CHAPTER 6 Diet and the Lifecycle
Growth has specific nutritional needs but is not a steady process, proceeding rapidly in early life, slowing in middle childhood and accelerating at puberty before linear growth ceases. With increasing age also come the physical and psychomotor maturation which influence activity and body composition and, through feeding skills and food choices, dietary intakes. Percentage body weight that is fat (% BF) increases rapidly to a peak between 6 and 12 months, followed by a period of natural ‘slimming’ until around 5 years, then by a second phase of relatively rapid fat deposition (the adiposity rebound) which continues in girls until growth ceases. In boys the adiposity rebound ceases with the rapid lean tissue deposition of late puberty.
Body weight for age (WFA) is frequently used as an indicator of nutritional status but weight is heavily influenced by height. Childhood nutritional assessment (see also chapter 12) commonly uses either weight-for-height (WFH) independent of age, or WFA in relation to height-for-age (HFA). Reference standards for growth and development do not distinguish the abnormal from the extremes of normal. Scores of <−2 or >+2 SD, or <3rd and >97th centiles, are often used as cut-off points for ‘normality’. Velocity of growth may be more informative than size attained.
Stunting is assessed by height-for-age or length-for-age, the same cut-off points as are used for normality. Growth retardation associated with socioeconomic deprivation is a significant problem in westernized as well as in less affluent societies and usually responds better with changes in psychosocial and/or economic environments than with specifically nutritional interventions.
In adults, body mass index (BMI: weight in kg/height in m2) is used to define underweight, overweight and obesity; however in children mean BMI varies non-linearly with age and so the use of this index is less simple. The International Obesity Task Force (IOTF) defines childhood overweight and obesity as the BMI Z score (ie SD score) at any age which, if maintained throughout childhood would achieve the adult overweight and obesity BMI cut off points of 25 and 30 kg/m2 at 18 years.
Growth from the fetus through infancy and childhood is clearly one aspect of physical maturation before the onset of puberty when rapid physical maturation occurs. The age at onset of puberty and the pubertal growth spurt vary widely between individuals. Secular trends towards increased height and weight and earlier age at puberty, attributed to positive changes in health and nutrition, have slowed or ceased in recent years in much of Europe and North America, but continue elsewhere.
The typical age for onset of the secondary sexual development characteristic of puberty is considered to be between 8 and 13 years in girls and 9 and 13.5 years in boys with similar mean age (11.5 years) in both sexes. In girls the growth spurt always occurs early in the progression of puberty with most rapid growth in height on average 0.7 years after the first signs of puberty and before menarche. Growth acceleration in boys occurs later in the pubertal process, with most rapid growth occurring on average 1.5 years after the first signs of puberty, and continues longer than in girls. Peak bone mass is achieved two years after cessation of growth (mean: girls 16 years; boys 18 years). Pubertal changes in body size and composition lead to greater differences in nutrient requirements between males and females than were present in earlier childhood. In adolescent girls the nutritional needs of pregnancy and lactation may have to be added to those of growth and menstruation. In adolescent boys increased lean body mass leads to greater nutritional demands per kg body weight (see Appendix 2).
The period of infancy (birth to 12 months) is one of almost total dependency on others for the provision of warmth, food, shelter and emotional needs. As children become more independent, they can make their wishes understood and learn to use food to manipulate those around them. Once at school, children also take their cues for food preferences from their friends and may be heavily influenced by advertising pressures. In adolescence, peer fashions can lead to haphazard eating and bizarre diets with risk of compromising the good quality diets needed to meet the demands of growth and maturation, and may be used to express independence of the family. Adolescents living away from home for the first time may lack the cooking skills required for a good diet. Lifestyles adopted in the adolescent years can continue into adult life. Adolescents (and, increasingly, younger children) may demonstrate psychiatric instability through anorexia nervosa or bulimia. These two conditions have profound, even fatal, effects (see chapter 8).
