Lactose in Cow Milk and Digestion in Humans

Are you lactose intolerant and know first-hand the discomfort that can come from it? Have you heard others talk about being lactose intolerant? Though many of us have heard lactose referred to when talking about dairy products, we may struggle to explain more than the general idea that lactose is in milk and sometimes causes problems for people. Let's take a look at what lactose is and how it may impact the lives of those who consume (or want to consume) dairy products.

What is Lactose?

Lactose is a disaccharide, meaning it is made up of two basic sugar molecules joined together – one glucose and one galactose. Glucose is the common sugar that circulates in the bloodstream after digestion of food, used to generate energy throughout the body. Galactose, which is remarkably similar to glucose in its chemical structure, can be transformed into glucose for energy, but it is also an important component of cell membranes.

Except for the milk of mammals, lactose is rarely found in other whole, unprocessed foods. Infants use it as an important energy source during their first year of life (Silanikove et al., 2015). It also supports the development of probiotic bacteria in an infant's gastrointestinal tract, which helps protect them from infections (Fassio et al., 2018).

In cows, lactose is synthesized in the mammary gland from about 20% of the glucose in the bloodstream. It makes up an estimated 4.7% of the total nutrient content in a cow's milk, typically more by weight than even fat or protein (Costa et al., 2019).

Figure 1 – Milk from cows is full of nutrients. Making up the bulk of the solid components are a unique mixture of proteins, a variety of fatty acids, and lactose, the predominant carbohydrate.

Though not common in unprocessed foods, lactose is commonly found in processed food products (e.g. baked goods, processed meats, breakfast cereals, potato chips), protein supplements, and pharmaceuticals because of its usefulness as a binder, filler, or browning agent. Interestingly, lactose is about six times less sweet than sucrose (Ugidos-Rodriguez et al., 2018), which is the common table sugar made up of one glucose molecule and one fructose molecule (not galactose) joined together.

How is Lactose Digested in Humans?

Lactose is digested by an enzyme commonly referred to as lactase (notice the slight but significant spelling difference). Lactase generally resides in the small intestine. Once lactose is broken down by lactase into its glucose and galactose components, they are readily absorbed and used throughout the body.

When lactose is not fully digested in the small intestine and proceeds to the large intestine, some of the resident bacteria cause it to ferment and produce lactate, small-chain fatty acids, and gases (e.g. hydrogen gas, carbon dioxide, methane). Humans experience a more favorable outcome in terms of comfort if digestion of lactose occurs in the small intestine, not in the large intestine.

Figure 2 – A simplified diagram of lactose breakdown in the small and large intestine of humans. Lactase activity in the small intestine is preferable compared to breakdown of lactose by bacteria in the large intestine, which can lead to uncomfortable symptoms.

What is Lactose Intolerance, and How Big of a Problem Is It?

As might be expected, lactase activity is typically highest at birth, and most infants can digest lactose. The big issue with lactose digestion is that almost three-quarters of the human population experience a gradual drop in lactase production, especially between 3 and 5 years of age (Ugidos-Rodriguez et al., 2018), with levels dropping to about 10% of what they were as a baby. Only about one-quarter of adults retain the ability to effectively digest lactose into adulthood (Szilagyi & Ishayek, 2018).

The key determinant for persistent production of lactase in an individual is not the frequency of lactose consumption, but rather the individual's genetic makeup. There is tremendous variability between ethnic groups and regions of the world. In Southeast Asia, more than 95% of adults are deficient in lactase, whereas deficiency is only found in about 10% of adults in Scandinavia (Forsgård, 2019). Lactose intolerance was found in only 1% of Dutch people, but it was prevalent in 100% of Native Americans (Silanikove et al., 2015). Given the ethnic composition of the U.S. population, roughly 35% of adolescents and adults in the U.S. are lactase non-persistent (Levitt et al., 2013), meaning they do not continue producing the lactase enzyme into adulthood.

There are known genetic variants in human DNA that regulate lactase expression. One homozygous genotype results in reduced lactase activity, whereas the alternative homozygous genotype is associated with normal, sustained lactase activity. Those with a heterozygous coding in their DNA (one of each allele) usually continue to produce lactase, but at lower levels (Forsgård, 2019). Conveniently, only 50% of full lactase activity is necessary for effective utilization of lactose in the diet, which would be the case with heterozygous individuals, though they may be more susceptible to intolerance during times of stress or infection (Lomer et al., 2008).

