Endocrine Abstracts

Background: In obese patients compared to non-obese subjects, serum immunoreactive parathyroid hormone (PTH) is higher and decreases with weight loss.

Objectives: To find out if obesity affects the vitamin D endocrine system and if the resulting secondary hyperparathyroidism is linked to a decrease in urine calcium.

Material and Methods: 25 healthy individuals were examined. Between the ages of 21-30 years, there were 13 obese people and 12 non-obese subjects. They were only allowed to consume purified water and were fed a steady diet of 300 mg of calcium, 800 mg of phosphorus, 15 meq of magnesium, 100 meq of sodium, and 60 meq of potassium each day. For the analysis of serum calcium, ionized calcium, phosphorus, magnesium, creatinine, Gla protein, 25-hydroxyvitamin D, 1,25-dihydroxy vitamin D, and immune-reactive PTH, fasting blood samples were taken. Calcium, phosphorus, salt, potassium, magnesium, creatinine, and cyclic AMP were measured in 24-hour urine samples.

Results: Those who were obese weighed an average of 105±5 kg, compared to non-obese (67±1 kg; P<0.02)\. While mean serum calcium, mean serum ionized calcium, and mean serum phosphorus were similar between the two groups, mean serum immunoreactive PTH (517±47 vs. 242±32 pg/ml, P<0.002), mean serum 1,25 dihydroxy vitamin D (36±1 vs. 29±1 ng/ml, P<0.02), and mean serum Gla protein (32±1 vs. 22±2 ng/ml, P<0.02) were significantly higher. Mean blood 25-hydroxyvitamin D levels were substantially lower in obese men and women than in nonobese individuals (7±2 s. vs. 21±1% ng/ml, P<0.002). Mean urine phosphorus was the same in both groups, however mean urinary calcium was considerably lower in the obese group (114±11 mg/d vs. 165±12 mg/d, P<0.02) and had higher mean urinary cyclic AMP (3.17±0.42 mg/dl vs. 1.83±0.24 mg/dl GF, P<0.02) and creatinine clearance (215±12 mg/d vs. 172±5 mg/d, P<0.02). Both urine cyclic AMP and serum immunoreactive PTH were significantly correlated with percentage of ideal body weight (r=0.523, P<0.02) and the percentage of ideal body weight (r=0.715, P<0.02), respectively.

Conclusion: Secondary hyperparathyroidism is linked to greater renal tubular reabsorption of calcium and elevated levels of circulating 1,25(OH)2D, is a feature of the vitamin D endocrine system change seen in obese people. They have lower levels of serum 25-OHD due to feedback regulation of the precursor’s synthesis in the liver brought on by higher levels of serum 1,25(OH)2D.