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Casein Curds in Stool: Everything You Need to Understand and What You Can Do About It

Trained in dentistry, Sree is currently pursuing lab sciences. She loves researching and sharing information on various health topics.

What is casein? How does the body normally react to casein?

What is casein? How does the body normally react to casein?

What Is Casein?

Casein is one of milk's major protein contents. Its molecular composition includes phosphorus. Casein proteins typically coagulate or form curds when exposed to pH 4.6.

Casein is found in the milk of all mammals, including human breast milk. The amount of casein varies among mammals. For example, in cow's milk, 82% of its milk proteins are casein. The other 18% of its total milk proteins is called whey or serum proteins.

There are also different subtypes of casein, according to Milk Facts. There are alpha-s1, alpha-s2, beta and 6. Each of these has its own genetic variations, functional properties and amino acid composition. Hence, each of these subtypes of casein has a different effect in the body.

Casein is high in the amino acid proline. It is also high in phosphorus. This enables casein to bind well with calcium and turn into calcium phosphate salts. This is one of the main reasons why milk contains high amounts of calcium. This phosphorus function from the casein molecule has a very important role in calcium absorption and utilization in the body.

There are many benefits from casein, according to an article by Mawer (2016). It is one of the complete protein sources. It contains all the essential amino acids that the body needs for growth, repair and development.

It is also a great source of macronutrients. A scoop (1.16 ounces/33 grams) of casein protein powder contains around 24 grams of protein, with 1 gram of fat and 3 grams of carbohydrates.

Aside from these, casein also has unique bioactive compounds and proteins with many great health benefits. These compounds come in two forms: micellar casein and casein hydrolysate.

Micellar casein is the slowly digestible form of casein. Casein hydrolysate is a pre-digested form. It is more rapidly absorbed than micellar casein.

How Does the Body Normally React to Casein?

Casein is one of the proteins that take a longer time to be digested. It spends more time in the gastrointestinal (GI) tract. It is even called a "time-release" protein because of its slow absorption rate.

This is a good thing for the body under normal circumstances. As it is slowly broken down, it supplies amino acids to the cells at low levels but for a prolonged period. This is good as it allows cells to fully utilize the amino acids, leaving very little excess that might be excreted.

This slow release also helps the cells in protein synthesis. This can help feed the cells with amino acid building blocks that are helpful during times of muscle breakdown. This type of breakdown usually happens when the body hasn't eaten for some time, such as during fasting periods.

This characteristic makes casein a preferred protein source for bodybuilders and fitness enthusiasts. It supplies low levels of energy that are better utilized compared to whey protein. This helps prevent muscle breakdown during intense activities and periods of not eating.

According to the article by Mawer (2016) in Healthline, there are several preliminary studies that found many benefits for casein. These include:

  • Immunity
  • Antibacterial effects
  • Effects on triglyceride levels
  • Reduces free radicals
  • Aids in fat loss
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However, some people experience problems with casein. Many health experts and health practitioners link casein as one of the key ingredients in milk allergies and milk intolerances.

Milk intolerance is the sensitivity of the body to the presence of certain sugars and/or proteins in milk. One of the most popular is lactose intolerance. Another is casein allergy.

In intolerance, a person experiences mild to moderate digestive issues after taking milk and certain dairy products. Discomforts commonly include bloating, cramps, diarrhea, constipation, gas and abdominal pain.

People with milk allergies experience more severe symptoms. These symptoms include other systems. For example, symptoms of milk allergy include digestive issues like diarrhea, bloating and cramps, with additional symptoms like rashes, difficulty breathing and wheezing.

Some people may not have any of these symptoms yet might be affected by milk intolerances or milk allergies. What they may notice is the presence of something resembling cheese curds, also called casein curds, in their stool.

How Do Casein Curds Form?

The incidence of casein curds passing out in the stools is more likely to be observed in infants than in adults.

In an article by Courtney (1912), studies on infant stools have varying ideas on whether these are casein curds or not. Some say that these are fats that the infant's digestive system cannot break down and absorb.

