Having examined the normal menstrual cycle and the most common hormonal modifiers, I want to discuss the topic of body composition for the next several blogs. There are two primary reasons I want to do this. The first is that the differences in body composition between women and men tend to underlie many of the differences that are seen in terms of apparent gender differences in fat gain, fat loss and exercise performance. Hormonal differences (and the changes that occur) clearly play a role but the difference in body composition tends to explain a great deal of the differences that are seen.
The second has to do with a topic that will take up a large portion of the blogs ahead, which relates to dieting and what I will for now call weight loss. I mentioned that women are generally more likely to be dieting than men and this is true whether the general population or an athletic population is being examined. There are still many long-held misconceptions and simply poor ideas about dieting and many of them relate to a misunderstanding of the differences between body weight, body fat and body composition.
Because while many who pursue dieting tend to still think in terms of weight loss itself, looking at body composition is not only far more accurate but far more important. This isn't to say that the scale doesn't have it's uses or that weight is irrelevant in all situations. But there are a number of potential problems with it by itself. To nobody's surprise, there are a set of issues that women face in this regard that men really don't. Understanding body composition, what it means, along with the differences between body weight and body fat, are a key aspect of improving women's results in everything from dieting in general to improving their athletic performance.
So what is a woman's body actually made of? The answer is a whole bunch of different things including bones, skeletal muscle, organs (heart, liver, kidney, brain, etc.), water, stored carbohydrate, blood, minerals and of course there is body fat. For simplicity's sake, these different components of the human body are typically divided into two categories. The first is fat and includes, well, all of the different types of fat that I will discuss in some detail below. Everything that is not fat will be called lean body mass (LBM) and you'll sometimes see this called Fat Free Mass (FFM). For all practical purposes they are interchangeable and I will use LBM throughout these blogs.
What is LBM and What is it For?
While many, especially in the athletic community, tend to equate LBM with muscle, this isn't really accurate. Rather, LBM refers to everything that isn't fat and this includes a number of distinct tissues which are structurally very different. The brain has a very specific structure as do the various organs (including a woman's reproductive organs). Bone is it's own tissue as is skeletal muscle. Water, minerals and carbohydrates are all distinct as well. Every type of LBM in the body tends to have a fairly specialized purpose. The heart pumps blood, kidneys filter waste, the liver is involved in tons of different biological processes, bones provide the body with a physical framework, skeletal muscle generates force for movement, reproductive organs exist for reproduction, etc. All are important although, as you'll see later, some are relatively more important than others in terms of short-term survival. They are all important but as I'll talk about below, only a few are really that relevant in terms of what can or cannot be impacted on by diet (or training) and what is really worth paying attention to in the short term.
While the amount of bone, or rather how dense bones are, is critical to women's health, the primary type of LBM that is important in terms of altering body composition is skeletal muscle. Other types of LBM can change, water and glycogen for example, but changes in the amount of muscle are key here. Skeletal muscle is made up of a number of different types of tissue. The actual muscle fibers are made of protein but this is only about 25% of the total in muscle. The rest is a combination of water, minerals, stored carbohydrate (called glycogen), intramuscular triglyceride (IMTG, fat stored within the muscle itself) and the various cellular machinery involved in muscular metabolism.
What is Body Fat and What is it For?
In contrast to LBM which is made up of a number of very distinct tissues, body fat tends to be fairly similar in its chemical structure with one exception. The technical term for body fat is adipose tissue and most types of body fat fall under description of white adipose tissue (WAT) although it's really more of a milky beige color. Whether they know it or not, when people want to lose "weight" or improve their appearance, it's WAT that they want to lose. All WAT is made up primarily of stored triglyceride (TG), and this makes up 85-90% of the total fat cell (the rest is water and cellular machinery). A TG is the combination of three fatty acids attached to a glycerol molecule. When people talk about saturated or unsaturated fats they are actually referring to the chemical structure of the fatty acid chains. The fat found in food is nothing more than TG and I'll talk about how women's bodies handle dietary fat in later blogs.
The exception to the above is what used to be called brown adipose tissue (BAT) but is now thought to be brite or beige adipose tissue in humans (the distinction isn't that important here and I'll call this BAT as well). Sort of a reddish color, BAT stores very little triglyceride and exists to burn other fuels for energy and to produce heat. It's currently not clear how much of a real world impact on calorie expenditure BAT has at this point. As well, since BAT tends to be primarily activated under conditions of chronic cold exposure, which most in the modern world try to avoid, the relevance of BAT is questionable.
So what is body fat for beyond making people unhappy about their appearance? The earliest ideas held that body fat was nothing more a relatively inert place to store energy and clearly that is certainly one of it's primary purposes. During certain types of exercise or when there is insufficient food (as in dieting or starvation), stored fat is mobilized to provide energy to the body. While carbohydrate can also provide energy, fat stores are especially suited to this role as they provide 9 calories per gram while carbohydrates only provide 4. As importantly, the storage of carbohydrate requires a large amount of water with 3-4 grams of water being stored for every gram of carbohydrate, while fat does not. A fairly lean individual might store 100,000 calories of fat, enough to sustain them for weeks or months without any food. To store that much energy as carbohydrate would be impossible and the actual stores of carbs (as glycogen in the muscle and liver) is fairly limited.
