Milk and Human Health
This page discusses human health issues that have been positively and negatively linked to dairy product consumption. The effect of diet is only one factor that affect human health. Influences of gender, genetics, and lifestyle patterns such as exercise and smoking are significant factors affecting the health of an individual. There is a considerable amount of variation among individuals and results from different studies on the same nutrient may be contradictory, and thus confusing to the consumer. Scientists continue to refine their study techniques as new information is discovered and scientific tools improve.
Human health topics covered below are the relationship between milk and allergies, arthritis, cancer, conjugated linoleic acid (CLA), coronary heart disease, dental caries, diabetes, hypertension, lactose intolerance, osteoporosis and bone health, probiotics, and raw milk consumption. There is a considerable amount of information available in the literature on these topics and an in-depth discussion is beyond the scope of this web site. References written by experts in the field that review and summarize the current medical and dietary studies are provided.
A food allergy is an immune response to proteins resulting in inflammatory symptoms involving the skin, respiratory system, and gastrointestinal tract. Some people think they are allergic to milk because they have symptoms of bloating, cramps, flatulence, and diarrhea after consumption of dairy products. This is not an allergic response but is a condition known as lactose malabsorption, which is discussed in a separate section below.
Allergic responses have different mechanisms, and some are mediated by immunoglobulins, specifically IgE. The major food proteins that trigger allergic reactions are milk, eggs, peanuts, tree nuts, fish, shellfish, soy, and wheat. Prevalence of milk allergy in the population is about 2% in children and 1% in adults. Onset of milk allergy may occur within about 1 yr of age and most children outgrow this allergy by 6 yrs of age.
There are 2 main categories of proteins in milk, the caseins and whey proteins, and there are multiple proteins within each category. Although the level of sensitivity to individual milk proteins vary, to date all of the milk proteins have been shown to contribute to milk allergy. Human reactivity to the diverse types of proteins in milk also suggests that people allergic to cow's milk are also likely to be allergic to goat, sheep and buffalo milk, although more research is needed to confirm this. Most therapy recommendations for milk allergies are to remove dairy products from the diet during the allergic period. Because many children outgrow milk allergies, dairy products can be reintroduced into the diet so that children may receive the nutritional benefits from consuming dairy products. Typical pasteurization conditions used for milk do not affect the protein content or structure and, therefore, have no effect on its allergenicity.
The area of food allergies is an emerging and evolving scientific field. For reviews of the scientific literature and more information on milk allergies see the following references: Crittenden and Bennett (2005), Sicherer and Sampson (2006), and Wal (2004).
Arthritis is characterized by damaged joints and leads to physical disabilities. There are many forms of the disease caused by degeneration of the joint (osteoarthritis), autoimmune diseases and inflammation (rheumatoid, psoriatic arthritis), infection (septic arthritis), and others.
It has been suggested that consumption of raw milk helps relieve arthritis. However, there are no studies in the literature that report an effect of drinking milk and relieving arthritis. There are currently no scientifically supported dietary recommendations to help relieve arthritis aside from maintaining a healthy weight and balanced diet. For more information please see www.arthritis.org.
Cancer is a disease characterized by malignant growth in the body. Different cancers have different causes, modes of progression, treatments, and possible risk factors. The area of cancer research is very active and there is a considerable amount of data in the literature. Because of the variation in cancer types, individual metabolism, and factors evaluated in any given study there may be conflicting results published between studies on the same topic. Factors affecting the risk of cancer includes genetics, lifestyle choices such as smoking and exercise, diet and other environmental causes.
Diets high in fat and calories and low in fiber and fruits and vegetables appear to increase the risk of some cancers. Diets high in dairy were thought to increase the risk of breast cancer because of the high fat content of some dairy products. However, there are conflicting research studies on the association between high fat (any type) intake and breast cancer. It used to be thought that dairy product consumption increased the risk of cancer, but we now know that there are several components in milk that are protective against cancer: calcium, vitamin D, conjugated linoleic acid (CLA), sphingomyelin, and whey proteins.
