We know that complete nutrition is necessary for healthy growth and development. But is it possible to identify single nutrients that are responsible for the health and function of a specific organ, a vitamin that determines good hearing, healthy joints, or efficient liver function? Not exactly. While researchers have indeed identified many different nutrients that play key roles in specific systems and physiological pathways, in nearly all cases, it is the interaction of many different nutrients that determine healthy growth and development.

Much of what we know about the impact of single nutrients has been learned through the study of deficiencies. The classic investigation of a deficiency, and one of the first scientific nutrition experiments, was conducted in 1747 by a physician in the British Navy, who believed that citrus could prevent scurvy, a disease of malnutrition that was a terrible scourge for sailors who spent long periods of time at sea without fresh fruit or vegetables. It would take decades before the significance of the experiment was widely appreciated, but eventually, the consumption of citrus on ships to prevent scurvy gave rise to the slang term "limey" for British sailors.

Other common deficiencies include pellagra, caused by a niacin deficiency; kwashiorkor, a form of malnutrition caused by insufficient protein; beriberi, caused by a thiamine deficiency and often a consequence of eating only polished white rice; rickets, the result of inadequate mineralization of bone caused by a vitamin D deficiency or defect in its metabolism; and iodine deficiency, which is the most common cause of preventable mental retardation and brain damage worldwide.

Anemia caused by insufficient iron, according to international health organizations, is currently the most common deficiency overall, affecting 700 million worldwide. In some developing countries the immediate cause of the deficiency may be blood loss, a consequence of hookworm infestation, for example, but in many other countries it is caused by not getting enough of this mineral in the diet.

Iron is also a good example of the balance that must be struck in a diet, particularly an infant's diet. There is very little iron in breast milk, but what is there is easily absorbed by the infant. While iron is indeed essential for the production of red blood cells as well as the development and regulation of other organ systems, iron can also fuel the growth of pathogenic bacteria in a baby's gut. Breast milk provides enough iron to meet the infant's needs, but not enough to encourage disease-causing microbes. And in fact, there is a key protein, lactoferrin, also present in breast milk, which binds to iron in order to maintain this balance.

The more researchers learn about infant nutrition, the more examples we have of the critical role played by micronutrients. Occasionally, says registered dietitian Julie Balay, this can be the source of debate. "There are two issues with breast milk that are controversial, and those are fluoride and vitamin D. Even if the mother drinks fluoridated water, it doesn't show up in breast milk. And often we find that vitamin D is at very low levels in mothers." So this raises the question: should we fortify breastfed babies with fluoride and vitamin D?

"It's controversial, because on one hand you think, well, it's breast milk. It must be perfect," says Balay. "But on the other hand, we now know a lot more about nutrition than we used to. We didn't use to live as long as we do now; maybe we need fluoride to make our teeth last 80 years." The issue with vitamin D, explains Balay, is trickier. "We know that vitamin D has hormonal effects on the body and that it is also critical for the proper absorption of calcium. The concern is whether all babies get enough in breast milk. Well, we now we have recommendations that all breastfed babies be given vitamin D."

A pregnent woman doesn't need to eat the same diet as a newborn to assure that all the right nutrients get to the baby. Some of the necessary nutrients are going to get shifted and shunted to the fetus. The same goes for breastfeeding. Take fat, for example. A nursing mother should still eat a moderate diet, not one in which 50% of the calories come from fat. However, not all nutrient gaps can be overcome through nutritional shunting. Careful consideration of dietary choices and supplementation may be necessary to optimize the health of the mother and developing infant.

Micronutrients are clearly critical. But what is not as obvious, explains Balay, is that both mothers and developing fetuses as well as infants may absorb a higher percentage of some micronutrients than would a nonpregnant woman or older child. In other cases, an infant might absorb less of a particular micronutrient. "That's the neat thing about human bodies and nutrition. We have these safety valves. When we are low in something we will absorb a lot more of it, and by and large, when we have enough, our absorption rate will go down. It's often a protective mechanism."

While it is impossible to completely isolate the impact of a single nutrient, the USDA's Nutritional Needs of Infants offers information on some of the roles and functions that specific vitamins and minerals play in infant growth and development.

Vitamin D: essential for utilization of calcium and phosphorous, formation of bones

Vitamin A: plays role in formation and maintenance of healthy skin, hair, and mucous membranes; proper vision; growth and development; healthy immune and reproductive systems

Vitamin E: protects vitamin A and essential fatty acids in the body, prevents breakdown of tissues

Vitamin K: necessary for proper blood clotting

Vitamin C: necessary for the formation of the protein collagen found in bones, cartilage, muscle, blood vessels, and other connective tissue; helps maintain capillaries, bones, and teeth; plays role in healing of wounds and body's ability to resist infections and absorb iron

Vitamin B12: necessary for healthy blood cells, proper functioning of the nervous system

Folate: essential for cell division, growth and development of healthy blood cells, formation of genetic material within cells throughout body

Vitamin B6 (pyridoxine): helps the body use protein to build tissues, aids in metabolism of fat

Vitamin B1 (thiamine): helps body release energy from carbohydrates during metabolism, plays vital role in normal functioning of nervous system

Vitamin B2 (riboflavin): helps body release energy in the metabolism of protein, fat, and carbohydrates

Niacin: helps body release energy during protein, fat, and carbohydrate metabolism

Calcium: necessary for bone and tooth development, blood clotting, and maintenance of healthy nerves and muscles

Iron: essential for growth and formation of healthy blood cells

Zinc: plays role in the formation of protein in the body and in wound healing, necessary for growth and maintenance of all tissues, taste perception, healthy immune system

Sodium: essential for the maintenance of water balance, regulation of blood volume, proper functioning of cell membranes