Studies in mice demonstrated that dietary phosphate (Pi) loading that mimic the level of US adult consumption leads to reduced spontaneous locomotor activity, exercise capacity, and reduced resting metabolic rate when in normal mice by impairing skeletal muscle mitochondrial function and fat oxidation. However, relevance of this findings in humans remains unknown.

The investigators will perform a randomized, crossover study to determine if lowering dietary Pi intake from 1,200 mg/day to recommended daily allowance (RDA) of 700 mg/day increases skeletal muscle ATP synthesis, raises exercise oxygen uptake (VO2) during cardiopulmonary testing. To ensure stable Pi intake prior to randomization, Pi consumption will be estimated by food recall during a run-in and washout phase for 2 consecutive days, using the Automated Self-Administered 24-Hour (ASA24®) Dietary Assessment Tool (https://epi.grants.cancer.gov/asa24/). After the run-in period, all participants will be maintained on a low Pi diet (700 mg/d) and a normal Na diet of approximately 3,128 mg/d throughout the study for 8 weeks. Subjects will also be randomized to receive Sodium Phosphate 2 capsules daily (containing a total of 500 mg of Pi, 372mg of sodium) for 4 weeks during the high Pi phase (total Pi intake 1,200 mg/d) vs 2 capsules of Sodium Chloride (NaCl, containing a total of 372mg of sodium) to match Na content to Sodium Phosphate without extra Pi daily for 4 weeks during the low Pi phase (total Pi intake = 700 mg/d). The total Na intake during the entire study will be at usual US consumption level of 3,500 mg/d. Investigational drug service at UT Southwestern will dispense Sodium Phosphate vs. NaCl tablets and the study subjects will be blinded to the type of supplement they receive.

During the periods of high and low Pi phases, the investigators will monitor 24-hr urinary Pi excretion (UPiV) to ensure adherence. Since approximately 70% of ingested Pi is absorbed and excreted in the urine, we expect 24-hr UPiV excretion to be approximately 840 mg during the high Pi phase and 490 mg during the low Pi phase. If 24-hr UPiV is < 800 mg during the high Pi diet or > 500 mg during the low Pi diet, the research dietitian will provide additional counseling to improve adherence to the menu plan. If needed, the research diet will be altered to be more compatible with the participant's preference.

24-hr urinary Pi, Na, K, Cr, and Ca excretion will be assessed after weeks 2 and 4 of the low and high Pi phases. Serum electrolytes and Pi will be monitored every 2 weeks. Serum FGF23, and soluble Klotho levels and cardiopulmonary testing and P31 MRS of quadriceps muscle will be obtained after 4 weeks on the study diet. The order of study supplement will be randomized. Subsequently, participants will stop the first study supplement and undergo washout for 2 weeks. After 2 weeks of washout, they will receive the 2nd study supplement. Measurement of skeletal muscle ATP synthesis and VO2 at rest and during exercise will be repeated in the same fashion.

Inclusion Criteria:

The randomized crossover trial will be performed in on otherwise healthy subjects without diabetes mellitus, chronic kidney disease (CKD), preexisting cardiovascular disease or treatment with any vasoactive agent that might alter cardiovascular responses to exercise.

Exclusion Criteria:

1. history of cardiopulmonary disease or chronic kidney disease,
2. treatment with antihypertensive medications,
3. estimated glomerular filtration rate (eGFR) < 60 mL/min/1.73m2, (4) diabetes mellitus or other systemic illness,

(5) pregnancy, (6) hypersensitivity to sodium phosphate, (7) any history of substance abuse or current cigarette use, (8) any history of psychiatric illness, (9) history of active malignancy, (10) serum phosphorus < 2.4 or > 4.5 mg/dL.

We will share de-identified data upon reasonable request.

12 months after study is published

Investigational drug service at UT Southwestern will dispense Sodium Phosphate vs. NaCl tablets and the study subjects will be blinded to the type of supplement they receive.