Calcium and vitamin D may reduce the risk of colon cancer. The protective effect of calcium and vitamin D was observed in numerous human studies in several countries. The amount of calcium consumed in most of these studies was at least 700 mg/d from either food sources or supplements. Sphingomyelin and whey proteins were shown to inhibit colon cancer cell growth in laboratory settings.
Calcium and vitamin D consumption have also been shown to have a protective effect against breast cancer in humans. Conjugated linoleic acid (CLA) is a very effective inhibitor in breast cancer development, as well as inhibiting skin, stomach, and colon cancers in laboratory settings. Whey proteins have been shown to inhibit breast cancer in laboratory settings. Some reports suggest that insulin-like growth factor (IGF-1) and growth hormones (GH) found in milk may promote breast cancer. However, the amount of IGF consumed in milk is very small compared to the amount produced by the human body, and bovine GH is not biologically active in humans. Therefore, these compounds do not contribute to breast cancer in humans.
The prospect of CLA, sphingomyelin, and whey proteins as anticancer agents looks promising, but must be confirmed by human studies. For reviews of the scientific literature and more information on milk and cancer see the following references: Gill and Cross (2000), Huth et al. (2006), Kritchevsky (2000), Miller et al. (1999), Parodi (2004, 2005).
Conjugated linoleic acid (CLA) is a fatty acid found in milk fat that has a specific chemical arrangement and unique health benefits. CLA is an 18 carbon fatty acid with 2 double bonds that are conjugated, meaning that the double bonds are on adjacent carbon atoms in the fatty acid chain, rather than being separated by one or more carbons. The double bonds can be in either the cis or trans configuration. The CLA family is made of many combinations, called isomers, of where the conjugated double bond is located along the carbon chain and whether it a cis and trans bond. CLA isomers include cis-9, trans-11 CLA, trans-10, cis-12 CLA, trans-7, cis-9 CLA, and trans-11, cis-13 CLA. The most common isomer is cis -9, trans -11 CLA, which accounts for over 70% of the CLA in milk fat. The cis-9, trans-11 and the trans -10, cis -12 CLA isomers are the most important to human health, as discussed below.
There is a growing understanding that different fatty acids are metabolized differently and contribute differently to human health, both positively and negatively. In general, trans fatty acids are considered bad for health and contribute to the risk of coronary heart disease. There is a movement in the U.S. and other developed countries to minimize consumption of trans fatty acids and their presence in the food supply. However, the trans fatty acids of concern are those that are generated during partial hydrogenation of vegetable oils to create desirable functional properties for use in margarines, baked goods, and many other processed foods. There is a difference in the health effects of trans fatty acids obtained by industrial processing of vegetable oil and trans fatty acids that naturally occur in ruminant animal fat found in milk and meat.
CLA was first found to have beneficial anticarcinogenic properties approximately 20 years ago. Consequently, this is a relatively new field of research and, while there are many studies ongoing, the health benefits of CLA need to be confirmed in controlled human studies. One confounding factor in CLA research is that there are many isomers of CLA, and only recently have scientists been able to identify and separate them. The different isomers of CLA have different physiological functions.
The anticarcinogenic properties of CLA have been studied in numerous laboratory settings. There is strong evidence that CLA inhibits breast cancer at the initiation and growth stages. CLA is also effective in inhibiting skin, stomach and colon cancers. The cis -9, trans-11 CLA isomer is responsible for the anticarcinogenic properties of CLA, although the mechanisms are still under study. Both the cis -9, trans -11 and trans -10, cis -12 isomers increase immune function and decrease inflammation response. Animal studies show a change in body composition towards decreased fat mass and increased lean mass with the trans -10, cis -12 isomer. Human studies on this effect have been inconclusive. It should be noted that the animals were usually in the growth phase accumulating mass and CLA was effective in shifting the balance toward more lean mass and less fat mass, whereas the human studies used adult subjects. A mixture of isomers was used in studies that showed CLA reduced plasma cholesterol and inhibited the progress of coronary heart disease.
