The maximal rate of fat oxidation is defined as the highest observed use of fat as an energy source during oxidative metabolism (1). The maximal rate of fat oxidation is typically determined during a graded exercise test to exhaustion (1,2,9). Information provided from this type of testing can be used to monitor changes to fat oxidation rates during training and to develop effective exercise prescriptions.
To date, most research has focused mainly on determining the maximal fat oxidation rates of men (1,2,6,11,18) and there has been less research examining women and the maximal fat oxidation rate (9,10). Friedlander et al. (9) found that women do not increase their rate of fat oxidation at the same relative exercise intensities as men, thus indicating that women and men have different fat oxidations at a given exercise intensity. Compared with previous research conducted on men, women had a higher fat oxidation rate at the same relative workload after training than men (9,10). These findings suggest that muscle and adipose tissue metabolism may be different in men and women (9). Consequently, the rates of fat oxidation and the corresponding exercise intensity determined from studies in men may not be generalizable to women.
Little is known about the contribution of body composition to the maximal fat oxidation rate. Elevated fat mass (FM) allows for increased fasting free fatty acid levels in the blood and increased uptake by the muscles, promoting higher rates of fat oxidation (19). Because of the increase in fat oxidation, it can be assumed that in the long-term maintenance of fat and energy balance, this increase would serve as a significant lipostatic factor (20). For example, a cross-sectional study of 106 obese women maintaining stable weight demonstrated that postabsorptive fat oxidation was positively correlated (r = 0.56, p < 0.001) with FM (20). Schutz et al. (20) also examined 24 obese women in an effort to clarify the long-term adaptation in fat oxidation resulting from body fat loss. The subjects lost an average of 12.7 kg of body weight and 9.8 kg of body fat. The reduction in fat oxidation was identical to the regression coefficient found in the cross-sectional study. The researchers concluded that changes in FM significantly affected fat oxidation and that this process may contribute to the long-term relationship between fat and energy balance in obese individuals (20).
Additional research is needed to determine if differences in maximal fat oxidation rates occur in women with different body fat levels. If the rate of maximal fat oxidation is affected by body fat, this information can help personal trainers and health practitioners prescribe more effective exercise protocols to women desiring to prevent weight gain and promote body fat and body weight loss. This in turn may help women effectively decrease their risk for health disorders, such as obesity, insulin resistance, diabetes, and cardiovascular disease. The purpose of this study was to use a graded exercise test to exhaustion to examine the hypothesis that body fat would affect the maximal rate of fat oxidation in young women. We hypothesized that higher body fat levels would result in a higher maximal fat oxidation rate.
Experimental Approach to the Problem
We used an experimental design to examine the influence of body fat levels on the determination of the maximal rate of fat oxidation in women. All subjects were tested on 2 separate occasions, with each session performed 48 hours apart. The subjects refrained from vigorous exercise 24 hours before testing and were tested after a 4-hour fast. To reduce the influence of previous food consumption on the substrate response during exercise, the subjects were instructed to maintain their normal diet throughout the study. On the first day of testing, the subjects reported to the Human Performance Laboratory where they were measured for height and weight. The subjects then performed a graded exercise test to exhaustion on a treadmill. On the second day of testing, the subjects reported to the laboratory where they were assessed for body mass and body composition. Immediately thereafter, subjects performed a treadmill fat oxidation test to determine the exercise intensity that elicits maximal fat oxidation. The test was stopped when the subjects reached a respiratory exchange ratio (RER) of 1.0 (2).
Fourteen active, healthy young women (age range, 21–31 years) participated in this study, which was approved by the Human Subjects Institutional Review Board at Miami University. All testing was performed after the subject signed an informed consent and completed a health history questionnaire in accordance with university guidelines on human experimentation. Women with greater than 25% body fat were considered part of the higher-fat group, and women with less than 25% body fat were considered part of the lower-fat group. The 25% body fat criterion measure for separation into groups was derived from data provided in the American College of Sports Medicine's Guidelines for Exercise Testing and Prescription manual (3), with the value of >25% approximating a poor level of body composition of individuals in the age range of 20–29 year. Subjects refrained from previous exercise on the days of testing. Inclusionary criteria were 15–35% body fat, weight stable (±2.3 kg) for at least 2 months before the start of the investigation, a normal menstrual cycle (≥8 cycles per year), and physically active as defined by participating in at least 3 aerobic exercise training sessions per week for at least 2 months before the start of the study. Menstrual cycle changes in estrogen and progesterone production during a normal menstrual cycle seem to only have a minor effect on free fatty acid mobilization; therefore, we only required a normal menstrual cycle and did not control for phase of cycle (13). Potential subjects were excluded from the study if they had 2 or more risk factors on the health history questionnaire, were using any medications that could affect exercise or metabolism, were a current smoker, or consumed a vegetarian diet.
