Disturbances in glutathione (GSH) and redox homeostasis contribute to the pathophysiological processes leading to neurodegenerative diseases , pancreatitis , and diseases associated with abnormal cell differentiation . Therefore, maintaining redox balance is important in the context of disease prevention. In a recent study, we showed that exposure to β-casomorphin-7 (BCM-7), a proline-rich opioid peptide derived from bovine β-casein, caused a decrease of intracellular GSH concentrations in cultured neuronal SH-SY5Y cells . This reduction in GSH was driven by a reduction in cellular uptake of cysteine, the rate-limiting precursor for GSH synthesis. Exposure to BCM-7 also reduced the ratio of reduced GSH to oxidized GSH (glutathione disulfide) as a marker of redox status, and the ratio of S-adenosylmethionine to S-adenosylhomocysteine as a marker of cellular methylation capacity. These results indicate that BCM-7 is a potential modulator of GSH.
Results of in vitro studies may not be reliably extrapolated to physiological outcomes in animals and humans. Several factors might influence the systemic concentrations of BCM-7 in animal and human studies, including the amount of BCM-7 produced from A1 β-casein contained in a typical serving of milk, the half-life of BCM-7 in the gastrointestinal tract, degradation by brush border dipeptidyl peptidase-4 (DPP-4) and the rate of transport of BCM-7 across the gastrointestinal tract into the systemic or potentially lymphatic circulation. Furthermore, other milk metabolites, notably those derived from whey proteins, have been reported to influence GSH concentrations by promoting cysteine absorption and providing a substrate for GSH synthesis [15, 16]. Therefore, it is necessary to conduct animal and human studies to examine if the effects of BCM-7 on GSH concentrations observed in vitro are also apparent in vivo following the consumption of A1 type protein and potential exposure to BCM-7.
In a preliminary in vivo study using rabbits, it was observed that consumption of a diet containing the A1 type of β-casein was associated with decreased cysteine concentrations in the ileum and decreased GSH concentrations in the blood, liver, and brain compared with consumption of the A2 type (Additional file 1: Figure S1). These data indicate that, if BCM-7 is not absorbed into the systemic circulation, GSH downregulation may occur because of limited intestinal cysteine uptake from the small intestine. Thus, it was speculated that a reduction in cysteine absorption into the circulation, secondary to downregulation of cellular cysteine uptake, contributed to the reduction in systemic GSH concentrations, which was coupled with reduced GSH concentrations in the brain. While these in vitro and in vivo results were consistent, it is necessary to extend this investigation to clinical trials in order to determine if the observed effects of the A1 and A2 types of β-casein impart corresponding effects on circulating GSH concentrations in humans.
Plasma BCM-7 concentrations were significantly greater in samples obtained during consumption of milk containing both β-casein types than at baseline during consumption of milk containing A2 β-casein (Additional file 1: Table S2). The BCM-7 concentrations were not significantly different between samples obtained after consumption of milk containing A2 β-casein and the washout/baseline phases.
The results obtained in this randomized cross-over study are consistent with the results of these prior studies. Eliminating the A1 type of β-casein from the milk diet allowed for greater increases in GSH synthesis, likely through eliminating the inhibitory effects of BCM-7 on cysteine uptake by the small intestine, as demonstrated in animal studies and in cells exposed to BCM-7 . This is also supported by the lower BCM-7 concentrations observed in the A2 β-casein periods compared with the A1/A2 β-casein periods in the present study.
It is also notable that, even though BCM-7 concentrations were proportionately greater after consumption of milk containing both β-casein types than after consumption of milk containing the A2 type (Additional file 1: Table S2), the concentrations were lower than those reported to elicit biological effects in cultured human gut epithelial or neuronal cells . Additionally, the concentrations were lower than those in individuals with neurological disorders or DPP-4 deficiency . This observation is indicative that the measurements of plasma BCM-7 may be misleading, because BCM-7 transport and subsequent systemic tissue exposure might be influenced by alternative transport mechanisms. Thus, the chaotrophic environment of peptide extraction procedures seemingly rules out sequestering or compartmentalizing of BCM-7 by blood borne molecules or cells, and opens up the possibility of endocytosis or binding of macrophages and subsequent uptake by dendritic cells through the lymphatic system. This mechanism was proposed by Martin et al. to explain the transport of macromolecules from the small intestine to other tissues in the body , and the observation implies that BCM-7 concentrations should be measured in other tissue components, such as lymph tissue, in future studies.
