In an infant's first year of life, the baby's brain will nearly triple in size. At birth the brain weighs 300-400 g (about 3/4 lb); it weights 550-650 g (1 1/3 lbs) at 6 months and 750-1000 g (about 2 lbs) at 1 year. During this period of rapid growth, the brain uses 60% of the total energy consumed by the infant. The rate of brain growth eventually slows. An average adult brain weighs around 1300-1400 g (or about 3 lbs). If a baby's brain continued to grow at the rate it develops in the first year, it would weigh over 120 lbs by the time the child entered kindergarten!

There are many nutritional factors that play important roles in neurological development. Vitamins and minerals, such as iron and iodine, for example, are essential. But one of the most exciting discoveries in the past generation of research involves the role of fatty acids, which are the building blocks of fats and oils found in nature. The fatty acids that have gotten the most public attention are the ones found in fish oil. These compounds are actually produced by algae, which is a vital part of the aquatic food chain and the reason that these fatty acids are eventually concentrated in oily fish. Research suggests that fish consumption is beneficial to cardiovascular health and is associated with a decreased risk of cancer and neurological damage.

There are many different fatty acids found in nature, however. Two fatty acids in particular are called "essential fatty acids," because the human body can't make them and must therefore get them through the diet. These are linoleic and alpha-linolenic acid, and they are found in many types of plant-seed oils. From these two fatty acids, which chemists describe as polyunsaturated fatty acids (PUFAs), the body can assemble long-chain polyunsaturated fatty acids (called LCPUFAs) such as docosahexaenoic acid (DHA) and arachidonic acid (ARA). Researchers believe that an infant's body, however, does not convert PUFAs into DHA very effectively or in sufficient quantity to meet the needs of the infant's rapidly growing brain.

The story of the role of DHA and ARA in infant nutrition has unfolded over the past three decades. Researchers discovered that these two fatty acids, which are found in cell membranes, become concentrated in the fetal brain during the last trimester of pregnancy, a time when there is an extraordinary increase in the growth of neurons in the brain. Researchers also found that fatty acids appear to play a role in the development of vision (studies in rats and monkeys showed that diets that excluded key fatty acids resulted in retinas that didn't develop or function properly).

"The importance of DHA in the diet came to the fore in the 1980s," explains Tom Brenna, professor of human nutrition at Cornell University, "when medical technology developed to the point where very premature infants, less than 2 lbs, began to survive at very significant rates. A baby that small is born at 6 months and has its last trimester after it is born. That last trimester is the developmental period when the brain starts depositing DHA exponentially. Those babies were in an incubator being fed a formula that did not include any LCPUFAs, which meant that the preterm infants had to make all of their DHA from precursor fatty acids."

The newfound ability to keep very low-weight preterm babies alive became an opportunity to see the importance of these fatty acids to the developing infant. "It occurred to some very forward-looking researchers," says Brenna, "that those babies were not getting the supply of DHA they would have been getting had they still been in the womb; there was no placenta to transmit that DHA to the baby. So they started to study DHA as a nutrient that ought to be included in the formula of those premature babies whose brains were growing so rapidly." This led to the first clinical trials of formulas supplemented with key fatty acids.

Researchers found that increasing the amount of DHA in the diet of the infant or in the diet of the mother while pregnant delivered more DHA to the baby. And this in turn could be measured in changes in the development of the visual system as well as cognition.

Studies have shown that DHA in cell membranes not only encourages nerve growth (neurogenesis) but also plays a key role in the development of the synapses, or connections, between nerves (synaptogenesis). Each neuron can have hundreds or even thousands of connections with other neurons. Synapses are the connections through which neurons signal other neurons in the brain as well as other cells such as muscles. In this way, synapses create the network of circuits within the brain, which gives rise to thought and perception, and throughout the central nervous system, which allows the nervous system to communicate with the rest of the body. And although synaptogenesis occurs throughout a healthy person's lifespan, there is an explosion of synapse formation during early brain development.

