Nutritional needs of the infant and how these are met by human and formula milks

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Adequate dietary intake and nutritional status among children are important for their own growth, development and function. However, there is clear evidence that childhood nutrition influences adult health. Research has shown that intrauterine nutrition influences adult morbidity and mortality, but childhood diet and nutritional status can modify the consequences of being born small. Diet in all the stages of childhood needs to be taken seriously because of its potential for producing normally developed children as well as determining their lifelong health and thus having an impact on a nation’s health. With many diets in transition owing to changing social, economic and environmental conditions, consideration of the impact of these changes on the diets of children is important. Particular issues of concern include the establishment of good dietary habits, together with adequate physical activity to prevent the development of overweight, adequate intakes of calcium to promote bone health and sufficient intakes of minerals and vitamins in the face of a culture often centered on fast food. There are many gaps in our current understanding of children’s food habits and influences on these. It is difficult to obtain good-quality information about these and, therefore, health promotion inputs may be missing their target.

Nutritional Needs

It must be assumed that the nutritional needs of the infant are ideally met by breast milk, when this is produced in sufficient quantity by a fully breastfeeding mother. For this reason, the UK Department of Health took the view that no dietary reference values (DRVs) were required for breastfed infants. Values were, therefore, set for formula-fed infants, which are based on the nutritional composition of breast milk and the average amounts consumed. In addition, some allowance is made in certain cases for poorer efficiency of digestion and absorption of the nutrients in formula milks.

Energy

Energy needs are determined primarily by body size and composition, physical activity and rate of growth. Infants have a high basal metabolic rate owing to the large proportion of metabolically active tissue and the large loss of body heat over a relatively great surface area. In the second half of the first year, the growth rate slows, but the level of activity increases as the child starts to crawl and then learns to walk around the age of 1 year. The total energy expenditure in infants has recently been measured using the doubly labelled water (DLW) technique, which has produced lower results than had been previously reported. Results from studies of energy intakes confirm these results.

Reference Nutrient Intakes for Selected Nutrients for Infants

Nutrient 0-3 months 4-6 months 7-9 months 10-12 months
Protein (g/day) 12.5 12.7 13.7 14.9
Thiamin (mg/day) 0.2 0.2 0.2 0.3
Riboflavin (mg/day) 0.4 0.4 0.4 0.4
Niacin (nicotinic acid equivalents) (mg/day) 3 3 4 5
Folate (µg/day) 50 50 50 50
Vitamin C (mg/day) 25 25 25 25
Vitamin A (µg/day) 350 350 350 350
Vitamin D (µg/day) 8.5 8.5 7 7
Calcium (mg/day) 525 525 525 525
Iron (mg/day) 1.7 4.3 7.8 7.8
Zinc (mg/day) 4 4 5 5

Protein

In infants, the role of protein is almost entirely to support growth. The infant requires more protein per unit weight than the adult and has a particular requirement for the essential amino acids histidine and taurine. Human milk provides a relative excess of some of the amino acids (glutamine, leucine and isoleucine) needed for tissue synthesis and relatively lower levels of others (arginine, alanine and glycine). This means that the neonate has to be very efficient in transforming some amino acids into others to fulfill needs for tissue synthesis. Adequate amounts of feed should be provided to allow the protein to be used for growth rather than to meet energy needs. Excessive amounts of protein are undesirable and may be harmful to the infant, as they increase the amounts of waste material to be excreted in the urine and might result in dehydration. In addition, immature kidneys cannot adequately filter high-molecular-weight proteins.

Fats

Fat should comprise 30%–50% of an infant’s energy intake; above this level, it may be poorly digested. In breast milk, fats supply 50% of the energy. Fats are an important part of an infant’s diet because of their energy density, that is, they provide a substantial amount of energy in a relatively small volume. The essential fatty acids found in milk, and particularly the long-chain n-3 fatty acids, are important for the development of the brain, vascular systems and retina in early months of life. In particular, docosahexaenoic acid may not be synthesized in sufficient amounts by the infant from precursors in the diet to meet the needs of tissue development.

