European Journal of Applied Physiology

, Volume 111, Issue 6, pp 1027–1034

No effect of menstrual cycle phase on fuel oxidation during exercise in rowers

Authors

  • Sille Vaiksaar
    • Institute of Sport Pedagogy and Coaching Sciences, Centre of Behavioral, Social and Health SciencesUniversity of Tartu
    • Institute of Sport Pedagogy and Coaching Sciences, Centre of Behavioral, Social and Health SciencesUniversity of Tartu
  • Jarek Mäestu
    • Institute of Sport Pedagogy and Coaching Sciences, Centre of Behavioral, Social and Health SciencesUniversity of Tartu
  • Priit Purge
    • Institute of Sport Pedagogy and Coaching Sciences, Centre of Behavioral, Social and Health SciencesUniversity of Tartu
  • Svetlana Kalytka
    • National University of Physical Education and Sports of Ukraine
  • Larissa Shakhlina
    • National University of Physical Education and Sports of Ukraine
  • Toivo Jürimäe
    • Institute of Sport Pedagogy and Coaching Sciences, Centre of Behavioral, Social and Health SciencesUniversity of Tartu
Original Article

DOI: 10.1007/s00421-010-1730-1

Cite this article as:
Vaiksaar, S., Jürimäe, J., Mäestu, J. et al. Eur J Appl Physiol (2011) 111: 1027. doi:10.1007/s00421-010-1730-1

Abstract

The aim of this investigation was to examine the effects of menstrual cycle phase on substrate oxidation and lactate concentration during exercise. Eleven eumenorrheic female rowers (18.4 ± 1.9 years; 172.0 ± 4.0 cm; 67.2 ± 8.4 kg; 27.7 ± 4.8% body fat) completed 1 h rowing ergometer exercise at 70% of maximal oxygen consumption (VO2max) during two different phases of the menstrual cycle: the follicular phase (FP) and the luteal phase (LP). Resting and exercise measurements of the whole body energy expenditure, oxygen consumption (VO2), respiratory exchange ratio (RER), substrate oxidation and lactate blood levels were made. Energy expenditure, VO2 and heart rate during the 1-h exercise were not significantly different (P > 0.05) among menstrual cycle phases. Resting RER and RER during the entire 1 h exercise period were not significantly different among menstrual cycle phases. There was an increase (P < 0.05) in RER in the transition between rest and exercise and a further increase in RER occurred after the first 30 min of exercise at both menstrual cycle phases. Blood lactate concentrations significantly increased in the transition between rest and exercise and remained relatively constant during the whole 1 h of exercise in both menstrual cycle phases. No menstrual cycle phase effect (P > 0.05) was observed for blood lactate concentrations. In conclusion, our results demonstrated no effect of menstrual cycle phase on substrate oxidation and blood lactate concentration during rowing exercise at 70% of VO2max in athletes. Normally menstruating female rowers should not be concerned about their menstrual cycle phase with regard to substrate oxidation in everyday training.

Keywords

Female athletesFemale hormonesMenstrual cycleSubstrate oxidationEndurance exercise

Introduction

The relative utilization of carbohydrates and lipids as fuel sources during aerobic exercise can be influenced by factors such as exercise intensity (Goedecke et al. 2000; Venables et al. 2005), training status (Jeukendrup et al. 1997), diet (Goedecke et al. 2000) and also the relative hormonal mileau during exercise (Devries et al. 2006; Horton et al. 1998). Some studies have demonstrated that females, compared with males, utilize less carbohydrate to fuel endurance exercise, as evidenced by a lower respiratory exchange ratio (RER) (Carter et al. 2001; Horton et al. 1998; Tarnopolsky et al. 1995; Venables et al. 2005), while other studies have shown a similar relative utilization of carbohydrates and lipids in females and males exercising at the same relative intensity (Marliss et al. 2000; Roepstorff et al. 2002; Romijn et al. 2000). It has been suggested that possible gender differences in the relative fuel utilization during endurance exercise are due to the differences in circulating estrogen (Bunt 1990; Devries et al. 2006).

