The protective effects of breastfeeding on chronic non-communicable diseases in adulthood: A review of evidence.

Chronic non-communicable diseases (NCDs), including cardiovascular diseases, cancers, chronic respiratory diseases, diabetes, etc., are the major causes of mortality in the world, notably in low- and middle-income countries. A growing body of evidence suggests that NCDs have a complex etiology resulting from the interaction of genetic factors, gender, age, ethnicity, and the environmental factors. It is well-documented that chronic diseases in adulthood origins in early life. In recent years, much attention has been focused on primordial and primary prevention of NCD risk factors. There are many biological and epidemiological studies on beneficial effects of breastfeeding during infancy on chronic diseases in adulthood, particularly on hypertension, obesity, diabetes, hypercholesterolemia, and cardiovascular diseases. This review article aims to summarize the current literature on the long-term effects of breastfeeding on prevention of NCDs and their risk factors. The current literature is controversial about these effects; however, a growing body of evidence suggests that breastfeeding has protective roles against obesity, hypertension, dyslipidemia, and type II diabetes mellitus during adulthood. In addition to its short-term benefits, encouraging breastfeeding can have long-term beneficial health effects at individual and population levels.

INTRODUCTION

Chronic non-communicable diseases (NCDs) are an emerging global health problem. These diseases have a long and slow process, and mainly include cardiovascular diseases (48%), cancers (21%), chronic respiratory diseases (12%), and diabetes (13%).[1] According to the 2008 World Health Organization (WHO) report, NCDs are responsible for 63% of all-cause mortality in the world, and are forecasted to reach 69% by 2020.[2] Currently, NCDs have a share of 80% of the burden of diseases in the developed countries, and 70% in developing countries.[345] Based on the WHO report in 2011, 90% of mortality in Iran has been because of NCDs.[1] It is well-documented that some factors such as genetics, gender, and age cannot be accountable for high prevalence of these diseases alone, and in addition to these risk factors, modifiable environmental and lifestyle factors also play an important role in this process.[6]

The major risk factors of chronic NCDs include smoking, hyperlipidemia, hypertension, hyperglycemia, obesity, and sedentary lifestyle. Because of the high costs of treatment of NCDs, prevention and early control of their risk factors would be much more efficient and cost-effective than their treatment. A growing body of evidence supports the origins of adult NCDs from early life. Therefore, in recent years, much attention has been focused on primordial and primary prevention of chronic NCDs and their risk factors.[78] Breast feeding is considered as one of the protective factors against NCDs. Some epidemiological and biological studies have documented the long-term beneficial effects of breastfeeding on chronic diseases.[9]

This review aims to summarize the current studies on long-term effects of breastfeeding on major NCDs and their risk factors.

Breastfeeding and hypertension

Hypertension has a crucial role on the process of coronary heart diseases and stroke.[10] Some factors in early life may influence the development of hypertension in adulthood.[111213] The effect of breastfeeding on hypertension has attracted much interest because of the differences between breast milk and artificial formulas, mainly in terms of their content of sodium and fatty acids. It is documented that breastfeeding can affect systolic and diastolic blood pressures[14] in adulthood.[1516] The effect of breastfeeding on blood pressure in adulthood can be partly explained through the following mechanisms: (1) reduced sodium intake in infancy,[17] (2) high content of long-chain unsaturated fatty acids in breast milk, which is an important component of the tissue membrane system, as coronary endothelial system,[18] (3) protection against hyperinsulinemia in infancy,[19] as well as prevention of insulin resistance in early life,[20] adolescence,[21] and adulthood.[22] However, conflicting results exist in this regard, and many studies did not confirm this effect.[23] In general, the results on the protective effects of breastfeeding on hypertension are still conflicting, and remain to be determined in longitudinal studies with long-term follow up.

Breastfeeding and obesity

Findings of studies on the association between breastfeeding and obesity in adulthood[24] showed that breast milk is a protective factor against obesity.[24252627] Compared to artificial formulas, this effect could be explained by existing differences in macro-nutrients,[2829] even though the accuracy of this relationship is still uncertain.[27] In addition to its various beneficial effects, encouraging the intake of breast milk could be part of an important general strategy in preventing the global epidemic of obesity and related health consequences. Various biological mechanisms some stress behavioral patterns, and some physiological influences may explain the protective role of breast feeding on obesity. In respect to behavioral patterns, it could be proposed that breastfeeding improves feeding method and affects infant's appetite and satiety, and eventually would enhance children's appetite in later years of life. Moreover, there are physiological differences between breast milk and artificial formulas in terms of their nutrients and hormone contents. For instance, protein content of baby formulas is higher than that of breast milk, and leptin exists in breast milk, but not in artificial formulas.[30] Due to their high fat and protein contents, baby formulas would lead to increased secretion of Insulin Growth Factor-type 1 (IGF-1), and subsequently to stimulation of adipocytes, which eventually result in excess weight.[31] Early differences in nutrients intake would lead to long-term effects on metabolic systems; this may be mediated through changes in appetite and metabolism.[30] In vitro studies have shown that factors, which specifically exist in breast milk as specific hormones like leptin, could change growth factors and prevent formation of adipocytes.[3233] Moreover, breastfeeding affects the intake of calorie and protein,[34] insulin secretion,[35] balancing fat reserves, and adipocyte size.[36] The effect of breastfeeding is found to be independent from dietary patterns and physical activity in adulthood.[27] Many studies show that only long-term breastfeeding influences obesity in adulthood. Therefore, it can be concluded that feeding in infancy can affect obesity and its subsequent diseases in adulthood. However, there are still conflicting results of studies in relation to breastfeeding and obesity, and some studies did not confirm such association.[37] This important issue deserves more attention in future birth cohort studies.

