Ten healthy young females (age 22.3 ± 0.2 y, V0₂ₘₐₓ > 40 ml·kg⁻¹·min⁻¹) volunteered to participate in a randomized, double-blinded study that examined the effect of both (A) the timing of PRO/CHO/FAT supplementation, pre- and post-endurance exercise, and (B), extra-energy supplementation ( ~ 400 kcal) on indices of protein metabolism. Each subject completed each of three, 7-day supplementation trials conducted at least 1 week apart: a pre-exercise trial (PRE), a post-exercise (PO) trial, and a post-exercise with extra energy (POE) trial. All females were eumenorheic and tested during the early (days 4-7), middle (days 8-11 ), and late (days 12-14) periods of the follicular phase of their menstrual cycle. Subjects were weighed prior to and following each trial to determine weight loss. During each trial, subjects would consume a checklist diet (days 1-3) and a prepackaged diet (days 4-6) that were isoenergetic, isonitrogenous, and matched for diet composition (carbohydrates, protein, and fat) to the individual's habitual dietary intake. Subjects cycled at 65% V0₂ₘₐₓ for 1 hr, according to their target heart rate on Days 1,3, and 4 and for 1.5 hrs on Day 6 with Days 2 and 5 being rest days. Expired gas and blood samples were collected on Day 6 during exercise (t=0,30,60,90 min) and post exercise for 2 hrs (Blood t=30,60,90,120, Gas t=10,20,30,45,60,90,120 min). On Day 5, expired gas samples were collected post- supplement consumption at the same timepoints as Day 6. 24 h-urine collections were performed on Days 5 and 6. A 75% V0₂ₘₐₓ performance ride to exhaustion was performed in a fasted state on the morning of Day 7. During each trial, subjects had consumed their daily meals at regular intervals by 1500 h prior to the exercise bout (1600 h). In the PRE trial, a PL can+ PRO/CHO/FAT Supplement (Results® + 0.5 g·kg CHO powder[Gatorade®]) was consumed with breakfast (BKFT), and a PL Supplement (I can+ powder) was consumed immediately following the cessation of exercise. In the PO trial, a PL Supplement (2 cans+ powder) was taken at BKFT and the PRO/CHO/F AT Supplement was consumed post-exercise. The POE trial was identical to the PO trial with the exception that subjects received an extra 400 kcal of energy·d-1 at BKFT (Boost®-250 kcal and an extra 150 kcal CHO powder). During the POE checklist diet (Days 1-3), the extra energy was given by the addition of extra items to their daily food consumption. Although plasma estrogen levels were significantly higher during the PO trial (P <0.05), plasma progesterone levels were not significantly different between trials, and both hormones indicated that subjects were in the follicular phase of their menstrual cycle. Significant main effects for time occured in which hematocrit, sodium, potassium, and lactate were observed to increase during exercise and decrease post-exercise. Glucose concentration increase was signiciant for time (P < 0.01) increasing post-exercise after consumption of the supplement. Significant main effects for insulin were observed across time (P < 0.000001) and for trial (P < 0.01) with the POE trial having the largest post-exercise insulin response. No significant differences were observed between the three trials for urinary creatinine (g·24 h⁻¹) and urea (g·24 h⁻¹ and g·g Cr⁻¹). Day 6 urinary 3-methylhistidine (μmol·g Cr⁻¹) was significantly different between trials (POE>PO>PRE)(P < 0.01). Similarly, no significant differences were observed between trials for resting metabolic rate, thermic effect of food, thermic effect of exercise, and excess post-exercise oxygen consumption. However, on Day 5, Apparent Nitrogen Balance trended toward signifcance for trial (P=0.086) becoming significant on Day 6 (P <0.005) wherein POE and PO trials were positive and PRE trial was negative. There was a trend towards increased performance times when comparing the POE and PO trials versus the PRE trial (P =0.074). Furthermore, weight loss was significantly lower with post-exercise supplementation (POE<PO<PRE)(P < 0.01). This study suggests that the protein balance of females regularly engaging in endurance activity may benefit from immediate nutritional supplementation following exercise. / Thesis / Master of Science (MSc)