PTH and secondary hyperparathyroidism: In a 1997 study, it was shown that a low protein diet (0.7 g/kg/day) but balanced in all other nutrients, including calcium, carried out in 14 healthy young women led to secondary hyperparathyroidism, with serum parathyroid hormone (PTH) levels increased 1.5–2.4-fold by day four and 1.6–2.7-fold by day 14 compared to values observed in subjects consuming moderate protein intake (1.0 g/kg/day) [79].