Breastfeeding and diabetes mellitus

The study of Bore et al.[38] showed that fasting blood glucose level is inversely proportional to long-chain polyunsaturated fatty acids in skeletal muscle membrane. These fatty acids are found in breast milk and not in artificial formulas.[39] It seems that changes in skeletal muscle membrane have an important role in development of insulin resistance, and subsequent hyperinsulinemia, which gradually leads to defects in beta cells and ultimately to type II diabetes.[40] Various studies[414243] have shown that infants fed with artificial formulas have higher levels of insulin than breast-fed infants; in turn it would lead to modulation in releasing glucagon and insulin. These changes also lead to early development of insulin resistance and type II diabetes. These two mechanisms are suggested to be the possible underlying mechanisms of the protective role of breastfeeding against type II diabetes.[44] The existing evidence proposes that breastfeeding during infancy may prevent the development of type II diabetes in later years of life.[45464748] Furthermore, composition of breast milk is vastly different from artificial formulas,[49] particularly because the protein and energy, as well as the volume of milk consumed by breast-fed infants are much lower than artificial formulas.[50] It is suggested that the protective effects of breastfeeding against the risk of obesity in adulthood could also influence the insulin and glucose metabolism.[51] The results of epidemiological studies are controversial in this regard.[5253] However, many studies have confirmed the protective role of breastfeeding against type II diabetes mellitus. This effect is considered to be because of the difference in composition of breast milk and the difference in hormones of insulin, motilin, intro-glucagon, neurotencin, and pancreatic polypeptide in breast milk and artificial formulas, which in turn would lead to lower subcutaneous fat deposition in breastfed infants.[54] Finally, according to the current evidence, this hypothesis is implied that type II diabetes is planned from early infancy.[49] Future longitudinal studies shall determine the clinical significance of such associations.

Breastfeeding and hypercholesterolemia

High concentrations of total cholesterol and low-density lipoprotein (LDL)- cholesterol is a risk factor for coronary heart diseases.[55] It seems that the feeding type during infancy affects their levels.[56] The cholesterol content of breast milk is evidently higher than many artificial formulas. This high intake of cholesterol from breastfeeding may have long-term effects on cholesterol endogenesis, this may be mediated through diminishing the regulation of hydroxymethyl glutaril liver coenzyme A.[57] This effect has been also observed in animal studies that high cholesterol level in infancy is associated with low cholesterol level in older ages.[58] The enzyme of hydroxymethyl glutaril coenzyme A is a restrictive enzyme in cholesterol biosynthetic pathway from acetate; its inhibitors have a blood cholesterol reducing effect.[59] It is documented that compared to formula-fed infants, the breast-fed ones have higher mean blood cholesterol in infancy, similar levels in childhood, and lower in adulthood.[60] These differences may be because early exposure to breast milk cholesterol affects long-term cholesterol metabolism.[61] In many studies, low blood cholesterol concentration observed in adulthood has been associated with breastfeeding in infancy. The composition of breast milk is different from artificial formulas in many ways, for instance, in addition to breast milk high content of cholesterol, it contains hormones, particularly leptin and tri-iodotronin.[62] It should be acknowledged that in addition to these mechanisms, breastfeeding may affect eating habits in later years of life, and this may be another influencing factor on cholesterol level in adulthood.[60]

Breastfeeding and cardiovascular diseases

Some studies have shown that breastfeeding in infancy can affect risk factors of cardiovascular diseases in adulthood.[6364] Breastfeeding may have protective effects on major risk factors of cardiovascular diseases, as elevated total cholesterol level, hypertension[6566] LDL level,[67] and obesity.[60] A large body of evidence shows that breastfeeding could affect high-density lipoprotein (HDL) cholesterol level in adulthood.[6869] The results are inconclusive in this regard. The duration of breast feeding is also important in its protective role against cardiovascular diseases (Ref); however, our study among adolescents did not confirm such association[70]; possibly these effects would develop over longer periods of time.

CONCLUSION

Generally, the importance of primordial and primary prevention of chronic NCDs is justified by the high prevalence of such diseases and their risk factors, which mainly origin from early life. The current literature is controversial about the association of breast feeding with NCDs and their risk factors; however, a growing body of evidence suggests that breastfeeding has protective roles against obesity, hypertension, dyslipidemia, and type II diabetes mellitus during adulthood. Therefore, encouraging breastfeeding can have short-term and long-term beneficial health effects at individual and population levels.