Lactose intolerance is specifically defined as a situation where physical symptoms arise due to an inability to digest lactose. Common symptoms of lactose intolerance include diarrhea, nausea, bloating, and abdominal pain. The symptoms usually develop 30 minutes to 2 hours after ingestion (Costanzo & Canani, 2018). It is possible to be deficient in lactase production without being truly lactose intolerant.

Some lactose-sensitive individuals, whether they realize it or not, can tolerate lactose consumption. They may be deficient in lactase and unable to digest lactose efficiently, but they can ingest limited amounts of lactose without significant symptoms. Not everyone who has lactose digestion issues experiences symptoms every time.

The Dietary Guidelines for Americans, updated in 2020, still recommend regular consumption of milk, yogurt, and cheese for most people because of the easily digestible nutrients that are important to include in the diet, including calcium. The guidelines acknowledge lactose digestion as a common health challenge, which is why the recommendations clarify that lactose-free products are a good option for those who are lactose intolerant. Dairy products, as well as some soy-based alternatives, are a recommended part of a healthy eating pattern, supplying important nutrients, regardless of the consumer's genetic makeup.

What Can Be Done to Counteract Lactose Intolerance?

Most people lacking persistence in lactase production have few, if any, symptoms after consuming lactose if the quantity consumed during a meal or snack event is less than 12 grams (Dekker et al., 2019). This can be achieved easily. Foods high in lactose can be consumed in small portions, or foods low in lactose can be consumed more liberally.

One strategy that sometimes works is to consume lactose with meals. Foods with significant fat content, especially, tend to slow down gastric emptying, thus exposing the small intestine to less lactose per unit of time, potentially lessening symptoms. Because of this, whole milk is better tolerated than other fluid milk options with lower fat levels (Ugidos-Rodriguez et al., 2018), even though the fat content is still less than 4%.

Some fluid milk and yogurt products have the lactase enzyme added during processing, which breaks down the lactose before it reaches the consumer's mouth. It changes the taste of the product, but it easily solves the lactose problem.

In yogurt and fermented milk products, probiotic bacteria often possess lactase, which can break down lactose. In one research study, ingestion of 18 grams of lactose in yogurt resulted in fewer symptoms for the people participating in the study than those consuming a similar amount of lactose in fluid milk. It was also reported that more than 90% of the lactose in yogurt was digested in the small intestine, which was aided by slow gastrointestinal transit time (Savaiano, 2014).

Lactose content diminishes over time in some dairy products, especially cheese. During the cheesemaking process, more than 90% of the lactose is separated off in the whey portion (Costa et al., 2019). Then, in cheeses that are aged for development of flavor and harder textures, the remaining lactose is broken down by the bacteria that aid in those aging processes. Aged cheeses have very little lactose.

One additional option is to take the lactase enzyme as a food supplement. It is often helpful, but the length of time it can be effective is brief. It typically needs to be taken 5-30 minutes before consuming a meal containing lactose (Fassio et al., 2018).

Figure 3 – The nutrition label from a container of plain, Greek yogurt made with whole milk. With 4 total grams of sugar per serving, 1 or 2 servings of this yogurt would be well below the suggested threshold of 12 grams, at which many lactose-sensitive individuals start to see symptoms develop. Since this yogurt was made with whole milk, the fat content may also aid lactose digestion.

Unfortunately, most nutrition labels do not give details specific for lactose, but looking at general sugar content and the list of ingredients can prove helpful. For the sake of comparison, milk that has not been treated with lactase or filtered to remove sugars has approximately 12 grams of lactose (sugar) per 1-cup / 8-ounce serving. Sugar, which is added to various products (e.g. chocolate milk, flavored yogurt), most likely does not include lactose.

Sources: UpToDate.com; Costanzo & Canani, 2019; Lomer et al., 2008

What Other Food Issues Might be Confused with Lactose Intolerance?

A survey study done by Casellas et al. (2016) in Spain revealed that subjective perception (a general feeling) of lactose intolerance influences a person's decision to avoid dairy consumption more than objective determination of intolerance. All 580 patients who participated in the study suspected lactose maldigestion, and 56% considered themselves lactose intolerant. After a standard diagnostic test, only 45% were classified as malabsorbers. Half of the patients who were classified as being able to effectively digest lactose after the diagnostic test had claimed to be lactose intolerant, and more than 60% of them had been consuming less dairy than they would prefer or avoided dairy altogether.

