Calcium and nutrients are transferred from mothers to fetuses or infants during pregnancy or lactation, respectively, promoting metabolic changes in the mother, many uncharacterized. To evaluate these changes, we undertook two parallel studies. In one we analyzed fourteen clinical cases of vertebral fragility fractures, at or before three months after partum, in mothers who breastfed their infants. In the other, we enrolled 79 additional pregnant subjects, some who chose to breastfeed and others who did not, and analyzed changes in bone metabolic status starting between 34 and 36 weeks of gestation and ending one month after partum. In the larger group, bone-resorbing and bone-forming parameters such as serum TRACP5b and osteocalcin, respectively, significantly increased after partum. Among parameters that changed after partum, serum PTH and the bone-resorbing markers serum TRACP5b and urine NTX were significantly higher in mothers who only breastfed infants compared to mothers who fed infants formula or a mix of both. However, bone-forming parameters were comparable between breastfeeding and non-breast-feeding groups after partum, suggesting that elevated bone-resorption occurs only in the breastfeeding group. Radiographic analysis after partum demonstrated that no subject among the 79 analyzed showed vertebral fractures, even those who breastfed exclusively. Among fracture cases analyzed, subjects exhibited significantly lower bone mineral density than did non-fracture cases in breastfeeding-only subjects. We conclude that bone metabolic status significantly changes over the period between pregnancy and post-partum lactation, and that low bone mineral density seen in a small subset of breastfeeding-only cases likely causes post-partum vertebral fragility fractures.
Pregnancy and lactation alter skeletal metabolism in females1. During pregnancy, nutrients, in particular calcium, are supplied to fetuses from mothers, and after birth, lactation continues to supply calcium to infants. Thus, mothers must undergo increased calcium resorption from the intestine to satisfy infants’ needs, and osteoclastic bone resorption as well as osteocyteic osteolysis are activated to enhance calcium supply1,2,3,4,5, resulting in decreased bone mass in women6. Case reports have described fragility fractures in maternal skeletons after partum1,7,8,9,10,11. These conditions are infrequent and pathophysiological mechanisms underlying them are unknown. During pregnancy and lactation, hormonal changes can perturb skeletal homeostasis. Serum levels of parathyroid hormone [PTH] or parathyroid-related protein [PTHrP] reportedly increase during lactation1. Elevated PTH levels promote osteoclast differentiation and activation, followed by increases in serum calcium levels due to activated osteoclastic bone-resorption, which in turn, reduces bone mass12,13. Women show well-documented declines in bone mass after menopause14. Estrogen deficiency due to the menopause activates osteoclastic bone resorption, leading to osteoporosis, which is frequently associated with bone fragility fractures15. Similarly, after partum, mothers are in a transient condition of “menopause”, which likely alters bone metabolism. Osteoclasts reportedly express the estrogen receptor [ER], and either loss of estrogen or lack of ER in osteoclasts activates bone resorption in these cells16,17. These changes may occur not only in menopausal but in pregnant or breastfeeding women.Introduction
Bone homeostasis is regulated by integrated activity of bone-resorbing osteoclasts and bone-forming osteoblasts18, a process termed “coupling”18. Increased bone-resorption activates bone-formation, and if unregulated leads to reduced bone mass frequently seen in skeletal disorders such as postmenopausal osteoporosis18,19. The receptor activator of nuclear factor kappa B ligand [RANKL] plays a pivotal role in osteoclast differentiation and activation20, and lack of either RANKL or its receptor RANK completely abrogates osteoclastogenesis21,22. Interestingly, the RANKL-RANK system is also required for mammary gland development during lactation23, and female mice lacking either RANKL or RANK can become pregnant but cannot lactate. Thus, osteoclast formation and lactation are also considered coupled.
Materials and Methods
Subjects
Fracture subjects were female post-partum vertebral fracture patients aged 31–44 years at the first visit, who had been referred to our hospital with fragility fracture[s] of unknown origin between February 2014 and July 2017.
A different group of subjects examined for bone turnover markers before and after partum were pregnant females who had visited our hospital to give birth between February 2014 and April 2017. A total of 86 subjects were invited to this study, written informed consent was obtained from all individual participants, and each completed a self-reported questionnaire regarding past history and drug usage. Both fracture and non-fracture participants were asked at their first visit to the hospital and at the first visit after partum, respectively, to describe the way they fed their infants, such as through breastfeeding exclusively or use of formula, or mix of breast and artificial milk. All fracture subjects continued to breastfeed exclusively until they experienced fractures. Seven subjects were excluded due to refusal to undergo a follow-up examination, moving to another hospital or perinatal death, leaving 79 subjects enrolled. All subjects were Japanese women living in the Tokyo area. Both studies were approved by an ethics committee at Keio University School of Medicine and were carried out in accordance with clinical study guidelines.
Measurements
Height, body weight, and body mass index [BMI] calculated from body weight and height data were assessed in all subjects. Sera and urine samples were collected from all subjects between 34 and 36 weeks of gestation and at one month after partum. Serum calcium [Ca], inorganic phosphorus [IP], creatinine, albumin, parathyroid hormone [PTH], TRACP5b and estradiol [E2] levels were assessed in all subjects. TRACP5b [Nittobo, Fukushima, Japan] and uDPD [Quidel Corporation, San Diego, CA] were analyzed by enzyme immunoassay [EIA]. PTH [Roche diagnostics, Tokyo, Japan], osteocalcin [Roche], P1NP [Roche], E2 [Roche], prolactin [Roche] and ucOC [Sekisui Medical, Tokyo, Japan] were analyzed by electrochemiluminescent immunoassay [ECLIA]. BAP [Beckman Coulter, Pasadena, CA] and uNTX [Alere Medical Co., Ltd, Tokyo, Japan] were analyzed by chemiluminescent enzyme immunoassay [CLEIA] and ELISA, respectively. 25[OH]D [DIAsource, Neuve, Belgium] was analyzed by radioimmunoassay. Bone metabolic parameters were examined at the first visit in fracture patients, and one month before and one month after birth in the group of pregnant subjects. Bone mineral density [BMD] was analyzed using a dual-energy X-ray absorptiometry [DEXA; GE Healthcare, Amersham Place, Little Chalfont, Buckinghamshire HP7 9NA, England] at the first visit in fracture patients, and one month after birth in pregnant subjects. Statistical analysis was undertaken using the unpaired two-tailed Student’s or Welch’s t-test [*p