The goal of this controlled-feeding cross-over diet intervention is to compare a beef diet to a vegetarian diet on muscle fatigue in older adults. The aims are:

AIM 1: To test the hypothesis that muscle fatigue is attenuated in older adults consuming beef compared to older adults consuming a plant-based diet.

AIM2: To test the hypothesis that daily beef consumption improves biochemical indicators related to muscle fatigue.

Participants will be randomized to consume either a beef-based diet or vegetarian diet under controlled-feeding conditions for 8 weeks. After a 2-week washout period, participants will cross-over to consume either the beef diet or vegetarian diet for 8 weeks. Body composition and functional muscle outcomes will be measured during each 8-week feeding period. Blood samples will also be collected.

Inclusion Criteria

• Ages 60-85 years
• Ability to eat study diet meals for a total of 16 weeks
• Ability to perform sit-to-stand chair and 6-minute walking tests based on results from PAR-Q questionnaire
• Ability to provide fasted blood samples
• Access to transportation
• Willing to abstain from alcohol consumption and/or use illicit drug use for each 8-week intervention period
• Ability to consent to participating in this study
• English-speaking
• Body mass index between 18.5 (normal) and 39.9 (obese) kg/m2
• Minimum total body weight of 110 pounds
• Non-anemia hemoglobin values: Females - >12.0 grams/dL; Men - >13.0 grams/dL
• Able to ambulate without assistance
• Physician's clearance for study participation (required prior to baseline testing)

Exclusion Criteria

• Inability or refusal to sign the informed consent
• Significant orthopedic limitations or other contraindications to exercise
• Inability or refusal to perform muscle fatigue tests
• Has dietary restrictions that prevent to consumption of beef, soy and/or wheat
• Has liver or kidney complications
• Live or work >25 miles from Bloomington study site or does not have transportation to the study site
• Anticipate elective surgery during the study period
• Diagnosis of progressive neurological disorders, musculoskeletal disorders, osteoarthritis, rheumatoid arthritis
• Fracture or serious injury in the last 6 months
• Stroke, revascularization, deep vein thrombosis, or pulmonary embolism in the past 12 months
• Severe cardiovascular disease, uncontrolled systemic hypertension, chronic pulmonary disease
• Plan to move residence or travel out of the local area during the study period
• Current use of anti-coagulants (e.g. Coumadin or Warfarin)
• Current use of prescription medications that affect heart rate or blood vessel dilation
• Psychological or social characteristics that would interfere with their ability to fully participate in the study