 1.15) | 0.75 (0.48, 1.20) | 0.46 (0.28, 0.76) | 0.83 (0.67, 1.03) |
Adjusted HR (95% CI) model II | 1 | 0.55 (0.36, 0.82) | 1 | 0.80 (0.46, 1.37) | 0.57 (0.33, 0.98) | 0.62 (0.39, 1.00) | 0.31 (0.18, 0.55) | 0.73 (0.56, 0.94) |
Adjusted HR (95% CI) model III | 1 | 0.59 (0.39, 0.89) | 1 | 0.87 (0.50, 1.52) | 0.57 (0.33, 0.98) | 0.69 (0.43, 1.13) | 0.34 (0.19, 0.60) | 0.73 (0.57, 0.95) |
Adjusted HR (95% CI) model IV | 1 | 0.77 (0.47, 1.25) | 1 | 1.16 (0.62, 2.19) | 0.77 (0.43, 1.41) | 0.82 (0.47, 1.41) | 0.47 (0.25, 0.89) | 0.80 (0.61, 1.04) |
While the lower risk in women who had ever breast-fed compared with women who had never breast-fed remained after BMI at the age of 25 years had been accounted for (HR 0.59; 95% CI 0.39, 0.89), adjustment for BMI and waist circumference at baseline moderately attenuated the association, which became—although still inverse—non-significant (HR 0.77; 95% CI 0.47, 1.25). Similarly, a longer duration of breast-feeding remained associated with a decreased risk of type 2 diabetes after anthropometric characteristics had been accounted for (HR per 6 months 0.80; 95% CI 0.61, 1.04). Women who had breast-fed for 6 months or longer had an HR of 0.47 (95% CI 0.25, 0.89) compared with women who had never breast-fed.
In sensitivity analysis (ESM Table
4), we stratified the data by number of children. Mothers with one child had a confounder-adjusted HR per 6 months of 0.83 (95% CI 0.39, 1.76), with two children they had an HR of 0.77 (95% CI 0.52, 1.15), and with three children the HR was 0.72 (95% CI 0.43, 1.19). We further analysed substrata according to the time lap between last birth and baseline examination (ESM Table
4). The inverse association between breast-feeding and diabetes appeared to be slightly stronger among women who gave birth less than 20 years before baseline (HR per 6 months 0.67; 95% CI 0.31, 1.46) than among women who gave birth to their latest child 20 years or longer before baseline (HR 0.73; 95% CI 0.55, 0.97).
To investigate the role of body mass in early adulthood as a potential confounder vs mediator, we used BMI at age 25 as a proxy measure and stratified the analysis by the age at which women had given birth to their first child (considering an age of either ≤24 or ≥26 to define subgroups) (ESM Table
4). Breast-feeding was associated with lower diabetes risk irrespective of the age at first birth, and adjustment for self-reported BMI at age 25 had no appreciable effect on the strength of association in both strata. In analysis stratified by education, an important confounding factor, we observed no major difference when evaluating breast-feeding categories (ESM Table
4). The HR for duration of breast-feeding per 6 months in women without a university degree was 0.68 (95% CI 0.50, 0.91), while there was no association observable among women with a university degree (HR 0.94; 95% CI 0.59, 1.51).
Further adjustment for biomarkers (Table
4) attenuated the association, with the strongest attenuation observed with adjustment for lipid biomarkers (HR per 6 months 0.85; 95% CI 0.66, 1.09) and adiponectin (HR 0.84; 95% CI 0.64, 1.10). Accounting for all biomarkers simultaneously in one model largely attenuated the association (HR per 6 months 0.89; 95% CI 0.68, 1.16; HR for ever vs never breast-fed 0.95; 95% CI 0.59, 1.53).