Body Mass and Composition
Height was determined using a stadiometer, and body mass was measured using a calibrated electronic scale. Body composition was assessed using air displacement plethysmography (Bod Pod; Life Measurement, Inc., Concord, CA, USA) according to the manufacturer's instructions. All subjects voided before testing and were measured wearing only a 1-piece nylon swimsuit and cap. Percent body fat (% fat) and body mass were used to calculate the amount of FM and fat-free mass (FFM) for each subject.
Maximal Oxygen Consumption Test
Each participant performed a graded exercise test to exhaustion to determine maximal oxygen consumption (V[Combining Dot Above]O2max). The subjects walked at 5.6 km·h−1 and 0% grade for 2 minutes. The treadmill was then increased to the subject's desired running speed (range, 8.0–10.8 km·h−1) at a 0% grade. The grade of the treadmill was then increased by 2% every 2 minutes until the subjects reached volitional exhaustion. Expired air was measured for oxygen and carbon dioxide concentrations at 1-minute interval using a Parvo Medics metabolic measurement cart. This system was calibrated before each test according to the manufacturer's instructions. A test was considered maximal if the subject achieved any 3 of the 4 following criteria: a plateau in oxygen consumption (<2.0 ml·kg−1·min−1) with an increase in exercise intensity, RER ≥1.10, a maximal heart rate within ±10 beats per minute of age-predicted values, and volitional exhaustion (7). All subjects reached at least 3 of the 4 criteria during testing.
Maximal Fat Oxidation Test
The participants performed a graded exercise test on the treadmill to determine individual maximal fat oxidation rates. The treadmill protocol used in this study was modified from previous research (2). Briefly, the participants walked at 3.5 km·h−1 at a 1% grade for 3 minutes. The speed of the treadmill was increased by 0.9 km·h−1 every 3 minutes until the subjects reached 9.3 km·h−1. Thereafter, the grade of the treadmill was increased by 2% every 3 minutes until the participants' RER reached 1.0 and the test was stopped.
Heart rate, RER, volume of oxygen consumed (V[Combining Dot Above]O2), and volume of carbon dioxide expired (V[Combining Dot Above]CO2) were recorded every minute using a Parvo Medics metabolic measurement cart. The metabolic cart was calibrated before each test according to the manufacturer's instructions. After the completion of testing, the V[Combining Dot Above]O2 and V[Combining Dot Above]CO2 values were averaged for the last 2 minutes of every stage (1,21). The values for fat and carbohydrate oxidation were determined using stoichiometric equations and appropriate energy equivalents (2,8). The results of the maximal fat oxidation test were used to construct a curve of fat oxidation (in gram per minute; in milligram per kilogram of FFM per minute) vs. exercise intensity, expressed as a percent of V[Combining Dot Above]O2max. The curves were used to determine maximal fat oxidation and the exercise intensity at which the maximal rate of fat oxidation was observed.
Diet Control and Analysis
Before the start of the study, participants were shown food samples and given detailed instructions for the accurate recording of food content and quantity. Participants recorded all food and fluid intake for 1 day before the testing session and were instructed to maintain their normal diet throughout the study. Dietary analyses were performed using commercially available software (Food Processor, version 11.0; EHSA Research, Salem, OR, USA). All subjects were tested after a 4-hour fast.
Experimental data are presented as mean ± SD unless stated otherwise. A series of analyses of variance were used to identify differences between the lower-fat and higher-fat groups for dietary analysis, maximal fat oxidation rates, and the exercise intensity that elicited the maximal fat oxidation rate. Pearson's correlations were used to identify significant relationships between percent body fat, FM, FFM, V[Combining Dot Above]O2max (absolute and relative), and maximal fat oxidation rates. A linear regression was run to determine if % fat, FM, FFM, or V[Combining Dot Above]O2max significantly predicted the maximal rate of fat oxidation during testing. The relationship between fat oxidation rates and relative exercise intensities for the lower-fat and higher-fat groups was estimated using a linear mixed effects model with a random intercept for each individual (15). Curves were fitted using the lme function within the nlme package in the statistical software R (16,17). For all analyses, statistical significance was accepted at p ≤ 0.05.
