Nutrient Composition of Human Milk

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International agencies and various US health organizations uniformly recommend breastfeeding as the preferred method of infant feeding for the entire first year of life and thereafter as long as is beneficial to the mother–infant dyad.1, 52 These recommendations are based on knowledge that term infants nursed by nutritionally adequate mothers are provided with sufficient energy and the proper profile of nutrients to support normal growth and development without any additional foods through the first 4 to 6 months of life. After 6 months, complimentary foods are needed to furnish nutrients likely to become limiting. Also, human milk furnishes an array of nonnutrient growth factors, immune factors, hormones, and other bioactive components that can act as biological signals and confer protection against illness in infancy and later in life.20, 43

From a nutritional perspective, infancy is a critical and vulnerable period. At no other stage of life is a single food adequate as the sole source of nutrition. This phenomenon occurs when immaturity in tissues and organs involved in nutrient metabolism (i.e., the gastrointestinal tract, liver, and kidneys) limits the ability of an infant to respond to excesses or deficiencies in nutrient intakes. Human milk is species specific, and many of the nutrients it contains are secreted as bound components that can offer protection from digestion and facilitate absorption and utilization.

In this article, the characteristic nutritional features of human milk and influencing factors are reviewed. Knowledge of human-milk composition and nutrient intakes of thriving human milk–fed infants are central to the understanding of infant nutritional requirements and the nutritional cost of lactation for mothers.

Section snippets

ASSESSING INFANT NUTRITIONAL REQUIREMENTS

Intakes of thriving breastfed infants often serve as the standard for infant nutrition, but other approaches for the assessment of infant nutritional requirements exist, including direct experimentation, metabolic balance studies, clinical observations of deficiencies, and theoretically based calculations.13 The latter approaches provide information that is useful for the formulation of human milk substitutes and for assessing whether maternal nutritional inadequacy is evident and when

MATERNAL NUTRITION

The nutritional requirements to support lactation are among the highest in human development. The production of 750 to 1000 mL/d of human milk represents 2100 to 2520 kJ/d transferred as energy-yielding macronutrients to nursing infants. Also, all vitamins and minerals required to support growth and development of growing infants are likewise transferred. Nutritional needs to support lactation are exceedingly high and often not fully appreciated. The estimated energy cost of 6 and 9 months of

COMPOSITION OF HUMAN MILK

Human milk is a complex biological fluid composed of thousands of constituents in several compartments: an aqueous phase with true solutions (87%), colloidal dispersions of casein molecules (0.3%), emulsions of fat globules (4%), fat-globule membranes, and live cells. Human-milk constituents can be broadly categorized according to their physical or physiologic properties. These categories of constituents include:

  • Proteins

    • α-Lactalbumin

    • β-Lactoglobulin

    • Caseins

    • Enzymes

    • Growth factors

Protein and Nonprotein Nitrogen

Human milk protein content [(total nitrogen − nonprotein nitrogen) × 6.25] is high in early secretions, 15.8 g/L, and slowly decreases to 8.0 to 9.0 g/L with the establishment of lactation.12, 27 The protein constituents of human milk serve diverse functions. Besides providing essential amino acids for growth, they provide protective factors (e.g., immunoglobulins, lysozymes, and lactoferrin), carriers for vitamins (e.g., folate, vitamin D, and vitamin B12 binding proteins), and for hormones

SUMMARY

A complex interplay of maternal homeostatic mechanisms influences nutrient transfer to nursing infants, and with a few exceptions, excess maternal intake or a moderate deficiency in the maternal diet does not appreciably alter nutrient transfer to infants unless it has persisted for some time. Milk vitamins D and K contents, even in apparently well-nourished women, may not always provide adequate amounts for infants. Investigations provide evidence that human milk possesses many unique

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    Address reprint requests to Mary Frances Picciano, PhD Department of Nutrition The Pennsylvania State University 126 Henderson Building South University Park, PA 16802 e-mail: [email protected]

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    Department of Nutrition, The Pennsylvania State University, University Park, Pennsylvania

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