Protein in dairy
Milk protein has more functions than previously known. The quality of milk protein is higher than previously estimated.
The traditional anabolic functions of protein remain important, yet recent research has identified additional metabolic and neutraceutical roles, many specific to milk protein. Common protein quality assessment methods underestimate the quality of milk protein. Protein intake recommendations for optimal health require rethinking.
The composition and functions of milk protein
Milk is an important source of protein in the human diet. The soluble whey constitutes about 20% of the protein and is rich in branched chain amino acids (leucine, iso-leucine and valine). The less soluble casein has a higher proportion of histidine, methionine and phenylalanine. Dairy protein contains a number of bioactive peptides, and the essential amino acids are well-represented.
Milk protein has numerous roles in human nutrition. All classical functions of proteins – anabolism, biological catalysts, plasma and membrane transport, movement, structure, protein folding, immunity, growth and differentiation – also apply to dairy protein, mainly through the provision of amino acids for protein turnover. The bioactive peptides of dairy have recently been shown to have unique functions, including involvement in nutrient transport, immunomodulation, gastrointestinal function and flora, cell signalling and antimicrobial and antiviral activity. In addition, milk protein plays an important role in metabolic health. Favourable effects on hypertension, dyslipidaemia and mild hyperglycemia have been associated with milk protein intakes. In the case of the effect of dairy on cardiometabolic risk factors, Pal and Radavelli-Bagatini narrowed this down to the whey component, highlighting the potential role of leucine in this regard. Dairy protein may indirectly aid weight management through its effect on satiety and body composition. It seems to function synergistically with physical activity and is a popular recovery drink after exercise. Osteoporosis and bone health are also related to dairy protein intakes and in the elderly milk proteins appear to prevent age-related muscle mass.
The biologically active components of dairy have been labelled “neutraceuticals” , resulting in milk and dairy being classified as functional foods.
Milk protein quality
Proteins differ in composition, indispensable amino acid profiles and the physico-chemical properties of the food matrix in which they are embedded. Generally speaking, protein quality refers to the ability of a food protein to meet the metabolic demand of the (human) body for amino acids and nitrogen (N). It follows that characteristics of the person consuming the food (age, health status and energy intake) are also related to protein quality, yet different criteria and markers are used to define dietary protein requirements. In addition to the composition of a food protein, physiological criteria such as digestibility and bioavailability are core in describing protein quality.
Protein digestibility refers to the proteolytic processing of proteins to release amino acids, which extends from the mouth to the small intestine (“ileal digestibility”) or the anus (“faecal digestibility”) and involves many progressive steps.
Bioavailability of an amino acid is the proportion of consumed protein that is absorbed in a chemical form for it to be used by the human body. This utilization can be influenced by food processing or by anti-nutritional factors, as well as the interaction between these compounds and the processing. Sometimes specific processes applied during processing may affect the various protein fractions differently, for example gelation through acidification or through renneting.
Numerous methods are available to assess protein quality. These range from nitrogen balance, biological value profiling, protein efficiency ratio, net protein utilization, to amino acids scores. The latter includes the so-called protein digestibility-corrected amino acid score (PDCAAS) and the digestible indispensable amino acid score (DIAAS). The former is widely used and still recommended by the WHO/FAO/UNU in spite of some limitations.
- The so-called truncation rule of PDCAAS states that the biological value may not exceed 100%. This means that proteins with extra essential amino acids (i.e. beyond those in the reference amino acid pattern) do not get due credit. So-called “SP values” (supplementation power values) have been published to correct for this. In the case of milk powder the SP values for lysine, sulphur amino acids, threonine and tryptophan are respectively 1.46, 1.22, 1.30 and 1.54, meaning that milk protein has the power to supplement the limiting amino acids from plant based proteins.
- Amino acid availability, anti-nutritional factors of plant food protein, and gastrointestinal factors are not taken into account.
In practice this means that proteins of high biological value, for example milk protein, are of even better quality than the original (uncorrected) PDCAAS method suggests. It has been recommended that a new expert consultation re-examines protein quality assessment methods. The DIAAS may address some of the limitations, but as yet international consensus is outstanding.
It has been suggested that the unique functions of a protein beyond providing amino acid building blocks should be taken into account when determining protein quality. As new research reveals more and more functions for milk protein, peptides and amino acids, researchers and practitioners need to integrate this in the interpretation and planning of diets. It follows that protein function and quality have become intricately interlinked.
Implications for protein requirements
Protein requirement is still defined as being “the lowest level of dietary protein intake that will balance the losses of nitrogen from the body and thus maintain the body’s protein mass in persons at energy balance with modest levels of physical activity”. New evidence challenges scientists to rethink this definition. The following points are food for thought:
- As protein quality decreases, the percentage of energy from protein needed to meet the requirements increases. This implies that more protein must be consumed to meet amino acid requirements when the bioavailability is lower. Similarly, quality-adjusted protein-energy ratios have been proposed for the recommended protein and amino acid intakes.
- Essential amino acid requirements per gram of protein for children are now considered to be higher than indicated in previous international recommendations. However, current protein intake recommendations still reflect the minimum, and are based on ideal conditions, whereas the reality is that children (in developing countries) live under conditions of repeated infections, chronic energy deficiency, poor sanitation and psychological stress.
- Cows’ milk protein is key in the treatment of severe acute malnutrition (SAM), because adding dairy protein improves protein quality, which makes it possible to reduce total protein content of a product or diet used in the treatment of SAM. Dairy protein in the treatment of SAM has metabolic advantages, including improving linear growth without excess body fat gain, and increased muscle mass and functional test scores. By using less soy and cereal in the treatment of SAM, the anti-nutritional effects of the plant proteins are reduced. The increased cost of using dairy in SAM, should, however, not limit the number of potential beneficiaries, and the ability of this vulnerable group to tolerate the lactose in dairy requires consideration.
- Current recommendations do not yet consider the “new” metabolic and neutraceutical functions of protein related to proteogenic and non-proteogenic pathways.
- Current evidence shows that the quality of milk protein is higher than previously acknowledged
The importance of the protein in milk has been known for centuries. This is appreciated now more than ever before, as current evidence shows that the quality of milk protein is higher than previously acknowledged; functions of dairy protein beyond the provision of amino acids and organic nitrogen are discovered. The new South African food-based dietary guideline “Have milk, maas or yoghurt every day” is a step in this direction.