In this large population-based study, we found that prolonged lifetime lactation was associated with a more favourable cardiovascular risk profile among women 50 years or younger. Parous women ≤ 50 years who had never lactated were more likely to have developed hypertension, obesity and diabetes than women who had the longest lactation duration. Furthermore, there were strong indications of a dose–response association between the total duration of lactation and a favourable cardiovascular risk profile. Although the largest difference was found for women who had never lactated compared to those who had ever lactated, our analyses showed that the associations remained significant also within the lactation categories. Among women older than 50 years, only waist circumference and possibly diabetes were associated with lactation duration.

Our findings are consistent with recent studies showing that the favourable effects of lactation on maternal metabolic health persist post weaning [7, 8, 11, 17–20] and thereby further supporting the notion that lactation may induce long-term beneficial effects on maternal blood pressure, weight [21], diabetes [6, 18], components of the metabolic syndrome [5, 19] and cardiovascular health [7, 11, 17]. Previous studies have shown that the beneficial effect of lactation on cardiovascular risk factors seems to wane with time since last birth [6, 11, 18]. Adjusting for this period did not change the estimate in our study. On the other hand, the stronger associations observed among women aged ≤ 50 years compared to those aged >50 years could possibly be due to the shorter period since last birth.

The present study was conducted in a large and unselected population with a wide age range and a high participation rate. Breastfeeding was common, and this observation is consistent with other studies showing that breastfeeding rates in Norway are among the highest in industrialised countries [22]. We therefore have a large sample size of women who have lactated. Combined with the standardised measurements of lipids, anthropometric measures and blood pressure, it provides a unique opportunity to study the association of lactation and cardiovascular risk factors and whether differences exist by duration of lactation.

However, the cross sectional study design calls for a cautious interpretation of the findings, as in all observational studies. It is possible that women who breastfeed their children have a better health status, healthier lifestyles and higher socioeconomic status than women who do not breastfeed [23]. Given the high rates of breastfeeding among Norwegian women, the group of women who had never lactated in our study was less than 4% of the entire sample. Thus, it is possible that the group of women who had never lactated in our study differed in major confounders than might be expected in populations where breastfeeding rates are lower.

A previous study among Norwegian women found that maternal age, education and smoking were among the most important factors associated with lactation duration [22]. In addition to these factors, we found significant differences across the lactation categories in the level of physical activity, marital status, and parity in our study. Although all of these factors are known to be associated with risk of cardiovascular disease, adjusting for them did not materially change the estimated associations. However, residual confounding due to unmeasured and unknown factors cannot be ruled out, such as pre-pregnancy and early postpartum health status. Women with gestational diabetes mellitus are at an increased risk of developing type 2 diabetes [24]. Furthermore, gestational diabetes mellitus may have a role in impacting breastfeeding initiation and success, and could thus act as a major confounder. In a recent study, longer duration of lactation was associated with lower incidence of the metabolic syndrome both among women with and without a history of gestational diabetes mellitus, and the findings were particularly striking for women who developed gestational diabetes mellitus during their pregnancy [10]. In our study, women reporting a diagnosis of diabetes prior to first pregnancy were excluded from our analyses. Nevertheless, the lack of data on the history of gestational diabetes mellitus during pregnancy is a limitation of our study.

Moreover, the potential for reverse causation must be considered when interpreting the results from the present study. Obesity [25, 26] and type 1 diabetes [27] have been linked to difficulties with lactation, and hence shorter lactation duration could be a marker for an already existing abnormal metabolic profile influencing whether the women lactate and for how long. Unfortunately, we did not have pre-pregnancy measurements of weight and height and could therefore not adjust for pre-pregnant body mass index. However, when we adjusted for body mass index measured at study participation in supplementary analyses, the adjustments did not change our estimated associations substantially, with the exception of HDL-cholesterol. Obesity may either precede [26] or follow lactation practices. Thus, one may argue that body mass index measured at study participation rather acts as an intermediate factor, and hence should not be adjusted for as a confounder in the analyses.

Diet accounts for much of the variation in coronary heart disease risk [28]. The HUNT2 study was not designed to measure dietary intake, and we had insufficient dietary data to adjust for dietary factors in our analyses. However, previous studies have found that the association between lactation and cardiovascular health persists even after adjustment for dietary intake [5, 7, 11, 17].

