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Gender Differences in Fat Metabolism
Chantal Vella, M.S. and Len Kravitz, Ph.D.

ThereThe incidence of obesity in America is currently on the rise. Approximately 25 percent of U.S. adult females and 20 percent of U.S. adult males are obese (National Heart, Lung, and Blood Institute, 2002). Over the last two decades, the number of cases of obesity in the U.S. has increased more than 50 percent (from 14.5 percent of the adult population to 22.5 percent). The primary cause of weight gain is an energy intake that constantly exceeds the amount of physical activity or energy expenditure of an individual. According to Blair and Nichaman (2002), a decrease in regular physical activity, and not an increase in energy intake, is responsible for the recent increase in obesity prevalence.

With the rising incidence of obesity there has been an increasing interest in investigating the determinants of fat metabolism (the complete breakdown of fat into usable energy) at rest and during exercise. Enhancing fat metabolism has become a key component in the battle of the bulge for many of our clients. Current research shows that, although exercise and training increase the amount of fat metabolized, there may be gender differences in the way we store and metabolize fat during rest and exercise. This article will provide an in-depth review on fat metabolism and explore the possible mechanisms involved in the differences in fat metabolism between men and women. Practical applications for prescribing exercise to maximize caloric expenditure and fat metabolism will also be presented.

How is Body Fat Stored?
Fat is stored in the body in the form of triglycerides. Triglycerides (TG) are made up of three free fatty acid (FFA) molecules held together by a molecule of glycerol (not a fat but a type of alcohol) (Robergs & Roberts, 1997). Most of our body fat is stored in fat cells which are called adipocytes. Typically, about 50,000 to 60,000 kilocalories (kcals) of energy are stored as TG in fat cells throughout the body (Coyle, 1995). Fat can also be stored as “droplets” within skeletal muscle cells. These fat droplets are called intramuscular triglycerides (IMTG) and they may hold 2000-3000 kcals of stored energy. In addition to the stores of fat, some TG travel freely in the blood. During exercise, TG in fat cells, muscle cells, and in the blood can be broken down (a process called lipolysis) and used as fuel by the exercising muscles.

Gender Differences in Fat Storage
It is well established that women generally have a higher percentage of body fat than men. A healthy range of body fat for women is 20-25%, and a healthy range of body fat for men is 10-15% (Robergs and Roberts, 1997). A body fat percentage over 20% for men or 30% for women is considered an indication of obesity. Body fat distribution varies among individuals and is a determinant of cardiovascular risk. Some people carry more of their body fat in and around the abdominal area. This type of fat deposition is called android, or apple body type and is most characteristic among males. The android body type is associated with a higher risk for cardiovascular disease. The body type most common among females is the gynoid, or pear body type. This body type is characterized by fat stores in the hip and thigh region (Robergs & Roberts, 1997). The scientific explanations for the dramatic difference in body fat distribution between men and women are largely unknown, although differences in hormones, hormone receptors, and enzyme concentrations play a contributing role. These possible mechanisms are discussed later in the section on epinephrine and lipolysis. See Sidebar 1 for two ways to determine your client’s body type and risk of cardiovascular disease.

Determining Body Type & Cardiovascular Risk
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A person’s body type is recognized as an important predictor of risk for hypertension (high blood pressure), hyperlipidemia (high cholesterol), coronary heart disease, type II diabetes, and premature death (ACSM, 2000). Individuals with more body fat in the abdominal area (android body type) are at increased risk of developing the above conditions compared with individuals who are equally fat, but have most of their fat in the hip and thigh regions (gynoid body type).

