ESPE Yearbook of Paediatric Endocrinology

In this review, Prof Zeev Hochberg presents the characteristics and the function of the juvenile stage of life of human beings from a broad evolutionary perspective.

Homo sapiens is unique in having four postnatal pre-adult life stages: infancy, childhood, juvenility and adolescence. Unlike humans, all other mammals (including the great apes) transit directly from infancy to juvenility and then to adulthood, without the childhood and adolescence stages. This review highlights the importance of these intermediary growth stages in human evolution with a specific emphasis on juvenility.

The onset of juvenility coincides with the eruption of permanent molars around 6 years, at the end of brain growth, and is highly associated with brain weight. Unlike the secular trend, the age of transition to juvenility has not changed throughout the 200 000 years of modern humans.

The juvenile stage of life is presented by three distinct endocrine and body composition changes: adrenarche (the onset of adrenal androgen generation), growth pattern (decelerating from a linear childhood growth velocity) and adiposity rebound (acceleration of body mass index).

Serum DHEA and DHEAS rise progressively throughout juvenility, is required to prepare the central nervous system for adolescence, both in its psychosocial sense and in setting the scene for pubertal maturation of the hypothalamic–pituitary–gonadal axis. Increased adrenal androgens are also related with enhanced GH-IGF-1 axis activity, increased muscle mass and bone mineral; and increased leptin and body fat.

Adiposity rebound may be the first clinical sign of juvenility, which corresponds with the second rise in the age-related BMI curve that occurs between ages 4 and 6. Increases in DHEAS levels correlate positively and coincide with increases in BMI. The aim of adiposity rebound in the transition to juvenility is linked to energy supply. The energy that was allocated to brain growth in childhood is stored as abdominal fat in juvenility, in order to support the energetically costly accelerated growth during the subsequent adolescence.

The growth declines in juvenility to the slowest rate after a period of constant growth rate in childhood. This accompanies a clear acceleration in lower limb growth. This pattern of growth is required for living within the social hierarchy as longer lower limbs relative to body mass reduce the energetic cost of human walking as the juvenile joined the hunter-gatherer adult society without posing the physical threat of a large body.

Finally, the timing of transition from childhood to juvenility has significant impact on body composition and metabolic adaptation. Given as an example is that an early adiposity rebound is observed in overweight children and is associated with an increased risk of being overweight. Another example is early juvenility and premature adrenarche seen in babies born small for gestational age. This juvenile adaptive response generates a thrifty phenotype characterized by ovarian hyperandrogenism, hyperinsulinemia, and dyslipidemia in the female later in life to enhance self-sufficient energy supplies, survival schedules and behavioral strategies that yield the highest fitness against environmental constraints.

Ze’ev introduced this new concept of juvenility to try to explain and understand the enigma of adrenarche. He described juvenility as part of a strategy in the transition from a period of total dependence on the family for provision and security to self-supply; it is assigned with a predictive adaptive response of body composition and energy metabolism. The juvenile stage offers opportunities to prepare for the physical and social complexity of adolescence as well as adulthood.