PROJECT SUMMARY Skeletal muscle loss and impairment – prominent features of the aging process – lead to physical disability. There is plausible evidence that altering components of the diet, mainly protein intake and acid/base balance, may improve indices of muscle health in older adults and thus translate to a reduction in physical disability. A growing number of studies suggest that increasing protein intake using a whey protein supplement may benefit muscle health in older adults. However, a main concern is that high protein results in a large dietary acid load from the breakdown of protein to acidogenic byproducts, which could in turn promote muscle degradation particularly in older adults with age-related declines in renal excretion of acid. Thus, our hypothesis is that the balance between the amount of protein in the diet (anabolic component) and the net acid load of the diet (catabolic component) in part determines whether the diet as a whole has a net anabolic or catabolic effect on muscle. Preliminary data from our group have suggested that a daily potassium bicarbonate (KHCO3) supplement improved lower extremity muscle power in postmenopausal women. Based on our preliminary data, we plan to conduct a randomized, double-blind, placebo-controlled, 2x2 factorial study in which sedentary men and women age 65 and older on baseline lower protein diets will be enrolled. Participants will assigned to one of four groups: either a whey protein supplement (to raise protein intake to 1.5 g/kg/d) with or without KHCO3 81 mmol/d or an isocaloric placebo supplement with or without KHCO3 81 mmol/d for 24 wks. The primary outcome is lower extremity muscle power. Secondary outcomes are lower extremity muscle endurance, physical performance, lean mass, and 24-hr urinary nitrogen excretion. Twenty-four-week changes in these outcomes will be compared in the 4 groups. Our central hypothesis is that higher protein intake plus a neutralizing alkaline salt will have additive effects on muscle performance and mass in older adults to either intervention alone. If successful, this research could result in a paradigm shift in dietary advice to older adults to reduce physical disability. As 1 in 5 US adults will be over age 65 by year 2040, a safe dietary intervention that reduces age-related musculoskeletal decline will be an important preventative health step for our aging society.