"A fat baby is a cute baby," says Professor Tom Brenna of Cornell University. "And the real question is, why do humans make fat babies? Chimpanzees don't, baboons don't. Humans are the only primates born with fat. And we put that fat on for very good reasons." We do it for our brains.

"Humans have larger brains compared to other primates," explains Brenna. "When we think about what else makes a cute baby, we realize that a cute baby is a baby with a big head." Our sense of aesthetics has, in fact, internalized something profound about our development. "If you think about the average 2-year-old," says Brenna, "the thing that's been growing over those 2 years faster than anything else is the head. If you look at him next to his mother, his head size is not that different even though his body size is greatly different. Why? Because the emphasis of perinatal growth is the brain. Humans have been called the animals with the runaway brains."

In the third trimester, brain growth accelerates tremendously. This extraordinary growth continues throughout the baby's first year of life, when brain size will nearly triple. And this, says Brenna, "is a compelling reason, a definitive reason, why the nutritional requirements in the perinatal period are different for babies than they are for adults. Babies are very energy intense. They need to take in a lot of energy to support growth. And they are growing various parts of their body at different rates. In the first couple of years of life, they are emphasizing the growth of their brain. Adults are not growing their brains anymore; we're just trying to hold onto the brain we have."

The brain requires an enormous amount of energy. "Measurements of the energy the body uses to run the brain are extraordinary," says Brenna. "Even as adults, it's more than a quarter of the energy that humans use." Infants, on the other hand, use 60% of the calories they consume for brain development. All that energy must be available on demand. The body can use fat for fuel in various ways that support the growing brain.

Fat is not just fuel for growth, it is also a critical building block. "The human brain is the fattiest organ in the body," says Brenna. "More than 50% of the dry weight of the human brain is fat. That surprises people, but it's true."

Perhaps not so surprising is that brain fat is not inert fat. "Unlike storage lipid, or adipose tissue," explains Brenna, "which many of us have around our abdomen, the fat in neural tissue is not storage lipid. It is structural fat contained in the membranes of neural cells and a key component of synapses, or connections, between neural cells."

Fat and Thermoregulation
Fat is also essential in thermoregulation, which is one of the most critical physiological functions for an infant during the transition to extrauterine life.

It is hard to imagine a cozier (98.6 degrees Fahrenheit) or more stable thermal environment than the womb. Even a 72 degree Fahrenheit room is cool for a newborn. For a preterm infant, who hasn't had the advantage of putting on fat during the third trimester, it is a shock. One researcher, according to Susan Tucker Blackburn, in Maternal, Fetal, & Neonatal Physiology, has estimated that the body temperature of a 2-1/2 lb preterm infant would fall 1.7 degrees Farenheight (1 degree Celcius) every 5 minutes if left unattended in a cool room.

The goal of thermoregulation is to maintain a fine balance between heat production and heat loss. Heat is produced by the body through normal metabolic processes and muscular activity. The amount of metabolic heat produced changes according to activity (whether the baby is fussing, sleeping, or just sitting). The organs that generate the most metabolic heat are the brain, heart, and liver.

Newborns can increase their metabolic rate by 200-300%, according to Blackburn, and the way they do this is largely through harnessing the power of brown adipose tissue (BAT). Brown fat is found in newborn animals of many species, as well as in animals that hibernate. In a full-term human newborn, BAT accounts for 2-7% of the infant's weight and is concentrated in the back and nape of the neck, as well as around the heart, lungs, esophagus, liver, and kidneys. Only very small amounts of BAT remain in adults, mostly located around the kidneys and aorta.

The major function of brown fat is heat production. In appearance and composition BAT is distinct from white fat, which replaces brown fat in adults. BAT is comprised of many small fat droplets and contains large numbers of mitochondria, which are tiny intracellular structures that function as powerhouses (and which also give BAT its brown color). BAT also has an extensive capillary blood supply and nerve network. These characteristics give this unique type of fat the ability to generate more energy than any other tissue in the body. But the energy that is created in BAT is not stored, it is immediately converted to heat.

The reason that brown fat is in such large supply in infants and short supply in adults is that infants can't shiver. And, in fact, the main mechanism of thermoregulation in infants goes by the name of nonshivering thermogenesis (NST). NST is the way in which an infant regulates its temperature for the first six months. As the baby matures, he or she will be able to rely more on physical methods of heat generation such as shivering and increasing activity, and with that development, brown fat gradually disappears.

More on this topic

Infant Nutrition (VIDEO)
Fueling Growth & Development
Milk Enters the Stomach
Milk Enters the Small Intestine
Some Key Nutrients
Gut & Immune Development
Respiratory Health
Skeletal & Muscular Development
Skin & Hair Growth
Cardiovascular Development
The Importance of Fat
DHA & ARA
Nervous System Development
Good Nutrition Builds Healthy Babies


Related Health Centers:

Infant Nutrition Health Center, Mother-Baby Bond Health Center, Mother’s Milk Health Center, Monthly Infant Development Calendar Health Center,Weekly Pregnancy Calendar Health Center