Estrogen may promote endurance performance by altering carbohydrate and lipid metabolism with progesterone often appearing to act antagonistically (Oosthuyse and Bosch 2010). Specifically, Oosthuyse and Bosch (2010) recently suggested that the magnitude of increase in the ovarian hormones between menstrual cycle phases and the ratio between estrogen and progesterone may be important factors determining an effect on substrate metabolism. It has been argued that relatively high progesterone concentration during the luteal phase (LP) of the menstrual cycle may have countered the benefits of an elevated estrogen concentration during endurance performance (McLay et al. 2007; Oosthuyse et al. 2005). Accordingly, several studies report that there appear to be only small differences in substrate metabolism during prolonged exercise due to the endogenous ovarian hormone fluctuations across the normal menstrual cycle (Campbell et al. 2001; Casazza et al. 2004; Devries et al. 2006; McLay et al. 2007; Oosthuyse et al. 2005; Suh et al. 2002). There are reports of no significant differences in RER (De Souza et al. 1990; Horton et al. 2002; Suh et al. 2002), or blood glucose (Devries et al. 2006; Horton et al. 2002; Suh et al. 2002), and blood lactate (De Souza et al. 1990; Devries et al. 2006; Horton et al. 2002; Nicklas et al. 1989) concentrations due to the ovarian hormone variations during menstrual cycle. In contrast, certain data suggest that there may be a greater carbohydrate oxidation and a lower lipid oxidation during submaximal exercise [<70% of maximal oxygen consumption (VO2max)] in the follicular phase (FP) compared to the LP of the menstrual cycle (Campbell et al. 2001; Hackney 1999; Hackney et al. 1991, 1994; Wenz et al. 1997; Zderic et al. 2001). However, most of the studies have been performed with habitually physically active women (Casazza et al. 2004; Devries et al. 2006; Hackney 1999; Hackney et al. 1991, 1994; Horton et al. 2002; Suh et al. 2002; Zderic et al. 2001) and only very few with highly trained women (Campbell et al. 2001; Goedecke et al. 2000). Accordingly, further clarification of the effect of menstrual cycle on substrate utilization during exercise in relation to exercise intensity and the training status in exercising women is needed.

Because limited data are available on substrate metabolism in response to menstrual cycle phase changes in female athletes, the purpose of the present investigation was to examine the effects of menstrual cycle phase on substrate oxidation and lactate concentration during 1 h rowing exercise at the intensity of 70% VO2max in national level rowers. This submaximal rowing exercise was designed to closely approximate a rower’s typical training session (Jürimäe et al. 2001, 2009b) to provide a physiological and hormonal environment representative of daily exercise stress for this population. To our knowledge, no studies have been performed, which have investigated the effect of menstrual cycle phase on rowing exercise in sport specific conditions. Rowers utilize relatively large body mass where all extremities and trunk muscles are involved compared to other endurance sports, which results in higher energy consumption during exercise (Jürimäe 2008; Mäestu et al. 2005). It was hypothesized that menstrual cycle phase would affect substrate oxidation, with elevated levels of the ovarian hormones in the LP of the menstrual cycle decreasing carbohydrate oxidation during rowing exercise.

Methods

Participants

Eleven eumenorrheic Caucasian female rowers (18.4 ± 1.9 years; height: 172.0 ± 4.0 cm; 67.2 ± 8.4 kg; body fat%: 27.7 ± 4.8%) with normal menstrual cycles (26–32 days) participated in this study. All participants were free of injuries and diseases, and were not taking any medication as determined by health history questionnaire. None of the rowers smoke or used any supplements. In addition, participants did not have any pregnancies before or during the study. Participants were required to have menstrual cycle duration of 24–35 days, with at least 6 months of documented menstrual cycles, and were not using the oral contraceptive pills for at least 6 months preceding the study (Casazza et al. 2004; Dean et al. 2003). Participants whose menstrual cycle occurred later than 35 days were excluded (Thong et al. 2000). All rowers competed at the national level. The study was conducted during the preparatory period for the competitive rowing season, where the training intensity was below anaerobic threshold for approximately 90% of the entire training time. The main goal of training during the preparatory period is to increase the aerobic base through aerobic extensive endurance training sessions (Jürimäe 2008; Mäestu et al. 2005). Written informed consent was obtained before commencing the study after a description of the study and advisement of the possible risks and benefits of participation. The study protocol was approved by the Medical Ethics Committee of the University of Tartu.