DHA and ARA are fascinating fatty acids because they represent such a tiny fraction of the total fat in human milk and infant formula and yet their purpose is so specific. DHA and ARA make up a third of the fatty acids found in brain grey matter and up to two-thirds of those found in the eye's retina. But researchers have also discovered that DHA levels in breast milk vary greatly in different regions. For example, levels in Australia have been measured at 0.23%, in the US at 0.17%, and in Japan at 1.0%. Among the lowest levels recorded was 0.07% in Sudan, and within a large country, China, levels ranged from 0.44% to 2.78%. The variation is believed to be due to differences in diet, especially fish consumption, with DHA levels in some areas declining significantly because of changes in traditional diets. One comprehensive review found a worldwide average of 0.32%.

Says Brenna,"The concentration of DHA in breast milk as a percentage of other fatty acids is generally between 0.2 % and 1.0%, so there's quite a wide range. But that still seems like a very small percent of the total. The thing to remember is that most of the components that we consume in food are burned. We use them for energy. We eat carbs and we burn them. We don't store them to much of an extent. We eat fat and most of the fat is burned. But very little DHA is burned. DHA is a structural component. DHA goes into the membranes. And in fact it is clear that the brain avidly retains DHA. So if the supply of DHA is low in the diet, the brain seems to hang on to it even more avidly."

OPTIMAL DHA CONSUMPTION

A natural question, says Brenna, is how much DHA is enough? "We produce a basal amount of DHA. That's clear. And we know this because we don't see deficiencies in people who don't eat any animal products at all. Those people have babies that are healthy. So it seems like our biochemistry is tuned to provide a more or less minimal amount of DHA to prevent obvious signs of deficiency. But our biochemistry also seems to rely on a supply of DHA to optimize our development."

So the related question, says Brenna, is how much DHA is optimal? "One of the observations that people have made about certain development periods is that when we compare consumption of DHA with consumption of only precursors, we find that babies who consume DHA develop a little faster, but babies not eating DHA end up at the same point eventually." If that is the case, and it seems to be with certain elements of development like visual acuity, is there any reason to be worried about DHA?

That's a difficult question to answer, acknowledges Brenna. "I'm personally compelled by arguments made by some psychologists that boil down to the concept of 'crawl, walk, run.' In a developmental sequence, we crawl first, then we walk, then we run. In that order. And you might say that even if you found a treatment that enhanced the development of the infant so that the infant crawled earlier, well, that really doesn't matter because eventually all infants crawl. Yes, but those who crawl first also walk first and those who walk first also run first."

It is something we call a ceiling effect, says Brenna. "Everybody eventually reaches the particular developmental milestone, but those who reach it a little bit earlier are also a little bit advantaged. Thus in studies that show an early advantage that may not persist at later ages, you can ask yourself the question, 'Would I rather have an early advantage or not have an early advantage?'"

Vision also provides a good example of one mechanism by which DHA acts. "In the retina, DHA is in highest concentration in what we call the photoreceptors," explains Brenna. "Photoreceptors are cells stimulated by light. The molecule in the photoreceptor that detects light, the energy of the photon, is a protein called rhodopsin. This molecule of rhodopsin is embedded in a membrane and surrounded by phospholipids that contain DHA. The DHA interacts with the rhodopsin molecule in a way that facilitates the transfer of the signal from rhodopsin to the other membranes of the cell, which then amplify that signal. This ultimately results in the transmission of the nerve signal indicating that a photon has been detected in that photoreceptor."

There is still a great deal to learn about exactly why and exactly how fatty acids in breast milk, like DHA and ARA, are important in development. Although DHA seems to be getting all the attention, says Brenna, we know that ARA also has many functions. "It is best known for its properties as a signaling molecule or as a precursor for lots of other signaling molecules. These signaling molecules are best known for blood clotting and they are also involved in inflammation. ARA is also found at very high levels in the brain." Not all of ARA's functions are completely understood, notes Brenna, but we do know that ARA is also part of the ongoing story of researchers trying to tease out the role and function of key nutrients in fetal and infant development. It is a story with many chapters yet to be written.