There are two ways to determine body type and health risk: waist-to-hip ratio and waist circumference. The waist-to-hip ratio is the circumference of the waist divided by the circumference of the hips. This measurement can be taken in inches or centimeters. To determine if your client has a healthy waist-to-hip ratio, use a measuring tape to measure the smallest part of the waist (usually above the belly button and below the chest) and the largest part of the hips. Make sure the measuring tape is horizontal all the way around the body when taking a measurement. When measuring the hip circumference have your client stand with their feet together. Take the measurement while standing next to your client. This will allow you to easily determine the largest and widest part of the hips. The standards for risk vary with age and sex. Ratios above .94 for young men and .82 for young women place the individual at very high risk of disease. For ages 60-69 years, ratios indicating very high risk are above 1.03 for men and .90 for women. Recently the expert panel on obesity and health risk developed the waist circumference measurement as an indicator of health risk. The waist circumference measurement is taken the same way as in the waist-to-hip ratio. A healthy waist circumference is below 102 cm (40 inches) for men and 88 cm (34 inches) for women (ACSM, 2000).

Mobilization and Metabolism of Fat
The mobilization of fat refers to the process of releasing fat from storage sites in the body, whereas, metabolism of fat is the complete biological breakdown or oxidation (which means loss of electrons) of fat into energy that can be used by the body. There are two main enzymes that regulate the mobilization of FFA: hormone sensitive lipase (HSL) and lipoprotein lipase (LPL). HSL is located directly in the fat cell and is stimulated by the hormone epinephrine. When HSL is stimulated, it acts to break apart TG in the adipose tissue and release three FFA and glycerol into the blood stream. This process is called lipolysis. Epinephrine, which is released by the sympathetic nervous system during exercise, is the primary stimulator of lipolysis (Rasmussen & Wolfe, 1999). Epinephrine binds to specific receptors on the fat cell, which in turn, activate HSL. An individual’s physiological state can affect the body’s sensitivity to epinephrine. For example, during aerobic exercise, HSL responsiveness to epinephrine is enhanced due to an increase in body temperature and a greater concentration of epinephrine in the blood stream when compared to rest. In an endurance-trained individual the HSL responsiveness to epinephrine is further enhanced, such that HSL can be activated by a lower concentration of epinephrine when compared to a non-endurance trained individual. Therefore a metabolic training effect of aerobic exercise is an enhanced receptiveness to mobilize and break apart TG for energy use. In contrast, obesity blunts the HSL responsiveness to epinephrine, meaning a higher concentration of epinephrine is needed to activate HSL in obese individuals (Rasmussen & Wolfe, 1999).

Once in the blood stream, the FFA molecules bind to albumin, a blood protein and the main transporter of FFA molecules. FFA molecules are not water soluble and thus require a protein carrier to allow them to be transported to cells and within the blood stream. Once the FFA molecules are transported to the muscle cell, they are released from albumin and carried across the muscle cell membrane by specific transporters. There are three main FFA transporters located on the muscle cell membrane: fatty acid binding protein (FABP), fatty acid translocase (FAT), and fatty acid transport protein (FATP) (Turcotte, 2000). These proteins bind the FFA molecules and transport them across the cell membrane and to the mitochondria for complete oxidation. The number of FFA transporters on the muscle cell membrane can increase with aerobic training, thus enhancing the ability to metabolize fat. The glycerol molecule released from lipolysis is circulated to the liver for oxidation and is either used as an intermediate in the breakdown of glucose or used to make more TG (Robergs and Roberts, 1997).

LPL, the second enzyme of lipolysis, is located on blood vessel walls throughout the body. Both adipose tissue and the liver have large quantities of this enzyme. LPL acts on TG within lipoproteins in the blood stream. Lipoproteins are special transporters that carry cholesterol and TG through the blood stream to fat storage depots and body cells for fuel and cellular life-support needs. The TG are broken down to FFA molecules and used as fuel by active tissues or diffuse into fat and liver cells where they are re-synthesized into TG and stored. LPL is often referred to as the “gatekeeper” that controls the distribution of fat in the various storage depots of the body (Pollock & Wilmore, 1990)

