The Effects of Leucine on Fatty Acid Oxidation in Chronically Active Males

Leucine is a branched-chain amino acid that is known to stimulate muscle protein synthesis and prevent muscle protein degradation (22). However, the effects of leucine on fat oxidation are still being determined. Thus, we investigated the effect of leucine on fat oxidation in chronically active males, following a similar study conducted by our laboratory on leucine and fat oxidation in sedentary, overweight subjects. Participants in this study were males, ages 25-55 years old, with BMI values of 18.5-27 kg/m², and participated in >180 minutes/week of vigorous-intensity physical activity. Participants were randomized to receive either a leucine-supplemented beverage or a placebo beverage three times/day for seven days, followed by a seven-day washout period, and then followed by a final seven days on the remaining treatment (leucine or placebo). Leucine supplementation totaled 2.25g/day. Each participant came in after a 12-hour fast and 24-hour abstention from exercise and his resting metabolic rate was obtained using a metabolic cart. Each participant provided 24 hours of collected urine for urea nitrogen analysis and calculation of respiratory quotient and substrate oxidation.

Respiratory quotient and fat oxidation did not change in response to placebo compared to baseline measurements. However, during the leucine treatment, respiratory quotient significantly decreased from 0.85 ± 0.032 to 0.81 ± 0.032 (p=0.000056). Fat oxidation during the leucine treatment also increased, but did not reach statistical significance (48 ± 13.19 g/day before treatment and 58.8 ± 20.14 g/day after treatment). Carbohydrate oxidation also exhibited a non-significant decrease during the leucine treatment from 118.6 ± 31.96 g/day to 75.8 ± 38.02 g/day. Our results indicate that leucine decreases the respiratory quotient and may increase fat oxidation in chronically active men when compared to a placebo.