About all

Can lactose intolerance develop later in life. Lactose Intolerance: Causes, Symptoms, and Management in Adults

Can lactose intolerance develop later in life. How is lactose intolerance diagnosed. What are the treatment options for lactose intolerance. Are there any alternatives to dairy products for lactose intolerant individuals.

Содержание

Understanding Lactose Intolerance: A Common Digestive Issue

Lactose intolerance is a digestive condition that affects millions of people worldwide. It occurs when the body is unable to properly digest lactose, a sugar found in milk and dairy products. This inability to process lactose can lead to uncomfortable symptoms and dietary challenges.

Contrary to popular belief, lactose intolerance is not a true allergy. Instead, it’s a digestive issue that can develop at any age, even in individuals who have previously consumed dairy products without problems.

What causes lactose intolerance?

The primary cause of lactose intolerance is a deficiency in lactase, an enzyme produced in the small intestine. Lactase is responsible for breaking down lactose into simpler sugars that can be absorbed by the body. When lactase production decreases, the undigested lactose ferments in the intestines, leading to various digestive symptoms.

Types of Lactose Intolerance: Primary and Secondary

There are two main types of lactose intolerance: primary and secondary. Understanding the differences between these types can help individuals better manage their condition and seek appropriate treatment.

Primary Lactose Intolerance

Primary lactose intolerance is the most common form of the condition. It typically develops over time as the body naturally decreases lactase production. This type of lactose intolerance can occur at any age, but it’s more commonly seen in adulthood.

  • Gradual onset of symptoms
  • No specific underlying cause
  • Often genetically predisposed

Secondary Lactose Intolerance

Secondary lactose intolerance is triggered by an underlying medical condition or injury to the small intestine. This type can be temporary or permanent, depending on the cause and treatment of the underlying issue.

  • Sudden onset of symptoms
  • Associated with other medical conditions
  • May be reversible with treatment of the underlying cause

Recognizing the Symptoms of Lactose Intolerance

Identifying the symptoms of lactose intolerance is crucial for proper diagnosis and management. While the severity and combination of symptoms may vary from person to person, there are several common indicators to watch for:

  1. Bloating
  2. Abdominal cramps
  3. Gas
  4. Nausea
  5. Diarrhea

These symptoms typically occur within 30 minutes to 2 hours after consuming dairy products. The intensity of symptoms can depend on the amount of lactose consumed and an individual’s level of lactase deficiency.

How quickly do lactose intolerance symptoms appear?

Lactose intolerance symptoms usually manifest within 30 minutes to 2 hours after consuming dairy products. However, the onset and duration of symptoms can vary depending on factors such as the amount of lactose consumed, individual sensitivity, and the presence of other foods in the digestive system.

Diagnosing Lactose Intolerance: Medical Tests and Procedures

If you suspect you may be lactose intolerant, it’s essential to consult with a healthcare professional for an accurate diagnosis. Several tests can be used to confirm lactose intolerance:

Lactose Tolerance Test

This test involves consuming a liquid containing high levels of lactose. Blood samples are then taken at intervals to measure glucose levels. If glucose levels don’t rise, it indicates that the body isn’t properly digesting and absorbing lactose.

Hydrogen Breath Test

During this test, you consume a lactose-containing drink, and then your breath is analyzed for hydrogen content. Elevated levels of hydrogen in the breath suggest that lactose is not being properly digested.

Stool Acidity Test

This test is primarily used for infants and young children. It involves examining a stool sample for lactic acid and other acids produced when undigested lactose is fermented by bacteria in the colon.

Managing Lactose Intolerance: Dietary Adjustments and Alternatives

While there is no cure for lactose intolerance, several strategies can help manage symptoms and maintain a balanced diet:

Reducing Dairy Intake

The most effective way to manage lactose intolerance is to reduce or eliminate lactose-containing foods from your diet. This may involve avoiding milk, cheese, yogurt, and other dairy products.

Lactose-Free and Lactose-Reduced Products

Many dairy alternatives and lactose-free products are available in the market. These include lactose-free milk, cheese, and yogurt, which have been treated with lactase to break down the lactose.

Lactase Enzyme Supplements

Over-the-counter lactase enzyme supplements can be taken before consuming dairy products to help digest lactose. These supplements come in various forms, including tablets, capsules, and drops.

