Before I begin, I should admit that the title of this post probably oversells the depth and breadth of the content that follows. In fact, this post is going to focus exclusively on the one breast-related issue on which I may be considered an expert, and that is the relationship between breast fat and metabolic risk (if it sounds like I’m bragging, I’m not). I should also assure people that while this post does contain pictures, they are all completely safe for work.
As you can see in the above figure, fat tissue makes up a large proportion of the tissue – often the majority – within the breasts. And from what I understand, differences in breast size are due primarily to differences in the amount of fat tissue, as opposed to differences in duct or lobule volume. Similarly, changes in breast size due to weight loss (which can be seen in the photo at the bottom of this post) are due to reductions in the volume of fat within the breast.
Breast fat is a fascinating topic, but before we get to that specific fat depot, we need to briefly review the major types of fat within your body. If you’ve been following our blog for a few years this issue will be old hat, so feel free to skip to the next section.
All body fat is not created equal
In contrast to what some advertisements might have you believe, the fat in our bodies is not simply sandwiched between other tissues like a layer of butter on a baguette. Instead, fat is stored within specialized cells called “adipocytes” (hence fat is referred to as “adipose tissue”). Regardless of your body size, everyone has adipocytes in their body (as Peter has discussed in the past, having too few fat cells is actually much worse than having too many). When you lose or gain weight you are primarily changing the amount of fat stored in your adipocytes, rather than adding or removing actual fat cells.
The image below is taken from my Master’s thesis, which illustrates the three main adipocyte depots. The top image is a cross section of an abdomen, while the two bottom images are cross-sections of a pair of thighs.
Visceral adipocytes - these adipocytes are found within the abdominal wall and surround the internal organs (e.g. the viscera).
Intermuscular adipocytes – these are the fat cells that are found in between your muscles. The marbling on a steak is fat stored in these intermuscular adipocytes.
Subcutaneous adipocytes – this is the fat that you can pinch directly underneath your skin. Love handles, breasts, and anything mentioned in the song My Humps fall into this category.
The importance of this distinction is that these 3 fat depots have very different associations with health risk. Visceral and intermuscular adipocytes are generally very bad places to store body fat. Research has consistently shown that the more fat you store in these depots, the greater your risk of death and disease.
In contrast subcutaneous fat in the legs and buttocks actually seems to be protective against heath risk. For example, this paper from my MSc found that people with more subcutaneous fat in their lower body are actually healthier than people with the same body weight but less subcutaneous lower body fat. Despite being counter-intuitive, this is a very consistent finding (it made up the bulk of Peter’s PhD thesis). It’s also the reason why an “apple” body shape (e.g. lots of abdominal fat with very little lower body subcutaneous fat) is associated with much more health risk than the “pear shape”, characterized by fat stored mainly in the hips and thighs.
I’ve borrowed the two images below from one of Peter’s slideshare presentations, which nicely summarize the contrasting effects of visceral and subcutaneous fat accumulation.
There are a number of reasons why visceral fat stores are so much worse than lower body subcutaneous stores – as they expand visceral adipocytes become insulin resistant and promote inflammation, while also releasing fat into the blood stream where it can do damage to other tissues like the heart and liver. In contrast, subcutaneous adipocytes in the legs and butt tend to hold onto fat very tightly (hence why they are sometimes viewed as a “problem area” that are so difficult to slim down), which is actually quite beneficial from a health perspective.
What is the health impact of breast fat?