Pregnancy is an anabolic state requiring energy for the products of conception, the foetal-placental unit and the increase in maternal tissues. Newly synthesised maternal tissues account for a 15-26% increase in metabolic rate in pregnancy (26). The total calculated energy cost for pregnancy is around 355 640 kJ (85 000 kcal) and this translates into an extra 1191.3 kJ (285kcal) a day (27,28). These theoretical energy costs, originally derived in the 1960s by Hytten and Leitch, have been confirmed by more recent physiological measurements (29).
Maternal physiology is highly adaptable and pregnancy can progress during times of extreme food deprivation and/or physical activity (30). Under adverse
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