Another limitation of the study is the lack of data on lactation intensity. Higher intensity of lactation has been associated with improved fasting glucose and lower insulin levels at 6–9 weeks postpartum in a previous study [29]. Data on lactation intensity could therefore possibly have strengthened our estimates of associations among women with higher, and attenuated the associations among women with lower, lactation intensity. Moreover, lactation was assessed retrospectively. Nevertheless, studies have shown that maternal recall of lactation is fairly valid and reliable [30], even after 20 years [31]. However, even if misclassification should exist, it is not likely to be differential according to cardiovascular risk factors. Our observed estimates are therefore likely to be conservative.

Furthermore, selection bias could have influenced our results. However, a non-responder study showed that the most important reason for non-attending the HUNT2-study in the age group 20–69 was lack of time/moved away, while in those aged 70 years or more, immobilising and frequent follow-up by medical doctor were important reasons [32]. We do not believe that reasons for non-attending were unevenly distributed across the lactation categories, and we find it unlikely that selection bias would have altered the results in our study.

During pregnancy the maternal metabolism is profoundly changed, and the changes that occur could theoretically increase women’s risk of metabolic disease. These changes include accumulation of adipose tissue stores [33], increased insulin resistance [34] and blood pressure, [35] as well as a change of the quantity and quality of circulating lipoproteins [36, 37]. By the end of the pregnancy, LDL cholesterol and triglyceride levels are two to three times higher compared with pre-pregnancy levels. In fact, some studies have shown that increasing parity may increase risk of cardiovascular disease [38, 39]. These studies do not, however, include data on lactation. Our findings of a more favourable cardiovascular risk profile associated with lactation seem to confirm the recent suggestion that lactation could affect risk of metabolic disease by facilitating a faster resetting of the maternal metabolism after pregnancy [40].

Lactation increases a mother’s metabolic expenditure by an estimated 480 kcal/d [41], and although the association between lactation and postpartum weight loss so far remains inconclusive [21, 42–45], lactation could reduce cardiovascular risk by mobilising accumulated fat stores. Furthermore, lactation provides a route for physiologic excretion of large amounts of cholesterol, which could explain the more speedy return of blood lipids to pre-pregnancy levels observed in lactating mothers [3]. Additionally, hormonal effects, such as those of prolactin and oxytocin, may affect maternal blood pressure [46]. Our data among women with an average time since last pregnancy of about 21 years suggest that these favourable changes are persisting on a long term scale and are not limited to the period of lactation. Among women older than 50 years, however, we found no similar linear trend in the association between lifetime duration of lactation and cardiovascular risk factors as in younger women. Still, women > 50 years who had never lactated had a significantly higher body mass index, wider waist circumference, higher lipid and glucose levels and higher prevalence of hypertension, obesity and diabetes compared to women who had lactated. Menopause appears to be a time of transition to increased cardiovascular risk, including adverse changes in serum lipid profile [47]. Hence, the cardiovascular risk alterations occurring during the menopausal transition may dilute the possible beneficial effects of lactation on maternal metabolic health as shown in previous studies [6, 11, 18].

Lactation may also improve insulin sensitivity and glucose tolerance. Insulin levels and insulin/glucose ratios are lower, and carbohydrate use and total energy expenditure are higher, in the lactating women compared to women who do not lactate [41]. Our data suggest a relation between lactation and glucose levels later in life. However, no statistically significant association with life-time duration of lactation could be found in either age group, although the association among women 50 years or younger were close to significant. In contrast, the association between lifetime duration of lactation and the prevalence of diabetes was strong and significant in the younger age group, although not among the older women, further supporting the notion that the possible effect wanes with time since last delivery. These mechanisms, together with our results, indicate that lactation helps women return to pre-pregnant metabolism more quickly post partum, which could in turn affect metabolic disease risk profile later in life.

Our results indicate that lactation may have a considerable impact on cardiovascular risk factors. The difference in systolic/diastolic blood pressure between women 50 years or younger who had never lactated and women who had lactated for 24 months or more is similar to the blood pressure-lowering effect of salt reduction (4/2 mm Hg) among normotensive individuals [48]. Furthermore, it has been estimated that a 10% reduction in serum cholesterol could halve the risk of ischaemic heart disease at age 40 [49], and hence the 5 % difference in total cholesterol levels observed between women 50 years or younger who had never lactated, and women who had lactated more than 24 months, could represent a substantial risk reduction. Also, the 17% difference in triglycerides between women 50 years or younger who had never lactated and women who had lactated 24 months or more must be added to this altered cardiovascular disease risk pattern.