Physical characteristics of the 14 subjects who completed the study are shown in Table 1. There were no significant differences between the lower-fat and higher-fat women in most of the anthropometrical variables (height, body mass, FFM, or V[Combining Dot Above]O2max). The groups did have significant differences in the % fat and FM (p ≤ 0.05), with the higher-fat group having higher values than the lower-fat group.
There were no significant differences in total kilocalories from fat between the lower-fat group (554.9 ± 100.3) and the higher-fat group (754.03 ± 233). The total grams per kilogram body mass of carbohydrate, fat, and protein consumed were also not significantly different between the lower-fat group (4.31 ± 0.49, 1.07 ± 0.22, and 1.26 ± 0.18 g·kg−1 BM, respectively) and the higher-fat group (5.36 ± 1.04, 1.27 ± 0.35, and 1.19 ± 0.26 g·kg−1 BM, respectively).
Maximal Fat Oxidation Rates
There was no significant difference in maximal fat oxidation rates between the lower-fat group (0.39 ± 0.10 g·min−1, 8.52 ± 2.69 mg·kg−1 FFM·min−1) and the higher fat group (0.49 ± 0.13 g·min−1, 10.81 ± 2.80 mg·kg−1 FFM·min−1) (Table 2). Maximal fat oxidation occurred at an exercise intensity of 55.7 ± 11.1% and 59.1 ± 5.4% V[Combining Dot Above]O2max for the lower-fat and higher-fat groups, respectively (Figure 1). There was no significant difference between the exercise intensity that elicited the maximal fat oxidation between the lower-fat and higher-fat groups (p ≤ 0.05). No significant differences were observed in fat oxidation rates at various exercise intensities for the lower-fat and higher-fat groups (Figure 1).
Because there were no significant differences in fat oxidation and carbohydrate oxidation between the 2 groups, the groups were collapsed for the correlation analysis. The maximal fat oxidation rate (absolute) was not significantly correlated with any of the descriptive variables (FFM, % fat, FM, or absolute and relative V[Combining Dot Above]O2max). Fat-free mass was significantly and positively related to the absolute (r (13) = 0.73, p < 0.01) and relative maximal rate of carbohydrate oxidation (r (13) = 0.58, p ≤ 0.05). There was no significant relationship between FM and the maximal rate of fat oxidation (Figure 2).
An important finding from this research is that there were no significant differences in the maximal fat oxidation rate and the exercise intensity that elicited that rate between the lower-fat and higher-fat groups of women. Another key result is that FM, FFM, % fat, and absolute and relative V[Combining Dot Above]O2max were not significantly correlated to maximal fat oxidation rates. These results indicate that maximal fat oxidation rates, as determined during a graded exercise test, are not affected by body fat levels in young physically active women with different % fat (range, 18.6–30.0%).
Substrate supply can affect whole-body substrate oxidation rates both during rest and exercise. Several investigations have demonstrated that body fat levels can specifically affect fat and carbohydrate oxidation. For example, Schutz et al. (20) investigated obese women maintaining stable weight and obese women losing weight and demonstrated that postabsorptive resting fat oxidation was positively correlated with FM, which led to the conclusion that FM significantly affected fat oxidation. Additionally, Astrup et al. (4) examined 24-hour energy expenditure (EE) and fat oxidation rates in women who were classified as obese (body mass index [BMI] > 30 kg·m−2) and women who were classified as normal weight (BMI < 25 kg·m−2). The authors found that obese women had higher levels of lipid oxidation compared with the normal weight women, and these differences could be explained by body composition and body size (4). When comparing the obese and normal weight subgroups, the obese women had significantly higher 24-hour EE, lipid oxidation, and carbohydrate oxidation. These results led the researchers to believe that higher fat availability increases substrate supply and can lead to increased fat oxidation (4,9).
It is important to note the FM, body mass, and % fat ranges were different for the current study when compared with the aforementioned research. Schutz et al. (20) examined women who ranged from 62.0 to 111.7 kg with percent body fat ranges from 31.0 to 44.7%, whereas Astrup et al. (4) examined obese women with an average mass of 93.2 kg and FM of 40.8 kg and normal weight women with an average mass of 61.0 kg and FM of 17.4 kg. In the present study, women had an average mass of 61.2 kg (range, 51.6–69.9 kg), an average FM of 15.4 kg (range, 10–20 kg), and an average percent body fat of 24.9% (range, 18.6–30%). The women in the current study fall into 1 of the 2 body weight categories, normal or overweight, with none of the women being classified as obese. The influence of increased FM on substrate oxidation is thought to be mediated by increased levels of free fatty acids (4). Because free fatty acid levels are mainly controlled by substrate availability, increases in FM tend to promote fat mobilization and ultimately increase fat oxidation (4,20). Because none of the women in the present study were obese, this could serve as a potential explanation for why the higher-fat women in the present study did not have significantly higher maximal fat oxidation rates than the lower-fat women. It is also important to note that there was no significant difference between FFM between the 2 groups. Muscle (FFM) is the metabolically active tissue that is involved in fat oxidation (5). If both groups in the present study have the same amount of metabolically active tissue, it can be hypothesized that the uptake of fasting free fatty acids could be the same, which could account for the lack of a significant difference in maximal fat oxidation rate between the 2 groups. Also worth mentioning is the fact that the lower- and higher-fat women did not have any differences in their intake of carbohydrate, fat, or protein consumed per kilogram body weight. Dissimilarities in macronutrient composition could potentially lead to differences in the maximal fat oxidation rate in young women.