The amount of CLA needed by humans to obtain these benefits currently is not known. The content of CLA in milk varies based on many factors including feed and cow-to-cow variation. In general, milk from cows on pasture has a higher CLA content than milk from cows fed on rations, although the ration diet may be supplemented to increase CLA content. CLA is found in the milk fat, therefore, dairy products that have higher fat content will have a higher CLA content.
For reviews of the scientific literature and more information on conjugated linoleic acid (CLA) see the following references: Kritchevsky (2000), MacDonald (2000), O'Shea et al. (2004), Parodi (2004), Terpstra (2004), and the web site on CLA maintained by the University of Wisconsin, http://www.wisc.edu/fri/clarefs.htm.
Coronary heart disease (CHD) is the leading cause of death in developed countries. Coronary heart disease is characterized by the accumulation of plaque inside the artery walls which narrows the arteries and reduces the flow of blood and oxygen to the heart muscle. There are many risk factors that contribute to CHD. Some risk factors cannot be modified such as being male, getting older, and a family history of CHD, while some risk factors can be modified such as smoking, high blood pressure, obesity, lack of exercise, and high blood cholesterol levels. The positive correlation of high blood cholesterol levels and increased risk of CHD is well-documented.
Total blood cholesterol consists of several components that have different effects on CHD. A high level of low density lipoprotein (LDL) has the strongest effect of increasing the risk of CHD, whereas a high level of high density lipoprotein (HDL) has a protective effect against CHD. Increased blood triglyceride, or triacylglycerol, levels also increase the risk of CHD. The blood cholesterol profile of an individual can be influenced by diet and thus modified, but there is also a strong genetic influence that cannot be modified. High intakes of total fat, saturated fat, and to some extent dietary cholesterol are associated with increased total and LDL cholesterol.
Research studies have shown that different fats have a different effect on blood cholesterol profile. As a category, saturated fatty acids have the largest effect on increasing blood cholesterol. However, chain length plays an important role in a fatty acid's influence on cholesterol. The short and medium chain fatty acids, 10 carbons and under in length, are metabolized by a different mechanism than the longer chain fatty acids, and have little effect on blood cholesterol levels. The 18 carbon (stearic acid) fatty acid also has little effect on blood cholesterol. The 16 (palmitic acid), 14 (myristic acid), and 12 (lauric acid) fatty acids have the most effect on increasing blood cholesterol. The monounsaturated fatty acids have a neutral effect on blood cholesterol and polyunsaturated fatty acids tend to decrease blood cholesterol levels. Trans -unsaturated fatty acids increase blood cholesterol. Individuals differ in their response to dietary cholesterol – it may increase blood cholesterol or have no effect, and may reduce the natural (endogenous) synthesis of cholesterol by the body.
Milk fat is considered to be a highly saturated fat, with approximately 65% of the fatty acids being saturated. Milk fat typically contains 10% of fatty acids that are 10 carbons or under, 4% lauric acid (12 carbons), 10% myristic acid (14 carbons), 27% palmitic acid (16 carbons), and 14% stearic acid (18 carbons). Milk fat contains approximately 30% monounsaturated and 5% polyunsaturated fatty acids. With respect to blood cholesterol, if only the palmitic, myristic and lauric acids are considered then 41% of milk fat contributes to increased blood cholesterol rather than the total 65% saturated fatty acids.
Consumption of nonfat or lowfat dairy products is recommended for cardiovascular health. For reviews of the scientific literature and more information on dairy products and CHD see the following references: Lovegrove and Jackson (2004), Mensink (2006), Miller et al. (1999), and Reddy and Katan (2004).
Dental caries occur when the enamel on the teeth loses its mineral content (demineralization) resulting in a pitted surface. Saliva protects teeth from caries by maintaining a balanced pH (approx. 7) in the mouth and by a continuous depositing of calcium and phosphorus on the enamel (mineralization). The formation of dental caries occurs when demineralization is greater than mineralization of the enamel. Fermentation of sugars by the bacteria in the mouth results in the formation of acids which then lower the pH (to 5.5) of the mouth and allows for mineral loss from tooth enamel. It has long been recognized that sugar consumption contributes to dental caries.