In the same article, there was a greater body of scholars who believed that these are actual casein curds encapsulated in fat. These curds are believed to be caused by the digestive system's inability to fully digest and absorb the casein protein from milk.

One likely reason for greater chances of seeing casein curds in infant stools is the maturity of the digestive system. An infant's digestive system is not yet well equipped and well-developed to handle more complex food components such as casein proteins.

Proteins are tougher compounds that require more digestive processes in order to be bioavailable. Infant digestive systems are not yet fully developed to handle these. Hence, they typically pass casein in the form of curds in their stool.

This might also explain why human breast milk protein percentage is comprised only of 40% casein. Human breast milk has developed to provide casein in smaller amounts that the infants' digestive tract can handle.

Remember, casein is important as it is a rich source of amino acids for brain development, as well as overall growth and development. An infant should still receive this nutrient despite possibilities of intolerances. The key here is to provide small amounts to allow the infant's GI tract to handle it with more ease.

This occurrence might be more likely to be observed in formula-fed infants. Infant formula is usually made with a high percentage of cow's milk. In many studies, such as one by Dalziel (2017), cow's milk contains 3.5% proteins. Of these, 80% is casein, and 20% is whey. Hence, drinking cow's milk has a larger possibility of causing casein-related digestive problems.

In adults, the slow movement of casein in the digestive tract is the likely reason for the formation of casein curds. It may also be linked to milk allergies and intolerances.

Casein is a protein that's slow to digest compared to most other proteins, even for adult digestive tracts. This places greater strain on the digestive system, making it work harder.

This is a likely contributor to weakness and easy fatigability experienced by people who suffer from milk allergies and milk intolerances.

According to Dalziel et. al. (2017), casein moves through the digestive tract much slower than whey. Casein stays far longer in the stomach and the small intestines. The longer casein stays in the digestive tract, the longer it is exposed to highly acidic environments.

The low (acidic) pH of the stomach and the small intestines causes the casein to coagulate and form curds. This is similar to adding vinegar, lemon or other acidic compounds to milk and making it curdle.

That same effect happens to casein when it is exposed for a prolonged period to the acidic digestive juices in the stomach and small intestines.

This coagulated form also further slows gastric emptying of casein. The curds pass through the stomach and small intestines because they are in undigested form, and since they are in curd form, they are even more difficult to digest. This slow movement worsens the situation and promotes more curd formation.

Normally, there will be no problems with the coagulated casein or casein curds formed in the stomach. Once it passes into the small intestines, several hydrolysis methods occur. Hydrolysis is the process that splits a compound and separates its components. Digestive enzymes act on the curds and break them down.

In the small intestines, the hydrolysis of casein releases an abundance of peptides. The curds are broken down, and the casein proteins are metabolized into different amino acids and peptides. These peptides have many different important functions in the body. Some of these peptides turn into antimicrobial peptides, opioids and angiotensin-converting enzymes (ACE).

The slower transit time in the small intestines can help in greater absorption of these peptides.

On the other hand, slower transit in the small intestines can contribute to the formation of casein curds. This can happen if there isn't enough hydrolysis happening in this section of the GI tract.

Hence, the casein curds remain as curds as they pass through the small intestines. When they reach the large intestines, these curds combine with fecal matter. That's when people see those small, bean-like materials that look like cheese curds.

Some people suffer from casein allergy. Some believe that this condition contributes to the formation of casein curds.

According to research conducted by Pal, (2015), a derivative of casein found in cows' milk contributes to the symptoms of lactose intolerance. The study found evidence that lactose intolerance is not always caused by lack of lactase enzyme activity.

Lactase enzyme is naturally produced by the body in order to digest lactose. Lactose is sugar found in milk. Lack of this enzyme results in improper digestion and absorption of lactose. This causes build-up of lactose in the GI tract and serves as food for gut microbes. The gut microbes overpopulate the GI tract. In an article by Rossi (2015), as the gut microbes feed on lactose, fermentation results. All these conditions lead to symptoms such as:

  • Bloating
  • Diarrhea
  • Pain
  • Flatulence

This link is believed to be one of the reasons why people think they have lactose intolerance but get negative results in diagnostic tests. This is also probably why some people still suffer symptoms despite removing lactose in their diet. They may have eliminated lactose but are still eating foods that contain casein.

