During preadolescent childhood, the growth of the mammary gland is isometric, keeping pace with the general growth of the child’s body.¹ At thelarche, the onset of secondary breast development occurring at puberty, the female mammary gland undergoes significant further development of its previously primitive ductal and lobular structures. It is the deposition of adipose tissue within the mammary gland, however, that accounts for the majority of the increase in breast size associated with puberty.² Body mass index (BMI) has been found to relate to both the timing of thelarche and the composition of the adult female breast.

Childhood Obesity and Thelarchal Age

Age at thelarche is known to vary by race and ethnicity. A recent prospective study of a cohort of more than 1200 girls in the United States reported the median onset of thelarche (Tanner stage 2) to be 8.8 years for African American girls, 9.3 years for Hispanic girls, and 9.7 years for both white non-Hispanic and Asian girls participating in the study.³ Excess childhood weight has also long been associated with earlier breast development, independent of race and ethnicity.⁴ Some studies have relied on visual inspection as a means to assess breast development, raising the potential for excess fatty tissue deposition among obese girls to be confused with true glandular tissue, thus confounding results. However, more recent studies have made use of palpation by trained examiners to determine the onset of glandular breast development, and these studies have likewise demonstrated correlation between higher BMI and younger age at thelarche. The Breast Cancer and the Environment Research Centers (BCERC) study found a significant correlation between higher BMI and onset of breast development with girls in all BMI categories above the 50th percentile progressively more likely to have reached thelarche than those below the 50th percentile, adjusting for race and ethnicity. The authors reported that while race accounted for 4.4% of the variance in thelarchal age, BMI was the strongest predictor of earlier age at thelarche of all covariates included in their statistical model, accounting for 14.2% of the variance.³

There is growing evidence of a decline in age at female puberty, specifically in the age at thelarche, over the past several decades.^5,6 The concurrent trend of increasing prevalence of childhood obesity in Western populations further suggests a relationship between childhood BMI and pubertal breast development. Childhood obesity may have a direct causal impact on the timing of pubertal development through, for example, obesity’s influence on endogenous hormonal levels. Studies in animal models have demonstrated causal links between feeding and pubertal development and support the possibility of a direct causal relationship between increasing obesity and earlier age at thelarche.⁷ However, other potential mechanisms for the association between childhood BMI and timing of, and trends in, pubertal development have been proposed. One possible such mechanism is population exposure to environmental “endocrine-disrupting chemicals” that have the potential to independently affect both obesity and pubertal timing.⁵ Certainly, further research exploring the underlying relationship between etiology of and sequelae of trends in prevalence of childhood obesity and the timing of female pubertal development is warranted.

Obesity and Breast Size and Composition

Adolescent BMI has been found to relate breast size and composition. Dual-energy x-ray absorptiometry scanning demonstrates that higher BMI correlates with greater breast area, greater breast volume, and lower breast density among adolescent girls.⁸ Analysis of data from the Nurses’ Health Study suggested that childhood body fatness is inversely associated with mammographic density in adulthood as well.⁹ The relationship between adolescent BMI and adult breast density is of particular interest given the well-documented association between breast density and risk of breast malignancy.¹⁰

The percentage of fat composition within the human female breast varies greatly. One study examining fresh mastectomy specimens found fat composition ranged from 7% to 56% by volume. There was no significant correlation between BMI and percentage of fat in the breast, although the sample size was small.¹¹ Other studies using magnetic resonance imaging (MRI) assessment, however, have documented a positive correlation between BMI and both overall breast volume and amount of breast adipose tissue within the breast of adult women.^12,13 As discussed in the section on obesity’s impact on successful breastfeeding, this additional adipose tissue may play a role in an altered breast function among obese and overweight women.

Structure and Role of Adipose Tissue Within the Breast

Many texts and figures have portrayed a distinct anatomical separation between the glandular epithelial tissue and the fatty tissues of the mature female breast. However, contemporary histological examination of fresh, fixed normal mastectomy specimens reveals that while subcutaneous fat is indeed present over the surface of the body of the breast and can be separated from the underlying breast parenchyma, there is a great deal of fat intermixed within the breast glandular parenchyma.¹⁴ In addition, sonographic examination of the breasts of lactating women has demonstrated significant and variable fat within the glandular tissue of the breast, with only minimal subcutaneous fat noted within a 30-mm radius of the base of the nipple. Figure 10-1 portrays the currently understood distribution of fat in the human breast, including notable amounts of fat intermixed within the glandular parenchyma.¹⁵

