Dose-response of myofibrillar protein synthesis to ingested whey protein during energy restriction in overweight postmenopausal women: a randomized, controlled trial

Background Diet-induced weight loss is associated with a decline in lean body mass, as mediated by an impaired response of muscle protein synthesis (MPS). The dose-response of MPS to ingested protein, with or without resistance exercise, is well characterised during energy balance but limited data exist under conditions of energy restriction in clinical populations.

Objective To determine the dose-response of MPS to ingested whey protein following short-term diet-induced energy restriction in overweight, postmenopausal, women at rest and post-exercise.

Design Forty middle-aged (58.6±0.4 years), overweight (BMI: 28.6±0.4), postmenopausal women were randomised to one of four groups: Three groups underwent 5 days of energy restriction (∼800 kcal/d). On day 6, participants performed a unilateral leg resistance exercise bout before ingesting either a bolus of 15g (ERW15, n=10), 35g (ERW35, n=10) or 60g (ERW60, n=10) of whey protein. The fourth group (n=10) ingested a 35g whey protein bolus after 5 days of an energy balanced diet (EBW35, n=10). Myofibrillar fractional synthetic rate (FSR) was calculated under basal, fed (FED) and post-exercise (FED-EX) conditions by combining an L-[ring-13C6]phenylalanine tracer infusion with the collection of bilateral muscle biopsies.

Results Myofibrillar-FSR was greater in ERW35 (0.043±0.003%/h, P=0.013) and ERW60 (0.042±0.003%/h, P=0.026) than ERW15 (0.032±0.003%/h), with no differences between ERW35 and ERW60 (P=1.000). Myofibrillar-FSR was greater in FED (0.044±0.003%/h, P