In contrast to intolerance of lactose, which is a carbohydrate, some people are allergic to proteins in cow milk. This is one of the most common forms of food allergy, particularly in the first few years of life (Costanzo & Canani, 2018). Cow milk allergies are oftentimes temporary and largely limited to children. However, there has been some research suggesting one form of the beta casein found in milk (A1) may trigger negative reactions in some people, and this may sometimes be confused with lactose intolerance (Pal et al., 2015; Szilagyi & Ishayek, 2018). More research is necessary to reach any major conclusions, but this has been a growing area of interest.

Irritable Bowel Syndrome (IBS) affects a significant percentage of the world population and often causes similar symptoms to lactose intolerance. However, the two health issues are distinctly different, and lactose intolerance does not lead to IBS (Lomer et al., 2008).

For diagnosing lactose intolerance or one of these other health issues, it is important to work with a professional health care provider. There are multiple ways to diagnose an individual's condition and determine the best approach to managing diet and lifestyle.

In summary, many people are hesitant to consume dairy products because of an expectation the result will be uncomfortable. This may or may not always be true. If true, lactose may or may not be the appropriate target for blame. Lactose intolerance due to genetics is certainly a real problem for a lot of people, but at least some of those individuals can still consume dairy products by choosing specific products their body can handle. There are many options available.

Where trade names appear, no discrimination is intended, and no endorsement by Penn State Extension is implied.

References

Casella, F., Aparici, A., Pérez, M.J., & Rodríguez, P. (2016). Perception of lactose intolerance impairs health-related quality of life. European Journal of Clinical Nutrition, 70, 1068-1072.

Costa, A., Lopez-Villalobos, N., Sneddon, N.W., Shalloo, L., Franzoi, M., De Marchi, M., & Penasa M. (2019). Invited review: Milk lactose – Current status and future challenges in dairy cattle. Journal of Dairy Science, 102, 5883-5898.

Costanzo, M.D. & Canani, R.B. (2018). Lactose intolerance: Common misunderstandings. Annals of Nutrition & Metabolism, 73(suppl 4), 30-37.

Dekker, P.J.T., Koenders, D., & Bruins, M.J. (2019). Lactose-free dairy products: Market developments, production, nutrition and health benefits. Nutrients, 11, 551-564.

Deng, Y., Misselwitz, B., Dai, N., & Fox, M. (2015). Lactose intolerance in adults: Biological mechanism and dietary management. Nutrients, 7, 8020-8035.

Fassio, F., Facioni, M.S., & Guagnini, F. (2018). Lactose maldigestion, malabsorption, and intolerance: A comprehensive review with a focus on current management and future perspectives. Nutrients, 10, 1599-1610.

Forsgård, R.A. (2019). Lactose digestion in humans: intestinal lactase appears to be constitutive whereas the colonic microbiome is adaptable. The American Journal of Clinical Nutrition, 110, 273-279.

Levitt, M., Wilt, T., & Shaukat, A. (2013). Clinical implications of lactose malabsorption versus lactose intolerance. Journal of Clinical Gastroenterology, 47(6), 471-480.

Lomer, M.C.E., Parkes, G.C., & Sanderson, J.D. (2008). Review article: Lactose intolerance in clinical practice – myths and realities. Alimentary Pharmacology & Therapeutics, 27, 93-103.

McDonald, S. (2020). Lactose intolerance. Penn State Extension.

Pal, S., Woodford, K., Kukuljan, S., & Ho, S. (2015). Milk intolerance, beta-casein, and lactose. Nutrients, 7, 7285-7297.

Savaiano, D.A. (2014). Lactose digestion from yogurt: Mechanism and relevance. The American Journal of Clinical Nutrition, 99(suppl), 1251S-1255S.

Silanikove, N., Leitner, G., & Merin, U. (2015). The interrelationships between lactose intolerance and the modern dairy industry: Global perspectives in evolutional and historical backgrounds. Nutrients, 7, 7312-7331.

Szilagyi, A. & Ishayek, N. (2018). Lactose intolerance, dairy avoidance, and treatment options. Nutrients, 10, 1994-2023.

Ugidos-Rodriguez, S., Matallana-González, M.C., & Sánchez-Mata, M.C. (2018). Lactose malabsorption and intolerance: a review. Food & Function, 9, 4056-4068.