Another notable finding is that BCM-7 was detected in baseline samples, which were obtained after a dairy washout phase. These findings indicate that A1 β-casein protein was not entirely eliminated from the diet, probably because dairy protein is present in non-milk foods, including processed foods, allowing the production of BCM-7 from dairy proteins or caseinates. Changes in the expression of DPP-4 could also influence the systemic BCM-7 concentrations.
These observations and prior findings support the proposition that BCM-7 may act by inhibiting cysteine absorption via gut epithelial cells, ultimately reducing the circulating GSH concentrations. Other dietary and physiological factors are also expected to influence the circulating GSH concentrations, and may explain the differences in GSH concentrations between the A2 β-casein phases. Although it was not possible to examine the direct impact of complete elimination of dietary BCM-7 on circulating GSH concentrations, the diet-dependent changes observed in the present study indicate that the residual BCM-7 concentrations detected in the A2 β-casein phase participants were comparable to the washout period without supplementary A1 β-casein consumption and did not adversely affect GSH uptake.
In conclusion, the results of this study suggest that daily consumption of commercially available conventional milk is associated with an increase in GSH concentrations, possibly as a consequence of increased supply of cysteine in whey protein. However, the magnitude of this increase in GSH concentrations was greater when the participants consumed milk containing the A2 type of β-casein as compared with milk containing both A1 and A2 β-casein. This elevation might be attenuated by the presence of the A1 type of β-casein relative to milk containing only the A2 type of β-casein. These results suggest that eliminating the A1 type of β-casein from milk may allow for a greater increase in GSH concentrations or maximize the potential for GSH production, and hence confer greater antioxidant capacity.
This analysis and the study by Jianqin et al. were supported by The a2 Milk Company Limited. The authors thank Nicholas D. Smith, PhD (Edanz Group Limited), for medical writing support, which was funded by The a2 Milk Company Limited. The authors would also like to thank S.P.R.I.M. Clinical Research Group for conducting the clinical trial. The authors would also like to acknowledge Prof. Sun Jianqin from Huadong Hospital affiliated to Fudan University, Dr. Xu Leiming from Xin Hua Hospital affiliated to Shanghai Jiao Tong University School of Medicine, Dr. Xia Lu from Endoscopic Center and Internal Medicine Department, Shanghai International Medical Center Co., Ltd; for the samples from the study.
Milk is a nutrient-rich, white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for infant mammals (including humans who are breastfed) before they are able to digest other types of food. Early-lactation milk contains colostrums, which carries the mother's antibodies to its young and can reduce the risk of many diseases. It contains many other nutrients including protein and lactose. Interspecies consumption of milk is not uncommon, particularly among humans, many of whom consume the milk of other mammals. As an agricultural product, milk, also called dairy milk, is extracted from farm animals during or soon after pregnancy. Dairy farms produced about 730 million tones of milk in 2011, from 260 million dairy cows. India is the world's largest producer of milk, and is the leading exporter of skimmed milk powder, yet it exports few other milk products. The ever increasing rise in domestic demand for dairy products and a large demand-supply gap could lead to India being a net importer of dairy products in the future. The United States, India, China and Brazil are the world's largest exporters of milk and milk products. China and Russia were the world's largest importers of milk and milk products until 2016 when both countries became self-sufficient, contributing to a worldwide glut of milk. Throughout the world, more than six billion people consume milk and milk products. Over 750 million people live in dairy farming households. Milk as a whole contains water, minerals (Ca, K, Na and trace metals), vitamins (A, D, K), carbohydrates, proteins and fats. The proportion of the sevaries from source to source. Average composition of milk from different sources is given ahead.
Casein (from Latin caseus\"cheese\") is a family of related phosphor proteins (αS1, αS2, β, κ). These proteins are commonly found in mammalian milk, comprising c. 80% of the proteins in cow's milk and between 20% and 45% of the proteins in human milk. The j Casein has a wide variety of uses, from being a major component of chees , to use as a food additive. The most common form of casein is 153554b96e