Experimental design

Preliminary tests and the main exercise experiment in rowers were carried out during the FP (determined as days 7–11 from onset of menstruation, mean day 9 ± 2 for the main experiment) and the LP (determined as days 18–22 from onset of menstruation, mean day 20 ± 2 for the main experiment) of the menstrual cycle (Casazza et al. 2004; Horton et al. 2002; Suh et al. 2002; Timmons et al. 2005). Preliminary tests included incremental rowing ergometer test that was followed by body composition measurements. Main exercise experiment consisted of 1 h endurance rowing ergometer session that was conducted on the following day after the incremental rowing ergometer test. Testing order was balanced with respect to the cycle phase and test time was standardized between 4.00 and 6.00 p.m. On the day before the exercise tests, no physical activities were allowed (Jürimäe et al. 2006, 2009b). Over the testing period participants were asked to maintain a regular and constant volume and intensity of training. In addition, participants were asked to record all foods and drinks consumed during the last two weekdays (i.e., Thursday and Friday) and one weekend day (Saturday) preceding both trials. Subjects were asked to maintain their usual dietary habits and everyday activities before both the trials. Daily energy intake was calculated as the average of 3 days (Jürimäe et al. 2009b). Our athletes were instructed by an experienced dietician, and their daily nutritional intake consisted of a high-carbohydrate diet with the composition remaining stable throughout the training season (Jürimäe et al. 2006). Daily energy expenditure during the same days was calculated according to the method of Bouchard et al. (1983). Participants were in a post-absorptive state having eaten a meal for about 2 h before each physical test (Jürimäe et al. 2006). Participants recorded the meal before the first test and they ate the similar meals before each test to reach nearly identical nutritional intake (Devries et al. 2006).

Information about previous menstrual cycles was used to identify the phases of the menstrual cycle (Dean et al. 2003). The length of the menstrual cycle was calculated from the first day of menses to the day preceding the next menses. Menstrual cycle phases were later confirmed by plasma estradiol and progesterone concentrations from the blood samples (Hackney 1999; Nicklas et al. 1989; Suh et al. 2002). The accepted concentration ranges for the ovarian hormones during both menstrual cycle phases were 85–220 pmol l−1 for estradiol and <3 nmol l−1 for progesterone during the FP, and 230–750 pmol l−1 for estradiol and >16 nmol l−1 for progesterone during the LP. Therefore, a resting level of plasma progesterone higher than 16 nmol l−1 was required to confirm LP (Landgren et al. 1980). As it has been suggested that estradiol must differ at least by twofold between menstrual cycle phases in order to produce a significant impact on substrate metabolism during endurance exercise (D’Eon et al. 2002), a twofold increase in estradiol concentration was also required for the inclusion of the participant’s data in the analysis (Oosthuyse et al. 2005). Estradiol and progesterone concentrations were determined in duplicate on Immulite 2000 (DPC, Los Angeles, CA, USA). The intra- and interassay coefficients of variation (CVs) for estradiol were 5.3 and 6.5%, and for progesterone 5.4 and 3.4%, respectively. The height (Martin Metal Anthropometer) and body mass (Medical Balance Scales, A&D Instruments Ltd, Oxfordshire, UK) of the participants were measured to the nearest 0.1 cm and 0.05 kg, respectively. Body composition was measured using dual-energy X-ray absorptiometry. Scans of the whole body were performed using a Lunar DPX-IQ densitometer (Lunar Corporation, Madison, WI, USA) and analysed for fat (FM) and fat free (FFM) masses. The CVs for body composition measurements were less than 2% (Jürimäe et al. 2009a).