Gradual Introduction of Dairy

Some individuals with lactose intolerance may be able to tolerate small amounts of dairy, especially when consumed with other foods. Gradually introducing dairy products in small quantities can help determine personal tolerance levels.

Nutritional Considerations for Lactose Intolerant Individuals

Maintaining a balanced diet while managing lactose intolerance is crucial for overall health. Here are some key nutritional considerations:

Calcium and Vitamin D

Dairy products are significant sources of calcium and vitamin D. Lactose intolerant individuals should seek alternative sources of these nutrients, such as:

  • Leafy green vegetables (spinach, kale, collard greens)
  • Fortified plant-based milk (soy, almond, oat)
  • Fatty fish (salmon, sardines)
  • Calcium-fortified orange juice
  • Tofu processed with calcium sulfate

Protein Sources

While dairy is a good source of protein, there are many alternatives available:

  • Lean meats and poultry
  • Fish
  • Eggs
  • Legumes (beans, lentils, peas)
  • Nuts and seeds
  • Soy products

Hidden Sources of Lactose: Reading Labels and Making Informed Choices

Lactose can be found in many unexpected food items, making it essential for lactose intolerant individuals to carefully read food labels. Some common hidden sources of lactose include:

  • Baked goods and desserts
  • Processed meats
  • Salad dressings and sauces
  • Instant soups and meal mixes
  • Certain medications

When reading labels, look for terms such as “milk solids,” “whey,” “curds,” and “milk byproducts,” which indicate the presence of lactose.

How can lactose intolerant individuals ensure they’re avoiding hidden sources of lactose?

To avoid hidden sources of lactose, individuals should:
1. Read ingredient labels carefully
2. Be aware of common terms indicating lactose presence
3. Choose lactose-free or vegan alternatives when available
4. Consult with a nutritionist or dietitian for personalized guidance
5. Use smartphone apps designed to identify lactose-containing products

Lactose Intolerance vs. Milk Allergy: Understanding the Difference

It’s important to distinguish between lactose intolerance and milk allergy, as they are two distinct conditions with different causes and treatments.

Lactose Intolerance

  • Digestive issue
  • Caused by lactase deficiency
  • Symptoms primarily affect the digestive system
  • Can be managed through dietary adjustments

Milk Allergy

  • Immune system response
  • Reaction to proteins in milk
  • Can cause severe allergic reactions, including anaphylaxis
  • Requires complete avoidance of milk and milk products

If you’re unsure whether you have lactose intolerance or a milk allergy, it’s crucial to consult with a healthcare professional for proper diagnosis and management.

Living with Lactose Intolerance: Tips for Social Situations and Dining Out

Managing lactose intolerance doesn’t mean you have to miss out on social gatherings or dining experiences. Here are some tips for navigating these situations:

  1. Communicate your dietary needs with hosts or restaurant staff
  2. Research menu options in advance
  3. Carry lactase enzyme supplements for emergencies
  4. Choose lactose-free alternatives when available
  5. Be prepared with safe snacks or meals

How can lactose intolerant individuals enjoy dining out without compromising their dietary needs?

Lactose intolerant individuals can enjoy dining out by:
1. Researching restaurant menus and lactose-free options beforehand
2. Communicating clearly with wait staff about dietary restrictions
3. Asking about ingredient substitutions or modifications to dishes
4. Opting for naturally lactose-free menu items
5. Carrying lactase enzyme supplements as a precautionary measure

Emerging Research and Potential Treatments for Lactose Intolerance

While there is currently no cure for lactose intolerance, ongoing research is exploring potential treatments and management strategies. Some areas of investigation include:

  • Probiotics and their role in lactose digestion
  • Gene therapy to restore lactase production
  • Development of more effective lactase supplements
  • Personalized nutrition approaches based on genetic profiles

As research progresses, new options for managing lactose intolerance may become available, potentially improving the quality of life for those affected by this condition.

What promising developments are on the horizon for lactose intolerance treatment?

Promising developments in lactose intolerance treatment include:
1. Advanced probiotics targeting lactose digestion
2. Gene therapy techniques to restore lactase production
3. Personalized enzyme supplements based on individual needs
4. Improved diagnostic tools for more accurate assessment
5. Novel food processing methods to reduce lactose content in dairy products

Mayo Clinic Q and A: Lactose Intolerance Can Develop at Any Age

  • By

    Liza Torborg

DEAR MAYO CLINIC: What would cause someone to become lactose-intolerant later in life? I’m in my 40s and have never had an issue with dairy, but, now, I can’t seem to have it without problems. Do I need to see a doctor to be tested for allergies, or should I just avoid dairy?

