The aim of this randomised-controlled trial is to examine whether the timing of oral contraceptive administration affects cognitive function, substrate metabolism and exercise performance.

Participants will attend the laboratory for a familiarisation session, followed by two main trials in which cognition and exercise metabolism and performance are measured. In one trial they will be given their oral contraceptive 60 minutes before testing begins, in the other they will be given a placebo. Following testing (~60 minutes), participants will consume the alternative pill.

Reproductive hormones, such as oestrogen and progesterone, are known to affect cognitive function (i.e. thinking, learning and processing information) and substrate metabolism (i.e. how the body uses carbohydrates and fats). Oral contraceptives contain synthetic versions of reproductive hormones and are used by approximately half of women of reproductive age. When oral contraceptives are ingested, the concentrations of these synthetic hormones spike in the blood for several hours, yet no research has explored the effect of this spike on how the body works, especially with regards to cognitive function and substrate metabolism. If this spike in synthetic hormone concentrations affects cognition or metabolism, this could influence the way in which women use oral contraceptives, such as altering the timing of ingestion to improve performance and health outcomes.

The proposed study will use participants that currently use a specific preparation of oral contraceptive (30micrograms ethinyl estradiol and 150mg Levonorgestrel) and compare cognitive function and substrate metabolism on an occasion when a placebo is consumed (lactose - no hormones), to an occasion when their oral contraceptive is consumed. It takes approximately 60-90 minutes for concentrations of hormones to peak in the blood following ingestion, therefore measurements of cognitive function and substrate metabolism will be taken at this time. Cognitive function will be measured using a battery of tests administered orally or via a laptop and substrate metabolism will be measured by collecting expired air via a mouthpiece at rest and during exercise on a treadmill, during which time perceived difficulty of the exercise (rate of perceived exertion) and feelings (felt arousal) will be measured. Venous blood samples will be drawn at baseline (immediately before consumption of first pill/placebo) and 90 minutes following this. The participants will consume either the placebo or oral contraceptive (whichever was not initially consumed) following the testing protocol to ensure that their contraceptive protection is maintained.

Inclusion Criteria:

• Female
• Aged 18-35 years
• The use of the same oral contraceptive (30 micrograms ethinyl estradiol and 150mg levonorgestrel) for at least 6 months
• Physically active and able to take part in treadmill-based exercise
• Speak fluent English

Exclusion Criteria:

• Use of any other type of hormonal contraception
• Aged < 18 or > 35 years
• Poly-cystic ovarian syndrome
• Endometriosis
• Pregnant
• Child birth or lactation in the previous 6 months
• Body mass index < 18.5 or > 30 kg.m2
• Any disorder known to affect metabolic health
• History of head injury/neurological disorders
• Use of any steroid-based medication or medications known to influence the central nervous system
• History of psychiatric disorders such as major depression or an anxiety disorder
• Smokers
• Suffer from (or have suffered from) a heart complaint
• Advised by medical professional to refrain from high intensity exercise
• Current muscle or joint injury

Random number generator, via technicians, concealed allocation