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The objective of this study was aimed to determine gender-based differences in fuel metabolism in response to long duration exercise. Another way of saying is through a simple question . . . Which gender metabolizes a greater amount of fats vs. carbohydrates at the same relative intensity level?
Fuel metabolism responses from exercise were compared in a sample of 14 men and 13 women during a 2 hour (40% of maximal of uptake) of cycling and a 2 hour post-exercise recovery period. One of the women was released form the experiment because of the onset of menstruation, which the increased flux of hormones would deviate her results. The 27 subjects in the sample also completed a separate control day in which no exercise was performed and testing was performed. It should be noted that the (seven) trained subjects from each gender were competitive cyclist, and that all of the untrained subjects performed aerobic exercise less then 90 hours a weeks. All subjects were of normal weight, were nondiabetic and nonsmokers, and were not suffering from any medical conditions or taking any sort of medication. All of the subjects had their body composition taken, VO2 max and resting VO2 determined, and a health and physical examination performed in advance of the experiment at the University of Colorado’s Health Sciences Center. The purpose of this was so the subjects could all perform at the same relative intensity level and thus the results could be compared cross-sectionally. In the prior three days before the experiment, as well as during it, all subjects were fed a controlled diet consisting of 30% fat, 15% protein, and 55% carbohydrates. The purpose behind all of this was so that the independent variable would be the 40% relative intensity level performed by all subjects and the dependent variable was the change in each of the subject’s metabolism.
Fuel oxidation was measured using indirect calorimetry using the metabolic cart and Hans Rudolf masks. Measurements were made continuously for periods of 30 min, with recalibration performed between measurements. Blood samples were drawn for the determination of circulating substrate and hormone levels every 30 minutes on exercise day, and every hour during control day. Results were obtained and conclusions were able to be drawn a follows.
Although a cross sectional study had been conducted, the trained and untrained aspects of the experiment resulted to be less significant than intended. In other words, in comparing trained men against trained women, and untrained men against untrained women, no significant differences could be drawn due to training. There was significant change in the dependent variable from the data obtained that conclusions were drawn from. The first is shown in Figure 1. The relationship shows the fat free mass of each gender against the subject’s resting metabolic rate. The resting metabolic rate on cycling and controlled days were averaged for all figures. For women, r=. 52 and for men, r=. 67. This already shows that women have a lower value and are more likely to have a greater lipid metabolism than men. As exercise intensity increases, the value of r increases toward a value of one. This is an anaerobic, carbohydrate-metabolizing zone. Figure 2A is a comparison of lactate levels against time for both genders. Men show a higher level of lactate throughout the entire timeline. Lactate, or lactic acid, is a result from the non-oxidative production of ATP, also know as glycolysis. The source for this is glucose, which comes form carbohydrates. It can inferred that men were using more CHO and less fat in their metabolism than women. Figure 2B shows that men had a significantly higher level of glucose in the blood over time. The reason for this is the same as mentioned above. Men were at a higher r-value than women and thus were closer to the value of one and were in the non-oxidative metabolism zone. More blood glucose means a higher r-value. In Figure 2C, women show a higher level of free fatty acids in the blood over the course of time. This is by a descent margin. Once again, this correlates with all of the other figures thus far. Women are using more fat as a source of fuel than men. Figure 3A and B deal with hormones found in both genders. Epinephrine and norepinephrine are hormones released by the body that stimulate the release of glucose into the blood from cells. Higher levels of glucose indicate that a higher metabolism of carbohydrate. Once again, this shows that women have a higher fat metabolism than men do when exercising at the same relative intensity. This study has produced a numbers of results that can lead to a conclusion on the effects of endurance exercise on metabolism depending on gender. During endurance exercise, there are many factors that all contribute to women having a lower r-value than men, and thus using more of a fat metabolism at the same relative intensity level.
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