Low mineral density of a weight‐bearing bone among adult women in a high fertility population

Abstract

Evolutionary theories of aging posit that greater reproductive effort causes somatic decline given a fundamental trade-off between investing energy in reproduction and repair. Few studies in high fertility human populations support this hypothesis, and problems of phenotypic correlation can obscure the expected trade-off between reproduction and somatic condition. This cross-sectional study investigates whether greater reproductive effort is associated with reduced calcaneal bone mineral density (BMD) among female Tsimane forager-farmers of lowland Bolivia. We also investigate whether female Tsimane BMD values are lower than sex- and age-matched US reference values, despite the fact that Tsimane engage in higher physical activity levels that can increase mechanical loading. To measure calcaneal BMD, quantitative ultrasonography was performed on 130 women (mean ± SD age = 36.6 ± 15.7, range = 15 – 75) that were recruited regardless of past or current reproductive status. Anthropometric and demographic data were collected during routine medical exams. As predicted, higher parity, short inter-birth interval, and earlier age at first birth are associated with reduced BMD among Tsimane women after adjusting for potential confounders. Population-level differences are apparent prior to the onset of reproduction, and age-related decline in BMD is greater among Tsimane compared to American women. Greater cumulative reproductive burden may lower calcaneal BMD individually and jointly with other lifestyle and heritable factors. Fitness impacts of kin transfers in adulthood may determine the value of investments in bone remodeling, and thus affect selection on age-profiles of bone mineral loss.

Calcaneal Quantitative Ultrasound Indicates Reduced Bone Status Among Physically Active Adult Forager-Horticulturalists

“…Most population prevalence data are not directly comparable to the current study (because DXA‐derived BMD of the spine, femoral neck, or total hip is used to diagnose osteoporosis in other populations), but the Tsimane prevalence of reduced bone status is within the range of age‐matched Americans with osteoporosis (9%) and low bone mass (49%) . A recent study comparing calcaneal qUS parameters of Tsimane and American women aged 15 to 75 years found lower values among Tsimane throughout adulthood even after adjusting for reduced Tsimane body mass . Calcaneal BUA also shows greater annual decline for older Tsimane adults (both sexes) compared to older Chinese, Dutch, and Germans .…”

“…Despite a lean diet and high fertility (total fertility rate = 9 births per woman) with prolonged on‐demand breastfeeding, Tsimane women's breast‐milk concentration of long‐chain polyunsaturated fatty acids is high relative to American women, and does not decline with parity or age . Higher parity and older age are, however, each associated with reduced bone status …”

Immune function in Amazonian horticulturalists

“…During growth, Amazonians also appear to prioritise weight gain over skeletal growth and height attainment (Blackwell et al, 2009; Stieglitz et al, 2015a), and they also seem to accumulate fat in central locations where it may be particularly important for fueling immune defenses (Urlacher et al, 2015). Tsimane also have high resting metabolic rates and total daily energy expenditures, as measured by doubly labeled water and respirometry (Gurven et al, 2016).…”

Conflict or congruence? Maternal and infant-centric factors associated with shorter exclusive breastfeeding durations among the Tsimane

“…Finally, calcium stores become increasingly depleted with age and successive bouts of lactation (Horst et al, 1997; Nordin, 1997). Tsimane women lose relatively more bone mineral density with age as compared to American women, and bone mineral density is further reduced by relatively high parity and short interbirth intervals (Stieglitz et al, 2015). With sustained calcium secretion at high parity, homeostatic mechanisms may downregulate milk supply to conserve essential maternal function.…”