There are many people that cannot consume large amounts of milk or any dairy products due to the adverse reactions related to the intestine, as well as changes in their mood and reduced cognitive functions.

It is known that consumption of lactose can cause allergies and, in many cases, affect the function of human memory, the clarity and the time of human reaction (2).

So far, there was a common belief that lactose is responsible for any gastrointestinal disorders while casein caused the rest of the reactions.

What is Lactose?
Lactose is a type of sugar found in milk. A lot of adults do not have the enzyme to digest lactose, which is why they have gastrointestinal disorders every time they consume milk.

However, in fermentation milk products, such as cheese of long maturation period, the microorganisms that are included in fermentation, break down lactose. Thus, even though somebody cannot drink milk, cheese such as parmesan or gruyere are usually easy to consume, without having any problems (1).

What is Casein?
Casein is a protein found in milk and is used in the process of coagulation during cheese making.
This substance has such powerful coagulation properties that is even used in the production of glue and paint.
Apparently, the consumption of this substance in large amounts cannot be beneficial for the intestine.
In cow milk, approximately 80% of protein is casein, which is rich in amino acids, calcium, and phosphorus.
It has been observed that people who are intolerant to lactose can consume goat milk more easily, as well as products made from goat milk.

However, goat milk contains both lactose and casein. Therefore, why is goat milk easier to digest than cow milk?

Why Goat Milk and not Cow Milk?
This is happening because some types of milk such as goat milk, sheep, and buffalo milk contain a different type of casein called A2, while cow milk contains type A1 casein.

The two types of casein are processed differently by enzymes found in the human gastrointestinal system. This appears to be the main reason of why A1 casein causes gastrointestinal disorders as well as a reduction in cognitive function (3).
During the digestion of casein A1 in the human intestine, a substance called b-casomorphin-7 (bmc-7) is released. B-casomorphin-7 is an opioid protein and acts in the central nervous system in a way similar to morphine.

Casein A1 and b-casomorphin-7 have been linked to numerous health problems such as allergies, gastrointestinal disorders, changes in the mood, autism, increased mortality by ischemic heart disease, syndrome of sudden death in infants, diabetes type 1, as well as a disorder in the function of the autoimmune system. (5-7). Although further research to confirm the relation of casein A1 to the health problems referred is required, we have seen from our clinical experience, that patients consume better, and are more tolerant to, dairy products made from sheep, goat or buffalo milk. Here in Greece, there is a long tradition in the consumption of goat and sheep milk. Thus, especially for people with autoimmune diseases, diabetes, autism, heart disease and allergies, it is far better to opt for this type of nutritional choices and ideally bought from free-range farming animals.

In the case we prefer to consume dairy products from cow milk, we had better choose long maturing cheese such as parmesan, gruyere, brie, and cheddar, in which the levels of b-caseomorphin-7 are not detectable (1), due to the disintegration of bmc-7, through the long procedure of fermentation. Having this in mind, we can consume dairy products, without depriving ourselves of the delicious and vast nutritional value of these specific foods, and without any side effects.

To your Health!

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References:

1. Formation and Degradation of Beta-casomorphins in Dairy Processing. Duc Doan Nguyen, Stuart Keith Johnson, Francesco Busetti, and Vicky Ann Solah. Crit Rev Food Sci Nutr. 2015 Dec 6; 55(14): 1955–1967. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4487594/ 
2. Milk Intolerance, Beta-Casein and Lactose Sebely Pal, Keith Woodford, Sonja Kukuljan, and Suleen Ho Nutrients. 2015 Sep; 7(9): 7285–7297. https://www.ncbi.nlm.nih.gov/pubmed/26404362 
3. Effects of milk containing only A2 beta casein versus milk containing both A1 and A2 beta casein proteins on gastrointestinal physiology, symptoms of discomfort, and cognitive behavior of people with self-reported intolerance to traditional cows’ milk Sun Jianqin, Xu Leiming, Xia Lu,corresponding author Gregory W. Yelland,4,5 Jiayi Ni,6and Andrew J. Clarke. Nutr J. 2016; 15: 35. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4818854/ 
4. Milk proteins and human health: A1/A2 milk hypothesis. Monika Sodhi, Manishi Mukesh, Ranjit S. Kataria, Bishnu P. Mishra, and Balwinder K. Joshii. Indian J Endocrinol Metab. 2012 Sep-Oct; 16(5): 856. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3475924/ 
5. Health implications of milk containing beta-casein with the A2 genetic variant. Bell SJ1, Grochoski GT, Clarke AJ. Crit Rev Food Sci Nutr. 2006;46(1):93-100. https://www.ncbi.nlm.nih.gov/pubmed/16403684
6. A1 beta-casein milk protein and other environmental pre-disposing factors for type 1 diabetes. Chia JSJ1, McRae JL1, Kukuljan S2, Woodford K3, Elliott RB4, Swinburn B5, Dwyer KM1,6,7. Nutr Diabetes. 2017 May 15;7(5):e274. www.ncbi.nlm.nih.gov/pubmed/28504710
7. Polymorphism of bovine beta-casein and its potential effect on human health. Kamiński S1, Cieslińska A, Kostyra E. J Appl Genet. 2007;48(3):189-98. http://bit.ly/2vCdg2g