Milk is the liquid produced by the mammary glands of mammals, including humans. Breast milk is the preferred food for infants, as it is well-tolerated while their digestive tracts develop and mature. Dairy milk may be introduced at later ages if tolerated well. Although dairy milk may come from any mammal, cows, goats, buffalo, and sheep are common producers. This section will focus on dairy milk from cows, and briefly discuss non-dairy plant milk alternatives.
Whole cow’s milk contains about 87% water. The remaining 13% contains protein, fat, carbohydrates, vitamins, and minerals. Processing techniques remove fat to produce lower fat varieties: “reduced fat” contains 2% milkfat, “lowfat” contains 1% milkfat, and “nonfat” or “skim” has virtually no milkfat. Cows are often pregnant while they are milked, so dairy milk contains hormones like insulin-like growth factor-1 (IGF-1), estrogens, and progestins. Some cows are given additional hormones to increase milk production.
Milk and Health
The Dietary Guidelines for Americans recommends including three 8-ounce servings of milk daily (or equal portions of other dairy foods like cheese or yogurt), which is justified to increase calcium intake and reduce the risk of osteoporosis and fractures. Marketing efforts such as the iconic “Got Milk?” campaign with celebrities donning milk mustaches spread this message as well. However, research has not shown a consistent benefit on bone health with high intakes of milk, and furthermore has suggested potential harm with certain conditions like prostate cancer. 
Research on milk often produces contrary findings. Some reasons may be the wide range of different nutritional qualities in milk and how milk intake is measured, as seen in the following factors:
- The amount of milk that is considered a “high” or “low” intake can vary among populations studied. For example, people from Japan tend to drink about less than half of the milk consumed in Western countries 
- Are different classifications of milk included, or just one type? Whole, reduced-fat, fat-free, or organic?
- The composition of milk (fat, protein from varying amino acids) may differ depending on the breed and feed of the cows.
- Are other factors in the diet considered, such as if the participants are eating plenty of fruits and vegetables, or large amounts of processed meat or refined carbohydrates, which can confound the true health effects of milk?
- Different forms of dairy foods, such as cheese, or yogurt, may have health effects different than milk.
The connection of milk and cardiovascular disease (CVD) is unclear. Whole milk contains saturated fat, which is known to increase total cholesterol, raising both LDL “bad” and HDL “good” cholesterol levels. However, the minerals in milk, specifically calcium and potassium may help to control blood pressure.
Comparison of milk with other foods in the diet can affect study results. For example, if comparing milk with high saturated fat meats, milk may show no difference in heart health. However, if dairy fat in milk is compared with unsaturated plant oils, nuts, or seeds, milk may appear to increase CVD risk.
- A study following three large cohorts of men and women found that dairy fat including milk was not associated with overall CVD risk. However, when dairy fat was swapped with an equal amount of calories from polyunsaturated or vegetables oils, there was a 24% and 10% lower risk of CVD, respectively. There was a 6% increase in CVD risk when dairy fat was replaced with other animal fats (e.g., from red meat). 
- Whole milk was associated with 1.5 times the risk of heart disease in women drinking 1-2 glasses or more a day. Drinking skim milk was associated with a lower risk. 
- A meta-analysis of 15 prospective cohort studies found that low-fat and fermented milk were moderately protective of stroke, possibly due to a reduction in blood pressure.  Milk contains potassium, magnesium, and calcium, which helps to regulate blood pressure. However, randomized trials of low-fat milk have not consistently found milk to lower blood pressure.
- A review of 17 prospective studies found that milk intake was not associated with increased risk of mortality (early death) from cardiovascular diseases. There was no association when looking specifically at high-fat versus low-fat dairy products. 
- However, another study following three large cohorts of men and women found that whole milk was associated with higher risks of total mortality including mortality from cardiovascular disease. 
Milk contains nutrients important for bone health: calcium, phosphorus, vitamin D, and protein. However, an association with milk intake and decreased hip fractures has not been made.
Interestingly, the countries with the highest intakes of milk and calcium have the highest hip fracture rates.  Yet it’s unclear that milk alone is responsible because these studies are epidemiological and find associations rather than direct causes. It is known that higher milk intake tends to increase height, and a taller height is strongly related to fractures of the hip and other bones, particularly in men. 
Two meta-analyses of prospective cohort studies did not find an association between milk intake and risk of hip fractures. [10, 11] This result was found even with high milk intakes of 3-4 glasses a day.
Although it is believed that high calcium intakes at preadolescent and adolescent years may protect against bone loss later in life, studies have not supported this. The Recommended Daily Allowance for calcium in the U.S. for preadolescents ages 9-13 years is 1300 mg daily, but the calcium recommendations in other countries such as Japan and the United Kingdom are only on average 750 mg daily. Drinking three servings of milk daily for 18 months in adolescent boy and girls with a low calcium intake did not effect bone mass.  For further nutritional guidance for children and adolescents, visit Harvard’s Kid’s Healthy Eating Plate.
