Impact of energy availability and physical activity on variation in fertility across human populations

Abstract

Human reproduction is energetically costly, even more so than other primates. In this review, we consider how the energy cost of physical activity impacts reproductive tasks. Daily energy expenditure appears to be constrained, leading to trade-offs between activity and reproduction expenditures in physically active populations. High workloads can lead to suppression of basal metabolic rate and low gestational weight gain during pregnancy and longer interbirth intervals. These responses lead to variation in fertility, including age at first reproduction and interbirth interval. The influence of energetics is evident even in industrialized populations, where cultural and economic factors predominate. With the decoupling of skills acquisition from food procurement, extrasomatic resources and investment in individual offspring becomes very costly. The result is greater investment in fewer offspring. We present a summary of age at first reproduction and interbirth interval trends across a diverse, global sample representing 44 countries and two natural fertility populations. While economic factors impact fertility, women in energy-rich, industrialized populations are capable of greater reproductive output than women in energy-stressed populations. Thus, energetic factors can be disentangled from cultural and economic impacts on fertility. Future research should focus on objective measurements of energy intake, energy expenditure, and physical activity in a broader sample of populations to elucidate the role of energetics in shaping reproductive outcomes and health.

Keywords: Fertility; Maternal energetics; Physical activity; Reproduction; Resource availability.

Fig. 1

Global distribution of n = 44 countries and n = 2 natural fertility populations. Countries shaded in blue are those included in the sample. Natural fertility populations (Shuar and Tsimane) are labeled with red diamonds

Fig. 2

Distribution of a AFR (years) and b IBI (months) by income group. Vertical dashed lines indicate modal values for color-matched income groups. a Middle-income countries and WIC participants in the US have the same modal AFR value (20 years), represented by the black, dashed line. b WIC participants and nonparticipants in the US have the same modal IBI value (21 months), represented by the black, dashed line. The upper IBI range has been limited to 120 months. Abbreviations for modal lines: NF, natural fertility; LI, low-income; MI, middle-income; HI, high-income; US WIC, US WIC participant; US NP, US WIC nonparticipant