Incremental exercise test

A stepwise rowing ergometer test was performed on a wind resistance-braked rowing ergometer (Concept II; Morrisville, VT; USA) to determine VO2max and target heart rate (HR) values for 1 h endurance rowing ergometer test. The rowers were fully familiarized with the use of the apparatus. Participants were instrumented and sat quietly for 1 min on the ergometer before starting to exercise at 40 W. Load was increased by 15 W every minute until maximal voluntary exhaustion was reached. Power and stroke rate were recorded continuously on the computer display of the rowing ergometer. The test was designed to reach the maximum in approximately 15 min in each participant (Hofmann et al. 2007). Subjects were strongly encouraged to achieve maximal performance. HR was recorded at every 5 s during the test using Sporttester Polar 725X (Polar Electro Oy Kempele, Finland). Respiratory gas exchange variables were measured throughout the test in a breath-by-breath mode using a portable open circuit spirometry system (MetaMax 3B, Cortex Biophysic GmbH, Germany) and data were stored in 10-s intervals. Oxygen consumption (VO2), carbon dioxide production (VCO2) and minute ventilation (VE) were continuously measured, and the mean respiratory exchange ratio (RER) and ventilatory equivalents of O2 (VE/VO2) and CO2 (VE/VCO2) were calculated from the recorded measurements. The analyzer was calibrated before the test with gases of known concentration according to the manufacturer’s guidelines. All data were processed by means of computer analysis using standard software (MetaSoft, Cortex Biophysic GmbH, Germany) and the system for HR analysis. To establish that VO2max was reached, the attainment of a plateau in VO2 with increasing work rate was used as a criterion. When this plateau in VO2 was not observed, a RER exceeding 1.1 and theoretical maximal cardiac frequency were used as a criterion. Anaerobic threshold (AT) determination was performed using linear regression turn point analysis (Hofmann et al. 2007). Turn points in HR, VE, VE/VO2 and VE/VCO2 were calculated as described previously (Hofmann et al. 2007). Two regression lines were calculated and the intersection point between both optimized regression lines was taken as the HR turn point and was used in the AT analysis (Hofmann et al. 2007). The suggested method has been found to be reliable in determining the individual intensity for aerobic–anaerobic transition in rowers (Hofmann et al. 2007).

Endurance exercise protocol

The exercise test consisted of rowing on a rowing ergometer for 1 h at the intensity of 70% VO2max. Target HR was set at the level obtained from the incremental test using a practical set ± 2 bpm of 70% VO2max (Jürimäe et al. 2001, 2009b). Rowers were asked to increase exercise intensity smoothly and the requested HR was achieved after the first 5 min. The participants were instructed to maintain the required HR steady state for the entire exercise session and to reduce exercise intensity to accommodate the required HR steady state as needed (Jürimäe et al. 2001, 2009b). Respiratory gas exchange variables were measured throughout the test in a breath-by-breath mode using a portable open circuit spirometry system (MetaMax I, Cortex, Germany) for 5 min at rest and during the 1-h exercise session as described above. 1 h exercise session was stopped for 1 min blood lactate measurements after each 15 min of exercise. Respiratory gas exchange measurements were computed at rest and for 10 min of each 15 min of exercise period after the first 5 min of stabilization period (Goedecke et al. 2000). Fat and carbohydrate oxidation and energy expenditure were estimated from the RER using stoichiometric equations (Frayn 1983), with the assumption that urinary nitrogen excretion rate was negligible (Venables et al. 2005). These equations have previously been used in females to assess submaximal exercise substrate oxidation, during which the RER was <1 (Devries et al. 2006; Roepstorff et al. 2002; Suh et al. 2002; Venables et al. 2005). Capillary blood samples for enzymatic determination of lactate (Lange, Germany) were collected before, and after 15, 30, 45, and 60 min of the start of exercise.

Statistical analyses

Statistical analyses were performed using SPSS software for Windows, version 13.0 (SPSS Inc, Chicago, IL, USA). Mean ± SD was determined. A two-way analysis of variance and LSD post hoc analysis tests were used to evaluate differences between measured variables. The level of significance was set at P < 0.05.