Several previous investigations have found significant positive correlations between FM and fat oxidation rates. The results of the present study showed no statistically significant correlation between fat oxidation rates and FM, FFM, % fat, and V[Combining Dot Above]O2max (in both absolute and relative values). The fact that all the women in the present study were either normal weight or overweight could explain why there are no significant correlations between these variables and maximal fat oxidation rates. When comparing obese women with their postobese state, Schutz et al. (20) found that fat oxidation fell 42% (2.00 ± 0.95 g·h−1 to 1.17 ± 1.08 g·h−1) with weight loss. This shows that there is a relationship between FM and fat oxidation when comparing obese women with normal weight women who have had significant weight loss. A possible explanation for the lack of a significant difference in the current study could lie in the level of plasma free fatty acids. It is known that obese women have higher resting levels of fatty acids in the blood compared with normal weight individuals (12), and this could likely contribute to higher fat oxidation rates. Once again, the fact that none of the subjects in the higher-fat group were obese could mean that plasma fatty acid levels were normal in all subjects. This would mean that substrate supply was similar in both groups. It is unfortunate that plasma fatty acid levels were not measured in the current study and this remains an area for future investigation.
The range of exercise intensity that elicited the maximal fat oxidation rate has been shown to be between 59.2 and 64.0% of V[Combining Dot Above]O2max (1,2); and these values are similar to the current study. Research has shown that one of the benefits of high physical fitness is a higher rate of fat oxidation during exercise (14). However, in a study by Kriketos et al. (14) that examined the effects of aerobic fitness on fat oxidation and body fatness, it was observed that V[Combining Dot Above]O2max and fat oxidation were significantly related in men but not in women. These findings suggest that physical fitness in women may contribute to substrate oxidation differently than men (14). This could be because of the fact that women have other factors that influence substrate oxidation other than body composition, such as fat oxidation capacity (FFM) or different hormonal levels (14). Further research in the difference between men and women and the regulation of fat oxidation is still needed.
The present study has some potential limitations. The study participants were all young women (21–31 years), nonobese (average % fat was 24.9%; range, 18.6–30%), and all physically fit (mean V[Combining Dot Above]O2max 63.2 ml·kg−1 FFM·min−1; range, 57.5–68.9 ml·kg−1 FFM·min−1). These factors make it difficult to generalize the findings to women who are not classified as normal or overweight with good levels of cardiovascular fitness. The range we used for categorizing the lower-fat and higher-fat groups could also have been a limitation. The cutoff point for separating the women into lower-fat and higher-fat groups was 25%. Some of the subjects had body fat percentages that put them on the border for each group (e.g., 24.5 and 25.5% fat). This small variability between some of the lower-fat and higher-fat women could also partially explain why there are no significant differences between the 2 group's maximal fat oxidation rates.
When working with physically active individuals, it is often important to identify the exercise intensity that elicits the maximal rate of fat oxidation. The identified intensity can then be used to prescribe an optimal exercise or training prescription that can promote the highest level of fat utilization for either promoting alterations in body composition, changes in body weight, or improvements in health or optimizing substrate oxidation for endurance activities. The findings of the present study suggest that body fat does not affect the maximal fat oxidation rate or the exercise intensity that elicits that rate in young, physically active, healthy women of normal and overweight status. These findings indicate that the substrate oxidation results derived from a maximal fat oxidation test may be used by strength and conditioning specialists or personal trainers to accurately prescribe an exercise intensity that will maximize the utilization of fat as an energy source if the women tested are of normal or overweight status. Exercise programs that target the maximal use of fat will help reduce overall body mass and promote fat loss. These body changes could help normal and overweight women reduce their risk for health disparities, such as obesity, diabetes, and cardiovascular disease.
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