Fat within the human breast is often thought of as an inert structural component, and discussions of breast development and function typically focus on mammary epithelial tissues rather than on the mammary fat pad in which they reside. However, there is a growing recognition of the active role that mammary adipocytes seemingly play in the regulation of mammary epithelial development and function. Breast adipose tissue is thought to serve not only as a local source of lipids for the mammary epithelium, but also as a reservoir and source of paracrine and endocrine molecules affecting both breast growth and its function during lactation.¹⁶ Although inconsistent, some evidence suggests that breast volume and the degree of breast adiposity may reflect the degree of metabolical fat elsewhere in a woman’s body and may potentially serve as a marker for risk of metabolic disease.^12,13

Lactation: The Normal Function of the Breast

Considerable attention in the medical literature is given to the breast in the context of its potential for malignant disease. However, there is a growing and important focus on issues related to the physiologic function of the breast organ: lactation. While the success of the breastfeeding relationship has the potential to significantly affect the health of all mothers and infants, it may be of special importance to obese and overweight mothers and their families.

The Importance of Breastfeeding for Infants

Exclusive breastfeeding has been well established as the normative and preferred method for the feeding of human infants. The American College of Obstetricians and Gynecologists (ACOG),¹⁷ the American Academy of Pediatrics (AAP),¹⁸ the American Academy of Family Physicians,¹⁹ and the World Health Organization (WHO)²⁰ all recommend exclusive breastfeeding for the first half of the first year of life, followed by continued breastfeeding with complementary feedings for longer periods, ranging from at least 1 year of age to at least 2 years of age. The use of artificial breast milk substitutes (formulas) at any time, the introduction of complementary foods before approximately 6 months of age, and the discontinuation of breastfeeding before 1 to 2 years of age are all considered suboptimal breastfeeding practices. The association between lack of breastfeeding or suboptimal breastfeeding and excess morbidity and mortality in children is well documented, even in populations in the industrialized world possessing substantial economic and health care resources. Infants not breastfed or breastfed for suboptimal durations are at higher risk for otitis media, respiratory tract infection, gastrointestinal tract infections, necrotizing enterocolitis, sudden infant death syndrome (SIDS), inflammatory bowel disease, type 1 diabetes, childhood leukemias, and obesity.^18,21

Breastfeeding as Childhood Obesity Prevention

It has been long established that breastfed infants exhibit different growth and weight-gaining patterns than their formula-fed peers, with breastfed infants overall leaner at 12 months of age.²² A more recent literature asserts a possible relationship between breastfeeding during infancy and a child’s weight beyond the breastfeeding period. Based on evidence of an association between suboptimal breastfeeding and risk of later obesity, in a 2010 statement the US Breastfeeding Committee recommended breastfeeding as a primary prevention strategy to reduce overweight and obesity and to promote the maintenance of a healthy weight throughout the life span.²³

Multiple plausible mechanisms for a causal relationship between breastfeeding in infancy and a lowered risk of later obesity have been purported. Among these are the exposure to components of breast milk during infancy providing determination of long-term metabolism and appetite signaling (“programming”) and the infant-driven process of receiving nutrition at the breast itself leading to the development of later optimal self-regulation of energy intake.^24,25 Given that the data evidencing a relationship between breastfeeding and lowered risk of later obesity are nearly entirely observational in nature, however, it is important to carefully control for confounding variables and methodological bias when seeking to delineate the nature of this relationship. A recent extensive and careful systematic review of the literature continued to find a modest reduction in the prevalence of overweight or obesity in children exposed to longer durations of breastfeeding,²⁶ and a recent large meta-analysis suggested that breastfeeding may be a significant protective factor against obesity in children.²⁷ However, questions related to methodological and data-confounding issues persist, and the evidence related to a significant causal relationship between breastfeeding and decreased risk of obesity remains somewhat conflicting and controversial.

The Importance of Breastfeeding for Mothers

Lactation is the physiologically normal postpartum state for the human mother. Lack of breastfeeding and suboptimal breastfeeding duration and intensity are associated with excess maternal morbidity. Mothers who do not breastfeed in the immediate postpartum period have increased postpartum blood loss and a slower involution of the uterus.¹⁸ Inverse relationships between lactation and later risks of breast malignancy, ovarian malignancy, depression, hypertension, diabetes, hyperlipedemia, and cardiovascular disease have also been documented.^18,21 The majority of evidence of these relationships arises from observational studies, however, and is therefore subject to concerns regarding methodological bias and confounding variables.