Contrary to widespread belief, research does not support that milk helps with weight control. Although a meta-analysis of 29 randomized controlled trials found that milk and other dairy foods were beneficial for body fat reduction in the short-term and if calories were restricted, no benefits on body weight were seen in the long-term and when calories were not restricted.  A later meta-analysis of 37 randomized controlled trials found similar beneficial effects of dairy intake on body weight and body fat when calories were limited, but without a calorie restriction, weight gain was likely. 
A large study of more than 12,000 adolescents looked to see if total milk helped to prevent weight gain. It found that dairy fat was not associated with weight gain, but a high intake of low-fat milk (more than 3 servings daily) was associated with weight gain and higher body mass index, mainly from the extra calories obtained. 
The association of milk and diabetes is unclear. Dairy foods are associated with a moderately lower risk of type 2 diabetes in cohort studies, but the benefit mainly appears with eating fermented milk products like yogurt rather than from milk. [16, 17] Although the action of yogurt in benefiting diabetes is unclear, it contains helpful bacteria that may reduce inflammation or improve the action of the body’s natural insulin.
Higher milk intake is associated with increased incidence of prostate cancer. The Physicians’ Health Study of 21,660 men found that an intake of more than 2.5 servings of dairy foods daily (compared with a half or less serving daily) was associated with a 12% increased risk prostate cancer. In men who consumed 1 or more servings of milk daily (compared with rarely consumed), skim milk was associated with an increased risk of early stage prostate cancer, and whole milk was associated with fatal advanced prostate cancer. 
A meta-analysis of 111 cohort studies by the World Cancer Research Fund found a decreased risk of colorectal cancer with higher milk intake but mostly in men. This is possibly due to the high calcium content in milk, a mineral found to be protective of colorectal cancer. 
Bottom line: The health benefits of milk appear to be stronger for fermented types like yogurt, which play a role in the gut microbiome. Milk possesses several individual nutrients that can affect blood pressure and bone health, but some of their health-promoting effects may be weakened by whole milk’s high saturated fat content. Although popular media articles have speculated that whole milk is not less healthful than skim milk, research has not supported this statement in regards to diabetes and heart disease, and a high intake of any type of milk can lead to weight gain due to the extra calories.
What is A2 milk?
The name A2 milk may sound like a futuristic space food, but it is a type of milk now sold in some stores. A1 and A2 are two types of gene variants of casein, a protein in cow’s milk. The digestion and breakdown of casein produces a peptide (BCM-7) that has been associated with inflammation, digestive discomfort, and even some chronic diseases.  Some animal studies have shown that A1 milk has 4 times the amount of BCM-7 than A2 milk. Certain cattle breeds possess more of the A1 gene variant than A2, and visa versa. 
Through epidemiological studies and animal studies, a high intake of A1 casein has been associated with increased risk of ischemic heart disease, increased incidence of type 1 diabetes, and some neurological disorders such as autism. 
However, a European Food Safety Authority (EFSA) published a summary report after conducting a review of the literature. Some of their findings:
- The effects of A1 and A2 genes on cardiovascular and neurological diseases and type 1 diabetes were inadequate to make health recommendations.  They noted inconsistent findings and weaknesses in study design that often did not adjust for confounders, included a small number of participants, or had a short intervention period.
- Fresh raw (unprocessed) milk from healthy cows does not contain BCM-7, and that in theory the fermentation process involved when producing fermented milks and cheeses may break down BCM-7. However little to no data are available on the actual BCM-7 content of fermented milk products.
A2 milk distributers market their product as beneficial for adults, children, and infants who experience milk intolerances, whether from milk allergy or lactose intolerance. Symptoms of these conditions include gas, bloating, constipation, or diarrhea. However, it is noted that the majority of clinical trials comparing A1 with A2 milk, with findings showing improved tolerance and less gastrointestinal side effects with A2 milk, are funded by the A2 Corporation, producer of the brand “a2 Milk” that is sold worldwide.
Bottom line: although milk with the A2 gene variant may be metabolized differently in the body than A1 milk, there is not enough evidence from research to show that these differences significantly affect health. If one has lactose intolerance or a milk allergy causing unpleasant symptoms, A2 milk may be tried. Plant-based milks are another option as they do not contain lactose or the milk protein casein.
Is raw milk safe to drink?
