HELEN A. GUTHRIE Professor of Nutrition Emerita Pennsylvania State University Over the past century recommendations for the appropriate age at which to introduce solid foods in addition to breast milk or formula, sometimes referred to as "Beikost", in the feeding of infants have ranged from cautiously adding scraped meat after one year of age (1900) or meat broth at eight to nine months (1917), to the use of pureed bacon and eggs as early as two to three weeks (1956). Since 1960, however, professionals have consistently advised that four to six months is the age at which the infant may benefit from a more varied diet and at which the risks are minimal (1). In spite of this recommendation studies show that from one third to one half of infants are given solid foods by two to three months of age and about three-fourth before four months. (2). This practice most likely reflects the mother's eagerness to watch her child progress and her unsubstantiated assumption that it would help the infant sleep through the night, rather than the health-care provider's concern that it is nutritionally or developmentally necessary. At a scientific level, the almost universal recommendation that the diet of breast milk or formula be supplemented no sooner than four months nor delayed beyond six months is based on our well established and emerging knowledge of:
There is a growing and evolving body of scientific data on the nutritional, immunological and psychological merits of breast feeding over formula feeding. However, there is no evidence that the choice of initial feeding regimen among breast, bottle or mixed feeding influences the timing of the subsequent introduction of a feeding pattern more varied in texture, flavor and /or feeding method. There is evidence, however, that breast-fed infants who are exposed to many flavors in mother's milk as her diet varies during lactation are more likely to accept new flavors once solid foods are offered in early infancy than are infants fed formula in which the flavor is more consistent fromfeeding to feeding(3). Physiological Maturity Although the infants' gastrointestinal and renal systems are relatively immature at birth, they are able to deal with the components of breast milk most of which are uniquely suited to the infant's physiological maturity. Cow's milk based formula modified to mimic the desirable qualities of breast milk such as a lower protein and sodium content, a higher lactalbumen to casein ratio and emulsified fat are well tolerated by most infants. The presence of enzymes and other bioactive components in breast milk that have beneficial effects in digestion and metabolism have not, as yet, been replicated in formula. In addition, breast milk contains enzymes that facilitate digestion. By four months of age the gastrointestinal and renal systems have matured to the point where the infant is more able to handle foods in addition to milk or formula. This maturation coincides with or precedes more obvious cues of eating readiness. The ability to digest lactose appears in the 34th to 38th week of gestation and thus the enzyme lactase is present at birth in full term infants. Infants also have the capacity to hydrolyze sucrose and can tolerate corn syrup composed of glucose and fructose, maltodextrin and glucose polymers some of which are partially hydrolyzed. Up until two months of age, the newborn infant produces relatively little saliva that in an older infant serves to lubricate food to facilitate swallowing and provides salivary alpha amylase the enzyme responsible for initiating the digestion of complex carbohydrates usually first introduced with the feeding of cereal. Human milk also contains a starch-splitting amylase that can compensate for any limitation in the amount of salivary amylase if starches are fed earlier. The enhanced flow of saliva is evident when the infant who has limited ennervation in the lip area to prevent drooling from the mouth. The pancreatic alpha-amylase important in carbohydrate digestion in the adult is present in very small amount up until 6 months of age. Similarly the release of hydrochloric acid in the stomach is is relatively limited until two to three months of age. The hydrochloric acid serves to digest foods and to inactivate foreign microorganisms that might enter the gastrointestinal tract from food or feeding utensils, and to prevent the conversion of nitrates naturally occurring in foods such as spinach, carrots beets and collards to potential carcinogenic nitrites. These functions may be compromised until the pH of the stomach falls below 4.0. Lack of hydrochloric acid is never a problem for infants fed breast milk or sterile formula. The relatively high fat content of both human milk and formulas (3.6-3.8 gm/dl) provides up to 50% of the calories for the newborn. Fat is digested through the action of lingual lipase secreted by the serous gland in the mouth on triglycerides. This enzyme is able to cleave the glycerol-fatty acid bond in the third position on the glycerol molecule. Human milk contains a lipase active throughout lactation which is stable at pH >3.5 including the duodenal pH of 8,0 to 9.5. In the duodenum where the lipase is activated by bile salts which also protect it from proteolysis, it is able to hydrolyze lipid to the fatty acids and glycerol stage. Pancreatic lipase capable of digesting triglycerides to diglycerides to fatty acids at a pH of 6.5 increases gradually to 20 times the birth level by nine months of age. The lower protein content of breast milk and modified cow's milk formula (0.9 and 1.4 gm/ 100 dl ) respectively compared to 3.2 gm /dl in cow's milk is advantageous, There is a lower urea synthesis when the excess protein is deaminated in the liver. Until the kidney tubules mature to the point they are able to reabsorb fluid and some of the electrolytes filtered out of the blood by the more mature renal glomeruli, the kidney has a limited capacity to concentrate urine. This may result in an excessive loss of body fluid and some electrolytes. This loss of fluid becomes a problem when the kidney is presented with a high renal solute load usually due to a high protein or extra sodium chloride intake. This potential excessive loss of fluid in the urine is an even greater problem when the infant may also be losing fluid through perspiration or diarrhea. Up to three months of age, the gastrointestinal tract lining allows the passage of some large intact protein molecules or partially digested proteins into the blood or lymph.. The presence of these 'foreign substances' stimulates the production of antibodies to the protein which may then trigger an allergenic response such as a rash, gastrointestinal or respiratory symptoms when the offending protein is consumed at a later time(4). For immunologically immature infants, a hypoallergenic diet involving the avoidance of potential allergens such as eggs, wheat cow's milk in early infancy is strongly advised to reduce the possibility of IgE mediated reactions in infancy. Intake of meat, eggs, nuts and cereals which if absorbed intact could initiate an allergenic response, especially in any infants under three months of age who have a genetic predisposition to react to specific allergens. Young infants have a limited capacity to absorb sorbitol, the reduced alcohol derivitve of glucose present in fruit juices such as apple, peach, plum and pear. When these are introduced early in life sorbitol proceeds unchanged to the colon where it exerts an osmotic effect, causing fluid from the intercellular compartment to enter the colon resulting in diarrhea. In general, it is best to delay giving fruit juices until the baby can drink from a cup and preferably until he is consuming sufficient food so that the calorie and nutrient intakes are not compromised if juice replace more calorie -dense foods. In addition, any juice given in a bottle has been implicated in the development of dental caries.The sugar content of the juice tends to collect around the teeth where it ferments to produce an acid which creates a cariogenic environment for the teeth. Nutritional Needs The nutrient composition and density of both human milk and infant formula are sufficiently high so that infants can consume enough milk to meet their need for calories and almost all other nutrients without taxing the limited capacity of the stomach or requiring more than five to six feedings a day. Breast-fed infants who have a limited exposure to the ultra-violet rays of the sun may need supplemental vitamin D, which can be provided by pediatric drops. The calorie density of 64 and 67 kcals/ dl for breast milk and formula respectively is sufficient to meet the currently estimated average energy needs of 108 and 98 Kcalories / kg/day for the first and second six months of life respectively. Studies using doubly labeled water to assess total energy needs suggest values 15 to 25% lower than current recommendations(5). Initially the introduction of solid foods contributes minimally to nutrient intake but is encouraged as the infant is stimulated to utilize its ability to swallow semi-solid nourishment and to accept food from a spoon. In addition, the baby is learning to accept a variety of flavors and textures in food. Once an infant is awake more hours and more active physically there is a need to supplement the energy intake from milk to support normal growth and to preclude the use of protein calories for energy rather than growth. By six or seven months food other than milk may contribute over 50% of the infants' caloric intake. The first major nutritional shortfall in a breast milk diet for older infants is iron. At four to six months the infant has depleted the iron stores present at birth and needs an exogenous source in addition to the highly bioavailable iron in breast milk. This can be provided through the use of iron-fortified formula throughout the first year of life and iron enriched cereals during or after weaning(6) While the provision of an iron-rich dietary ingredient is important to prevent further depletion of iron reserves, it is important to avoid an excessive intake of a combination of iron fortified formula, cereals and supplements. Physical Readiness At birth, the infant with a receding chin and fat pads in the cheeks is endowed with a strong rooting and sucking reflex especially suited to obtaining milk from the breast. It also has a strong extrusion reflex leading to the use of the tongue to reject hard objects such as a spoon or a lump of food. With physical maturation, the fat pads diminish, the routing reflex is replaced with the ability to signal interest in food by moving the head forward and the extrusion reflex disappears permitting more successful feeding from a spoon. The reflex is replaced by the ability of the tongue to form a semi-solid or solid food into a bolus and to thrust it to the back of the mouth from where it can be swallowed. The disappearance of the extrusion reflex eliminates the problem, which may arise earlier in life when caregivers frequently interpret the innate tendency to push out with the tongue as a rejection of the food being offered. During this same period the jaw takes on