Protein ingestion impairs insulin‐stimulated net muscle carnitine uptake in healthy young men (LB812)

Abstract

Carbohydrate ingestion promotes insulin‐mediated muscle carnitine accretion, but the effect of protein ingestion is unknown. Six healthy males (age 24 ± 5 yrs, BMI 23 ± 3 kg/m²) ingested 80g carbohydrate (CHO), 40g whey protein + 40g CHO (PRO), or flavoured water (CON) beverages (500 ml) 1 hr following ingestion of 3g L‐carnitine (0.1% ²H3‐L‐carnitine) on 3 randomised 3‐hr visits. The rate of disposal (Rd) and arterio‐venous (AV) forearm carnitine balance (determined from arterialised‐venous and venous plasma ²H3‐carnitine and carnitine concentrations), and brachial artery blood flow (BAF; Doppler ultrasound) were measured, together with serum insulin concentrations, at baseline and 20 min intervals for 3 hrs. Carnitine Rd at 100 min was 4.4 fold greater in CHO compared to CON (0.3 ± 0.1 vs 0.1 ± 0.1 µmol/min; P<0.05), which coincided with peak serum insulin concentration in CHO (64 ± 10 mU/L). However, despite similar insulin responses in CHO and PRO (3‐hr area under curve [AUC] was 4.6 ± 0.8 and 3.9 ± 0.7 U/L•min, respectively) compared to CON (0.4 ± 0.04 U/L•min; P<0.001) and increases in BAF (15 ± 3 and 12 ± 5%) above CON (P<0.05), net carnitine balance AUC increased in CHO (P<0.01), but not PRO, compared to CON (9.3 ± 4.0 and ‐3.9 ± 5.7 vs ‐1.2 ± 1.5 µmol•min, respectively) over the 2 hrs following beverage ingestion. Carbohydrate stimulates insulin mediated muscle carnitine balance, but protein may antagonise this process.