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Adipose tissue The adipose “organ” is composed of fat cells known 
as adipocytes (Ailhaud and Hauner, 1998). Adipocytes are distributed 
throughout the body in various organs and tissues, although they are largely 
clustered anatomically in structures called fat depots, which include a large 
number of adipocytes held together by a scaffold-like structure of collagen 
and other structural molecules. In the traditional view of the adipocyte, the 
cell provides a storage structure for fatty acids in the form of triacylglycerol 
molecules, with fatty acids being released when metabolic fuel is needed 
(Arner and Eckel, 1998). While adipocytes play this critical role, they are 
also involved in a number of endocrine, autocrine, and paracrine actions 
and play a key role in regulating other tissues and biological functions, for 
example, immunity and blood pressure, energy balance, glucose and lipid 
metabolism, and energy demands of exercise (Ailhaud and Hauner, 1998; 
Frühbeck et al., 2001). The role of adipocytes in regulation of energy bal-
ance and in carbohydrate and lipid metabolism and the potential effects 
of physical activity on adipocyte function are of particular interest here, 
given growing concerns related to pediatric and adult obesity (Ogden et al., 
2012) and the associated risk of cardiometabolic disease (Weiss et al., 2004; 
Eisenmann, 2007a,b; Steele et al., 2008). Metabolic differences among 
various fat depots are now well known (Frühbeck et al., 2001), and there 
is significant interest in the distribution of adipose tissue, the changes that 
occur during childhood and adolescence, and their clinical significance.
Adipocytes increase in size (hypertrophy) and number (hyperplasia) 
from birth through childhood and adolescence and into young adulthood 

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Educating the Student Body: Taking Physical Activity and Physical Education to School
Relationship to Growth, Development, and Health
 
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to accommodate energy storage needs. The number of adipocytes has been 
estimated to increase from about 5 billion at birth to 30 billion to 50 bil-
lion in the nonobese adult, with an increase in average diameter from about 
30-40 
µ
m at birth to about 80-100 
µ
m in the young adult (Knittle et al., 
1979; Bonnet and Rocour-Brumioul, 1981; Chumlea et al., 1982). In total 
the adipose organ contains about 0.5 kg of adipocytes at birth in both 
males and females, increasing to approximately 10 kg in average-weight-
for-height males and 14 kg in females (Malina et al., 2004). There is wide 
inter individual variation, however, and the difficulty of investigating chang-
es in the number and size of adipocytes is obvious given the invasiveness of 
the required biopsy procedures; understandably, then, data on these topics 
are scarce in children and adolescents. Also, since only subcutaneous depots 
are accessible, results must be extrapolated from a few sites. 
Based on such information, the average size of adipocytes has been 
reported to increase two- to threefold in the first year of life, with little 
increase in nonobese boys and girls until puberty (Malina et al., 2004). A 
small increase in average adipocyte size at puberty is more obvious in girls 
than in boys. There is considerable variation in size across various subcuta-
neous sites and between subcutaneous and internal depots. The number of 
adipocytes is difficult to estimate. Available data suggest that the cellular-
ity of adipose tissue does not increase significantly in early postnatal life 
(Malina et al., 2004). Thus, gain in fat mass is the result of an increase in 
the size of existing adipocytes. From about 1-2 years of age and continuing 
through early and middle childhood, the number of adipocytes increases 
gradually two- to threefold. With puberty the number practically doubles, 
followed by a plateau in late adolescence and early adulthood. The number 
of adipocytes is similar in boys and girls until puberty, when girls experi-
ence a greater increase than boys. 
The increases in the number of adipocytes during infancy and puberty 
are considered critical for enlargement of the adipose tissue organ and 
for the risk of obesity. Since size and number are linked, the number of 
adipocytes can potentially increase at any age if fat storage mechanisms are 
stimulated by chronic energy surfeit (Hager, 1981; Chumlea et al., 1982). 
Energy expenditure through regular physical activity is a critical element in 
preventing energy surfeit and excess adiposity. While cellularity undoubt-
edly is strongly genetically determined, regular physical activity, through its 
contribution to energy expenditure, can contribute to less adipocyte hyper-
plasia by limiting hypertrophy.