So, back to the casein link to milk intolerance.

In the study by Pal, (2015), cows' milk contains A1 beta-casein. This is found in higher amounts in milk from European-origin cattle. This compound is not found in significant amounts in purebred African or Asian cattle.

In the body, A1 beta-casein slows down the transit time of milk in the GI tract. From the above discussion, slow transit time promotes casein coagulation that produces casein curds. In addition, this slowed transit time also promotes dipeptidyl peptidase-4 production and an inflammatory marker called myeloperoxidase.

The slowed transit time can interfere with lactose metabolism, according to this same study by Pal (2015). It increases the opportunity for gut microbes to ferment lactose. The fermentation process naturally occurs as gut microbes feed on sugars. If lactose passes through the GI tract at normal speed, there is limited time for lactose fermentation. If the movement of lactose through the GI tract is slowed, lactose fermentation is prolonged. This fermentation process can promote imbalance in gut microflora. It can also produce compounds that can lead to bloating, diarrhea and other symptoms related to lactose intolerance.

When A1 beta-casein is digested, it releases a compound called beta-casomorphin-7 (BCM-7). In the article by Pal (2015), this BCM-7 compound promotes inflammation. This can contribute to commonly reported symptoms of milk intolerances such as bloating, gas and stomach pain.

The presence of BCM-7 (again, from the digestion of A1 beta casein or casein from European-origin cows' milk) seems to affect the production of lactase. It may also alter the activity of lactase, which can lead to the development of lactose intolerance symptoms.

In this same study, it was found that A1 beta-casein is the only casein form that produces all these discussed events, such as casein curd formation and slower transit time.

Hence, this can be considered a likely explanation as to why some people complain of symptoms after drinking cows' milk but not when they take milk from other mammals such as goats and sheep.

Furthermore, A1 beta-casein is found in significant amounts in the milk of European-origin cows, according to the Pal (2015) article. This form is not found in African and Asian purebred cows.

This prompted an initiative to find the feasibility of breeding European cows with African and Asian breeds in an attempt to lower the amount of A1 beta-casein in cows' milk.

What Is Casein Allergy?

Casein allergy is also another reason for curds forming and passing out in the stool.

Casein curds indicate an inability of the GI tract to properly digest casein protein. Undigested casein can cause inflammation in the gut. Inflammation can cause the walls of the intestines to become more permeable, letting larger molecules to pass through uncontrolled. One of these can be undigested casein molecules.

This event can spell trouble for the body.

Casein build-up has been linked to the development of nervous system conditions such as autism, ADHD and ADD, according to a WebMD article reviewed by Felson (2018). Eating casein can worsens symptoms and cause lethargy, brain fog and excess mucus production.

When casein passes through the intestines in undigested form, it triggers an immune reaction. The body mistakes this natural protein for a harmful compound. The response is a release of large amounts of IgE or immune cells responsible for allergic reactions. This is a protective response to keep allergens from destroying the cells.

In this case, casein is not an actual allergen. However, because it is not in the form required by the body since it has not been properly digested, the body sees it as a threat.

Once the IgE reacts with casein, histamine release is triggered. Once histamine is released, symptoms will appear:

  • Swelling of throat, face, tongue, lips or mouth
  • Skin reactions like red itchy skin, rash or hives
  • Respiratory symptoms such as sneezing, wheezing, coughing and/or runny nose
  • Itchy eyes

An allergic reaction can worsen and turn into an anaphylactic reaction. This condition can be life-threatening if not addressed properly and in a timely manner. Anaphylactic reactions can include:

  • Chest pain
  • Swelling inside the mouth that can lead to airway obstruction
  • Difficulty breathing
  • Hives

How to Avoid Casein Curds in Stool

One of the best ways to avoid this is to remove casein-containing foods from the diet.

