What trainers need to know about subtle differences in female physiology.
When it comes to optimal endurance exercise performance, fuel source and utilization play a major role in success. The contribution and expenditure of fats and carbohydrates for the synthesis of ATP (adenosine triphosphate) during exercise are regulated by several factors, including activity, duration and intensity, as well as the person’s age, training status, diet and gender. Proteins contribute a minor 1%–8% of fuel needs during submaximal exercise (Isacco, Duché & Boisseau 2012).
Before puberty, there is no difference between males and females when it comes to substrate usage during exercise (Isacco, Duché & Boisseau 2012). This is not the case during adulthood, when women rely more on fats than men do for the same relative exercise intensity. Read on for an in-depth exploration of how metabolism, energy expenditure and hormonal changes affect women during exercise.
Hormones help cells communicate with each other. Two major female sex hormones—estrogen and progesterone—play essential roles in a woman’s reproductive system. Both derived from cholesterol, these hormones are often called ovarian hormones because they are produced and secreted primarily by the ovaries (D’Eon & Braun 2002). Estrogen, which is also secreted to a lesser extent by the adrenal glands, is actually a class of 18-carbon steroid hormones. A steroid hormone affects the growth and development of sex organs.
The most biologically active estrogen is 17ß-estradiol, or estradiol. Other estrogens include estrone and estriol, but these are less biologically active than estradiol (D’Eon & Braun 2002). McCarthy (2008) notes that estradiol is at its highest level in the brain prenatally, or during the first few days of life, suggesting that this estrogen has potent and wide-ranging effects on the developing brain. Hormones from the hypothalamus and pituitary gland regulate and control the release of estrogen and progesterone.
Metabolism represents all chemical reactions that sustain the life of cells, and thus of the organism. The process can be divided into two categories: catabolism, or the breakdown of molecules to obtain energy; and anabolism, the synthesis of compounds needed by the cells.
Carbohydrates, proteins and fats provide a variety of substances essential for the building, upkeep and repair of body tissues, and for the efficient functioning of the body.
- Carbohydrates come in three forms: starch, sugar and cellulose. Starch and sugar are essential energy sources for humans. Cellulose is an indigestible carbohydrate known as dietary fiber, which provides volume or bulk in food. Cellulose has no caloric value, which makes it a popular bulking agent in diet foods.
- Proteins are the chief tissue builders of every cell in the body. Proteins help make the blood’s hemoglobin, which carries oxygen to the cells; form antibodies to fight infection; and supply nitrogen for DNA and RNA genetic material.
- Fats are considered concentrated sources of energy, providing more than twice as much of it as carbohydrates and protein do. Fats make up part of the structure of cells, form a protective cushion and heat insulation around vital organs, carry fat-soluble vitamins and provide reserve storage for energy.
A major role of carbohydrate metabolism in a resting state is to maintain a constant supply of glucose to the brain. Carbohydrate in the form of glucose traveling in the bloodstream is the fuel that powers the brain. Glucose is the only fuel that brain cells normally use. Neurons, the cells that send bioelectric messages to one another, have a large demand for glucose because these cells are always in a state of metabolic activity.
Even during sleep, neurons are at work repairing and rebuilding their structural components. Because neurons cannot store glucose, they depend on a constant supply of this precious fuel. From a health standpoint, repeatedly overloading the bloodstream with sugar (e.g., by consuming too many soft drinks) places extra demand on the pancreas to keep secreting more insulin. This condition, where excess levels of insulin are circulating in the blood, is called hyperinsulinemia, and it may lead to insulin resistance (Alemany 2011). Insulin resistance is a condition in which normal amounts of insulin are inadequate for transferring blood glucose into cells, and the risk of type 2 diabetes progressively increases.
The energy needs of the muscle are what primarily determine how quickly and in what quantity carbohydrates (glucose and glycogen, the stored form of glucose in the liver and muscle) are used during rest and exercise. Blood glucose and muscle glycogen are essential for vigorous and prolonged strenuous exercise. Development of hypoglycemia (low blood glucose) and muscle glycogen depletion highly influence exhaustion during exercise.
In respect to female use of glucose and glycogen during endurance exercise, the research consensus suggests that women have lower rates of glucose appearance and disappearance than men do (Tarnopolsky 2008). According to current findings, Tarnopolsky observes, this may be due to lower activation of the sympathetic nervous system (responsible for the fight-or-flight response) in women, and not due to ovarian hormones. Tarnopolsky adds that the slightly inhibited carbohydrate use observed in women during endurance exercise also reflects the influence of hormones on fat metabolism.
Exercise intensity is the primary factor determining the degree of fat or carbohydrate utilization during exercise. During very light exercise, most energy fuel in the form of ATP is derived from fat. However, fat oxidation increases as intensity rises—until it reaches about 65% of VO2max (Jeukendrup 2002). Beyond this intensity, notes Jeukendrup, a gradual decline in the rate of fat oxidation is observed until intensity goes above 80% of VO2max, at which point fat oxidation rapidly declines. Training can reduce this decline in fat utilization (in both women and men), however—thus demonstrating that with continual endurance training, the body becomes more efficient at using fat as a fuel.
Most studies show that females store slightly higher levels of intramuscular fat, which is stored in small lipid droplets in muscle, than men do; and women also have more lipid droplets as opposed to larger lipid storage depots (compared with men) (Tarnopolsky 2008). During moderately vigorous exercise that is sustainable for 90 minutes or longer (approximately 55%–75% of VO2max), there is a progressive decline in muscle glycogen use and greater reliance on fat oxidation (breakdown) for ATP synthesis (Holloszy, Kohrt & Hansen 1998). This adaptation during endurance exercise is often referred to as glycogen sparing. Most research indicates that total-body lipolysis (fat breakdown) is higher in women than in men, as women have higher levels of glycerol (the backbone molecule of triglycerides) than men during sustained endurance exercise (Tarnopolsky 2008; D’Eon & Braun 2002).
