Somatic versus reproductive energy allocation in Papua New Guinea: life history theory and public health policy.

A fundamental evolutionary problem faced by organisms is how to allocate energy to somatic and reproductive functions in ways that optimize fitness. Given that energy is limited in all environments, energy allocation necessarily involves physiological tradeoffs between such factors as growth and reproduction, reproduction and condition, and current reproduction and future survival. Ultimately, the "decisions" that are made about energy allocation among growth, survival, and reproduction determine life history patterns and trajectories of organisms. For humans, knowing how energy allocation to reproduction will likely impact other aspects of the somatic well-being of individuals may also have practical implications for public health policy. This article reviews the evidence for energy tradeoffs between somatic and reproductive functioning in a range of human societies. It also seeks to corroborate the results of earlier work in Papua New Guinea on lactation-related maternal energy depletion using an independent measure of maternal energy reserves, tetrapolar bioelectrical impedance analysis. The current analysis shows that maternal energy reserves decline over the course of lactation and that a cumulative parity-specific decline in maternal energy reserves also exists. A longitudinal follow-up of five women over 11 years shows the decline to amount to about 3 mm of subcutaneous fat per round of pregnancy and lactation. The results corroborate predictions from life history theory and have applied public health implications. In particular, It is suggested that policies such as lactation advocacy that encourage enhanced energy allocation to reproduction in order to promote child health may have the unintended result of compromising maternal well-being, particularly in nations of the developing world. Consequently, it is recommended that nutritional support of mothers be implemented in concert with lactation promotion.