The milk from cows' mammary glands has long been associated with good health, making it one of the most consumed beverages throughout the US and Europe.
The majority of people worldwide are not able to digest lactose, the sugar in milk, beyond infancy (i.e. when they stop breastfeeding). The ability of a small number of humans to digest lactose beyond infancy is first thought to have evolved in dairy farming communities in central Europe around 7500 years ago.
An estimated 15% of people of northern European descent, 80% of blacks and Hispanics, and up to 100% of Asians and First Nations people do not produce lactase, the enzyme needed to digest lactose. The resulting symptoms of lactose intolerance can include abdominal pain, bloating, gas and flatulence and watery stool.7
Advertising from the dairy industry has brought certain phrases into popular use, such as "Milk: it does a body good" and "Got Milk?" The presence of these slogans in mainstream media has further propelled the notion of milk being a healthful choice.
Milk is a white liquid produced by the mammary glands of mammals. All mammals, including humans, will normally produce milk to feed their offspring, weaning those offspring onto solid food as they get older. Some humans choose to consume milk intended for baby cows, sheep and goats, as well as other animals, often continuing this practice into adulthood. Many more people now choose instead to consume one or more of the numerous "milk alternatives" available, such as soy milk, almond milk, coconut milk, hemp milk and others.
In an attempt to maintain the popularity of cow's milk, manufacturers have created many new products including flavored varieties like strawberry or chocolate, lactose-free milks, milk with added omega-3s, hormone free or organic milks and reduced fat milk.
This MNT Knowledge Center feature will focus solely on cow's milk and is part of a collection of articles on the health benefits of popular foods. If you have an allergy or intolerance to cow's milk, or are considering avoiding cow's milk for ethical or environmental reasons, we have an article covering some of the milk alternatives that you may want to consider.
Possible health benefits of consuming milk
We will look individually at some of the possible health benefits of milk consumption.
Milk and bone health
Cow's milk can be a source of calcium, a mineral that is important in the development and maintenance of healthy bones and teeth.
Adequate calcium and vitamin D alone are not enough to prevent osteoporosis. Regular physical activity and strength training, along with not smoking and eating a diet low in sodium and high in potassium also contribute to overall bone health and a decreased risk of osteoporosis.
There is also some evidence that the acidifying effect of animal proteins in the diet (such as from cow's milk) could have a negative effect on bone health by causing the body to pull calcium from the bones to restore optimal blood pH.10 As such, the net benefit of calcium in cow's milk may be much lower than generally claimed.
Milk and heart health
Cow's milk is a source of potassium, an increased of which has been associated with vasodilation and reduced blood pressure.
An increase in potassium intake along with a decrease in sodium intake is the most important dietary change that a person can make to reduce their risk of cardiovascular disease, according to Mark Houston, M.D., M.S., an associate clinical professor of medicine at Vanderbilt Medical School and director of the Hypertension Institute at St. Thomas Hospital in Tennessee.3
In one study, those who consumed 4069 mg of potassium per day had a 49% lower risk of death from ischemic heart disease compared with those who consumed less potassium (about 1000 mg per day).3
Unfortunately, according to the National Health and Nutrition Examination Survey, fewer than 2% of US adults meet the daily 4700 mg recommendation.3
Potassium-rich foods include cow's milk, oranges, tomatoes, lima beans, spinach, bananas, prunes and yogurt. A dramatic increase in potassium intake can have risks however (including heart problems), so be sure to consult a physician before making major dietary changes or taking potassium supplements.
It should also be noted that cow's milk contains a high amount of saturated fat and cholesterol, which have been linked to an increased risk of heart disease.
Milk and cancer
The risk of dying from colorectal cancer is highest in geographic locations that receive the least amount of sunlight. Some research suggests that one reason for this is that vitamin D might play a role in cell growth regulation and cancer protection.
According to the National Cancer Institute, "research results overall support a relationship between higher intakes of calcium and reduced risks of colorectal cancer, but the results of studies have not always been consistent."2
Some studies have suggested an increased intake of calcium and lactose from dairy products may help to prevent ovarian cancer.2
Milk and depression
Adequate vitamin D levels support the production of serotonin, a hormone associated with mood, appetite and sleep. Vitamin D deficiency has been associated with depression, chronic fatigue and PMS. Cow's milk and other foods are often fortified with vitamin D.
Milk and muscle building
Cow's milk contains protein, which supports muscle growth and repair.
Cow's milk is designed to help baby cow's grow fast, which is why it makes sense that humans who drink milk intended for baby cows can also bulk up quickly. Cow's milk is a rich source of high-quality protein (containing all of the essential amino acids), and whole milk is also a rich source of energy in the form of saturated fat, which can prevent muscle mass being used for energy.
Maintaining a healthy amount of muscle is important for supporting metabolism and contributing to weight loss and weight maintenance, and sufficient dietary protein is needed to preserve or increase lean muscle mass. Protein found in dairy can support muscle growth and repair. According to Today's Dietitian, a recent analysis of over 20 clinical trials suggested that an increased milk intake can boost muscle mass and strength during resistance exercise in both younger and older adults.6
Cow's milk does not seem to significantly help with weight loss. One recent analysis of studies found that increased consumption of cow's milk in the short-term and without calorie restriction had no benefit for weight loss, with only modest benefits seen in long-term studies with energy restriction.11
Recent developments on the possible health benefits of drinking milk
A glass of milk a day' may delay knee osteoarthritis in women - knee osteoarthritis currently has no cure but researchers say drinking milk every day has been linked to reduced progression of the disease. Their research was published in the American College of Rheumatology journal Arthritis Care & Research.
Although some research suggests that women with osteoporosis can benefit from drinking cow's milk, other studies and analyzes have repeatedly shown no benefit. A review published in the journal Pediatrics in 2005 concluded that milk consumption does not improve bone integrity in children.12 In a seven-year study that tracked the diets and physical activity of adolescent girls, researchers concluded that dairy products and calcium did not prevent stress fractures.13
The extensive data collected on more than 72,000 women through the Harvard Nurses' Health Study, found that milk consumption had no protective effect on fracture risk over the 18 years of the study.14
On the next page we look at the nutritional breakdown of milk and possible concerns and precautions associated with drinking milk.