Milk contains the sugar lactose, but lactose may not have as much effect on dental caries as other sugars. Consumption of milk products, particularly cheese, has been shown in research studies to have a protective effect against dental caries. Although the mechanism by which cheese protects against dental caries is still not clear, milk proteins, calcium and phosphorus all contribute to this effect. Milk protein contains a large amount of calcium and phosphorus, thus increasing the concentration of these minerals in the mouth during the remineralization process. It is also thought that consumption of milk and cheese may have a buffering effect on mouth pH that prevents the pH from dropping to the conditions favorable for demineralization to occur (pH 5.5). Pasteurization of milk does not effect the function of milk protein, calcium, or phosphorus.
Diabetes is a disease of carbohydrate metabolism. Type I diabetes is caused by an inability to produce insulin and is controlled by insulin injections. Type I diabetes is often associated with childhood onset. Type II diabetes is caused by an inability to respond to insulin that typically occurs in adults, and can be controlled through diet and exercise.
There were some studies that suggested that milk consumption contributed to childhood diabetes. However, these data have not been confirmed in larger studies, and more definitive research on the effects of dairy product consumption on Type I diabetes is needed. There is also a lack of studies that directly evaluate the effects of dairy products in Type II diabetes. Several studies have evaluated the effect of whole diets on diabetes and conclude that people with higher dairy consumption have a lower incidence of Type II diabetes. Although the mechanism of this action is still unknown, it is possible that calcium, which also protects against hypertension, is active in protecting against diabetes.
Hypertension is characterized by high blood pressure. High blood pressure increases the risk of coronary heart disease, stroke, and kidney disease. Factors that contribute to high blood pressure include obesity, a sedentary life style, and high intake of sodium in some individuals.
Dietary factors that have been studied and shown to reduce blood pressure are adequate intake of calcium, potassium, and magnesium. Calcium and potassium were identified as important nutrients for the reduction of blood pressure in the 1980s. Milk is a good source of calcium, potassium, and magnesium. Lowfat dairy products have been used successfully in dietary intervention studies as a combined source of these minerals to reduce blood pressure.
Angiotensin-I-converting enzyme (ACE) is an enzyme in the body that regulates fluid and electrolyte balance which affects blood pressure. Inhibition of ACE activity helps control blood pressure. Recent studies have identified bioactive peptides from milk casein and whey proteins that inhibit ACE activity and reduce blood pressure. Milk proteins must be broken down into peptides (short amino acid chains) before they are bioactive, and this is accomplished by enzymes in the digestive tract or during the fermentation of milk.
For reviews of the scientific literature and more information on the benefits of milk calcium, potassium, magnesium, and bioactive peptides for blood pressure control see the following references: Huth et al. (2006), Miller et al. (1999), Miller et al. (2000).
Problems with lactose digestion result in bloating, abdominal cramps, flatulence, and loose stools. Many people consider these symptoms to reflect an intolerance to lactose, but there are several concepts surrounding lactose intolerance that need to be clarified. Lactose is a sugar that is unique to milk and requires the enzyme lactase (ß-galactosidase) for digestion. All humans, with a very small number of exceptions, are born with the ability to generate adequate amounts of the enzyme lactase in their digestive system. Milk is the only food newborns receive so their bodies must be able to digest lactose in order to obtain the energy required for growth and development. As humans age, the ability to produce the enzyme lactase decreases, a condition called lactase nonpersistence. Most people, even into adulthood, maintain some lactase activity in their gut. People that experience the symptoms described above after eating dairy products typically have exceeded the activity of the lactase available in their system and this is called lactose malabsorption. The term lactose intolerance is used to describe the above symptoms in response to a defined amount of lactose consumed, usually tested in a clinical setting.