Table 4
HRs (95% CI) for type 2 diabetes by duration of breast-feeding with adjustment for biochemical mediators, EPIC-Potsdam study
n cases | 49 | 177 | 49 | 31 | 38 | 66 | 42 | 226 |
Model I | 1 | 0.77 (0.47, 1.25) | 1 | 1.16 (0.62, 2.19) | 0.77 (0.43, 1.41) | 0.82 (0.47, 1.41) | 0.47 (0.25, 0.89) | 0.80 (0.61, 1.04) |
Model I + HDL, LDL, triacylglycerols | 1 | 0.88 (0.55, 1.42) | 1 | 1.38 (0.71, 2.69) | 0.93 (0.51, 1.67) | 0.91 (0.53, 1.57) | 0.55 (0.29, 1.02) | 0.85 (0.66, 1.09) |
Model I + CRP | 1 | 0.73 (0.45, 1.18) | 1 | 1.11 (0.59, 2.08) | 0.72 (0.39, 1.32) | 0.77 (0.45, 1.33) | 0.47 (0.25, 0.88) | 0.81 (0.62, 1.06) |
Model I + fetuin-A, GGT | 1 | 0.78 (0.48, 1.26) | 1 | 1.24 (0.66, 2.33) | 0.78 (0.43, 1.42) | 0.83 (0.48, 1.42) | 0.47 (0.24, 0.90) | 0.81 (0.62, 1.06) |
Model I + adiponectin | 1 | 0.91 (0.57, 1.45) | 1 | 1.64 (0.90, 3.00) | 0.91 (0.51, 1.64) | 0.93 (0.54, 1.59) | 0.58 (0.31, 1.07) | 0.84 (0.64, 1.10) |
Model I + HDL, LDL, triacylglycerols + CRP + fetuin-A, GGT + adiponectin | 1 | 0.95 (0.59, 1.53) | 1 | 1.74 (0.91, 3.32) | 1.00 (0.55, 1.83) | 0.91 (0.53, 1.58) | 0.62 (0.33, 1.16) | 0.89 (0.68, 1.16) |
Discussion
In the present study, breast-feeding was associated with a lower risk of type 2 diabetes. This association was independent of potential confounding sociodemographic, lifestyle and reproductive risk factors. Furthermore, meta-analysis of cohort studies indicated an inverse association. Adjustment for BMI at baseline had little effect on this relationship, while accounting for an at-risk metabolic profile in adult life, reflected by several biomarkers, largely explained the association between breast-feeding and type 2 diabetes.
A systematic literature search and meta-analysis including results from EPIC-Potsdam as well as findings from previous prospective cohort studies [
4,
5] suggest that longer breast-feeding duration may be associated with a lower risk of type 2 diabetes. Included studies showed good quality characterised by their prospective design, a comparable breast-feeding assessment, detailed adjustment sets and low rates of loss to follow-up (ESM Table
1). However, there was high heterogeneity between the studies, which complicates the drawing of general conclusions. The women in these four cohorts showed quite different breast-feeding patterns. German women breast-fed for a comparatively short time. Further limitations come from the imprecision of assessing breast-feeding behaviour on the basis of self-reports. Some studies found good validity of self-reported breast-feeding history after more than 20 years compared with medical records [
19,
20]. Others criticise the retrospective collection of breast-feeding data via questionnaires [
21]. Factors such as socioeconomic status and the number of children could affect the mother's memory [
22‐
24]. Furthermore, breast-feeding was self-reported, irrespective of additional feeding, and therefore was not stratified as exclusive or non-exclusive breast-feeding, which may be of importance when evaluating short breast-feeding periods [
25].
We cannot exclude residual confounding. Breast-feeding is highly related to sociodemographic factors. Adjusting for marital status, educational level and occupation and stratification for educational level had no influence on the association in our study. However, we cannot rule out the possibility that other correlates of breast-feeding, e.g. related to income, could explain our observation.
In addition, there is the possibility that cases remained undiagnosed during follow-up and were therefore misclassified as false-negative. However, this misclassification should not bias the associations given that false-positive case definitions should have been rare in most cohorts because of the verification procedures (EPIC-Potsdam) or the high accuracy of self-reports among nurses (US cohorts) [
26].
Finally, the Egger test provided evidence of publication bias. We addressed this observation by using fixed-effects models in the meta-analysis [
27]. However, only a few studies were available for this meta-analysis, and exclusion of one study that contributed the lowest weight but showed the strongest associations did not alter the overall result of the meta-analysis (data not shown).
The underlying mechanisms by which breast-feeding could lower diabetes risk are still unknown, although several hypotheses of dependence on weight development and metabolic pathways have been suggested.
