Relevance of Milk Protein for Muscle Hypertrophy, Fermented Milk Products for Gut Microbiome, and Milkfat for Cardiovascular Health: Facts and Controversies

Milk was designed by evolution to provide superior nutrition for support of growth and development of mammalian young. When humans domesticated dairy cattle about 10,500 years ago, they also adopted milk for adult consumption and learned to separate and utilize its constituent parts, milk proteins whey and casein for muscle growth, milk fermentation to make kefir, yogurt, and cheese, and milkfat to make butter. Research on how consumption of these different milk products affects human health has generated much factual data and some uncertainties and controversies about the extent it can be used to improve adult human body and overall metabolic health. This is important in the context of global burden of high cardiovascular morbidity and concerns about any impact of milkfat consumption on cardiovascular (CVD) and coronary heart diseases (CHD). The first theme of this review examines the involvement of milk proteins whey and casein on skeletal muscle hypertrophy (MHT). The major contribution of resistance training (RET) to MHT is contrasted to the lesser, but still important, contribution of protein supplementation (PS) and uncertainties about the efficacy of the milk proteins relative to plant proteins, along with dose, training status, and timing of PS relative to RET in producing MHT. The exceptionally rich concentration of essential and branched-chain amino acids makes whey protein and casein highly effective but not essential for MHT which can also be achieved with higher quality plant PS and is not critically dependent on either the timing of PS, the training status, or the age of users. The second theme examines the nature and importance of milk fermentation in production of full-fat and low-fat yogurt, kefir, and cheese in terms of bacteria involved, their metabolism in the gut, their beneficial influence on the gut microbiome (GM) and on overall as well as cardiovascular health. Lastly, milkfat as influence on cardiovascular health is discussed both from the perspective of its effects on blood lipids and cardiovascular physiology, but also as a component of the complex dairy matrices. As part of a rich nutrient matrix, milk products provide benefits to cardiovascular health because of their biologically active proteins and fatty acids which exert anti-inflammatory, anti-carcinogenic, antioxidative, and other beneficial actions, despite their high fat content and level of fat saturation. Fermentation usually lowers CVD and CHD risks of full-fat milk and its products, but health benefits often are greater when their fat content is reduced. Butter does not benefit from the biological activities of the milk proteins and is not fermented, so when consumed in large quantities, the balance of cardiovascular benefits shifts toward higher CVD and CHD risk. Three knowledge gaps need to be corrected for a better understanding of health benefits of consumption of milk products. Individual nutrient components in dairy food matrices need to be measured and recognized. Their identity needs to be linked to a better understanding of how they influence atherogenic lipoproteins and protein synthesis. And maximal consumption limits need to be defined for full-fat milk products to assure the benefits that their biologically active components offer, but also to reduce their detrimental effects on cardiovascular risk factors. Overall, as a food category, milk products justify acceptance as a healthy natural source of nutrition that was evolutionarily designed to support early growth and development of mammalian young but need to be prudently implemented for their lifelong consumption in adulthood.