ANSWER: Lactose intolerance isn’t a true allergy, and it can develop at any age. In some people, lactose intolerance may be triggered by another medical condition, such as Crohn’s disease. In others, it develops without a specific underlying cause. It would be a good idea to have your condition evaluated by your doctor to confirm that what you’re dealing with truly is lactose intolerance.

Lactose intolerance results from a problem with the carbohydrate lactose, a type of sugar found in dairy products. When you eat or drink dairy products, enzymes in your small intestine digest lactose, so the body can make energy. In people with lactose intolerance, a certain enzyme, called lactase, is missing from the body. When those people eat dairy products, the body has no way to break down the lactose. This leads to fermentation of the sugar in the intestines and triggers symptoms, such as diarrhea, nausea, abdominal cramps, bloating and gas.

Sometimes, lactose intolerance develops when the small intestine’s production of lactase decreases after an illness, injury or surgery involving the small intestine. This is called secondary lactose intolerance. Among the diseases associated with this kind of lactose intolerance are celiac disease, bacterial overgrowth and Crohn’s disease. Treatment of the underlying disorder may restore lactase levels and improve symptoms.

More common than secondary lactose intolerance is primary lactose intolerance. People who develop primary lactose intolerance start life making normal levels of lactase — a necessity for infants, who get all their nutrition from milk. As children replace milk with other foods, lactase production normally decreases. It stays high enough, though, to digest the amount of dairy in a typical adult diet. In primary lactose intolerance, lactase production declines below normal at some point for reasons that are unclear. The low amount of lactase then makes milk products difficult to digest and leads to lactose intolerance symptoms.

Your doctor can confirm a diagnosis of lactose intolerance with a clinical test. One that’s often used is a lactose tolerance test. It assesses your body’s reaction to a dose of lactose. After you consume a drink containing lactose, a sample of your blood is taken to measure glucose levels. If your glucose level does not rise, it means your body isn’t properly digesting and absorbing the lactose.

Alternatively, another test called the hydrogen breath test may be used. This test also requires you to consume a drink that contains high levels of lactose. Your doctor next measures the amount of hydrogen in your breath. Normally, very little hydrogen is detectable. However, if your body doesn’t digest the lactose, the fermentation reaction in your colon releases hydrogen and other gases. Your intestines absorb those gases, and you exhale them. Larger-than-normal amounts of hydrogen measured during this test are a sign that your body isn’t fully digesting and absorbing lactose.

If you have lactose intolerance, there isn’t a way to cure it. The most effective way for people with lactose intolerance to get relief from symptoms is to lower the amount of dairy products they eat. You may be able to use dairy products that have reduced levels of lactose or are lactose-free. Some people who have lactose intolerance benefit from taking lactase enzyme supplements, as well.

If test results do not point to lactose intolerance, your doctor may recommend additional tests to check for another condition that could be causing your symptoms, such as a milk allergy, intestinal disorders or other problems within your digestive tract. — Dr. Rohit Divekar, Allergic Diseases, Mayo Clinic, Rochester, Minnesota

Related articles

Mayo Clinic Minute: What is a breast medicine specialist?

Many people think that a breast health specialist only treats cancer, but the truth is that these doctors handle a wide range of breast-related conditions.
Dr. …

By Sonya Goins • July 7, 2023

Mayo Clinic Q and A: Proper posture and body alignment

DEAR MAYO CLINIC: My grandmother lives with me, and she is always nagging me to “stand up straight” and “put your shoulders back.” She says good …

By Mayo Clinic Staff • July 7, 2023

Can You Get It as You Get Older?

Written by Susan Bernstein

Do you get gas, bloating, or even diarrhea after you eat dairy foods? You may be lactose intolerant. This is when you can’t digest lactose, the type of sugar found in dairy products.

Lactose intolerance isn’t a serious health condition, but symptoms can be uncomfortable. You may have stomach upset or gas about 30 minutes to 2 hours after you eat or drink any dairy food.