Results

There were no significant differences (P > 0.05) in the energy intake (FP: 2,723 ± 397 vs. 2,683 ± 383 kcal d−1) or energy expenditure (2,887 ± 467 vs. 2,814 ± 565 kcal d−1) over the days preceding the FP and LP of the menstrual cycle. Rowers were weight stable throughout the study period, with no significant changes in body composition or VO2max values between menstrual cycle phases (Table 1). Plasma estradiol and progesterone concentrations confirmed the menstrual cycle phases, with a fourfold increase in estradiol (P < 0.05) and a 17-fold increase in progesterone (P < 0.05) in the LP compared with the FP (Table 1).
Table 1

Mean (±SD) subject characteristics in studied rowers (n = 11)

Variable

FP

LP

Body mass (kg)

67.2 ± 8.4

69.3 ± 8.3

Body fat%

27.7 ± 4.8

27.9 ± 4.6

Fat mass (kg)

18.0 ± 3.9

18.2 ± 3.8

Fat free mass (kg)

47.7 ± 6.0

48.0 ± 5.9

VO2max (l min−1)

3.18 ± 0.52

3.15 ± 0.42

VO2max (ml min −1 kg−1)

47.5 ± 8.3

45.2 ± 5.7

VO2AT (l min−1)

2.69 ± 0.44

2.68 ± 0.38

AT (%VO2max)

84.9 ± 6.6

85.3 ± 9.5

Estradiol (pmol l−1)

110.0 ± 31.3

460.9 ± 108.0*

Progesterone (nmol l−1)

1.5 ± 0.6

25.8 ± 10.2*

Ratio of estradiol to progesterone

85.2 ± 36.4

20.3 ± 9.7*

* Significantly different from FP; P < 0.05

Energy expenditure, VO2 and HR during 1 h rowing ergometer exercise were not significantly different (P > 0.05) among menstrual cycle phases (Table 2). On the average, athletes rowed at 69.3 and 69.8% of VO2max during the FP and LP, respectively. HR was not significantly different at any point between the trials (data not shown). Resting RER and RER during the entire 1-h exercise period were not significantly different between menstrual cycle phases (Fig. 1). However, there was an increase (P < 0.05) in RER during the transition between rest and exercise at both menstrual cycle phases. In addition, a further increase (P < 0.05) in RER occurred after the first 30 min of exercise at both menstrual cycle phases. At rest, about 50% of the energy was derived from carbohydrate sources during both menstrual cycle phases, but no significant phase effect was observed (Table 3). During exercise, at least 70% of the energy used was derived from carbohydrate sources (FP: ≥7.4 kcal min−1; LP: ≥7.6 kcal min−1). The contribution of the amount of carbohydrate to energy expenditure decreased (P < 0.05) after the first 30 min of exercise, while the contribution of the amount of lipid to energy expenditure was not significantly changed (P > 0.05) throughout the 1-h exercise in both menstrual cycle phases. However, no significant menstrual cycle phase effect was observed (Table 3). Blood lactate concentrations increased (P < 0.05) during the transition between rest and exercise and remained relatively constant during the whole 1-h exercise in both menstrual cycle phases (Fig. 1). No menstrual cycle phase effect (P > 0.05) was observed for blood lactate concentrations.
Table 2

Mean (±SD) measured physiological and energy expenditure values during submaximal exercise

Menstrual phase

HR (bpm)

VO2 (l min−1)

% of VO2max

RER

EE rate (kcal min−1)

Total EE (kcal 60 min−1)

Follicular phase

152.8 ± 5.6

2.2 ± 0.4

69.2 ± 2.2

0.89 ± 0.06

10.4 ± 1.3

627.8 ± 89.6

Luteal phase

153.1 ± 6.7

2.2 ± 0.5

69.8 ± 2.3

0.93 ± 0.05

10.8 ± 1.6

649.7 ± 100.2

https://static-content.springer.com/image/art%3A10.1007%2Fs00421-010-1730-1/MediaObjects/421_2010_1730_Fig1_HTML.gif
Fig. 1

