Breastmilk is an important energy source in early life. Various components of breastmilk have been ascribed bioactive capacities to be responsible for the superiority of breastmilk, compared with standard infant formulas. The superiority of breastmilk may not only refer to its diversity but have also been ascribed more physiologic and psychologic aspects of the suckling situation itself.
The close contact between the mother and her infant is described to be important as well. Several components and mechanisms of effect have been extensively studied, with high evidence for some, whereas others remain to be demonstrated.
Major components are described that are naturally present in breastmilk and that are important for the development of newborn infants and young children.
Milk protein consists broadly of two main types, whey protein and caseins. These are present in mature milk in the ratio 60:40 for whey and casein, respectively. In early lactation this ratio is approximately 90:10. Caseins form micelles with calcium, phosphate and magnesium to efficiently carry high quantities of these minerals in milk. Whey protein remains soluble and is therefore more readily digested.
Proteins consist of shorter or longer amino acid chains that are bound together. Twenty different amino acids are needed in the body, though more are present. Nine of the amino acids are essential and needed through the diet. Essential amino acids include histidine, leucin, isoleucine, valine, tryptophan, phenylalanine, methionine, threonine and lysine.
Proteins are needed by the body for growth and maintenance of body functions. When proteins are broken down into amino acids they are used as precursors for co-enzymes, hormones, immune responses, cellular repair, nucleic acids as RNA and DNA, and other important molecules.
Bioactive milk proteins have multiple functions that include binding of vitamins and minerals, anti-microbial effects, suppression or modulation of immune responses, enzyme inhibitory activity as well as effects on hormones and growth factors. The most abundant bioactive proteins in whole milk include; immunoglobulins, α-Lactalbumin, Lactoferrin, β-lactoglobulin, nucleotides, alkaline phosphatase, growth-factors, Lysozyme and Superoxide dismutase.
Fat is the lipid fraction and is the most variable macronutrient in maternal milk. The lipid fraction is mainly composed of triglycerides that account for 98% of the lipid content. Phospholipids, cholesterol and free fatty acids are also present. The lipids serve as major energy source and are contained within membrane-enclosed milk fat globules. Milk fat globule membranes consist of phospholipids, cholesterol and proteins and the core of these fat globules consists of triglycerides.
Polyunsaturated fatty acids contain a group of fatty acids that are important compounds. They need to be ingested through the diet, as the body is not able to naturally make those. Essential fatty acids include alpha-linolenic acid (ALA), an omega-3 fatty acid, and linoleic acid (LA), an omega-6 fatty acid. Synthesis of docosahexaenoic acid (DHA) and gamma-linolenic acid (GLA) are dependent on the presence of ALA and LA, respectively. DHA and GLA are therefore semi-essential fatty acids. Fatty acids are ascribed different functional properties, dependent on their structures. Poly unsaturated fatty acids, for instance are important for development of the vision and brain of infants and young children.
Carbohydrates are saccharide molecules that contain sugar, starch and cellulose. Carbohydrates can be categorized according to the number of glycoside-bound saccharide molecules; simple sugars (1-2 saccharides), oligosaccharides (carbohydrates with 3-10 saccharides) and more complex polysaccharides (carbohydrates with more than 10 saccharides). Lactose is the most abundant carbohydrate followed by oligosaccharides. Monosaccharides are the major source of energy in human metabolism.
Human milk oligosaccharides (HMO) are present in considerable amounts in human milk. More than hundred different HMO structures have been identified and ascribed specific characteristics. Several independent but overlapping biological functions are potential for the utilization of HMOs in infants. HMOs do not serve a nutritional role yet are considered prebiotics as they are resistant to digestion by enzymes and cause selective fermentation of beneficial bacteria in the colon. HMOs can bind directly to pathogenic bacteria, which prevent some pathogens from binding to the gut epithelium and cause damage as for instance diarrhea. HMOs, hence, have a major role in regulation of the gut microbiota, maturation of the gut and its immune development.
Human milk contains several micronutrients as vitamins, minerals, trace minerals and electrolytes that are additional to macronutrients. Individual vitamins and minerals contribute with different functionalities that include for instance antioxidant properties, co-enzymes for enzymes being involved in metabolism, contribution to normal function of the immune system and much more.