Breastfeeding in Relation to Maternal Weight and Metabolic Disease

Pregnancy is associated with numerous metabolic changes, including insulin resistance, visceral fat accumulation, and elevation in lipid and triglyceride levels. Accumulating evidence demonstrates an inverse relationship between lactation and the risk of subsequent maternal metabolic disease; therefore, breastfeeding may be of special importance for the obese mother. Stuebe and Rick-Edwards proposed the concept of lactation providing a physiologically normal and important “resetting” of the maternal metabolism after pregnancy, leading to a more rapid and more complete return to baseline metabolism.²⁸

The literature relating to the effect of lactation on maternal postpartum weight loss and return to prepregnancy weight is inconclusive. Inherent confounding variables such as physical activity, dietary intake, and gestational weight gain are difficult to control. A recent systematic review found insufficient evidence to suggest a direct association between breastfeeding and postpartum weight change, although a subset of the reviewed studies the authors determined to have “higher methodologic quality” did demonstrate a positive association.²⁹ A recent study investigating the association between adherence to 2005 AAP recommendations for exclusive and total breastfeeding duration and maternal weight 6 years postpartum found a significant association between complete adherence to these recommendations and maternal weight among women who were obese before pregnancy compared to those who had not breastfed. No such relationship was noted among normal-weight and nonobese overweight women.³⁰

Growing evidence suggests breastfeeding may play a role in postpartum maternal metabolism and related morbidities regardless of any effect it has on maternal postpartum weight specifically. Analysis of data from the Women’s Health Initiative supported an inverse relationship between lactation duration and subsequent development of hyperlipidemia, hypertension, and cardiovascular disease in postmenopausal women.³¹ In addition, breastfeeding has been associated with a reduced risk of subsequent development of type 2 diabetes mellitus in mothers both with and without a history of gestational diabetes mellitus (GDM), and mothers with GDM who breastfeed have improved lipid and glucose metabolic profiles for at least the first 3 months postpartum.^18,32

Studies using computerized tomography to assess the amount of the highly metabolically active visceral fat that typically accumulates during pregnancy demonstrated an association between breastfeeding and lowered abdominal adiposity, even years after pregnancy and lactation. Controlling for known risk factors for cardiovascular disease, including BMI, premenopausal mothers who had not lactated had significantly more visceral adiposity than mothers who had lactated, while mothers who had lactated for 3 or more months after each birth had no more visceral fat than women who had never been pregnant.³³

As with other literature related to the effects of breastfeeding, however, the observational nature of the vast majority of the studies demonstrating a relationship between breastfeeding and maternal morbidity limits the ability to assign causality. While it is plausible that suboptimal breastfeeding has a causal role in increased maternal metabolic morbidity via a failure of a physiologically normal return to baseline metabolism in the postpartum period and beyond, the relationship may also be explained by confounding variables not adequately controlled for via statistical methodologies. Alternatively, the relationship between breastfeeding success and maternal metabolism may indeed be causal, but in the opposite direction; as discussed in the following material, an inherent altered baseline metabolic state may interfere with lactation success. Thus, a woman’s unsuccessful breastfeeding may be the result of the same underlying physiologic abnormalities that lead to eventual metabolically associated morbidities, rather than being a cause of these morbidities itself. Future research should attempt to further define the likely complex nature of these relationships.

The Impact of Maternal Obesity on Successful Breastfeeding

Multiple systematic reviews of the literature demonstrated an overall inverse association between maternal BMI and breastfeeding initiation and duration.^34,35 Most recently, Turcksin and colleagues reviewed prospective studies evaluating the relationship between prepregnancy maternal obesity and lactation; these authors used more stringent inclusion criteria than those used in previous reviews. They found that, overall, even when adjusting for potentially confounding factors, maternal obesity was associated with reduced intention to breastfeed, reduced initiation of breastfeeding, shortened duration of breastfeeding, less-adequate milk supply, and delayed onset of copious milk supply (lactogenesis II).³⁶ Although this and other reviews have focused on the relationship between prepregnancy BMI and breastfeeding, other evidence indicated that excessive weight gain during pregnancy may be associated with suboptimal breastfeeding regardless of prepregnancy weight status.³⁷

The relationship between higher maternal BMI and decreased lactation initiation and success may not be a factor in all populations. In the United States, black women have both the lowest breastfeeding initiation and continuation rates and the highest rates of obesity compared to their white and Hispanic counterparts.^38,39 However, as detailed in the reviews previously mentioned, some of the associations between higher BMI and negative breastfeeding outcomes were noted among white or Hispanic women, but not among African American women. It is noteworthy that despite the diverse international locations of the reviewed studies, most included a majority of women of white European origin as subjects. Certainly, further research is warranted to explore and understand the seemingly complex relationships between race, obesity, and breastfeeding.

