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Why Human Babies Are Fattest (and Smartest)

Newborn Baby Fat Protects the Brain During Critical Stages
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Feb. 20, 2004 -- The next time you see a plump baby, compliment the parent on how smart their child will be. A new study shows that baby fat is linked to the size of babies' heads and future intelligence.

Researchers say human infants have long been recognized as the fattest newborns, much fatter than other animals that need extra fat for insulation. In fact, most mammals, including nonhuman primates, don't start accumulating fat until after birth.

Answering the evolutionary question of why human babies are born plump has puzzled scientists for years, and many explanations have been offered. But Portuguese researchers say they now have evidence to back up their theory.

Fatter Babies Have Bigger Heads

In their study, published in the current issue of the American Journal of Human Biology, researchers studied 1,069 human newborns. They found that the fatter the baby was, the bigger his or her head was likely to be, even after accounting for other factors such as gestational age and birth length.

They say the finding shows that newborns need that extra fat to feed their brains during the critical early stages of development.

Previous studies have shown that the brain needs high amounts of energy in order to function and grow, and the bigger the organ is, the more energy it needs.

Researchers say this is especially true during infancy when the brain goes through its maximum growth stage. For example, a newborn's brain can expend as much as 60% of the total energy produced by the body.

Because human babies are not capable of fending for themselves, fatness as a newborn may have developed as an evolutionary mechanism to protect the brain as it develops. Malnutrition in early life can negatively affect the brain's development and is a major cause of infant death.

Researchers say prior studies have also shown that birth weight and head size at birth are associated with head size and mental abilities in childhood and later life. Therefore, newborn fatness would have been favored by natural selection and explain why the trait has endured throughout human evolution.

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