Epinephrine & Lipolysis
Epinephrine is the primary hormone that stimulates lipolysis (Rasmussen & Wolfe, 1999). Epinephrine binds to receptors on various cells throughout the body, such as adipocytes and muscle cells, and can either activate or inhibit HSL (Blaak, 2001). The two main types of epinephrine receptors are alpha and beta receptors. Epinephrine can stimulate lipolysis through the beta receptors and can inhibit lipolysis through the alpha receptors (Blaak, 2001). The type of receptor available and its sensitivity to epinephrine will determine the response of HSL in any given tissue. It is interesting to note that alpha and beta receptors can be located on the same cells, however, depending on which receptor is more abundant and available for epinephrine binding determines the response of HSL. For example, research has shown that abdominal adipocytes are more sensitive to beta receptor stimulation by epinephrine than hip and thigh adipocytes in both men and women (Braun & Horton, 2001). This finding suggests that fat around the abdominal area is easier to mobilize than fat located in the hip and thigh areas. In addition, women tend to have a greater number of alpha receptors in the hip and thigh regions (Blaak, 2001). This would tend to favor the storage of fat, as opposed to the mobilization of fat, in the hip and thigh region. The differences in the type and number of cell receptors may be one of the mechanisms contributing to the differences in fat distribution between men and women (Blaak, 2001). Another mechanism contributing to the differences in fat distribution between men and women is the concentration of LPL in various tissues. Women have a higher LPL concentration and activity in the hip and thigh region compared to the abdominal region (Pollock & Wilmore, 1990).

Estrogen & Lipolysis
The female hormone estrogen may have a positive effect on resting and exercise fat metabolism. Although there appears to be a connection between estrogen and increased fat metabolism, the mechanisms are not fully understood. Research has suggested that estrogen may aid in the mobilization of fat from adipose tissue. There are several proposed mechanisms for this increase in fat mobilization. First estrogen has been found to inhibit the hormone LPL (Ashley et al., 2000). Remember that LPL is responsible for the breakdown of TG in the blood stream for storage in adipose tissue or fuel for active tissues. Second, estrogen has been shown to enhance epinephrine production. A higher concentration of epinephrine would increase the activity of HSL, the hormone responsible for adipose tissue lipolysis.

Estrogen has also been reported to stimulate the production of growth hormone (GH). Growth hormone inhibits the uptake of glucose (carbohydrate) by active tissues and increases the mobilization of FFA from adipose tissue (Robergs & Roberts, 1997). GH works by inhibiting insulin production from the pancreas and stimulating HSL (Ashley et al., 2000). Insulin is the main hormone that promotes glucose transport into muscle cells to be used as energy, and it is a potent inhibitor of HSL. Estrogen may enhance fat metabolism by increasing the production of GH and inhibiting the production of insulin. In turn, this would decrease glucose metabolism and increase FFA utilization (Ashley et al., 2000).

Another factor that could promote a higher fat metabolism in women is an increase in blood flow to adipose tissue, especially during exercise (Braun & Horton, 2001). Estrogen has been shown to cause a vasodilation (widening) in blood vessels, but it is not yet known if this vasodilation is specific to adipose tissue perfusion (flow of blood into the tissue) or a general effect on the entire vasculature in the body. Estrogen also increases the production of the hormone Nitric Oxide (NO). NO, which is produced by cells that line the blood vessels, causes a relaxation of the smooth muscle that surrounds blood vessels leading to vasodilation. If women maintained a higher blood flow to the adipose tissue, interaction between epinephrine and adipose tissue beta receptors would be increased. Additionally, this could enhance FFA transport from adipose tissue to active muscles during exercise.

Fat Metabolism at Rest
The level of fat metabolism at rest is positively correlated with the size of fat cells in the body, with larger fat cells having a higher lipolytic (causing TG splitting) activity (Blaak, 2001). In earlier research it was hypothesized that women may have a higher resting fat metabolism due to typically higher body fat stores when compared to men. However, recent research has found that resting fat metabolism (adjusted for differences in lean body mass) is actually lower in women than in men (Nagy et al., 1996; Toth et al., 1998). Although the mechanisms are unclear, this finding suggests that a lower resting fat metabolism may contribute to the increased fat storage in women as compared with men.