Carbohydrates

Carbohydrate, predominantly in the form of lactose, supplies 40% of the energy in an infant’s diet. Lactose yields glucose and galactose on digestion; the latter is essential in the development of the brain and nervous system. Undigested lactose is fermented in the digestive tract to lactic acid and lowers the pH. This is beneficial as many of the pathogenic organisms that can cause gastroenteritis do not thrive in an acidic environment. Infants can also digest and utilize sucrose, although this sugar is sweeter tasting than lactose and can induce a preference for sweet foods in the infant. The ability to digest starch is limited.

Fluid

Because of their relatively small total body water content, babies have a vital need for fluids. Their small body weight/surface area ratio makes them susceptible to dehydration, for example, in hot weather and illness. As an absolute minimum, the normal infant requires between 75 and 100 mL of fluid per kilogram body weight daily and should be provided with 150 mL/kg, to ensure that all needs are met. Under normal circumstances, this amount of fluid is provided by the milk feed and no additional water is required.

The infant loses water through the skin and respiratory tract, through sweating in warm environments and through the urine and faeces. The volume of urine produced is dependent on the fluid intake and on the amount of solutes to be excreted. An adult kidney is able to concentrate solutes and reduce water loss, if fluid intake is low, but a baby’s kidneys initially lack this ability. Thus, feeding a diet with a high ‘solute load’, in particular with high protein and sodium contents, results in increased water loss via the kidney. Under normal circumstances, fluid intake should be sufficient to cope with this. However, difficulties may arise if a baby is given an over concentrated feed:

  • Amounts of feed are very small (due to illness)
  • There is fluid loss via other routes (vomiting, diarrhea, sweating)
  • Solids are given at a very young age (below 2 months)

In each of these cases, additional water should be given to avoid dehydration.

Minerals

Babies require a wide range of minerals in their diet. These include calcium, phosphorus and magnesium for bone development, iron and copper for red blood cell formation and zinc for cell division and growth, together with other trace elements. The iron content present at birth has usually been used in red blood cell formation by 4–6 months, and an additional source of iron is needed at this stage. Calcium and phosphorus are present in equimolar quantities in human milk, which matches the ratio in the body. An excessive intake of phosphorus can dangerously lower calcium levels. This is a particular problem in premature infants and those fed on unmodified formula. The minerals in human milk are associated with the protein or fat fractions of the milk, which probably facilitates their availability.

Vitamins

The vitamin content of milk is generally adequate, with the exception of vitamins D and K. Human milk is low in vitamin D, and the UK Department of Health recommends that breastfeeding mothers should take a vitamin D supplement of 10 μg/day to ensure adequate levels in their milk, especially in the winter months. Formula-fed infants receive adequate levels of the vitamin. Breastfed infants are also at risk of low vitamin K intakes. It has been a routine practice to give newborn infants a dose of the vitamin in the first days of life by intramuscular injection or oral dose.

Meeting Nutritional Needs

 A baby’s nutritional needs are generally met either by the use of human milk from the breast or formula derived from cows’ milk, modified to a composition resembling that of human milk. The continued development of formula milks ensures that they come closer to the content of human milk than ever before. In Western societies, mothers are free to make the choice between breastfeeding and bottle-feeding, without fear that their baby will be disadvantaged in any way as a result of their decision. The professional consensus is that breastfeeding is better for the baby and possibly confers benefits to the mother. In many poor areas of the world, the use of infant formula may increase health problems rather than solving them.

Breastfeeding or Bottle-feeding?

