Racial/Ethnic Differences in Calcium Metabolism in Response to Dietary Sodium

Experimental design: Seventy-six adults aged 18-50 years (19 white, 19 Black, 19 Hispanic, and 19 Asians) will be recruited to complete 16 in each group and tested on two levels of dietary salt (lower = 2300 mg/Na/d and higher = 3700 mg Na/d) in randomized order. The primary outcome measures are fractional calcium absorption and urinary calcium excretion. Secondary outcome measures are urinary sodium excretion and whole blood serum ionized calcium and magnesium concentrations.

Participants will be recruited by poster or by passed out flyers on and near campus, through presentations to organizations where potential participants attend (YMCA, fraternities, churches, etc.), and advertising through newspaper ads, social media, and others. Respondents will be invited for a screening visit at the Exercise and Nutritional Sciences Department to determine eligibility and to sign a consent form.

Eligible participants will have 3 of 4 grandparents self-identified as the racial/ethnic group. They will not have taken any vasoactive or calcium metabolism-altering medications (statins, antihypertensives, aspirin, steroids, bisphosphonates, thiazide diuretics, hormone replacement therapy, and antidepressants), experienced a bone fracture within the past 6 months, had diabetes, cancer, liver disease, gastrointestinal disease (e.g.: Crohn's disease, irritable bowel syndrome, malabsorption syndrome), decreased kidney function, or have smoked cigarettes within the past 12 months before the study. Eligible subjects will also not have had habitually consumed > 1000 mg Ca/d from food or supplements.

Upon providing written consent, applicants will complete a brief questionnaire on medical history, physical activity, and supplement use and a 4-day diet record to assess usual dairy intake as well as nutrients known to influence calcium and sodium metabolism i.e. calcium, sodium, magnesium, protein, fiber. Waist circumference, height, and weight will be measured to determine the body mass index (BMI) and a fasting blood sample will be collected to assess blood biochemistries and metabolomics. A DXA will be performed to determine total body calcium as a covariate.

Enrolled subjects will be randomized to a sequence of 2 treatments of a high (3700 mg sodium) or low (2300 mg sodium) diet for 2-days each separated by a 4-week washout period during which they will consume self-selected diets. All food and beverages will be provided to control for the nutrients known to influence calcium metabolism with salt being the main difference between the periods. Menus will be devised to control for energy, calcium, sodium, macronutrients, and fiber. Portions will be adjusted for energy needs of the participant. They will be asked to discontinue taking any dietary supplements 30 days prior to the study and to abstain from caffeine, alcohol, and strenuous exercise for 24 hours before each test session. Participants will be asked to collect urine for 48 hours to measure sodium, calcium, and magnesium excretion and a stool sample for measuring fecal metabolomics. On the following day. Participants will be asked to abstain from all foods and beverages not provided until the last blood draw. Blood pressure will be measured on the last day of each treatment.