There is increasing evidence of a relationship between fetal and early infant growth and nutrition with health and disease in adulthood (fetal origins hypothesis). Low birth weight (LBW), particularly when there is rapid catch-up growth postnatally, is associated with increased prevalence of coronary heart disease and Type 2 diabetes mellitus in adult life.
Food allergy and intolerance and the maturation of the immune system in relation to dietary components are discussed in chapter 10. Infants are born with unchallenged and immature immune systems. Gastrointestinal resistance to invasion by foreign proteins relies in part on protective substances such as immunoglobulins (eg IgA and IgM) and enzymes which destroy histamine and active substances in the gut. Low levels of secretory IgA (sIgA) and lack of specifically sensitized immunoglobulins make young infants more at risk of sensitization to foreign proteins which cross the mucosal barrier.
Infant eczema has been attributed to foreign proteins either in maternal milk, infant formula, or early weaning foods. Maternal allergen exclusion diets during lactation may reduce the prevalence of eczema in breast fed infants, at high risk of atopy (allergic hypersensitivity), in the early months of life. The nature and pattern, rather than simply the timing, of solid feeding are important for subsequent development of food allergy.
Digestion and absorption in breast fed infants are promoted by many specific components in breast milk, such as lactose, lipase and lactoferrin. Immaturity of gastrointestinal enzymatic function makes digestion and absorption less efficient with infant formula than with breast milk in the first months of life. Fat absorption is less in formula fed infants than in breast fed infants. Pancreatic lipase, amylase and bile salt pool size are low in the newborn compared with older infants. Lactase levels in the newborn are quite low, increase as milk feeding begins and may decline later as milk ceases to be the predominant feed. Low lactase levels and lactose intolerance are common in older African and Asian children and adults but less common in Caucasian children and adults.
Young infants cannot dilute or concentrate their urine as much as older children and adults. This makes them particularly susceptible to fluid overload and to overload from other substances which have to be excreted via the kidneys. The unmodified cow milk formulas used before 1972 gave young infants difficulty excreting sufficiently concentrated urine to expel the necessary solutes. The ensuing intracellular hyperosmolality, especially in the brain, had disastrous, often fatal, consequences.
The high phosphate content of unmodified cow milk based formulas precipitated falls in plasma calcium, hypocalcaemic tetany and convulsions in otherwise healthy infants around 7–10 days old. UK legislation in the early 1970s lowered the acceptable levels of sodium, phosphate and protein in infant formulas and this was followed by similar EC directives. Changes to low phosphate infant formula have virtually eliminated the problem of tetany.
There is now almost universal consensus that breast milk is the best food for normal infants with healthy mothers, because of its composition, digestibility, and anti-infective properties. Despite this consensus, most one-month-old infants in UK have received some infant formula and by 10 weeks 64% of infants are wholly formula fed. It is proving difficult to improve ‘breast feeding statistics’ in UK despite widespread education and publicity promoting breast feeding.
The composition of human milk is variable. The first milk, colostrum, is low in volume and high in proteins, especially immunoglobulin A, as well as vitamin A and zinc. As the volumes of milk secreted increase, milk composition modifies to ‘transitional’ and then ‘mature’ milk. Table 6.1 outlines the biochemical composition of colostrum, human milk, modern infant formula and cow’s milk. Volumes of milk produced and precise composition of breast milk vary between women, over time and by time of day. Human milk contains cells (macrophages, lymphocytes, neutrophils) and humoral components, eg sIgA, which protect infants against infection in the first months of life. Lactobacillus and Bifidobacterium spp promote lactic and acetic acid production from lactose, which discourages growth in the large bowel of potential pathogens such as E. coli and Shigella spp.
Lactose, the main carbohydrate in milk, accounts for approximately 40% of total milk energy and facilitates calcium absorption. Human milk protein is 30–40% casein and 60–70% whey. Whey proteins include lactalbumin, sIgA, lactoferrin and lysozymes, whereas casein is a mixture of proteins bound with calcium. Human milk casein forms smaller micelles with looser structure than the casein of cow’s milk, which facilitates digestion. Nutrient binding proteins in milk such as lactoferrin (which binds iron) facilitate absorption of some specific nutrients. The quantities of fat in human and cow’s milk are similar, but human milk fat is higher in unsaturated fat, particularly the essential fatty acids linoleic and α-linolenic acids, and also contains the long chain polyunsaturated fatty acids arachidonic, eicosapentaenoic and docosahexaenoic acids (22:6ω3), which are important for neurological development (see chapter 4).