Etiology of the Impact of Maternal Obesity on Breastfeeding

Maternal obesity may have a negative impact on breastfeeding success through a number of mechanisms, and the relationship is likely complex and multifactorial. Successful lactation necessitates multiple steps, including (1) the appropriate structural development of the mammary gland during childhood and puberty, (2) further differentiation of the mammary epithelial cells under the influence of a complex of reproductive and metabolic hormones during pregnancy, (3) the initiation of copious milk secretion (lactogenesis II) triggered by a rapid decline in progesterone in the setting of prolactin and an appropriate hormonal milieu after delivery, and (4) frequent breast stimulation triggering milk synthesis and milk ejection via the release of prolactin and oxytocin, respectively, along with frequent and continued milk removal from the breast.^40,41 In addition to the hormones noted, cortisol, insulin, growth hormone, and the thyroid hormones are involved in establishing and maintaining lactation.^40,41,42

Delayed onset of lactogenesis II, defined as onset of copious milk production after 72 hours after delivery of the placenta, is associated with and predictive of decreased breastfeeding success, including lowered rates of exclusive and any breastfeeding at 1 month postpartum,⁴³ and is often cited as a proxy for decreased lactation success. However, disruption at any step of the process outlined can have a negative impact on eventual successful lactation. Possible anatomic, physiologic, and psychosocial etiologies of the suboptimal breastfeeding success observed in obese and overweight women are discussed next and diagramed in Figure 10-2.

Anatomic Differences—Anatomic differences in women with higher BMIs may play a direct role in decreased breastfeeding success. Some have noted that large-breasted women may have flattish nipples, possibly related to an excess of periareolar adipose tissue.^44,45 This, in combination with larger breasts and an overall altered upper body anatomy, may create mechanical challenges in positioning and latching a nursing infant or pumping with standard-size pump flanges. Such difficulties could lead to suboptimal frequency and extent of milk transfer and significantly interfere with lactation success, especially in the early days and weeks of breastfeeding when frequent, effective feedings are essential to the development and maintenance of an optimal milk supply. In addition, mothers who have undergone breast reduction surgery in the past to alleviate the physical and emotional sequelae of their large breasts have decreased lactation performance, likely due to the anatomical disruption of fibroglandular tissue and ductal structures.⁴⁶

Breast adiposity—The increased breast adiposity noted among obese and overweight women^12,13 may exert a negative effect on lactation through nonmechanical means. As noted in the section on obesity and breast development and structure, breast adipose tissue is thought to serve not only as a local source of lipids for the mammary epithelium but also as a reservoir and source of paracrine and endocrine molecules affecting both breast growth and its function during lactation.¹⁶ The extra adipose tissue found within the breasts of obese women may thereby play a role in the altered lactation function seen among obese and overweight women.

Maternal Systemic Physiology and Comorbid Conditions—Underlying maternal systemic physiologic abnormalities that occur in association with, as a cause of, or as a result of maternal obesity may have a negative impact on breastfeeding success. A growing body of literature shows evidence of a relationship between abnormalities in glucose metabolism and suboptimal lactation physiology.^35,47 Polycystic ovary syndrome (PCOS), metabolic syndrome, and thyroid disease have all been associated with decreased breastfeeding success.^42,48,49,50

Some research has attempted to identify specific alterations in the endocrinologic physiology of obese women to explain decreased breastfeeding success. Controlling for confounding variables such as mode of delivery, early interaction with the infant and use of analgesia have often proven difficult.⁵¹ One study found overweight and obese women had similar progesterone levels but a blunted prolactin response to infant suckling in the first week postpartum compared to their normal-weight peers.⁵² A study of mothers with PCOS found the degree of metabolic derangement to be inversely associated with breastfeeding and breast size increment during pregnancy. However, neither androgen level nor randomized treatment with metformin was related to breastfeeding.⁴⁹ As underlying physiologic abnormalities can be lifelong, their impact on breastfeeding success may occur through affecting breast structure during development, through affecting physiologic function during the lactation process itself, or both. Further research is necessary to elucidate the nature of the relationship between abnormal maternal physiology and breastfeeding success.