Fat Metabolism During Exercise
Intramuscular triglycerides (IMTG) are an important source of fuel during moderate to high intensity exercise. It’s estimated that up to 50% of fat oxidized during moderate to intense exercise is derived from IMTG (Robergs & Roberts, 1997; Coyle, 1995). The majority of the rest comes from adipose tissue and the least comes from TG in the blood stream. The process of IMTG lipolysis is similar to adipose tissue lipolysis. During exercise, increasing levels of epinephrine activate HSL to begin IMTG breakdown. The FFA molecules that are released from IMTG are located within the muscle cell, therefore, they can be transported directly to the mitochondria for oxidation. The glycerol molecule released is either transported to the liver for oxidation or recycled to form additional IMTG stores (Robergs & Roberts, 1997).

The majority of the research shows that women derive a greater proportion of their energy expenditure from fats during low to moderate intensity exercise, relative to men. Research is still discerning the possible mechanisms leading to these gender differences.

Gender Differences in Fuel Selection
One of the most common methods used to determine fuel selection is the respiratory exchange ratio (RER). The RER is a numeric index of carbohydrate and fat utilization based on a ratio of carbon dioxide produced to oxygen consumed. A lower RER is an indication of a greater fat metabolism, whereas a higher RER is an indication of a greater carbohydrate metabolism. Current studies show that during low to moderate intensity exercise women maintain a lower RER when compared to men. In a study by Tarnopolsky et al. (1990), male and female subjects were matched for training status and performance experience. These researchers reported significant gender differences in RER values during moderate intensity exercise. Throughout the 90 minute run at 65% VO2max, females had significantly lower RER values compared with males, indicating an increased reliance on fats as fuel. The calculated energy expenditure (EE) from fat was 428.4 kcals for the women (42% of total EE) and 242.1 kcals for the men (20% of total EE) (See Figure 1). This data was supported by the muscle biopsy data that showed greater muscle glycogen depletion in male subjects when compared to female subjects. In a similar study by Horton and colleagues (1998) significant gender-based differences in fat metabolism during exercise were also reported. Women had significantly lower RER values compared with men during 2 hours of exercise at 40% VO2max. The percent of fat metabolized during exercise averaged 43.7% for the men and 50.9% for the women. Blatchford and colleagues (1985) studied gender differences in fat metabolism during 90 minutes of treadmill walking at 35% VO2max in untrained men and women. Women had significantly lower RER values compared with men at both 45 and 90 minutes of exercise. Both groups gradually increased the percent of fat metabolized during exercise, with the 90-minute values being 59% for the men and 73% for the women.

Froberg & Pedersen (1984) reported that women subjects exercised for a significantly longer period of time than age- and training-matched male subjects at 80% VO2 max. The women also had significantly lower RER values during exercise when compared to the men. These researchers concluded that the greater performance in women was due to a greater reliance on fats as fuel during exercise and a sparing of muscle glycogen.

What Exercise Intensity Burns the Most Fat?
During low intensity exercise the majority of energy (kcals) comes from fat. As exercise intensity increases, the percent of energy derived from fat decreases. However, the absolute amount of energy derived from fat is actually increased! As exercise intensity increases, so does total energy expenditure (caloric expenditure). Even though a smaller percentage of the energy expenditure is coming from fat, more kcals of fat are burned because there is a greater absolute energy expenditure. Therefore, expressing energy derived from fat as a percentage of energy expenditure without considering the total energy expenditure is misleading.

Another consideration is the effect that exercise has on energy expenditure after exercise. Following a high intensity exercise bout, the rate of metabolism is elevated for a slightly longer period of time (when compared to a lower exercise bout), and more energy is expended as your body returns to homeostasis (resting conditions). With regular aerobic exercise, this post-exercise energy expenditure will positively contribute to weight loss goals.

Gender Differences in Muscle Glycogen Depletion
Muscle glycogen concentration is another common technique used to determine fuel utilization during exercise. Muscle glycogen is the storage form of carbohydrate that is located within the muscle cells. Tarnopolsky and others (1990) compared the muscle glycogen depletion patterns of both trained males and females (matched for training status and performance experience) during a 90-minute run at 65% VO2 max. Although muscle glycogen levels were similar between males and females prior to the exercise bout, post exercise biopsy (removing of tissue for analysis) data indicated a significant difference between genders. Glycogen depletion was 25% greater in males compared with females. This was in agreement with the lower RER data reported for females, indicating a greater reliance on fats as fuel during submaximal exercise.

