Milk and a healthy digestion. The facts
The food scientist and researcher Dr Anabel Mulet-Cabero is an expert on the influence of Milk structures on stomach behaviour and nutrient digestion. She reveals the facts about milk and how milk is digested in the human body.
Is milk healthy?
Cow's milk is recognised as an excellent source of nutrition and contains essential proteins, lipids, carbohydrates, vitamins and minerals, especially calcium. Apart from intolerances that exist, milk is often problematised for its saturated fat content and linked to cardiovascular disease. However, the general scientific evidence shows that the intake of milk and some dairy products does not increase the risk of cardiovascular disease. In fact, there appear to be beneficial associations 1. The statement can also be attributed to the approach of assessing the health benefits of foods by their individual components rather than by the form and structure of the whole food. The health benefits of dairy products are due to a combined effect of the different components when consumed as a whole.
Milk proteins are nutritionally superior. They are highly digestible and easily absorbed, as well as one of the richest sources of the nine essential amino acids that we need to consume daily because our bodies cannot produce them themselves. The main proteins in milk are caseins and whey proteins. Studies have been conducted at the Quadram Institute and Teagasc Food Research Centre (Ireland) to understand the mechanisms behind this difference in behaviour and how the formulation and processing of milk affects nutrient digestion.
What happens to milk in the stomach?
Milk undergoes major structural changes in the stomach. The liquid milk turns into solid lumps with a mozzarella-like consistency. This happens due to the coagulation of caseins by stomach acid and the action of protein-degrading enzymes. In fact, this is very similar to the process of cheese making! This digestive behaviour of whey proteins and caseins and different mixtures has been studied with an advanced digestion model that can simulate the main processes in the human stomach3.
What is certain is that the structural changes occurring in the stomach limited the absorption rate of the various milk proteins. Larger mozzarella-like lumps remained in the stomach longer before they could be broken down and emptied into the small intestine. This helped to explain the slow and fast amino acid absorption of caseins and whey proteins, respectively, observed in the clinical study.
What happens during milk processing?
Raw milk is heat treated to kill bacteria and ensure that it is safe to consume. The most common heat treatments are pasteurisation and ultra high temperature (UHT) treatment. UHT involves heating the milk to 135-150°C. This milk can be stored for months without refrigeration. These processes have a profound effect on our nutrient digestion when we drink this milk4. UHT-treated milk, unlike pasteurised milk and raw milk, results in a faster release of proteins and lipids. This is due to the formation of "softer" lumps in the stomach model, which were emptied more easily into the intestine.
Cow's milk is unique in its composition and digestion. Changes in the initial matrix of milk through processing or formulation can alter the way nutrients are digested. This in turn could affect the physiological effects of proteins, such as muscle synthesis and appetite control.
Cow's milk vs. plant milk
Cow's milk has been increasingly replaced by plant-based drinks made from soy, almond, oats and rice in recent years. These drinks may of course be preferred for a variety of personal reasons. However, it is also important to understand the nutritional and health values of these different products. Especially if cow's milk is to be substituted with special attention to protein, minerals and vitamins. The nutritional quality of the proteins in plant-based drinks is generally lower than in cow's milk. Only soy contains all nine essential amino acids. It should be noted, however, that plant proteins are generally more complex to digest and are not as easily absorbed. Plant drinks have little or no intrinsic calcium, vitamins A, B2 and B12, so these drinks need to be fortified with them.
The Quadram Institute is investigating how these plant-based drinks are digested and how their structure can be modulated to better mimic the unique properties and health benefits of cow's milk.
References:
- Givens, DI (2017). Saturated fat, dairy and health: a strange paradox? Nutrition Bulletin, 42 , 274-282.
- Boirie, Y., Dangin, M., Gachon, P., Vasson, M.-P., Maubois, J.-L. & Beaufrère, B. (1997). Slow and fast dietary proteins differentially modulate postprandial protein accretion. . Proceedings of the National Academy of Sciences, 94 , 14930-14935.
- Mulet-Cabero, A.-I., Torcello-Gómez, A., Saha, S., Mackie, AR, Wilde, PJ, & Brodkorb, A. (2020). Influence of casein and whey protein ratio and lipid content on in vitro digestion and ex vivo absorption. . Food Chemistry , 126514.
- Mulet-Cabero, A.-I., Mackie, AR, Wilde, PJ, Fenelon, MA, & Brodkorb, A. (2019). Structural mechanism and kinetics of in vitro gastric digestion are influenced by process-related changes in bovine milk. . Food Hydrocolloids, 86 , 172-183.