Another clarification that needs to be made is that fresh milk does not contain lactase. Lactase may be present in dairy products, but it comes from lactic acid bacteria that are either added specifically to milk for fermentation or through airborne or other contamination. It also should be noted that pasteurization does not affect lactose, and pasteurized milk is neither more nor less digestible, nor has a different lactose content than raw milk.
Individuals vary in their ability to digest lactose, the severity of their symptoms, and their perception of their symptoms. Considerable research has been conducted on these topics. There is a greater incidence of lactose malabsorption in adults from countries that traditionally do not have a strong dairy industry. The prevalence of lactose malabsorption is very high (almost 100%) in Asian countries, above 50% in South America and Africa, and is approximately 53% among Mexican Americans and 80% among African American (Vesa et al., 2000).
The issue of lactose intolerance and maldigestion is a very emotional one. Although many people consider themselves to be lactose intolerant, they are often able to digest a limited quantity of lactose. Some people are more sensitive to the discomfort experienced with bloating and abdominal cramps and may perceive the symptoms to be more severe. Symptoms incorrectly attributed to poor digestion of dairy products may be caused by other bowel disorders. Many people may consider themselves lactose intolerant and therefore avoid dairy products, when in fact they could include dairy products in their diets. Long term avoidance of dairy products, due to perceived lactose malabsorption, can result in decreased consumption of calcium, potassium, magnesium and other minerals and vitamins that can lead to problems with bone health, hypertension and other disorders.
There are several strategies available for people with lactose maldigestion to allow them to enjoy dairy products with minimal discomfort. Consuming small quantities of dairy at a time will help to keep the lactose load manageable for the enzymes available. Consuming dairy products with a meal helps because the other foods in the meal will prolong gastric emptying, which means the stomach empties slower and therefore the amount of lactose reaching the small intestine is spread out over a longer period of time, thereby not overloading the lactase enzymes. Continued consumption of small quantities of dairy will not increase lactase activity, but the environment of the digestion system will begin to adapt to the presence of lactose and reduce unpleasant symptoms. Fermented dairy foods contain lactic acid bacteria that have lactase present, providing additional active enzymes to assist with human digestion. Fermented dairy products usually have less lactose present because the lactose has been partially used by the bacteria to produce the desirable flavors and textures of products like yogurt and cheese. There are also lactose-reduced dairy products on the market. Lactase enzyme can also be taken in pill form prior to consuming dairy products to ease digestion.
For reviews of the scientific literature and more information on lactose malabsorption and intolerance see the following references: Miller et al. (1999), Vesa et al. (2000), and Savaiano et al. (2006).
Osteoporosis is a disease that is characterized by decreased bone mass and deterioration of the bone tissue leading to an increased risk of fracture. Factors in osteoporosis include genetics, gender, race, exercise, and diet, particularly during growth years. Each individual has a genetically determined peak bone mass, that is the maximum amount of bone mass that one can possibly have. The higher the bone mass accumulated, the lower the risk of osteoporosis. The majority of bone mass is accumulated by age 20, and continues up to age 30. The size of the skeleton influences the risk of osteoporosis. In general, women are more prone to osteoporosis because they have smaller skeletons than men, and caucasians and Asians have smaller skeletons than people of African decent, who are less prone to osteoporosis. Estrogen has a protective effect on bone loss and women experience accelerated bone loss during early menopausal years because of decreased estrogen production at this time. Weight bearing exercise throughout life increases the load on the skeleton making it stronger and reducing the risk of osteoporosis.
Bone is approximately 50% protein and 50% calcium phosphate. Bone serves as a reservoir for calcium in the body. Blood levels of calcium are tightly regulated to remain at a constant level for proper body function. Bone is constantly undergoing a remodeling process where minerals are removed from bone and added to bone. If dietary intake is insufficient to replace the minerals then bone loss occurs. Bone remodeling is a complex process that involves hormones, protein, calcium, phosphate, vitamin D, and other vitamins and minerals. There is a large variation among individuals with respect to peak bone mass, bone loss, and fracture rates at different sites on the body. Many studies have been conducted to understand this complex behavior, and our knowledge about these relationships continues to change.
