The breast is composed of different tissue types, and its composition is linked to health and disease risk. Adipose tissue (fat) is a major component, playing a role that goes beyond providing physical volume. The percentage and health of this fat tissue are important for medical professionals, particularly when evaluating images used for cancer screening. Understanding the function and behavior of breast adipose tissue is relevant for gaining a clearer picture of breast health.
The Structure and Primary Function of Adipose Tissue
Breast tissue is a mixture of glandular tissue, fibrous connective tissue, and adipose tissue. Glandular tissue consists of lobules and ducts, while fibrous tissue provides a supportive framework. Adipose tissue fills the remaining spaces, giving the breast its softness, shape, and size.
The primary function of this fat tissue is to serve as a local energy reserve. It consists mainly of white adipocytes, which store energy in large cytoplasmic lipid droplets. During puberty, rising estrogen levels stimulate the accumulation of these fat cells, contributing to adult breast development.
Adipose tissue is also involved in the development and maturation of the mammary glands. This tissue is metabolically active. The amount of fat in the breast can vary widely, sometimes making up between 70% and 90% of the total composition.
Understanding the Clinical Significance of Breast Density
Breast density reflects the ratio of fibroglandular tissue to fatty tissue, as seen on a mammogram. Fibroglandular tissue (glandular and fibrous elements) appears white, while fatty tissue appears dark or transparent. A higher proportion of fibroglandular tissue results in a denser breast.
Density is clinically significant for breast cancer screening. First, dense tissue can mask a tumor because both dense tissue and cancerous masses appear white on a mammogram. This makes it difficult for radiologists to detect small cancers.
Second, high breast density is an independent risk factor for breast cancer. Women with dense breasts have a higher likelihood of developing the disease, and the risk increases with the degree of density.
Radiologists use the four-category Breast Imaging-Reporting and Data System (BI-RADS) to classify density:
- Category A indicates almost entirely fatty tissue.
- Category B shows scattered areas of density.
- Category C is heterogeneously dense.
- Category D is extremely dense.
Categories C and D are considered “dense breasts” and represent about half of all women over 40.
Factors Driving Changes in Breast Fat Volume
The volume of fat tissue changes throughout a person’s lifetime in response to systemic and hormonal factors. Systemic weight changes have a direct impact, as the breast is a storage site for body fat. Weight gain increases the total volume of breast fat, while weight loss decreases it.
Hormonal shifts related to aging are a major driver of composition changes. Before menopause, the breast contains a higher ratio of glandular tissue. As a woman ages and estrogen declines, the glandular tissue often atrophies.
This process, known as involution, results in the glandular tissue being replaced by adipose tissue. This causes a decrease in overall density and an increase in the relative amount of fat.
Pregnancy and lactation also cause rapid, temporary changes. Rising hormones cause the milk-producing ducts and lobules to enlarge, increasing glandular volume. White adipose tissue can even transdifferentiate into “pink adipose tissue,” which has milk secretory potential.
Adipose Tissue, Inflammation, and Health Risk
Adipose tissue is recognized as an endocrine organ that actively secretes hormones, growth factors, and signaling molecules, rather than a passive energy depot. This metabolic activity creates a local microenvironment that influences breast health and disease progression. A specific concern is the link between breast fat and inflammation, which can promote cancer initiation.
When fat cells become enlarged, often due to obesity, they can trigger low-grade chronic inflammation within the breast tissue. This inflammation is characterized by “crown-like structures,” which are immune cells clustering around stressed or dying fat cells. These inflammatory cells and the fat tissue secrete pro-inflammatory chemicals, creating a pro-tumorigenic environment.
The fat tissue is also a major source of estrogen, especially in postmenopausal women. An enzyme called aromatase, found in the adipose tissue, converts androgens into estrogens. Increased estrogen levels, stimulated by a larger volume of fat tissue, can promote the growth of hormone-sensitive breast cancers.
Fat cells also secrete adipokines. Leptin is associated with increased breast cancer risk, while adiponectin can be protective.