The fats in human milk are more readily digested and absorbed than those of cow’s milk. Most infant formulas now contain mainly vegetable oils with rather different proportions of fatty acids than those found in human milk fat, which partly depends on maternal dietary fatty acid content. Human milk has a high level of cholesterol and of carnitine, which is involved in mitochondrial oxidation of fatty acids. Premature infants and those undergoing very rapid (catch-up) growth may be unable to synthesize carnitine at a sufficiently rapid rate to meet demand.
In 1994 the Department of Health for the UK stated that ‘Breast fed infants under six months do not need vitamin supplementation provided the mother has an adequate vitamin status during pregnancy. From the age of six months infants receiving breast milk as their main drink should be given supplements of vitamins A and D’. Plasma vitamin K levels are low in the newborn because they lack colonic flora synthesizing vitamin K. Vitamin K deficiency can cause minor bruising, blood loss, or major haemorrhage in the brain. All term newborn infants in UK should receive prophylactic oral vitamin K (1 mg) at least once. Breast fed infants should be offered four further oral doses at two-weekly intervals.
All infant formulas are now highly modified from their base of cow’s milk or soya protein (Table 6.1). Many formulas are available and differ according to content, eg long chain polyunsaturated fatty acids, taurine, carnitine. Since the 1970s a series of government reports have made recommendations on the composition and promotion of infant formula. In 1991 an EC directive on the composition, labelling and marketing of infant and follow-on formulas was incorporated into the UK Infant Formula and Follow-on Formula Regulations. New EU regulations came into force in January 2008 but in the UK the labelling legislation will not be in force until 2010.
Most infant formulas are for non (exclusively) breast fed infants from birth until the age at which neat cow’s milk is introduced (not before 12 months). Other more specialized formulas fulfil various purposes, eg
Weaning, also known as complementary feeding, has been defined as ‘The process of expanding the diet to include foods and drinks other than breast milk or infant formula’. Since the term ‘weaning’ is also used to indicate complete cessation of breastfeeding, WHO recommends that the terms ‘weaning’ and ‘weaning foods’ are avoided. The term complementary feeding is used here to embrace the use of all foods and liquids other than breast milk or infant formula.
Maternal milk output averages 650 ml/day at one month of lactation, 750 ml/day at 3–4 months’ lactation and peaks at about 900–1000 ml/day at 4–5 months’ lactation. Average infants would need 850 ml and 1450 ml of breast milk to meet energy requirements at 6 and 12 months respectively on exclusive breast feeding. From six months, and probably before this for some infants, additional sources of energy and nutrition are needed to complement breast milk. The World Health Organization and the UK Department of Health have formally adopted the policy:
Breast milk is the best form of nutrition for infants. Exclusive breastfeeding is recommended for the first 6 months (26 weeks) of an infant’s life, as it provides all the nutrients a baby needs. Breastfeeding (and/or breast milk substitutes, if used) should continue beyond the first 6 months along with appropriate types and amounts of solid foods. Mothers who are unable to, or choose not to, follow these recommendations should be supported to optimize their infants’ nutrition.
Depending on the choice of food, complementary feeding can provide necessary extra energy and micronutrients. It encourages development of feeding techniques and ability to eat with the family, but is less likely to be sterile and early introduction can lead to allergic reactions to foods.
