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 Dietary Supplements: Overview of Diet and Activity as Modifiers of Growth and Adolescent Development 
 
Alan Rogol D. MD, PhD ©
The normal linear growth of a child is an expression of adequate nutrition and freedom from major illness; however, there is a remarkable range of what is considered normal. At each growth period-infancy, childhood, and adolescence--the growth rate results from the dynamic interplay of nutrition, physical activity, and hormonal processes upon the genetically determined template. Linear growth velocity decelerates rapidly from 30 cm/year during the first few months of life to approximately 9 cm/year at age 2 to 7 cm/year at age 5. Linear growth then continues at approximately 5.5 cm/year before slowing slightly just before puberty (preadolescent "dip"). For an average girl, the growth velocity increases sharply at approximately age 10, reaches a peak of approximately 10.5 cm/year at age 12, and decelerates toward zero as epiphyseal fusion occurs around age 15. For males, who follow a typical growth curve, the pubertal spurt begins around age 12, reaches a peak velocity of 12 cm/year at age 14, and then decelerates toward zero around age 17. The total growth at puberty is approximately 25 cm for girls and 28 cm for boys. If one adds the 2 extra years of prepubertal growth for boys, one has the 13 cm (5+5+3) difference in the mean height between men and women.

The overall contribution of heredity to adult size and body configuration varies with environmental circumstances, and the two continuously interact throughout the entire period of growth. The genetic control of the tempo of growth is apparently independent of that for size and configuration.

There has been a secular trend toward additional height and earlier sexual development documented at least over the past 150 years, with children of average economic status increasing their height by 1 to 2 cm per decade. During the 20th century, the age of menarche in Western Europe and the United States has decreased 2 to 3 months per decade, now averaging 12.8 years in middle class communities.1

Infancy
Size at birth is determined more by intrauterine and placental factors and maternal nutrition than by genetic growth potential. Although length at age 2 and adult height have a correlation coefficient of 0.80, the correlation is but 0.25 at birth, reflecting those factors noted above.2 Growth during the first 2 years is characterized by gradual deceleration in both linear growth velocity and rate of weight gain. Most infants will cross growth percentile lines as they "catch-up" or "lag-down" toward their genetically determined target.3 In addition, body shape changes toward a more linear one as fat accumulation wanes and the child becomes more muscular.

Childhood
Growth during childhood is a relatively stable process as the infancy shifts in growth channel are complete and the child follows the trajectory previously attained and grows at an average rate of 5 to 6 cm/year." 1,4 During this stage, growth primarily depends on the thyroid hormones, those of the GH/IGF-I axis and insulin.

Pubertal Growth
The onset of puberty corresponds to a skeletal (biological) age of approximately 11 years in girls and 13 years in boys.5 Most methods for detecting skeletal age use a single radiograph of the left hand and wrist. On average girls enter and complete each stage of puberty earlier than boys, but there is significant intra- and interindividual variation in the timing and tempo of puberty. One of the hallmarks of puberty is the adolescent growth spurt, often preceded by slowing of prepubertal growth, also known as the "pre-adolescent dip." Girls gain an average of 25 cm and boys 28 cm during pubertal growth.6,7 Boys enter puberty 2 years later than girls; the longer duration of prepubertal growth in combination with greater ~ of 13 cm between men and women.

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