Lactose intolerance is caused by low levels of lactase, an enzyme that helps you digest lactose in dairy foods, says Laura Acosta, a registered dietitian at the University of Florida in Gainesville.

“Some people are able to tolerate lactose throughout their lives, but others become more lactose intolerant as they age,” usually caused by your genes, she says. “Some people stop producing lactase, or produce less of it, as they reach adulthood and beyond.”

Lactose intolerance is more common among people of Southeast Asian, East Asian, West African, Native American, Hispanic, or Italian ancestry because they’re more likely to carry the gene mutation that causes the condition, says Vijaya Surampudi, MD, assistant professor of medicine at the University of California, Los Angeles.

“We have the highest amounts of lactase when we are young because as babies, we feed on milk. But we often make less and less lactase as we age,” she says. “About 65% of people may experience lactose intolerance at some point in their lives.”

There’s a wide range of lactose intolerance, with some people having no tolerance for dairy, while others can manage to digest foods like hard cheeses or yogurt, she says.

It’s fairly common to notice the signs of lactose intolerance appear as you get older, says Christine Lee, MD, a gastroenterologist at Cleveland Clinic in Ohio.

“This enzyme production can decrease over time in some people, so most people can experience some degree of lactose intolerance as they get older,” Lee says. You can also become lactose intolerant if you have an illness, infection, or take a medication that affects the bowels or intestines.

These conditions can also cause you to develop lactose intolerance later in life:

  • Crohn’s disease
  • Ulcerative colitis (UC)
  • Celiac disease
  • Injury or trauma to your small intestines

Foods. Any dairy foods or drinks can have lactose, so read food labels carefully. These foods or ingredients may trigger symptoms:

  • Cheese
  • Whey
  • Milk by-products
  • Dry milk
  • Butter
  • Curds
  • Ice cream
  • Heavy cream
  • Cottage cheese
  • Cheese spreads
  • Yogurt

Look for milk in ingredients lists in baked goods, chocolate candies, sauces, instant mashed potatoes, pancakes, or milk-based meal replacement or protein shakes. Even non-dairy coffee creamer or whipped toppings may contain some lactose.

Medications. Some medications like birth control pills or antacid tablets also contain lactose, but these may only cause symptoms in people with severe lactose intolerance. Check the labels of your prescription or OTC medications to see if they contain lactose. Ask your doctor or pharmacist if it’s OK for you to take them.

You may not have to give up all the dairy foods you enjoy if you develop lactose intolerance.

Some people with lactose intolerance may be able to eat or drink small amounts of dairy or switch to skim milk and have fewer symptoms. Some people can eat yogurt because it contains live bacteria cultures that help you produce the lactase enzyme to break down lactose.

“Cheese actually has very little lactose. In general, the harder the cheese, the less lactose it has. So many people with lactose intolerance can still tolerate cheese like Parmesan and Swiss in small amounts. This depends on an individual’s sensitivity,” Acosta says.

Talk to your doctor to identify what’s causing your lactose intolerance so you can make the right changes to your diet, Lee says.

“If it’s due to decreased production of lactase, then you can choose to take lactase tablets or drops, enzymes sold in most grocery stores without a prescription, or buy lactose-free milk products,” she says. Supplements are taken before a meal with dairy to help you digest the lactose.

If your lactose intolerance is due to a bowel disease or other health conditions, treat that illness first, she adds. “Always play it safe and discuss your symptoms with your physician.” Unexplained weight loss, bleeding, anemia, nausea, or vomiting may be signs of a more serious medical condition, not just lactose intolerance, she says.

As you age, you still need to get enough calcium even if you develop lactose intolerance. Calcium helps you maintain strong bones and teeth, and healthy muscles and nerves.

“There are many calcium-rich foods people can eat if they’re avoiding lactose,” says Allie Wergin, a registered dietitian/nutritionist with Mayo Clinic Health System. “These include lactose-free dairy products like Lactaid or Dairy Ease milks, where the lactose is predigested or hydrolyzed. This means the milk protein has already been broken down and is easier to digest.”

