Lactation and Cardiovascular Health in Mothers: The Nord-Trøndelag Health Study (HUNT), Norway
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Background Cardiovascular disease is the most common cause of death in women in all European countries and accounts for 26% of deaths before the age of 65. Thus it is important to identify behaviors that may modify women’s risk of cardiovascular disease. Lactation is a factor unique to women that involves hormonal changes and may affect maternal metabolic health. Short-term studies have reported a more speedy return of the pregnancy-induced raised lipid levels in lactating mothers after delivery. Still, few studies have examined the association between lactation and maternal long-term cardiovascular health. Furthermore, most of these have relied on maternal recall of breastfeeding history. Aims The aims of this thesis were to investigate:The association of lifetime duration of lactation with various risk factors for cardiovascular diseaseThe association of lifetime duration of lactation with cardiovascular mortalityThe accuracy of long-term maternal recall of breastfeeding durationMethod In paper I, Cross-sectional data from more than 20,000 women attending the second Nord-Trøndelag Health Study (HUNT2) was used to study the association of lifetime duration of lactation with various risk factors for cardiovascular disease. These data were furthermore linked to the Cause of Death Registry, enabling us to study the association of lifetime duration of lactation with cardiovascular mortality in a longitudinal study in paper II. In paper III, we studied the accuracy of long-term maternal recall of breastfeeding duration by comparing recorded information on breastfeeding initiation and duration collected prospectively during the child’s first year of life with recalled data obtained from mailed questionnaires some 20 years later. Results Among parous women aged 50 years or younger, we found an inverse association between lifetime duration of lactation and body mass index, waist circumference, systolic and diastolic blood pressure, triglycerides, total cholesterol and LDL cholesterol (Paper I). Furthermore, in the same age group, parous women who reported no lactation had almost two-fold higher odds ratio for hypertension, three-fold higher odds ratio for obesity and six-fold higher odds ratio for diabetes than those who reported lactating ≥ 24 months. Among women older than 50 years there were no clear associations. In paper II we reported that among parous women younger than 65 years, cardiovascular mortality was almost three times higher among those who had never lactated compared to women who had lactated 24 months or more. No clear associations were observed among women 65 years or older. In paper III, we observed that Norwegian women recall their breastfeeding duration fairly accurately 20 years after delivery, with a median overestimation of about 2 weeks. The high agreement between recalled and recorded breastfeeding supports the use of recalled breastfeeding duration as an exposure variable in epidemiological studies on lactation and health. Conclusion In conclusion, parous women who did not lactate had an unfavourable cardiovascular risk profile several years after weaning. Furthermore, parous women who did not lactate had excess mortality rates from cardiovascular disease. If our findings are causal, it could imply an important preventive health measure in women’s health.