Growth Hormone (GH)
Growth hormone is involved in many processes in the body but a primary one in adults is the mobilization of fat. Women show higher levels of GH than men along with an larger increase in response to exercise. This is probably part of why women use more fat for fuel under some conditions.
Insulin
The hormone insulin is one about which there is a great deal of confusion and mistaken information. While all too frequently blamed for being the cause of fat gain it's better to think of insulin as a general storage hormone. It impacts on fat metabolism, stimulating fat storage and inhibiting fat mobilization and burning but it also increases the storage of carbohydrate in skeletal muscle and liver and is involved in skeletal muscle growth. In general, women have lower levels of insulin along with better insulin sensitivity than men. This is more pronounced during the follicular phase when estrogen increases insulin sensitivity and is decreased in the luteal phase when progesterone causes some degree of insulin resistance.
Thyroid
Almost everyone knows about the thyroid hormone although, like so many things, there is a lot of confusion and misinformation about it. Among its other effects in the body, thyroid hormones are a primary controller of metabolic rate, interacting with other hormones (such as the catecholamines discussed below). Thyroid also impacts on fat cell metabolism (helping to mobilize fat out of the cells) and skeletal muscle.
There are two primary types of thyroid hormone called T4 (thyroxine) and T3 (triiodothyronine) which are released from the thyroid gland in a ratio of about 80% T4 to 20% T3. T4 is primarily a storage hormone which is converted to the more active T3 in other tissues in the body, especially the liver. The conversion of T4 to T3 is important as this process goes down while dieting, leading to lower levels of T3 and a lowered metabolic rate. Reproductive hormones interact with T3, estrogen decreases levels of active thyroid hormone while progesterone increases them. This may be part of the metabolic rate increase during the luteal phase. T3 can also be converted inside of tissues to T2 which has its own metabolic effects there.
Women are significantly more likely to be hypothyroidal (meaning that their thyroid gland releases insufficient levels of thyroid hormones) having more issues when iodine intake is insufficient. They are also three times as likely to suffer from thyroid cancer as men and this is probably related to the direct effect of estrogen on thyroid hormone metabolism. Women are also more likely to suffer from depression and, while far from the only cause, low thyroid levels are an often undiagnosed cause.
All medical cases of hypothyroid are treated with thyroid hormone to replace the hormone that is not being produced normally. Either T4 or a combination of T4 and T3 are typically given and some women report better results from a type of thyroid called Armour. Determination of hypothyroid of any sort must be made through blood tests and medication should be used under the care of a health practitioner. Certain diet and training practices can interact with thyroid and women's levels can be impacted significantly, this effect being both large and rapid. Women's overall dietary choices can also negatively impact on thyroid hormone levels (and by extension their metabolic rate).
The Catecholamines
The catecholamine hormones refers to adrenaline and noradrenaline in America and epinephrine and norepinephrine everywhere else in the world Adrenaline (epinephrine), as its name suggests is released from the adrenal gland into the bloodstream and has effects throughout the body. In contrast, noradrenaline (norepinephrine) is released from nerve terminals and only has effects very locally where it is released. Many of you may have heard these hormones referred to as the fight or flight hormones although, they appear to have slightly different effects in women.
Released in response to a variety of stressors, the catecholamines raise heart rate, blood pressure, mobilize fuel for energy and have many other functions. In general women show lower levels of the catecholamines at rest along with a generally lower response to exercise compared to men. While women initially increase levels during exercise to the same degree as men, they rapidly adapt to exercise and no longer match men's levels. As with the differences in GH described above, these differences are probably part of why women's overall nutrient utilization patterns at rest and during exercise are what they are.
The catecholamines also interact with thyroid hormone in controlling metabolic rate along with fat mobilization. Thyroid hormones sensitizes the catecholamine receptors (meaning that the hormones send a more potent signal) while the catecholamines stimulate conversion of T4 to T3 in the liver. Like thyroid, levels of the catecholamines drop during dieting and these two factors are a large part of the overall decrease in metabolic rate.
Leptin
Leptin is a hormone released primarily from fat cells and it's discovery in 1994 changed the field of obesity research forever. Not only did it indicate that fat cells were far more than just an inert storage space but would lead to the realization that they produced numerous hormones involved in regulating the body's metabolism. Relative to body weight and body fat specifically leptin helped to fill in a number of gaps in previous research in terms of how body fat and body weight were regulated. Early research had suggested that there was some way that the body or brain could "know" how much fat an animal was carrying or how much it weighed so that metabolism and food intake could be adjusted. Although the system is much more complicated, leptin provided a mechanism for how this could occur.
Leptin levels turn out to be related to two primary factors. The first is the amount of body fat someone is carrying with higher levels of body fat leading to higher leptin levels. The second is the amount someone is eating over several days with leptin levels rapidly changing in response to alterations in food (especially carbohydrate intake). In the early days of leptin research, it was thought that leptin acted to prevent obesity but this is now known to be false. Rather, leptin acts primarily as an anti-starvation signal with decreasing levels (in response to food restriction or fat loss) signaling the body to lower metabolic rate, increase hunger/appetite along with other effects. Decreasing leptin levels is also involved in menstrual cycle dysfunction.
Perhaps surprisingly, women turn out to have higher levels of leptin than men. While some of this is simply due to having more total body fat, women's fat cells also produce more leptin than men's due to the effect of estrogen. While estrogen acts to generally sensitize the brain to leptin, women do tend to show relatively more leptin resistance than men overall, meaning that it cannot send as potent of a signal. As well, although leptin levels increase during the luteal phase (possibly in response to the surge of estrogen right before ovulation), leptin resistance increases further. Finally, in response to dieting and exercise, women's bodies show a different response than men's and it looks like woman's brains may respond differently to changing leptin levels than men's. All of this adds to a woman's general physiological tendency to store and hold onto body fat as it means that women's bodies may fight back harder and adapt more quickly to dieting or exercise, slowing fat loss to a greater degree than would be seen in men.
Next we will look at what can modify a womans physiology
Coach HB
The above information is taken from the The Woman's Book by Lyle Mcdonald with Eric Elms