Greater protein intake emphasizing lean beef does not affect resistance training-induced adaptations to skeletal muscle and tendon in older women: A randomized controlled feeding trial

Background While experimental research supports that resistance training (RT), especially with greater dietary protein intake, improves muscle mass and strength in older adults, comparable research on tendons is needed. Objective We assessed the effects of a protein-rich diet emphasizing lean beef, compared to two control diets, on RT-induced changes in skeletal muscle and tendon size and strength in older women. Methods We randomly assigned women [age: 66±1y, body mass index (BMI): 28±1 kg/m2] to groups that consumed 1) 0.8 g total protein/kg body weight/day from mixed food sources (normal protein control, n=16); 2) 1.4 g/kg/d protein from mixed food sources (high protein control, n=17); or 3) 1.4 g/kg/d protein emphasizing unprocessed lean beef (high protein experimental group, n=16). Participants were provided with all foods and performed RT 3x/week, 70% of 1-repetition maximum (1RM) for 12 weeks. We measured quadriceps muscle volume via magnetic resonance imaging (MRI). We estimated patellar tendon biomechanical properties and cross-sectional area (CSA) using ultrasound and MRI. Results Dietary intake did not influence RT-induced increases in quadriceps strength (p<0.0001) or muscle volume (p<0.05). We noted a trend for a RT effect on mean tendon CSA (p=0.07), with no differences among diets (p>0.05). Proximal tendon CSA increased with RT (p<0.05) with no difference among dietary groups (p>0.05). Among all participants, mid-tendon CSA increased with RT (p≤0.05). We found a decrease in distal CSA in the 0.8 g group (p<0.05) but no change in the 1.4 g groups (p>0.05). Patellar tendon MRI signal or biomechanical properties were unchanged. Conclusions Our findings indicated that greater daily protein intake, emphasizing beef, did not influence RT-induced changes in quadriceps muscle strength or muscle volume of older women. While we noted trends in tendon CSA, we did not find a statistically significant impact of greater daily protein intake from beef on tendon outcomes. Clinical Trial Registry ClinicalTrials.gov (NCT04347447)