OVERFEEDING          If intakes of energy and specific nutrients meet requirements, the rate of growth of organs and tissues will be limited only by the infant's genetically determined potential for growth.⁴ Growth cannot be further increased by supplying additional energy or nutrients. By contrast, growth of adipose tissue and deposition of fat is not tightly regulated by genetic factors and gain in body fat will increase with increases in energy intake. Moreover, a small percentage increase in energy intake may result in a large percentage increase in fat deposition. "Obesity" in infancy is defined as weight for length above the 95th centile value; "overweight" is defined as a weight for length between the 90th and 95th centiles.⁴ In industrialized countries, overfeeding is more common than underfeeding.
          There can be little doubt that family characteristics,³⁵ probably based on genetic factors,^{36, 37} are of great importance to obesity. Using self-reported data of 540 individuals from the Danish Adoption Registry, Stunkard et al demonstrated that there was no relation between the body mass index of the adoptees and their adoptive parents, but a strong correlation between the body mass index of adoptees and their biological parents.           Recently, genes responsible for obesity in mice and humans were identified.³⁸ These genes secrete certain proteins or "satiety factors" in the fat tissue that are believed to enter the blood-stream and send a "stop eating" signal to the hypothalamus, the part of the brain that controls appetite. A single defect in the obese gene results in obesity because the gene fails to send or receive that signal. This preliminary finding suggests that obesity may be an organic problem, not simply a behavioral problem.
          In the late 1960s and early 1970s, some scientists believed that there was a critical period for adipocyte proliferation during the preweaning period, after which new adipocytes were no longer formed. According to the widely held hypothesis at the time, major increases in adipocyte cell number could occur in response to stimuli (including overfeeding) operating in early life, whereas stimuli operating after infancy were likely to increase cell size but to have little effect on cell number.^{39, 40, 41} Knittle advised early intervention "before immutable hypercellularity occurs,"⁴² and Mack and Kleinhenz, noting that adipocytes appear to proliferate during the early months of life, speculated that "early infant overfeeding may lead to adipose hypercellularity, with lifelong sequellae."⁴³
          The hypothesis that adipocyte number increases only during early infancy has now been abandoned,⁴⁴ and, in fact, in the human^{45, 46} and in the baboon,⁴⁷ the preweaning changes in adipocytes consist of an increase in adipocyte size but not in adipocyte number. Overfeeding the baboon during the preweaning period was found to be associated with a major increase in body fat and in adipocyte size but not in adipocyte number.⁴⁷

PRACTICAL APPLICATIONS          The emphasis on eating in moderation during infancy is based on a belief that habits of eating established during infancy may carry over to later life.⁴⁸ With this in mind, parents can avoid underfeeding or overfeeding their infant by paying attention to their baby's hunger and satiety cues.⁴⁹

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