Respiratory exchange ratio (RER) (a) and blood lactate concentrations (b) at rest and during 1 h ergometer rowing at an intensity of 70% VO2max at follicular (FP) and luteal (LP) phases of menstrual cycle in rowers (n = 11). *Significantly different from 0 min; P < 0.05. Significantly different from 15 min; P < 0.05. #Significantly different from 30 min; P < 0.05

Table 3

Energy expenditure values during rest and submaximal exercise

Variable

Menstrual phase

Rest

Exercise

0–15 min

16–30 min

31–45 min

46-60 min

CHO EE

Follicular

0.8 ± 0.4

8.2 ± 2.1*

7.9 ± 2.4*

7.4 ± 2.3*†#

7.4 ± 2.4*†#

(kcal min−1)

Luteal

0.8 ± 0.5

8.4 ± 2.2*

8.1 ± 2.0*

7.7 ± 2.1*†#

7.6 ± 2.1*†#

Lipid EE

Follicular

0.7 ± 0.5

2.4 ± 1.8*

2.6 ± 2.0*

2.9 ± 1.9*

2.9 ± 2.0*

(kcal min−1)

Luteal

0.8 ± 0.4

2.5 ± 1.8*

2.7 ± 1.8*

2.9 ± 1.8*

3.0 ± 1.9*

Total EE

Follicular

1.5 ± 0.4

10.6 ± 0.4*

10.5 ± 1.3*

10.3 ± 1.4*

10.3 ± 1.4*

(kcal min−1)

Luteal

1.6 ± 0.5

10.9 ± 1.5*

10.8 ± 1.5*

10.6 ± 1.3*

10.6 ± 1.3*

* Significantly different from resting conditions; P < 0.05

Significantly different from 0–15 min; P < 0.05

#Significantly different from 16–30 min; P < 0.05

Discussion

The present investigation determined the effect of the normal menstrual cycle on resting and exercise substrate oxidation. This study was unique in that it used rowing exercise, where all extremities and trunk muscles are involved, which produces higher energy expenditure during exercise and rowers present relatively large body mass values in comparison with other endurance athletes (Jürimäe 2008; Mäestu et al. 2005). To our knowledge, no studies before have compared the response of moderate-intensity long-duration rowing exercise across different phases of the menstrual cycle characterized by different estrogen and progesterone concentrations: FP, low estrogen and low progesterone; and LP, elevated estrogen and elevated progesterone. In contrast to what was hypothesized, no significant differences were observed in the whole body carbohydrate and lipid oxidation at rest and during 1 h rowing exercise at 70% of VO2max in FP and LP in national level athletes. In addition, there were no significant differences in plasma lactate concentrations during exercise at both menstrual cycle phases.

The findings of the present investigation that resting RER and energy expenditure values were not significantly different between FP and LP are in line with other studies (Bailey et al. 2000; Horton et al. 2002; Suh et al. 2002). In addition, about 50% of the energy was derived from carbohydrate sources at rest at both menstrual cycle phases without any phase effect similarly to the other studies (Suh et al. 2002). The present data also agree with the previous findings of no significant differences in resting blood lactate concentrations at different phases of the menstrual cycle (Devries et al. 2006; Horton et al. 2002; Suh et al. 2002). These results together suggest that normal cyclic variations in ovarian hormones are without significant effects on resting substrate metabolism in female athletes.