Complementary foods may be home prepared or commercially produced. Initially one small feed is introduced per day but feed frequency can increase quite quickly. Whilst breast milk (or formula) remains the main source of energy early in complementary feeding, cereal based complementary foods, with energy density enhanced by additional fat source, should be introduced early. Rice preparations are usually recommended since rice is gluten free. With wheat based foods there is slight risk of malabsorption from either temporary gluten intolerance following gastrointestinal infection, or permanent gluten intolerance in coeliac syndrome. Fats increase the energy density of foods, thus facilitating energy sufficiency and optimum infant growth with the relatively small volumes of food tolerated by infants’ small gastric capacities. Fats are also sources of fat-soluble vitamins, essential fatty acids and exogenous cholesterol and enhance taste and food texture and therefore palatability. Provided breast milk, formula, or (later) cow’s milk intakes are around 500 ml/day, protein intakes are likely to be adequate even if complementary feeds are low in protein and amino acid variety (eg in diets with a single plant staple).
About 40% of home prepared complementary foods have an energy content lower than breast milk and are lower in fat, iron and vitamin D and higher in sodium than commercially prepared infant foods. UK legislation specifies a range of nutritional contents for commercially produced infant foods. Thus infants receiving commercial complementary foods may have more balanced nutrient intakes than those fed home prepared foods. If only commercial fruit, vegetable and pudding products are offered as complementary foods, energy needs are unlikely to be met as the foods displace breast milk and formula in the diet but are usually less energy dense.
Early foods offered are semisolid. Infants quickly learn to cope with solid and lumpy foods and ultimately foods which require chewing prior to swallowing. This progression is important. Prolonged bottle feeding (beyond one year) can lead to failure to thrive due to the ‘comfort’ aspect of sucking and the low energy density of fluids proffered. Infants should be moved from fluids fed by bottle to predominantly fluids fed by cup over the second six months of life. Current UK recommendations are that cow’s milk should not be given as a drink to infants under one year, but when it forms part of family recipes, small amounts may be safe before this age.
Vegan mothers have higher levels of unsaturated fatty acids in their milk than omnivore mothers. This may be advantageous to the infants. Soya-based infant formulas (supplemented with micronutrients and appropriately balanced energy and protein) can be used as alternatives to breast milk as breast milk output declines, and continued beyond infancy. Vitamin deficiencies in infants born to deficient mothers (particularly for B1 and B12) may be exacerbated by low breast milk vitamin content.
Digestion and absorption in preschool children enable them to consume the same foods as adults but nutrient needs and feeding skills are different. Children’s small stomachs limit the amounts of food taken at any one meal. They should therefore be fed three meals a day and perhaps two between-meal snacks, with one snack or meal close to bedtime. Recommendations for adults to consume <35% dietary energy from fat do not apply to young children. The transition from >50% dietary energy derived from fat provided by exclusive breast feeding to <35% energy derived from fat should spread over the first five years of life. Similarly adult recommendations for fibre intake should not apply in early childhood since high fibre content lowers food energy density and phytates reduce absorption of micronutrients. Diets with <30% energy derived from fat are quite common amongst preschool children who consume large quantities of ‘juice’ and sweets instead of meals of varied content. They are likely to lead to failure to thrive if prolonged. Persuading children to eat family meals is not always easy. Children are often reluctant to eat green leafy vegetables, partly due to inexperience with chewing.
Failure to thrive (FTT) is failure to gain in weight and height at the expected rate. Resolution through catch-up growth can be very rapid if the cause is treatable and extra nutrients are provided. Where the precipitating cause cannot be resolved, increasing the energy and nutrient density of the diet can lead to improved growth rates. Psychosocial deprivation may be the commonest cause of FTT in the UK today although often unrecognized. Overt cases come from homes where the nurturing environment is in some way deficient in the love, warmth, enjoyment and stimulus which enable normal growth. Changing the adverse environments so as to provide more positive nurture rapidly normalizes hormone levels and results in catch-up growth.
Childhood obesity (see also chapter 8) is of major public health concern. Psychological distress and the physical handicap of being obese contribute to underachievement at school. Type II diabetes mellitus (previously considered only an adult disease) shows an increased prevalence in children and adolescents in Western Europe (including the UK) and North America. The vast majority of obese children have no recognizable underlying medical cause for their obesity. Around 80% of obese children have one obese parent and 20–40% have both parents obese.