An undisputed fact is that accumulation of maximum peak bone mass during childhood and early adulthood is the best protection against osteoporosis. Adequate calcium intake during this time is critical. Osteoporosis can be considered a disease of the elderly but conditions are set in childhood and early adulthood. Adequate calcium intake throughout life provides the minerals needed for bone remodeling and slows or reduces overall bone loss. The most important dietary source of calcium is dairy products. Calcium also may be obtained from other sources, such as leafy greens, fortified products, and supplements. However, fortified dairy products have the benefit of providing vitamin D, which regulates calcium absorption into bones, and other important components of bone metabolism such as phosphorus, protein, magnesium, and zinc. Dairy products contain a larger amount of calcium per serving than other foods. Many studies have shown that people with higher calcium consumption have a lower fracture rate, and often the calcium comes from dairy products. The importance of dairy as a source of calcium has led to dietary recommendations of 3 servings per day.
For reviews of the scientific literature and more information on dairy products and bone health see the following references: Bonjour (2005), Cashman (2006), Heaney (2000), Heaney and Weaver (2006), Huth et al. (2006), Miller et al. (1999), Nicklas (2003), and Prentice (2004).
Probiotics are defined as live bacteria that provide a health benefit when consumed in adequate amounts. Probiotic bacteria can be consumed as powders, capsules, or as part of foods, which may be called “functional foods.” Many probiotic bacteria are lactic acid bacteria, meaning that they use lactose as an energy source, and hence are associated with dairy products. Yogurt is often associated with probiotics because the bacterial cultures used to ferment milk into yogurt are probiotic cultures, and it is a nutritionally complete food for the maintenance of other probiotics that are added for further health benefits. Most yogurt is made with milk that is pasteurized before fermentation and, therefore, the bacteria are alive at consumption. Some yogurts may be pasteurized after fermentation and this would inactive the probiotic organisms. An important goal of the use of probiotics is that the organisms reach their target location in the body alive and in sufficient numbers to be effective. However, the number of bacteria needed for health benefits is unknown in many cases, because the literature on the survival and persistence of organisms in the human body is sparse and further research is needed. The number of live organisms may or may not be stated on functional foods and probiotic supplements, at the discretion of the producer.
The majority of the benefits of probiotics are seen in the digestive system. Probiotics improve the general health of the gastrointestinal tract, reduce the symptoms of lactose intolerance, help in the treatment of diarrhea, inflammatory bowel disease, and irritable bowel syndrome, and help prevent colon cancer. Probiotics also enhance the immune system and help reduce some allergic reactions. There is also an indication that probiotics may play a role in inhibiting Heliobacter pylori infections which cause ulcers, and in maintaining vaginal health.
Many different bacteria have been identified as probiotics. However, the benefits provided are often specific to the general type of bacteria and may be specific even to individual strains within a bacterial type. Streptococcus thermophilus and Lactobacillus bulgaricus are the primary organisms used in yogurt production and produce lactase which helps with lactose digestion in people with lactose malabsorption (intolerance). Lactobacillus species such as L. acidophilus, L. bulgaricus, L. casei, L. rhamnosus, L. reuteri, and Streptococcus thermophilus and Sacchromyces boulardii reduce the duration and severity of diarrhea associated with rotavirus and antibiotic use in children, and traveler's diarrhea. Lactobacillus and Bifidobacteria may help to treat the abdominal pain, flatulence and irregular bowl movements associated with irritable bowel syndrome, and the inflammation associated with inflammatory bowel diseases such as ulcerative colitis and Crohn's disease. The mechanisms behind these benefits are as varied as the diseases, and in many cases are still unknown. Probiotics may help by reestablishing the balance of the intestinal microflora and stimulating the beneficial organisms, changing the pH of the gastrointestinal tract, producing lactase and stimulating other enzyme activity, producing antimicrobial substances such as bacteriocins and organic acids, changing the permeability of the intestinal lining, and stimulating the immune system and antibody responses.