Gender Differences in Epinephrine Concentrations
Studies examining the hormonal responses to exercise have reported greater epinephrine concentrations during submaximal exercise in men when compared to women. Assuming a lower RER response in women during exercise, these findings indicate that women may be more sensitive to the lipolytic actions of epinephrine and are able to metabolize fat more effectively. Tarnopolsky et al. (1990) reported lower epinephrine concentrations during submaximal exercise in females when compared to equally trained males. This was in addition to lower RER values during exercise and less glycogen depletion post exercise in females. Horton and colleagues (1998) also reported that epinephrine levels were significantly lower in women than in men during exercise at 40% VO2max, again suggesting a greater sensitivity to the lipolytic action of epinephrine in women.

Gender Differences in Free Fatty Acid and Glycerol Concentrations
Plasma (fluid portion of blood) concentrations of FFA and glycerol are both indicators of adipose tissue lipolysis. As adipose tissue lipolysis increases, plasma concentrations of FFA and glycerol increase. Several investigators have studied the gender differences in plasma FFA and glycerol concentrations, in response to submaximal exercise. Blatchford et al. (1985) reported significant gender differences in FFA and glycerol concentrations when males and females (matched for training status) exercised at 35% VO2max. At both 45 and 90 minutes of exercise, plasma FFA values were higher in females than in males. In addition, at 45 minutes plasma glycerol levels were significantly higher in females than in males. Horton and colleagues (1998) also found significant gender-based differences in FFA and glycerol concentrations during exercise. Plasma FFA and glycerol concentrations were reportedly higher in females than in males during submaximal exercise.

Tarnopolsky et al. (1990) reported significantly lower RER values in females during exercise at 65% VO2 max, however, this was not accompanied by an increase in plasma concentration of FFA or glycerol in these subjects. These researchers hypothesized that the increase in fat metabolism in women was due to a higher utilization of IMTG (which do not increase plasma concentrations of FFA or glycerol), as opposed to a greater adipose tissue lipolysis. The above findings on gender differences in FFA and glycerol concentrations indicate that, in women, beta receptor sensitivity for lipolysis may be increased, alpha receptor sensitivity may be decreased, or IMTG contribute to a higher percentage of fat metabolism during exercise.

In addition to the above findings, it has been reported that IMTG stores are higher in women than in men (Blaak, 2001; Braun & Horton, 2001). This finding suggests the possibility that a higher IMTG oxidation may contribute to the increased fat oxidation and glycogen sparing in women during exercise. It has also been reported that women have a higher expression of FFA transport proteins (FATP, FABP, FAT) in skeletal muscle cells (Blaak, 2001). With an increase in FFA transport proteins the amount of FFA entering the muscle cell is augmented and the FFA available for oxidation in the mitochondria (organelle of cell responsible for energy production) is increased. An increase in FFA transport into the muscle cell could also contribute to an increased FFA storage into IMTG.

The Bottom Line
There are distinct differences in the mobilization, metabolism, and storage of fat between genders (summarized in Table 1). Most of the current research is finding that the proportion of energy derived from fat is increased during low to moderate intensity exercise in women as compared to men. Although there is a handful of research on this topic, additional research is needed to determine the exact mechanisms involved in this difference between genders and why the increase in fat metabolism is evident during exercise but not at rest. Differences in percent body fat, distribution of body fat, hormonal responses to exercise, and hormone receptor type and sensitivity may all contribute to gender-related differences in fat metabolism.

New Implications for Designing Aerobic Exercise Programs
Often times a review of literature will uncover fresh findings, introduce new ideas for research, or indicate modern opportunities for practical application. From this review on gender differences in fat metabolism, some cardiorespiratory training implications for optimal fat metabolism are presented.

