Potential Side Effects of the Casein Protein in Your Milk

Casein protein powder is a popular choice for athletes and fitness enthusiasts. Filling and nutritious, it's ideal before bedtime, as it fuels your muscles over several hours. Manufacturers claim that it helps reduce muscle breakdown, promotes hypertrophy and boosts overall strength. There are cases, though, when the casein in milk can do more harm than good.

Nutrient timing plays a key role in sports performance. Protein, carbs and fats — the macronutrients in food — are metabolized differently. That's why most athletes plan their meals around their workouts.

Your body needs protein to build muscle, recover from training and function optimally. Protein supplements have been shown to accelerate muscle repair and improve physical performance. Furthermore, protein timing has a direct impact on metabolism and post-workout recovery.

Read more: 15 New Ways to Use Protein Powder That Will Blow Your Mind

Most athletes consume casein at bedtime to preserve lean mass and reduce catabolism. Unlike whey protein, casein is slowly digested and has long-lasting effects. This nutrient occurs naturally in the milk of all mammals; cow's milk protein, for example, is about 82 percent casein. Cheese, kefir, yogurt, cream and other milk derivatives all contain casein.

The casein in milk may not be as healthy as it was once thought. This nutrient is comprised of several proteins called casein micelles, which include αs1-, αs2- and beta-caseins (A1 and A2). Each has distinctive biochemical properties.

According to a 2016 study published in the Nutrition Journal, milk containing both A1 and A2 beta-caseins may cause greater digestive discomfort and gut inflammation than A1 beta-casein milk. Furthermore, it may increase digestion time and affect cognitive function. A2 beta-casein alone had none of these side effects.

Read more: 7 Signs Your Gut Is Out of Whack

Until recently, it was believed that dairy casein may lead to cancer. These claims originated from Dr. T. Colin Campbell's bestseller, The China Study, which states that casein is a known carcinogen. As The World Journal of Men's Health points out, this compound does promote prostate cancer cell growth but under serum-free conditions (a controlled-culture medium without serum or a test-tube study); its effects on cancer cells in vivo (in a living organism) are unknown. More research is needed to confirm the link between dairy casein and cancer. If you have concerns about casein and prostate cancer, talk to your doctor for recommendations before you cut milk out of your diet.

Clinical evidence points out that casein may cause adverse reactions in some people, especially those with lactose intolerance. In a 2014 study featured in the European Journal of Clinical Nutrition, subjects who consumed A1 beta-casein milk daily for two weeks reported more severe abdominal pain and gastrointestinal symptoms than those drinking milk containing only A2 beta-casein.

A1 beta-casein delays intestinal transit and may cause constipation. Men and women who drank A1 beta-casein milk also experienced bloating and gas. It appears that A1 beta-casein stimulates the production of BCM-7, an opioid that triggers inflammation.

Some people are allergic to casein and can experience severe reactions after eating yogurt, cheese or milk and foods containing these ingredients. Casein protein side effects may include wheezing, coughing, itchy skin, hives and swelling of the face and throat. Another potential reaction is anaphylaxis, which requires emergency treatment. If you suspect that you may be allergic to casein, your doctor will recommend blood and stool tests to make a diagnosis.

Approximately 65 percent of people cannot digest lactose properly. This condition is known as lactose intolerance or lactose malabsorption and has genetic causes. Its symptoms include nausea, bloating, gas, digestive discomfort and abdominal pain.

Both lactose and casein occur naturally in milk and its derivatives, but not all milk products contain lactose. Most stores nowadays offer lactose-free milk, yogurt, ice cream and other similar foods. Casein and lactose are not one and same — if you have lactose malabsorption, it doesn't mean you can't drink casein shakes or eat cheese (assuming that it has little or no lactose).

However, dairy casein, especially A1, may affect the body's ability to digest lactose, according to a 2015 study published in the journal Nutrients. This milk protein takes longer to digest and affects gastrointestinal motility, which may trigger or exacerbate lactose intolerance symptoms, with constipation being the most common complaint.

Casein is found in all types of milk and dairy products, especially those with a high protein content. Mozarella cheese, kefir, yogurt, cow's milk, goat's milk and cream are just a few to mention. Any product made with milk will contain casein in varying amounts. Let's see some examples:

• Cream soups
• Ice cream
• Cheesecake
• Premade tuna salad
• Pizza
• Cheeseburgers
• Processed meats
• Commercial sauces and salad dressings
• Milk chocolate
• Baked goods
• Breakfast cereals, without milk
• Pastries
• Infant formulas
• Protein powder
• Coffee drinks, such as lattes and frappes
• Milkshakes
• Pudding
• Custard

Food manufacturers use casein as a binding agent in sausages, hot dogs and other processed meats. Some products, such as breakfast cereals, may contain dried whey and modified milk ingredients. Popular recipes, including pasta Alfredo, quiche and scrambled eggs with cheese, contain this protein too. If you're allergic to casein, eating out can be a problem. To stay safe, ask the waiter about the ingredients used to prepare your meal.

Read more: Foods That Contain the Protein Casein

Make a habit of reading food labels. Watch out for statements like "processed in a facility that also processes milk and dairy" or "contains milk ingredients." Beware that nondairy doesn't mean milk free. Manufacturers often replace the term "casein" with sodium caseinate, for example. Also, natural flavorings used in processed foods may contain milk, which is the primary source of casein.

Unless you're allergic to casein or milk, there's no reason to cut this protein from your diet. Moderation is the key. Some people are prone to its side effects, such as bloating and stomach upset, while others experience no adverse reactions — it all comes down to how your body reacts. Casein has been linked to gut inflammation, but more studies are needed in this regard.

Don't overlook the potential health benefits of casein. A 2014 study published in the British Journal of Nutrition found that active young men who consumed whey protein, casein or carbs before bedtime experienced a greater increase in metabolism the next day compared to the placebo group. A faster metabolism equals more calories burned and faster weight loss.

Casein may also protect against diabetes and make weight loss easier. This nutrient has been shown to improve enteroendocrine cell health and proliferation as well as GLP-1 (glucagon-like peptide-1) secretion. GLP-1 is a gut-derived hormone that helps slow down sugar absorption into your bloodstream. At the same time, it sends satiety signals to your brain, making you feel full faster.