For women especially, it's clear that lower body provides a fuel source during pregnancy and for breastfeeding. As I mentioned before, women's hip and thigh fat is actually used preferentially for this purpose being stored in preparation for pregnancy and being used for energy in the later stages of pregnancy and during breastfeeding. Relatedly to this, at least part of a woman's body fat distribution is probably related to sexual selection and attraction, providing the curves and other female characteristics that are found to be sexually appealing. But there is far more to body fat than that.
Body fat turns out to be crucial in both immune system function and inflammation with both too little and too much body fat causing problems. Too little fat means that the immune system may not function as well as it could while too much means that the immune system is overactive. Excessive body fat also causes an inflammatory state. Body fat may also play a role as a physical cushion in the body or act for insulation against cold (and women do handle heat and cold differently than men). Body fat is also a place where the body stores glucose; in a variety of disease states, it becomes impossible for the body to store incoming carbohydrate in muscle and fat cells can take up the slack at least for a little while.
Fat cells are also a place where local metabolism of hormones can occur. A great deal of women's estrogen is actually made from the conversion of testosterone within the fat cell (in postmenopausal women, almost all of her estrogen is made this way). Fat cells also can impact on cortisol (a stress hormone) metabolism, converting active cortisol to inactive cortisone and vice versa and there are other numerous effect occurring with more being discovered almost continuously. Perhaps one of the most newly recognized aspect (newly here means since the mid 1990's) is that fat cells, produce a host of chemicals and hormones that drastically impact on physiology. Leptin, which I mention before and which I will talk about in great detail later, was the first to be discovered and the list continues to grow almost weekly.
My point here is primarily to point out that, as much as people dislike body fat (for appearance reasons) and while excessive amounts certain cause health problems, fat cells are critical for overall health and function. Too little can be just as bad as too much and in odd disease states, where people make no fat cells, a number of health problems crops up. It's simply an issue where thinking of fat cells as "good" or "bad" is mistaken.
Types of White Adipose Tissue
While the above applies to all types of WAT generally, it turns out that fat stored in different parts of the body can act very differently. Different distributions of fat (i.e. upper versus lower body) can impact on overall health and there are large differences in the rate of blood flow through the fat cells, how easily or not the fat cells store fat along with how easily or not they release that fat back into the bloodstream. There are also clear gender differences that I will discuss in a later blogs. For now let me look at the different types of WAT in the body.
The first type of fat is essential fat, fat in the body that is essential for both life and normal function. This includes fat around the brain, around the internal organs (different from visceral fat, discussed next), in the nervous system (sheaths around nerves are made of fat) and in the brain. In general, essential fat is taken as 3-4% for men and 10-12% for women with the difference being attributable to what is called sex- specific fat (breast tissue is included here). You can't lose essential fat and if you did, you'd be dead.
Visceral fat, which many people have probably heard of, refers to a type of fat found primarily in the gut that surrounds the organs (it is different than essential fat, though). Visceral fat is highly metabolically active meaning that, while it stores fat fairly easily, it also releases fat easily. This is probably to provide a rapid source of energy to the body but excess amounts of visceral fat is associated with insulin resistance and increased heart disease risk. Visceral fat is deep within the body and not really visible outside of making the stomach stick out (and often feel quite hard to the touch). When it is lost, while the stomach may be flatter (or easier to suck in) there is no major change in appearance. Testosterone tends to promote visceral fat accumulation and between having low testosterone and elevated estrogen, women do not generally store much. However women with PCOS/hyperandrogenism or who become very overweight tend to store visceral fat. After menopause, visceral fat levels increase if HRT is not begun which contributes to the increased risk of heart disease seen in women under those conditions.
Subcutaneous fat is fat found underneath the skin which makes it visible in a way that essential and visceral fat is not. Whether they know it or not, when people talk about losing fat (or even weight to some degree), they are really talking about losing this type of fat. While subcutaneous fat used to be considered a single type of tissue, it's now known that fat in different parts of the body are physiologically distinct. Upper body fat is more similar than not and represents everything above the waist including fat on the face, shoulders, chest (except breast fat), upper and lower back and abdominal area (which can be further subdivided into deep and superficial and upper and lower). Lower body fat refers to everything below the waist including the glutes/hips, thighs and calves.
Since they have less visceral fat, women tend to carry more subcutaneous fat with more of the total being stored in the lower body (PCOS/hyperandrogenism, obesity and the postmenopausal woman on HRT tend to carry more upper body fat). Relative to visceral fat, subcutaneous fat is more difficult to lose although this depends on the area being examined and whether women or men are being examined. Subcutaneous fat is less metabolically active than visceral fat which means that it tends to have less of an impact on disease risk. Carrying more fat in the lower body, which is particularly metabolically inactive, lowers heart disease risk which is why women are typically protected until after menopause.