The main finding of the present investigation, in contrast to what was hypothesized, was that no significant mean differences in the whole body carbohydrate and lipid oxidation during moderate-intensity long-duration exercise at different phases of the menstrual cycle were observed in rowers. This is in agreement with other studies, which have measured exercise RER at the intensity of 45% (Suh et al. 2002), 50% (Horton et al. 2002), 65% (Devries et al. 2006; Suh et al. 2002), 70% (Bailey et al. 2000), 75% (Hackney et al. 1994), and 80% (De Souza et al. 1990) of VO2max in physically active healthy women. In contrast, other studies have found greater lipid oxidation and lower carbohydrate oxidation in LP compared to FP of the menstrual cycle with exercise intensity higher than 50% of VO2max (Campbell et al. 2001; Hackney 1999; Zderic et al. 2001). However, Hackney et al. (1994) reported greater LP lipid oxidation only when exercise was performed at low (35% of VO2max) and moderate (60% of VO2max) intensities, but not at the intensity of 75% of VO2max in healthy eumenorrheic women. In that study, women performed a 30-min treadmill run in which the exercise intensity was made more difficult in every 10 min (35, 60, and 75%) (Hackney et al. 1994). In another study, Wenz et al. (1997) also found greater LP lipid oxidation during cycle ergometer exercise at the intensities of 30 and 50% of VO2max but not during cycle ergometer exercise at the intensity of 70% of VO2max in healthy eumenorrheic women. The reasons for these inconsistencies with respect to the menstrual cycle phase effects on exercise fuel oxidation could be the discrepancies in the exercise mode and protocol, the RER measurements during the exercise, pretrial diet and exercise control.

It could be argued that the differences in the findings of substrate oxidation at different phases of the menstrual cycle between current study and the other study utilizing endurance-trained athletes (Campbell et al. 2001) could be explained in part by the differences in exercise duration and mode as both studies used exercise intensity of 70% of VO2max. Athletes in Campbell et al. (2001) study cycled for 2 h, while athletes in our study rowed for 1 h. In cycling, only lower limbs are exercising, while whole body muscles are involved in rowing causing higher exercise energy expenditure (Jürimäe 2008; Mäestu et al. 2005). Recently, the effect of exercise mode on substrate oxidation has also been demonstrated (Cheneviere et al. 2010). Another explanation could be the fact that athletes in Campbell et al. (2001) study exercised in a fasted state, while rowers in the present study were in a post-absorptive state having eaten a meal for about 2 h before the test to replicate the usual everyday conditions during training sessions (Jürimäe et al. 2006). Indeed, when endurance-trained athletes in Campbell et al. (2001) study were adequately supplied with carbohydrate throughout the exercise, the cycling levels of estrogen and progesterone during menstrual cycle had only a minimal effect on substrate oxidation during exercise. In addition, it has been found that estrogen supplementation has no effect on exercise RER (Carter et al. 2001). Accordingly, it could be suggested that changes in estrogen and progesterone across the normal menstrual cycle do not appear to be of sufficient magnitude to affect exercise substrate oxidation in post-absorptive state athletes as observed in the current study and by others (Campbell et al. 2001). However, additional studies with highly trained athletes are warranted before any conclusions can be drawn.

The current observation of no menstrual cycle phase differences in blood lactate concentrations during exercise is similar to what has been found in other studies (Campbell et al. 2001; De Souza et al. 1990; Devries et al. 2006; Nicklas et al. 1989; Suh et al. 2002). However, we cannot confirm the results of Jurkowski et al. (1981), who observed lower blood lactate concentrations at the LP compared to FP after exercise at 66 and 90% VO2max in exercising women. Jurkowski et al. (1981) argued that the primary factor related to lower blood lactate accumulation during LP was a decreased production of lactate. It has also been proposed that variations in blood lactate concentrations in menstrual cycle phases during exercise may occur only in a glycogen-depleted state (Lavoie et al. 1987) and/or in untrained females (Lynch et al. Lynch and Nimmo 1998). The subjects in the present study were trained and performed a steady state exercise in a fed state. However, measurement of substrate turnover rates should be performed to accurately estimate the relative contribution of glucose kinetics during exercise at different phases of the menstrual cycle in order to appropriately answer this question in rowers.

In conclusion, the results of present investigation demonstrated no significant effect of menstrual cycle phase on substrate oxidation and blood lactate concentration during rowing exercise at 70% of VO2max in endurance-trained athletes.

Acknowledgments

This study was supported by Estonian Science Foundation Grant GKKSP 6638.

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