The foundational research on the development and maintenance of cardiorespiratory fitness recommends performing endurance exercise, 3 to 5 days per week, on an exercise mode that involves the major muscles groups (in a rhythmic nature) for a prolonged time period (ACSM 2000). This includes physical activities such as step aerobics, aqua exercise, cardio kick-boxing, rowing and walking. The ACSM recommends an intensity of exercise between 55/65% to 90% of maximum heart rate (or 40/50% to 85% of oxygen uptake reserve), with a continuous duration of 20 to 60 minutes per session. Inherent in the exercise prescription is the concept of individualizing the program for each person’s fitness level, health, age, personal goals, risk factor profile, medications, behavioral characteristics, and individual preferences. The ACSM recommendations appropriately serve as the framework for the cardiorespiratory fitness prescription for healthy males and females that follows.

Expounding from this review of literature, it appears a contemporary approach, with regards to fat metabolism, may be suggested. Initially, the concept of periodizing aerobic training programs, that has become so popular in resistance training, is advocated. Periodization training is based on an inverse relationship between intensity (how hard) and volume (total repetitions) of training (Stone et al, 1999). With aerobic exercise, intensity can be individualized with %heart rate max, %VO2 max, or ratings of perceived exertion, where as volume is differentiated by the duration of the session, as well as the frequency of sessions.

Here are some specific periodization suggestions from which to individualize the prescription for optimizing fat metabolism, during aerobic exercise:
1) Regularly incorporate cardiorespiratory workouts that are low intensity for a longer duration. Rationale: The majority of the research shows that women derive a greater proportion of their energy expenditure from fats during low to moderate intensity exercise, relative to men. Thus, this will improve fat metabolism, particularly for females.
2) Incorporate some cardiorespiratory workouts that are of higher intensity for a shorter period of time. This may best be realized with high intensity continuous training or perhaps with interval training. Rationale: As exercise intensity increases, the percent of energy derived from fat decreases. However, the absolute amount of energy derived from fat is actually increased, for males and females. As exercise intensity increases, so does total energy expenditure (caloric expenditure). Even though a smaller percentage of the energy expenditure is coming from fat, more kcals of fat are burned, because there is a greater absolute energy expenditure.
3) Incorporate various modes of training, often referred to as cross-training (Kravitz & Vella, 2002). Rationale: The theory of multi-mode training implies that by training on different modes of exercise, the body is averted from getting overly fatigued and from overuse of the same muscles in the same movement patterns. This helps to thwart the occurrence of musculoskeletal system stress, aiding in the prevention of muscle soreness and injuries. Therefore, theoretically, a person will be able to safely do more work, more frequently, which equates to higher total energy expenditure and fat utilization.
4) Vary the above workout designs regularly! Endeavor to find a satisfactory method for each client, or students in a group-led class, where cardiorespirtory workouts vary either within each week, weekly, bi-weekly, or any combination of all. Rationale: Similar to the above, varying the workouts provides a new stimulus to the body’s cardiorespiratory system in an effort to avoid the consequences of overuse exercise fatigue.

Adipose Tissue Information
Adipose tissue is a form of connective tissue composed of cells (adipocytes) that are separated by a matrix of collagenous and elastic fibers. Body fat accumulates by filling existing adipocytes causing an increase in size (hypertrophy) and by the formation of new fat cells (hyperplasia). Normally, fat stores increase from birth to maturity by a combination of hypertrophy and hyperplasia. Obese adults typically have 60 to 100 billion fat cells, compared with 30 to 50 billion for non-obese adults (Pollock & Willmore, 1990). Early research indicated that fat cell number increased markedly during the first year of life, increased gradually until puberty, and then increased markedly again for a period of several years, with the maximum number of cells becoming fixed by adulthood. Current evidence suggests that fat cell size and number can be increased at any age. The exact mechanism for hyperplasia is still unknown, however, it is hypothesized that fat cells have a certain capacity and once that capacity is reached a new cell will be formed (Pollock & Willmore, 1990). Interestingly, fat cells can increase or decrease in size, but once a fat cell develops it is a permanent cell in your body, except for way of liposuction.

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