Casein is also found in many non-dairy food products. This can be one of the reasons why people complain of symptoms despite cutting off all milk and dairy in their diet.

Avoid all foods with ingredients listed as:

  • "contains milk ingredients"
  • "processed in a facility that also processes milk products"
  • "made with milk ingredients"

Not all dairy-free foods are casein-free. Most of these only have lactose or whey removed, but not casein.

These are casein derivatives usually added to many packaged foods like energy bars and drinks.

There are some foods that also contain casein, which is used as binders:

  • Sausages
  • Lunch meat
  • Hotdogs

Some people are highly sensitive to casein that even minute amounts can trigger allergies. Avoid anything that has come in contact with milk and dairy products. That includes deli meats. The knife used in slicing cheeses might be used in slicing the meats. Cross-contamination can occur and introduce casein in the meats.

Casein Food SourcesCasein Forms in Processed Foods

Ice cream

Sodium casein


Magnesium casein

Some margarine, even those labelled dairy-free

Potassium casein


Casein hydrolysate


Calcium casein

Sour cream

Rennet casein


Whey hydrolysate, Whey

Creamed soups

Lactalbumin, Lactaglobulin




Lactoalbumin phosphate



What Can You Eat?

For one, a casein-free diet is not always necessarily a dairy-free diet. As mentioned in the study by Pal (2015), curds are more likely to occur with A1 beta-casein. This is more abundant in milk from European cows. Eating dairy products and milk from the milk of Asian and African purebred cows may not carry this same risk.

Milk and milk products from other mammals are also good alternatives, such as sheep's milk and goat's milk, as well as cheeses made from these.

Other alternatives are also available. Among these are:

  • Vegetable margarine instead of butter, but check labels to be extra sure
  • Coconut milk instead of creams
  • Italian ices like gelato
  • Dairy-free sorbet
  • Soy milk, rice milk
  • Almond milk and other nut milk

But what about calcium?

It was mentioned earlier in this article that phosphorus in casein molecular structure binds with calcium, making milk an excellent rich calcium source. If milk is to be eliminated, then what other calcium sources are there?

Again, try milk from other mammals. If unsure or prefer to cut out milk entirely from meals, there are other excellent calcium sources. Kale, for one, is very high in calcium.


Cutting out casein from the diet is not always a good thing, especially without a doctor's recommendation.

Remember, casein has an important role in growth and development. It contains essential amino acids that the body needs for growth, repair and development. There are only a few complete dietary sources of these essential amino acids, so don't be too eager to swear off casein.

A good, healthy gut is also key to helping the digestive system function well. Take probiotics to keep the gut microbiome balance and healthy. Avoid junk foods and artificial ingredients that can promote inflammation.

Again, casein curds in the stool might indicate an underlying health issue, but proper treatment is always done under a doctor's supervision.

Tip: What About Probiotics?

Milk allergy treatment combines probiotics. These are excellent sources of beneficial microbes to help restore balance in the GI tract. In fact, there are many studies that showed infant gut bacteria were able to develop tolerance to cow's milk. Some of these studies were discussed in an article on the UChicago Medicine website (2015).

Supplementing the diet with probiotics such as Lactobacillus rhamnosus GG (LGG) help increase cow's milk tolerance.

Accurately Diagnosing Casein Intolerance or Allergy

Casein curds in stool might indicate an inability to digest casein protein properly. There are many factors that contribute to the formation of curds. One is prolonged stay of casein in the highly acidic environment of the stomach and the small intestines. Another reason is that there isn't enough hydrolysis in the small intestines to break down casein.

If casein curds are indeed in the stool, it is very important to work more closely with a doctor to find out what's really going on. Further tests will have to be done to accurately diagnose casein intolerance or allergy. Tests will also have to be done to determine if those are indeed casein curds.


This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.


Anarchus on February 24, 2019:

It's more expensive, but many stores in the US are now selling a2 Milk, which doesn't contain the a2 protein.

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