Across the world, there are programmes and activities to promote breastfeeding, since it is recognized as nutritionally the best way of feeding the newborn infant. The World Health Organisation (WHO) and the United Nations Children’s Fund jointly promote the Baby-Friendly Hospital Initiative, which encourages hospitals to put in place a number of measures that will facilitate the initiation and continuation of breastfeeding. At least 21,000 hospitals (approximately 27% of maternities worldwide, and 8.5% of maternities in industrialized countries) have been certified as baby friendly around the world.

Formula Milk and Breast Milk Compared

The composition of formula milks available in Britain is governed by a directive from the European Commission and Statutory Instrument. Derived from cows’ milk, they are classified as ‘casein dominant’, based on the entire protein fraction or ‘whey dominant’, containing the dialyzed whey protein. Modifications include the addition of lactose, maltodextrins, vegetable oils, various vitamins and trace elements and reductions in the level of protein, electrolytes and some minerals such as calcium.

Bottle-feeding, if carried out correctly, with due attention to hygiene, appropriate concentrations and closeness during the feeding, can provide most of what the infant needs. However, the unique composition of human milk, with more than 200 constituents and with a varying content, will probably never be matched by a manufactured formula feed. The composition of breast milk is not constant between women and within the same woman for different lactations and even during the day. The milk secreted towards the end of a feed (hind milk) is richer in fat and, therefore, higher in energy value than the fore milk, at the start of the feed. This may play a part in appetite control, with the richer hind milk providing a feeling of satiety. Obviously, this cannot happen with a formula feed.

Proteins in Milk

The proteins in human milk are predominantly whey proteins including alpha-lactalbumin, lactoferrin and various immunoglobulins; casein forms only 30%-40% of the total protein. Although the lactalbumin is a major source of amino acids, the other whey proteins have a non-nutritional role, in particular, as protective agents. In cows’ milk, casein comprises 80% of total protein, which can form tough, leathery curds in the stomach and be more difficult to digest. In the formula milks based on whey, the casein content is reduced (from 27 g/L in cows’ milk to 6.0 g/L). Beta-lactoglobulin, which is normally found in cows’ milk and is a potential allergen, is also absent from these formula milks.

Human milk also contains non-protein nitrogen compounds, including taurine, urea and a number of hormones and growth factors. Their functions are still uncertain but may well help with the normal development of the infant. Until their function is clearly defined, it is unlikely that these substances will be included in formula milks.

Carbohydrates in Milk

Lactose concentrations in human milk are greater than in cows’ milk, although levels in formula are similar. Formula milks may also contain maltodextrin as a source of carbohydrate. Lactose enhances the absorption of calcium as a result of the lower pH resulting from fermentation to lactic acid, which makes the calcium more soluble.

Fats in Milk

Although the total fat contents of human and cows’ milks are similar, the fatty acid compositions are quite different. Modified milks contain added oils to increase the unsaturated fatty acid content towards that of human milk. Nevertheless, there remains a much greater diversity of lipids in human milk, which contains cholesterol, phospholipids and essential fatty acids. Digestion and absorption of fat from human milk is aided by the presence of lipase within the milk secretion, which starts the process of digestion before the small intestine is reached. Some milk has been reformulated to include more essential and long-chain fatty acids.

Vitamins

The levels of the water-soluble vitamins in milk reflect the maternal levels and thus rely on a sufficient intake by the mother. In the West, it is rare for vitamin levels to be deficient in milk due to maternal under nutrition. Human milk also contains binding factors for folate and vitamin B12, which facilitate their absorption. Most formula milks contain levels of the vitamins greater than those found in human milk. However, apart from vitamins D and K, for which intake may be too low from human milk, there appears to be no advantage in this.

Minerals

Levels of many minerals are modified in the manufacture of formula from cows’ milk. This is because their concentrations would generally be too high for the human infant to cope with. In particular, this applies to calcium, phosphorus and the electrolytes. Many of the minerals are associated either with proteins or fat globules, and this appears to facilitate their absorption. Specific binding factors have been identified for iron and zinc, which make the absorption of these minerals from human milk much greater than that from formula.

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