Background: Creatine is a recognized ergogenic agent used by athletes to improve performance. Recently it has been investigated as beneficial for older people due to positive effects on glycemic control, lipid profile, functioning of the musculoskeletal system, and possible neuroprotective action. In this research, the aim was to determine the effects of supplementation with creatine and guanidinoacetic acid on muscle strength, muscle mass, serum biomarkers, and indicators of tissue bioenergetics, quality of life and cognitive functioning of older people. Materials and Methods: A total of 21 subjects participated in this trial (69.6±4.9 years; 162.7±7.7cm; weight 73.3±12.4kg, 13 women). Supplementation protocol lasted for 8 weeks as a double-blind placebo controlled crossover type of study. Experimental group ingested combination of creatine (Cr) + gunidinoacetate acid (Cr+GAA) (4 gr) once a day before breakfast, diluted in water (250 mL); placebo group consumed equivalent of inulin (4 gr). At baseline and after 8 weeks following measurements were performed: antropometry (upper arm, thigh, waist and hip girth, triceps and abdominal skinfold), body composition using bioimpedance, biochemical analyses, muscleand brain proton magnetic resonance spectroscopy, 5x Sit to Stand test, Timed up and Go Test, 4-m Speed Gait Test, and following questionnaires – The Short-form 36 Ouestionnaire, Sarcopenia and Quality of Life, International Physical Activity Questionairre – Short Form and Montreal Cognitive Assesment. Results: Statistically significant differences between groups following supplementation were found for 5x Sit to Stand test (at baseline 14.3±3.0s vs. experimental group 11.8±2.1s, control 12.1±2.4s; p<0.001), Timed-Up and Go Test (at baseline 10.4±2.0s vs. Cr+GAA 9.1±0.0s, control 9.61±1.9s; p<0.001), right (at baseline 30.5±2.9cm vs. Cr+GAA 31.9±2.8cm, control 31.6±3.4cm; p<0.001) and left upperarm girth (at baseline 30.1±3.0cm vs. Cr+GAA 31.8±3.1cm, placebo 31.9±0.01cm; p<0.001), blood glucose level (at baseline 4.7±0.6mmol/L vs. Cr+GAA 5.1±0.7mmol/L, control 4.9±0.5mmol/L; p=0.02), total serum protein (at baseline 78.9±5.7g/L vs. Cr+GAA 73.2±3.8g/L, control 76.6±4.4g/L; p=0.002), glomerular filtration (at baseline 84.4±10.5ml/min vs. Cr+GAA 71.9±12.0ml/min, control 75.3±12.9ml/min; p<0.001), serum creatine level (at baseline 12.9±4.5umol/L vs. Cr+GAA 22.9±12.2umol/L, control 18.3±4.2umol/L; p=0.01), GAA blood concentration (at baseline 1.9±0.8umol/L vs. Cr+GAA 2.8±0.8umol/L; control 2.6±1.4umol/L; p=0.002), serum creatinine level (at baseline 68.5±17.4umol/L vs. Cr+GAA 110.1±27.4umol/L, control 118.3±24.7umol/L; p<0.001) and diastolic blood pressure (at baseline 78.4±9.3mm/Hg vs. Cr+GAA 86.9±8.6mm/Hg, control 89.7±10.3mm/Hg; p<0.001). Proton magnetic resonance spectroscopy of the vastus medialis muscle showed a significant increase in muscle creatine level (at baseline 28.1±7.0mM vs. Cr+GAA 36.9±9.2mM; p=0.002) and significant sex differences (46.3±0.3mM vs. 30.7±4.6mM; p=0.008). N-acetyl aspartate (NAA) brain level significantly increased in right frontal lobe white matter (at baseline 7.2±1.9mM vs. Cr+GAA 8.1±0.7mM; p=0.03), choline showed no significant changes except between men and women in midline right parietal region grey matter (2.1±0.4mM vs. 1.1±0.1mM; p=0.02). Creatine brain levelincreased in almost all of the voxels but significantly just in the right frontal lobe white matter (at baseline 5.8±0.5mM vs. Cr+GAA 6.3±0.4mM; p=0.002). Significant differences were observed for men and women in the Cr+GAA group in the right parietal lobe white matter (5.5±0.3mM vs. 6.3±0.5mM; p=0.01), left (11.3±0.4mM vs. 8.0±0.9mM; p=0.01) and right midline parietal region grey matter (11.3±1.7mM vs. 8.2±1.4mM; p=0.03). The Short form 36 Quality of Life questionnaire physical component summary (PCS) (at baseline 52.0±4.6 vs. Cr+GAA 55.7±9.1, control 54.6±6.7; p=0.001) and mental component summary (MCS) (at baseline 55.0±3.8 vs. Cr+GAA 66.4±7.1, control 63.7±9.9; p<0.001) showed significant differences post administration in Cr+GAA group. Conclusion: Creatine and GAA 8-week administration demonstrated positive effects on functional abilities of the elderly, having the potential to benefit particular components of health. Improvements in brain and muscle tissue bioenergetics may lead to metabolism promotion preventing cerebrovascular and cardiovascular diseases of older people.