Although fans of raw milk believe it tastes better and offers more nutrients than processed milk, raw milk is not pasteurized and may harbor harmful bacteria including Salmonella, Listeria and E. coli. Currently, twenty states prohibit the sale of raw milk. According to the Centers for Disease Control and Prevention, unpasteurized milk is 150 times more likely to cause foodborne illness than illnesses related to pasteurized dairy products. Between 1993 and 2006 more than 1,500 people in the U.S. became ill from drinking raw milk or eating cheese made from raw milk.  Raw milk and soft cheeses made from raw milk like Brie, Camembert, Queso Blanco, and Queso Fresco are especially dangerous for older adults, those with weakened immune systems, pregnant women, and children. In pregnant women, Listeria can cause miscarriage, or illness or death of a fetus or newborn.
For Your Health and the Planet’s Health
The production of dairy foods places considerable demand on land, water, and other natural resources, and dairy-producing ruminant animals like cattle, sheep, and goats generate methane—a powerful greenhouse gas. In identifying a dietary pattern both healthy for people and sustainable for the planet, the “planetary health diet” sets the target for dairy foods at 250 grams per day (with a range of 0 to 500 grams per day), which is about one (8-ounce) cup of milk.
Milk is often sold in cartons or opaque containers because too much exposure to light can cause a loss of vitamin A and B2. Choose a carton with the latest sell-by or use-by date (indicating it is the freshest). Most milk sold in supermarkets is pasteurized and homogenized, processing techniques that use heat to kill most of the bacteria present and break down fat molecules so that texture of milk remains smooth and creamy.
Plant-based milks contain no lactose so may be better tolerated than dairy milk in some people. Also, plant-based milk has no cholesterol, and most have little saturated fat. However the nutritional content varies widely, so be sure to read the food label to ensure you are getting the desired nutrients. This chart compares dairy milks to a sample of plant-based milks. As you will see, nutrients vary across types of plant-based milks, as well as among different brands selling similar options. Specific brands may contain more or less of the nutrients depending on if products are fortified, or if added flavors or sweeteners are included. Generally, plant-milks labeled “original” will include added sugars, so look for unsweetened options and be sure to check the Nutrition Facts label and ingredients list so you know what you’re buying.
|Type of Milk
[all entries for 1 cup
(8 fluid oz/240 mL)]
|Sugar (g)||Fat (g)||Calc. (mg)||Pot.
|Cow||Whole milk (3.5% fat)||149||7.5||12/0||4.5||2||0.5||276||322||0|
|Low fat milk (1% fat)||102||8||12.5/0||1.5||0.7||0.1||305||366||0|
*Some sugars are created in the processing of oats to make oat milk, which are listed as “added sugars” even if no other sweeteners are added.
Milk requires refrigeration at a temperature below 40 F. If it has been stored at room temperature for two hours or longer, it is recommended to discard it. Although pasteurization kills much of the bacteria in milk, any remaining bacteria can grow quickly in milk at room temperature or warmer. Once milk is opened, it will last about 3-5 days after the sell-by date on the label. Spoiled milk has a strong, sour odor and lumpy texture caused by excess bacteria producing lactic acid, which curdles the protein in milk and produces off odors.
Store milk towards the rear of the refrigerator rather than the front or side shelf door, where the temperature varies the most. Don’t forget to close the carton or recap the bottle to prevent the milk from absorbing the odors and flavors of other foods in the refrigerator.
Lactose-free milk undergoes pasteurization and the addition of an enzyme lactase, which breaks down the milk sugar lactose, so it generally lasts longer than regular milk. If refrigerated properly, lactose-free milk can last about 7 days after the sell-by date once it is opened.
Milk is not just for drinking by the glass or splashing onto cold cereal. Adding milk to foods can boost one’s intake of calcium, vitamin D, protein, and other nutrients.
- Blend 1 cup of milk, 1/2 cup of fresh or frozen berries, and 1 small banana for an easy breakfast drink or snack.
- Overnight oats. In a mason jar (you can also reuse a clean jelly or salsa jar), add 1 cup of milk, 1/2 cup of rolled oats, 1 tablespoon chia seeds, 3 tablespoons of chopped nuts, ½ sliced banana, and ¼ cup fresh or frozen berries. Secure the lid and shake the jar well until all the ingredients are mixed. Refrigerate overnight.
- Hot oatmeal. Cook old-fashioned rolled oats in milk (the ratio is generally ½ cup oats to 1 cup milk).
Did You Know?
Several other animals produce milk including sheep, goat, and yaks. These types of milk are more popular in European, Middle Eastern, and Asian countries than in the U.S. Sheep’s milk can be made into various cheeses like feta and ricotta, and goat’s milk produces a popular cheese called goat cheese or chevre.
How do they compare nutritionally? Sheep, goat, and yak milks contain about the same if not more calcium than cow’s milk. The amount of protein and carbohydrate are about the same. They all contain some lactose but less than found in cow’s milk, so they may be easier to digest for people with lactose sensitivity.
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