Research indicates that estradiol plays a role in why women use more fat and less carbohydrate to fuel exercise at any given submaximal cardiovascular intensity (Tarnopolsky 2008). Also, women are not as efficient in carbohydrate-loading protocols that result in supercompensation of glycogen storage; this may be a direct result of higher levels of estradiol (Tarnopolsky 2008) and may slightly compromise women’s performance in marathon and ultra-marathon–like events.
Estrogen and progesterone levels fluctuate during the menstrual cycle, with inferences for exercise performance (Oosthuyse & Bosch 2010). Some studies show that endurance performance is affected by the menstrual cycle, while others report no difference (Oosthuyse & Bosch 2010). Nevertheless, the increase in estrogen concentrations relative to progesterone levels during menstruation’s luteal phase suggests that endurance performance can readily improve during the midluteal phase.
In addition, during menstruation’s late follicular phase, which is characterized by a surge in estrogen and suppression of progesterone, endurance performance may potentially be enhanced as well. Oosthuyse & Bosch state that estrogen alters fat, carbohydrate and protein metabolism, improving performance, whereas progesterone appears to act in opposition to estrogen. The researchers propose that increases in estrogen enhance glucose appearance and utilization by the slow-oxidative type I muscle fibers. The high estrogen content during the luteal phase augments glycogen storage, which additionally has a meaningful influence on exercise performance.
Lastly, Oosthuyse & Bosch note that both estrogen and progesterone suppress gluconeogenesis output during exercise.Gluconeogenesis is a metabolic pathway that results in the synthesis of glucose from substrates such as lactate, amino acids and odd-chain fatty acids. It is a primary mechanism the body uses to keep blood glucose levels from dropping too low during sustained exercise bouts. This suggests that toward the end of some ultralong endurance events, some women may need to enhance their fuel availability via energy (glucose) replacement supplements (Oosthuyse & Bosch 2010).
Resistance training is essential for the maintenance of musculoskeletal health. It also lowers the risk of bone deficiencies such as osteopenia (below-normal mineral density) and osteoporosis, and reduces the risk of bone fractures. Decreases in bone density and/or bone mass become a greater risk with age, especially for women. According to a review by Kaufman et al. (2013), women who have low bone mineral density have a lifetime fracture risk of up to 50%, as opposed to 13%–25% for men.
Estrogen plays a key role in bone metabolism. Estrogen deficiency tends to have a negative impact on bone turnover markers and bone mineral density (Moghadasi & Siavashpour 2013). Implementing a resistance training program may help increase levels of hormones associated with bone formation, especially in women of reproductive age. Women who are postmenopausal have lower estrogen levels, which can lead to negative effects on bone mass (Moghadasi & Siavashpour 2013).
Resistance exercise can also affect bone formation by promoting changes in growth hormone, insulin, insulin-like growth factor I, leptin and parathyroid hormone. Moghadasi & Siavashpour investigated the hormonal effects of resistance training in 20 sedentary women, aged 25 ± 3.2 years (all having normal menstrual cycles). Over 12 weeks, the women participated in a 50- to 60-minute resistance training routine 3 days per week. The program included 2–4 sets (8–12 repetitions) of eight exercises performed at 65%–85% of 1-repetition maximum (1-RM). Exercises included chest press, leg extension, shoulder press, leg curl, latissimus dorsi pull-down, leg press, arm curl and triceps extension. Before the 12 weeks, and after the study was completed, hormonal blood profiles were taken during the follicular phase of the menstrual cycle.
Results indicated significant increases in muscular strength by the end of the 12-week program, as well as increases in growth hormone, estrogen, testosterone and parathyroid hormone. The authors concluded that sedentary women who engaged in a multiset total-body resistance training program (similar in design and intensity to the present study) would experience an increase in bone formation hormones and gain other musculoskeletal health benefits.
The higher utilization of fat as a substrate in women decreases at menopause. According to Isacco, Duché & Boisseau (2012), this is most likely attributable to a decline in plasma estrogen (and progesterone) concentrations and a drop in fat-free muscle mass. However, the researchers note that obese, postmenopausal women training at approximately 50% of aerobic capacity use fat more efficiently than men of a similar age. These results were observed despite similar basal plasma estradiol concentrations in the women and men, and higher plasma free fatty acid levels in men. Thus the exercise-induced increase in plasma estrogen concentrations that is observed in premenopausal women induced may partially occur in postmenopausal obese women as well. More research is needed in this area to fully explain and understand the mechanisms involved.
An extensive review of the literature shows clearly that compared with men, women oxidize more fat and less carbohydrate at various levels of submaximal endurance exercise. This research suggests that personal trainers should incorporate a combination of the following training programs to most eﬀectively promote fat utilization for their female clients:
- low-to-moderate, long-duration, submaximal steady-state cardiovascular endurance exercise
- fast, continuous, maximal steady-state training (tell the client to train at her maximal steady-state pace)
- interval training with moderate and high-intensity intervals during the work bouts
- total-body resistance training (it improves several hormonal responses and adaptations that impressively improve muscloskeletal health)
Exercise reduces the risk of heart disease and diabetes for women while increasing stamina and improving numerous health outcomes. No other intervention can make such a strong health claim. Keep your clients moving!