Abstract:
Natural secretions of oestrogen and progesterone vary according to menstrual phase in
eumenorrhoeic women and have an affect on physiological systems that could
consequentially influence exercise performance. In a series of menstrual phase
comparative investigations (considering mainly the early follicular (EF), late follicular
(LF) and mid-luteal (ML) phase), I aimed to elucidate the physiological relation of
oestrogen and progesterone to fat metabolism and ventilation during endurance exercise
and exercise performance. I measured plasma free fatty acid (FFA) kinetics during
prolonged exercise from an intravenous infusion of K+[1-13C]palmitate in
eumenorrhoeic women and found the change in palmitate rate of appearance and
disappearance between menstrual phases to be related to the oestrogen/progesterone
(E/P) ratio. Overall the results from this study suggest that oestrogen promotes
increases in FFA availability during exercise, while progesterone mitigates this
response. Therefore, a high oestrogen concentration and E/P ratio in the ML phase is
required in order to produce a favourable FFA metabolic response for endurance events.
In order to estimate plasma FFA oxidation rate using carbon-FFA tracers, the acetate
correction factor which accounts for carbon-label retention in secondary metabolic
pools, must be applied. When I derived the acetate correction factor by measuring
fractional recovery of carbon-13 in expired CO2 from a constant infusion of Na+[1-
13C]acetate during submaximal exercise we found the acetate correction factor to be
significantly lower in the ML than EF phase (average change from EF phase -1.8±
0.5%, p<0.05). Failure to account for the lower correction factor in the ML phase will
result in a 6% underestimation of plasma FFA oxidation rate. Furthermore, since human
serum albumin (HSA) routinely used as a carrier for parenteral delivery of FFA tracers
has various disadvantages, I tested the feasibility of using 2-hydroxypropyl-b-
cyclodextrin (HP-b-CD) as a possible alternative carrier. A comparison of expired CO2
enrichment following HSA-FFA (7.4±2.0 %o) and HP-b-CD-FFA (8.6±2.1%o) infusion
during exercise showed that the HP-b-CD does not compromise natural in vivo
behaviour of the FFA tracer (p=0.4). Progesterone-induced hyperventilation is
occasionally reported during exercise in the luteal phase. I found that the change in ventilatory parameters (minute ventilation and respiratory rate) during exercise from EF
to ML phase is related to both the oestrogen and progesterone concentration in the ML
phase. However, the associated increase in respiratory rate throughout prolonged
exercise in the ML versus EF phase did not increase metabolic demand and therefore
could not be expected to exacerbate fatigue. When exercise performance was evaluated
by means of a cycling time trial, I found a trend for best performance in the LF phase
versus the EF phase (8 of 11 subjects improved by 5.2±2.9%, p=0.027), while no
differences occurred between other menstrual phases. Metabolic and performance
benefits of oestrogen may be concealed in the ML phase by the coincident increase in
progesterone in this phase and thus a significant effect is often only evident with a high
E/P ratio in the ML phase. The transient LF phase, characterised by the pre-ovulatory
surge in oestrogen, reveals the maximum benefits of oestrogen on metabolism and
performance during submaximal exercise.