It should be noted that the area of probiotic research is an active one and further research in the areas described above is needed. Clinical studies to conclusively determine the effectiveness of a particular organism on a particular disease are ongoing. For reviews to the current scientific literature and more information on probiotics see: Fondén et al. (2004), Kirjavainen (2004), Parvez et al. (2006), Santosa et al. (2006), and Shanahan (2004) .
Raw milk is often consumed by milk producers on their farms because of convenience and a preference for the taste of raw milk. Although not scientifically proven, people that grow up drinking raw milk often do not have adverse health reactions to raw milk consumption. However, there are many instances of farm visitors and others getting ill from raw milk and products made from raw milk. These are covered in the Disease Outbreaks section of this web site. Raw milk may be used to make cheese that has unique flavors, but raw milk cheeses are required by law to be aged for 60 days to reduce the likelihood of illness from disease causing organisms (pathogens) that may be present in the raw milk.
The risk in drinking raw milk is real, and those that choose to do so should make informed decisions about the risks and benefits of drinking raw milk. Pathogens come from the farm environment and may be present in raw milk, even from healthy cows and in milk produced under sanitary conditions. The types of pathogens and their prevalence in raw milk is discussed in the Microorganisms of Concern section of this web site. While adults in good health may consume raw milk without adverse affects, that same milk may cause severe illness in the young, elderly, and people with compromised immune systems. A particular concern is the consumption of raw milk by children who are not able to make the risk-benefit decision for themselves. There have been several outbreaks of E. coli O157:H7 in children who drank raw milk that resulted in hemolytic uremic syndrome, which causes kidney failure and may lead to life long problems or death. These illnesses could have been prevented.
There is a movement to consume raw milk because of perceived health benefits of raw milk compared to pasteurized milk. It is suggested that raw milk is easier to digest, particularly for people that are lactose intolerant, because enzymes present in raw milk are inactivated by pasteurization. However, as discussed above in the section on lactose intolerance, there is no lactase present in fresh milk. Lactase may be present in lactic acid bacteria that are added purposefully to milk for fermentation or from airborne or other contamination. Enzymes used to digest food are found in the mouth, stomach, and primarily in the intestines of humans – enzymes present in the food play an insignificant role in their digestion.
It is suggested that the pasteurization process changes the nutritional properties of milk fat, protein, lactose, vitamins, and minerals. Typical pasteurization conditions used for milk do not affect the nutritional or functional properties of milk fat, protein, lactose, minerals, and fat soluble vitamins, but there is a small reduction in the quantity of heat sensitive water-soluble vitamins. It should be noted here that milk is an insignificant source of vitamin C, so the small reduction that occurs during pasteurization is not important in the overall diet. In some products, like shelf-stable canned milk, the heat treatments need to be more severe in order to sterilize the milk and there may be changes in the nutritional qualities of these products.
It is suggested that raw milk "sours" and pasteurized milk "rots." Milk sours because of lactic acid bacteria that are present in the milk through airborne or other environmental contamination. These bacteria are not present in fresh milk, except in cows with an udder infection called mastitis. One possible explanation for is that as sanitation improves in milk processing there is less contact of the milk with air and, hence, less lactic acid bacteria are present in pasteurized milk.
It is suggested that raw milk has an antibacterial system that protects against pathogens that is inactivated by pasteurization. Lactoperoxidase is an enzyme present in milk that has antibacterial properties when activated by the addition of hydrogen peroxide, which is not present in milk. This system is discussed in more detail in the Antibacterial Properties of Milk section of this web site.
Some of the topics covered on this page are cited as benefits of drinking raw milk, but are based on studies that were conducted in the 1900s to1950s when dietary and medical research was much less advanced than it is today, and pasteurization was not common. As the U.S. began to industrialize and people moved from farms to the cities, milk processing became more widespread and pasteurization was necessary to reduce health risks and keep milk from spoiling. Pasteurization was not widely used in the U.S. until the 1950s and 1960s. Pasteurization greatly reduced the incidence of milkborne illnesses such as typhoid fever and diarrheal diseases.