Other foods that are good sources of calcium, according to Wergin:

  • Calcium-fortified cereals and fruit juices
  • Tofu prepared using calcium sulfate
  • Canned salmon or sardines with bones
  • Plant-based soy, almond, oat, or rice milks
  • Leafy green vegetables like spinach, kale, bok choy, or collard greens
  • Broccoli
  • Almonds
  • Beans like garbanzo, kidney, or navy

“We recommend that you get calcium through your diet. There are different calcium supplements, including calcium carbonate, which is best absorbed if you take it with food, and calcium citrate, which you can take with or without food,” Surampudi says. Only take a calcium supplement if your doctor says you need one, she says.

Top Picks

Lactose intolerance | Symptoms, complications, diagnosis and treatment

People with lactose intolerance are unable to fully digest the lactose in milk. As a result, they develop diarrhea, gas, and bloating after eating or consuming dairy products. The condition, also called lactose malabsorption, is usually harmless, but its symptoms can be uncomfortable. Most people with lactose intolerance can manage the condition without giving up all dairy products.

Lactase deficiency, an enzyme produced in the small intestine, is usually responsible for lactose intolerance. Many people have low lactase levels but can digest dairy products without problems. If you are actually lactose intolerant, lactase deficiency leads to symptoms after you eat dairy products.

Signs and symptoms of lactose intolerance usually begin 30 minutes to two hours after eating or drinking foods containing lactose. General signs and symptoms include:

  • Diarrhea
  • Nausea and sometimes vomiting
  • Abdominal cramps
  • Inflate
  • Gases

Make an appointment with your doctor if you often experience symptoms of lactose intolerance after eating dairy products, especially if you are worried about getting enough calcium.

Reasons

Lactose intolerance occurs when the small intestine does not produce enough enzyme (lactase) to digest milk sugar (lactose).

Normally, lactase converts milk sugar into two simple sugars, glucose and galactose, which are absorbed into the bloodstream through the intestinal lining.

If you are lactase deficient, the lactose in food moves to the large intestine instead of being processed and absorbed. In the colon, normal bacteria interact with undigested lactose, causing the signs and symptoms of lactose intolerance.

There are three types of lactose intolerance. Various factors cause lactase deficiency underlying each type.

Primary lactose intolerance

This is the most common type of lactose intolerance. People with primary lactose intolerance begin their lives by producing large amounts of lactase, a must for babies who get all their nutrients from milk. As children replace milk with other foods, their lactase production usually decreases but remains high enough to digest the amount of dairy in a normal adult diet.

In primary lactose intolerance, lactase production drops dramatically, making it difficult for adults to digest dairy products. Primary lactose intolerance is genetically determined, which occurs in a significant proportion of people of African, Asian, or Hispanic ancestry. This condition is also common among Mediterranean or South European ancestry.

Secondary lactose intolerance

This form of lactose intolerance occurs when the small intestine reduces its production of lactase after illness, injury, or surgery involving the small intestine. Diseases associated with secondary lactose intolerance include celiac disease, bacterial overgrowth, and Crohn’s disease. Treating the underlying disorder can restore lactase levels and improve symptoms and signs, although this may take some time.

Congenital or developing lactose intolerance

This disorder is passed from generation to generation in a form of inheritance called autosomal recessive. Premature babies may also be lactose intolerant due to insufficient lactase levels.

Factors that may make you or your child more likely to become lactose intolerant, include:

  • Growing up. Lactose intolerance usually appears in adulthood. This disease is rare in children and young children.
  • Ethnos. Lactose intolerance is most common in African, Asian, Hispanic, and American Indian people.
  • Premature birth. Babies born prematurely may have low lactase levels because the small intestine does not develop lactase-producing cells until late in the third trimester.
  • Diseases affecting the small intestine. Small intestinal problems that can cause lactose intolerance include bacterial overgrowth, celiac disease, and Crohn’s disease.
  • Some treatments for cancer. If you’ve had radiation therapy for abdominal cancer or intestinal complications from chemotherapy, you’re at increased risk of lactose intolerance.

Lactase deficiency in adults

Lactase deficiency is a violation of the breakdown of lactose due to a deficiency of the lactase enzyme of the small intestine mucosa, accompanied by clinical symptoms.

Lactose is milk sugar. The enzyme that breaks down lactose is called lactase.

Adult-type lactase deficiency develops after a period of breastfeeding. This is due to a gradual decrease in lactase activity with age. After taking dairy products or whole milk, intestinal disorders (diarrhea, flatulence) usually appear. Prevention and treatment consists of following a low-lactose diet or eliminating it completely.

