| Nutrition, life cycle and metabolic adaptation
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| Physiological states, as well as disease, change the demand for nutrients and the energy expenditure. Pregnancy, lactation, and growth (in particular the intensive growth in utero, during infancy and the adolescent growth spurt) are the three most important physiological states associated with increased demand for nutrients.
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| Pregnancy is an example of the type of metabolic adaptation termed expansive adaptation. Here the body of the mother adapts to carrying the fetus and supplying it with nutrients. Around the time of conception mother's body prepares for the metabolic demands of the fetus. In the early pregnancy the mother sets up the 'supply capacity', and later in pregnancy the actual supply takes place. Ninety percent of fetal weight is gained between the 20th and 40th week of pregnancy and the steepest growth takes place between the 24th and 36th week. The total amount of energy stored during pregnancy is about 70 000 kcal (293 090 kJ), amounting to approx 10 kg of weight.
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| Nutrient intake changes during the life cycle. After delivery there is a transition from feeding through the placenta to breastfeeding, and then, gradually, to diet. Up to the breastfeeding stage nutrition is controlled by substrates and the infant is entirely dependent on the mother for nutrition. Later, the growth hormone assumes a major role in directing development. At the school age, new eating and activity patterns emerge as a child learns to be independent from parents. This continues during adolescence on the background of the accelerated growth. At this stage, sex hormones begin to play a prominent developmental role.
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| In adulthood, muscle mass increases between the age of 20 and 30 years, and at that point the level of physical activity stabilizes. Thereafter, muscle mass starts to decline and the fat mass starts to increase. This accelerates after the age of sixty. The bone mass also declines with age.
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| When nutrients are in short supply, i.e. both in the instances of increased nutritional need and the reduced availability of food, the so-called reductive adaptation occurs. In such instance the metabolic rate falls and the desire to eat increases. This limits the weight loss.
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