All mammals are protected by the same thing -they can't physically eat food until they are physiologically ready to digest it. For humans this means picking up the food, placing it in their mouth, gumming it, moving it back with their tongue, and swallowing it. The most obvious of course it the tongue thrust that newborns have -this reflex actively keeps food out of their body until they can digest it. But the other steps all have safeguards as well.
For healthy, full term infants the ability to eat food develops around 6-9 months. In recent years there have been numerous studies looking at the risks of certain things (allergies, asthma, anemia, etc) in relation to when solids are started and almost all have shown that the lowest risks are when solids are started between 6-9 months.
However it should also be noted that babies with allergies may refuse solids for up to a year, and that breastmilk is nutritionally complete for at least the first year of life despite earlier statements that it is not. An unpleasant feeling in the mouth is often a first sign of allergy and may cause babies to spit out rather than swallow allergenic foods. This is a very useful safeguard that should not be overridden.
http://www.kellymom.com/nutrition/solids/delay-solids.html
The following organizations recommend that all babies be exclusively breastfed (no cereal, juice or any other foods) for the first 6 months of life (not the first 4-6 months):
* World Health Organization
* UNICEF
* US Department of Health & Human Services
* American Academy of Pediatrics
* American Academy of Family Physicians
* American Dietetic Association
* Australian National Health and Medical Research Council
* Royal Australian College of General Practitioners
* Health Canada
http://www.borstvoeding.com/voedselintroductie/vast_voedsel/rapley_guidelines.html#choke
It appears that a baby's general development keeps pace with the development of his ability to manage food in his mouth, and to digest it. A baby who is struggling to get food into his mouth is probably not quite ready to eat it.
http://www.borstvoeding.com/voedselintroductie/vast_voedsel/rapley_guidelines.html#early
The babies who participated in the research were allowed to begin at four months. But they were not able to feed themselves before six months. Some of the younger babies picked food up and took it to their mouths; some even chewed it, but none swallowed it. Their own development decided for them when the time was right. Part of the reason for this study was to show (based on a theory of self-feeding) that babies are not ready for solid food before six months. It seems that we have spent all these years working out that six months is the right age and babies have known it all along!
It seems reasonable to predict that if parents choose to provide babies with the opportunity to pick up and eat solid food from birth they will still not be able to do it until around six months. The principle is the same as putting a newborn baby on the floor to play: he is being provided with the opportunity to walk but will not do so until about one year – because his own development stops him. But: everything depends on the baby being in control. Food must not be put into his mouth for him. Since it is very tempting to do this, it is probably safer to recommend that babies should not be given the opportunity to eat solid food before six months.
http://www.borstvoeding.com/voedselintroductie/vast_voedsel/rapley_guidelines.html#choke
Many parents worry about babies choking. However, there is good reason to believe that babies are at less risk of choking if they are in control of what goes into their mouth than if they are spoon fed. This is because babies are not capable of intentionally moving food to the back of their throats until after they have learnt to chew. And they do not develop the ability to chew until after they have developed the ability to reach out and grab things. Thus, a very young baby cannot easily put himself at risk because he cannot get the food into his mouth in the first place. On the other hand, the action used to suck food off a spoon tends to take the food straight to the back of the mouth, causing the baby to gag. This means that spoon feeding has its own potential to lead to choking – and makes one wonder about the safety of giving lumpy foods off a spoon.
Why not cereal?
http://www.kellymom.com/nutrition/solids/first-foods.html
Cereal is not at all necessary, particularly the baby cereals. Regular (whole grain) oatmeal is more nutritious for your baby.
http://www.askdrsears.com/faq/ci2.asp
The truth is, there is nothing special about these foods that makes them better to start out with. Babies don't actually even need rice cereal
http://www.llli.org/llleaderweb/LV/LVDec99Jan00p130.html
Meat provides additional protein, zinc, B-vitamins, and other nutrients which may be in short supply when the decrease in breast milk occurs. A recent study from Sweden suggests that when infants are given substantial amounts of cereal, it may lead to low concentrations of zinc and reduced calcium absorption (Persson 1998). Dr. Nancy Krebs has shared preliminary results from a large infant growth study suggesting that breastfed infants who received pureed or strained meat as a primary weaning food beginning at four to five months, grow at a slightly faster rate. Dr. Krebs' premise is that inadequate protein or zinc from complementary foods may limit the growth of some breastfed infants during the weaning period. Both protein and zinc levels were consistently higher in the diets of the infants who received meat (Krebs 1998). Thus the custom of providing large amounts of cereal products and excluding meat products before seven months of age may not meet the nutritional needs of all breastfed infants.
Meat has also been recommended as an excellent source of iron in infancy. Heme iron (the form of iron found in meat) is better absorbed than iron from plant sources. In addition, the protein in meat helps the baby more easily absorb the iron from other foods. Two recent studies (Makrides 1998; Engelmann 1998) have examined iron status in breastfed infants who received meat earlier in the weaning period. These studies indicate that while there is not a measurable change in breastfed babies' iron stores when they receive an increased amount of meat (or iron), the levels of hemoglobin circulating in the blood stream do increase when babies receive meat as one of their first foods.
http://www.westonaprice.org/children/nourish-baby.html
Finally, respect the tiny, still-developing digestive system of your infant. Babies have limited enzyme production, which is necessary for the digestion of foods. In fact, it takes up to 28 months, just around the time when molar teeth are fully developed, for the big-gun carbohydrate enzymes (namely amylase) to fully kick into gear. Foods like cereals, grains and breads are very challenging for little ones to digest. Thus, these foods should be some of the last to be introduced. (One carbohydrate enzyme a baby's small intestine does produce is lactase, for the digestion of lactose in milk.1)
[...]
Babies do produce functional enzymes (pepsin and proteolytic enzymes) and digestive juices (hydrochloric acid in the stomach) that work on proteins and fats.12 This makes perfect sense since the milk from a healthy mother has 50-60 percent of its energy as fat, which is critical for growth, energy and development.13 In addition, the cholesterol in human milk supplies an infant with close to six times the amount most adults consume from food.13 In some cultures, a new mother is encouraged to eat six to ten eggs a day and almost ten ounces of chicken and pork for at least a month after birth. This fat-rich diet ensures her breast milk will contain adequate healthy fats.14
Thus, a baby's earliest solid foods should be mostly animal foods since his digestive system, although immature, is better equipped to supply enzymes for digestion of fats and proteins rather than carbohydrates.1 This explains why current research is pointing to meat (including nutrient-dense organ meat) as being a nourishing early weaning food.
http://www.informaworld.com/smpp/content~content=a713793510~db=all
The results indicate that in a group of healthy, well growing 12-month-old Swedish infants one-quarter is iron-depleted, although iron deficiency anaemia is rare, and one-third may be zinc-depleted. The high cereal intake of Swedish infants from 6 months of age may have limited the bioavailability of both iron and zinc from the diet.
http://www.jpgn.org/pt/re/jpgn/abstract.00005176-200201000-00009.htm;jsessionid=HW2Ny1WpvFRtf9h3hRTlGQMdLxXhWm20yJYNjLFZJCF2wkfjvTRn!1071114923!181195629!8091!-1
Conclusions: These results confirm that meat as a complementary food for breast-fed infants can provide a rich source of dietary zinc that is well absorbed. The significant positive correlation between zinc intake and exchangeable zinc pool size suggests that increasing zinc intake positively affects metabolically available zinc.