Russian synonyms

Congenital adult lactase deficiency (delayed onset type), primary adult lactase deficiency, hypolactasia, lactose intolerance, lactose malabsorption.

English synonyms

#OMIM 223100, Lactose intolerance, adult type, Hypolactasia, adult type, Disaccharide intolerance III.

Symptoms

Lactase deficiency is characterized by the development of intestinal symptoms after taking milk and dairy products: flatulence, pain and rumbling in the abdomen, diarrhea, bloating. Possible nausea. The stool is liquid or mushy, light yellow in color, with a sour smell.

General information about the disease

Lactase deficiency, or primary lactase deficiency, is a violation of the breakdown of lactose due to a deficiency of the lactase enzyme of the small intestine mucosa, accompanied by clinical symptoms. It is caused by a genetically determined decrease in its production, which manifests itself in the inability to absorb milk sugar (lactose).

Lactase activity manifests itself from the 12-14th week of intrauterine development and reaches its maximum values ​​by the time of birth (at the time of 39-40 weeks). After birth, lactase is produced in large quantities, but by the end of the first year of life, its production decreases. This is the primary, or congenital, late-onset lactase deficiency, which is inherited in an autosomal recessive manner.

Non-specific abdominal symptoms (bloating, upset stool, nausea) develop after consumption of foods containing lactose, mainly whole milk and dairy products. But today lactose is added to other foods (for example, meat, confectionery). The pharmaceutical industry also uses it as an excipient.

The severity of hypolactasia symptoms depends on individual physiological characteristics of a person, intestinal microflora, diet, and psychological factors. Conscious avoidance of milk, which is an important source of calcium, can lead to a deficiency of this element and, as a result, to osteoporosis. This should be remembered and compensate for the daily requirement through other foods rich in calcium. This is especially important for postmenopausal women.

Primary lactase deficiency should be distinguished from secondary (acquired), which occurs when the mucous membrane of the small intestine is damaged due to any acute or chronic disease. Such damage is possible with infectious (intestinal infection), immune (intolerance to cow’s milk protein), inflammatory processes in the intestine, atrophic changes (with celiac disease, after a long period of complete parenteral nutrition, etc.).

Also, this disorder should not be confused with congenital lactase deficiency, a rare genetic disorder whose symptoms appear immediately after birth and are associated with the onset of breastfeeding.

There is a genetic diagnosis of late-onset primary lactase deficiency. The MCM6 gene region is one of the important regulatory elements of the lactase gene. The genetic marker MCM6 (C(-13910)T) is associated with lactose intolerance.

Who is at risk?

  • Persons with relatives who are intolerant to milk and dairy products.
  • Certain ethnic groups. Lactose intolerance is most common in North America, Africa, Southeast Asia (the incidence of lactase deficiency is 70-100%) In the Russian population, lactase deficiency is observed in approximately 16%.
  • The condition rarely occurs in the first year of life, the development of lactose intolerance is associated with growing up.

Diagnosis

The diagnosis can be assumed on the basis of genealogical data, results of coprology (increase in starch, fiber, iodophilic microflora, decrease in fecal pH less than 5.5), determination of carbohydrates in feces. The diagnosis uses a hydrogen breath test. The concentration of hydrogen in the exhaled air is determined before and after lactose loading. In patients with lactase deficiency, an increase in the hydrogen content is recorded, which is associated with increased bacterial breakdown of lactose in the colon. That is, lactose, not completely absorbed in the small intestine, reaches the large intestine, where it is broken down by anaerobic microflora.

Important in the diagnosis is molecular genetic testing for predisposition to lactase deficiency. The analysis will help in the differential diagnosis of the causes of lactose malabsorption and in the selection of an appropriate diet.

With severe clinical manifestations, refusal of milk, it is also important to determine the calcium content in the blood serum and urine.

Treatment

Diet therapy: restriction of products containing lactose.

It is possible to prescribe enzyme preparations that break down lactose.

It is allowed to use fermented milk products with a reduced amount of lactose (yogurt, curdled milk), cottage cheese, butter, hard cheeses, as well as lactose-free.

It is important to take into account the possible lack of calcium intake on a dairy-free diet, which is recommended to be compensated by other foods rich in it or medications.

In young children, often lactase deficiency is combined with intolerance to cow’s milk proteins.