more adult configuration and the lips become enervated and capable of controlling the drooling which characterizes the initial increased production of saliva. The ability of the infant to control head movements and maintain an upright position with or without support permits an interaction between the infant and the care-giver in the feeding situation. By four months by moving his head toward a spoon when interested in food and turning away when satisfied or disinterested, the infant is able to communicate satiety or its need for food. Risk of chronic disease Although there is a significant body of information concerning the relationship between dietary practices during adulthood and the risk of chronic diseases such as coronary heart disease, hypertension, cancer and osteoporosis, there is little evidence to relate dietary intakes of infants to the risk of developing these conditions. On the contrary there is concern that if infants are subjected to the reduced or low fat diets suggested for adults that they will suffer from a physical growth delay as a result of caloric restriction. The role of essential fatty acids and cholesterol in the development of the central nervous system suggests the importance of providing ample amounts of these nutrients associated with dietary fat during the first two to five years of life when the nervous system is growing to its ultimate adult size. As a result of recent studies indicating a relationship between the intake of intact cow's milk protein and Insulin Dependent Diabetes Mellitus in infants susceptible to IDDM, the American Academy of Pediatrics recommends breast feeding and the avoidance of whole-cow's milk until 12 months of age.. Additionally they support a delay in the introduction of non-milk feedings until six months of age. Principles of initial feeding It is strongly recommended that the first food offered should be a single as opposed to a mixed food of more than one ingredient. Although the sequence in which foods are introduced is relatively unimportant as long as the food is nutrient dense and contributes to the child's experiences with a variety of flavors and textures, the food of choice is usually rice cereal ( iron enriched) since seldom, if ever. is it allergenic. In addition it can be prepared with breastmilk or formula (in a ratio of no less than 1:6) to a consistency that requires a swallowing rather than a sucking action important in stimulating the child to develop the ability to swallow. It should be fed for several days to a week before introducing a second single component food, preferably another cereal. By introducing foods one at a time it is possible to identify an offending food that may be causing problems for the child, This could be an indication of either an allergy or more commonly a food intolerance. To maximize the likelihood of acceptance, food should be offered prior to a milk feeding when the infant is hungry. Foods, carefully prepared in the home can be nutritionally equivalent to commercially prepared foods and, if prepared with aseptic care and refrigerated after preparation, they should be equally safe. If fresh fruits such as banana, papaya or avocado, are used they should be thoroughly ripe and prepared by mashing. Canned foods with added salt or sugar should be avoided or used sparingly as should food containing honey for the first year of life since the latter has been associated with some cases of infant botulism. In practice, cereals and fruit are the first foods offered followed by vegetables and finally meat or meat and vegetable mixtures by five or six months. Some authorities suggest that vegetables be offered before fruits on the theory that most infants prefer the sweetness and other flavor attributes of fruit and may reject more bland vegetables when offered after they have developed a liking for fruit flavors. Later Feedings/ After a range of pureed single item foods have been introduced and accepted, as the infant matures it will progress to eating foods with coarser textures. Foods that require different feeding techniques such as finger foods are appropriate once the child shows an ability to grasp items and carry them to her mouth. Similarly, once the teeth erupt at five or six months it is desirable to provide foods that utilize the infant's capacity to chew. Dry, but not flint-like foods such as crisp bacon or triscuit-like biscuits which tend to splinter and cause choking, biscuits or toast are most frequently used. Although complex desserts and casseroles, which have been mixed to provide a texture suitable for the infant are available commercially and widely used, there is no nutritional rationale for recommending them. There is no evidence that the child who is fed these 'gourmet' items is a more sophisticated eater later in lifethan the one who is encouraged to enjoy the flavors of a wide variety of single item foods. These are usualluy less expensive. Summary: In summary, while official recommendations suggest that the appropriate time for the introduction of solid foods is no earlier than four months and no later than six months of age, within that period the best time to add other foods will depend on a variety of factors including nutritional needs, growth rate and rate of physiological and neurological maturation. Part 2 will deal with the consequences of starting beikost too early and too late, and the issues relating to specific foods and food ingredients including fruit juice, nitrates, MSG, sodium, salt, sugar, modified starches, eggs etc. References
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