High prepregnancy BMI and extreme weight gain during pregnancy have been associated with early termination of breast-feeding [
28,
29]. Therefore, a shorter duration of breast-feeding may be a consequence, rather than a cause, of increased body weight, supporting the hypothesis that BMI acts as a confounder. However, other studies suggest that weight changes may mediate the association between breast-feeding and incident type 2 diabetes [
6]. Breast-feeding is associated with higher energy requirements [
7]. These requirements are compensated for by a higher energy intake and less physical activity during the first 3 months after delivery. Later on, women begin to mobilise fat stores accumulated through pregnancy [
30,
31]. During breast-feeding periods, significantly higher lipolysis in the femoral region has been observed [
32]. Therefore femoral adipose tissue acts as an important source of energy during lactation. Previous studies have obtained controversial results on the importance of weight change. Butte and Hopkinson [
8] reviewed 17 prospective studies conducted in industrialised and developing countries. Most studies demonstrated no difference in weight change between breast-feeding and non-breast-feeding women. However, participants were only followed-up for up to 6 months post partum. While some studies support these findings [
33], others found breast-feeding to be associated with lower post partum weight retention [
10,
34]. In our study, adjusting for self-reported BMI at the age of 25 showed only a slight effect on the relationship. Although adjustment for BMI and waist circumference assessed at baseline had stronger attenuating effects, an inverse association between breast-feeding and diabetes remained, especially in the longest-duration breast-feeding category. It can be discussed whether BMI assessed many years after breast-feeding acts as a confounder or a mediator in this context. Although less weight gain with breast-feeding is a plausible mediator, BMI at baseline may also partly reflect prepregnancy weight status or other confounding factors associated with body fatness and which may have insufficiently been controlled for by using lifestyle confounders and retrospectively self-reported BMI at age 25.
While adjustment for biomarkers revealed an attenuation of results in our analysis, another cohort study found no clear dose–response associations between breast-feeding and maternal glucose and lipid metabolism or inflammatory markers 3 years post partum [
35].
Still, prolonged breast-feeding seems to affect lipoprotein profiles. In a 3-year prospective study, breast-feeding women had higher HDL-cholesterol levels than non-breast-feeding mothers [
9]. Others reported a decline in triacylglycerols after delivery, which was more rapid in breast-feeding mothers [
36]. However, at the end of the breast-feeding period, blood lipids had again reached their baseline level [
37]. We found a weak association between breast-feeding duration and triacylglycerols as well as with HDL-cholesterol in adult life in covariate-adjusted linear regression models. Accounting for them in Cox models attenuated the association of breast-feeding with diabetes.
In addition, we investigated mediating effects related to insulin sensitivity during adulthood. Thereby we saw a positive association between cumulative breast-feeding duration and adiponectin levels. Although we observed an attenuation by accounting for adiponectin in our mediator analysis, others did not find a linear association between breast-feeding duration and adiponectin levels 3 years post partum [
38]. One study found lower fasting insulin levels in breast-feeding than non-breast-feeding women at 6 months post partum, although there was no difference in fasting glucose levels [
7]. Others observed a trend for increased fasting insulin levels and HOMA-IR in non-breast-feeding compared with breast-feeding mothers [
9]. Animal studies support this. During lactation, the insulin sensitivity changes are tissue- specific [
39] as a result of alterations in signal transmission after binding of insulin to its receptor [
40]. For instance, the number of insulin receptors of mammary epithelial cells is increased in mice [
41]. Therefore the mammary gland is more insulin-sensitive than adipose tissue or muscle to ensure an adequate supply of nutrients during lactation [
39,
42].
The long time lag (>20 years) between the last breast-feeding period and blood collection at the baseline examination might mask favourable effects of breast-feeding on metabolic variables in our analysis. However, adjustment for biomarkers still largely attenuated the association between breast-feeding and diabetes in our study. This suggests that breast-feeding leads to an overall more favourable metabolic risk profile in the long term. Previous studies discussed an effect of breast-feeding on long-term weight development as the main factor for decreased diabetes risk. However, our study revealed that BMI at baseline examination only partly explains this association.
In conclusion, the evidence from this study and previous studies, summarised by means of meta-analysis, suggests that longer breast-feeding duration may be related to lower maternal type 2 diabetes risk. However, the role of body weight as a mediator or confounder remains uncertain.