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Milk 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. Source Of 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. [1] 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: Cardiovascular disease 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. Bone health 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. [8] 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. [9] 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. [12] For further nutritional guidance for children and adolescents, visit Harvard’s Kid’s Healthy Eating Plate.Weight 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. [13] 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. [14] 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. [15]Diabetes 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.Cancer 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. [18] 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. [19] Bottom line: The health benefits of dairy foods 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