Is milk healthy, or is it not? If the grocery store isles could talk, they would suggest that consumers are moving towards popular dairy alternatives such as almond milk, rice milk, soy milk, cashew milk, hemp milk and coconut milk.

The shift seems to be caused in part by the large amount of the recent popularity of plant-based diets, the prevalence of cow dairy intolerance and recent research on the potential long-term consequences of dairy consumption (1).

Initially, the health benefits of cow dairy seemed to be quite positive for those who could tolerate it. Research supported a positive long-term role of cow dairy in health. To this day, this research is reflected in North American recommendations for daily dairy consumption. A 2016 review of this evidence, suggested that intake of milk and dairy products was associated with (2):

Reduced risk of childhood obesity.
In adults, intake of dairy products was shown to improve body composition and facilitate weight loss during energy restriction.
Intake of milk and dairy products was associated with a neutral or reduced risk of type 2 diabetes and a reduced risk of cardiovascular disease, particularly stroke.
The evidence suggested a beneficial effect of milk and dairy intake on bone mineral density but no association with reduced risk of bone fracture.
Among cancers, milk and dairy intake was inversely associated with colorectal cancer, bladder cancer, gastric cancer, and breast cancer, and not associated with risk of pancreatic cancer, ovarian cancer, or lung cancer, while the evidence for prostate cancer risk was inconsistent.
Consumption of milk and dairy products was not associated with all-cause mortality.

But along with these studies was conflicting evidence. Then came along data from the large data sets from the ‘Nurses’ Health Study, Nurses’ Health Study II, and the Health Professionals Follow-up Study’ which found (3):

Dairy intake was positively associated with risk of mortality such that moderate intake was associated with a slightly lower risk, whereas high intake was associated with an increased risk.
Total dairy intake was associated with higher risk of cancer mortality.
Intake of skimmed or low-fat milk was associated with slightly higher risk of total mortality and cardiovascular mortality, and with a lower risk of colorectal cancer.
Whole milk intake was significantly associated with higher risk of total mortality, cardiovascular mortality, and cancer mortality, including lung cancer, ovarian cancer, and prostate cancer.
Cheese and yogurt intake was not associated with risk of total or cause specific mortality.
Further adjustment for smoking pack years and other dietary factors such as red meat did not appreciably change the results.

These authors also found that:

Substituting one serving per day of total dairy for nuts and legumes was associated with a 14% lower risk of total mortality
Substituting one dairy serving per day for whole grains was associated with an 11% lower risk of total mortality.
Substitution for red and processed meat was also associated with a 5% higher risk of total mortality.

Reflection on the findings in this massive review noted that previous studies may not have taken into account that individuals who chose to drink milk may also have had other health promoting habits which could have affected the data. For example, those who consumed higher amounts of dairy foods also drank less alcohol, were more likely to be physically active and were less likely to be current smokers.

Another recent review article, Milk & Health, published in the New England Journal of Medicine (4), discusses that a number of our original assumption about cow dairy and health are conflicting. Specifically, they detail research which found that:

Cows have been bred to produce higher levels of insulin-like growth factor I (IGF-I), which greatly increases levels of progestins, estrogens, and has been implicated in various cancers (as discussed above) such as breast, endometrial, prostate and other cancer..
That existing data do not support high intakes of milk during adolescence for prevention of fractures later in life and suggest that such intakes may contribute to the high incidence of fractures in countries with the greatest milk consumption.
Findings of prospective cohort studies and randomized trials that do not show clear effects of milk intake on body weight in children or adults.
Intake of dairy products has been associated with a modestly lower risk of type 2 diabetes in some cohort studies. On the other hand, in large meta-analyses, dairy consumption was not associated with or was only weakly associated with lower risk.
Some reports suggest that milk consumption may exacerbate asthma, eczema, and food allergies.

The Milk & Health review article concluded that the current American recommendation to greatly increase consumption of dairy foods to 3 or more servings per day does not appear to be justified. However, the authors make an important point that ‘The optimal intake of milk for an individual person will depend on overall diet quality.’ If diet quality is low, especially for children in low-income environments, dairy foods can improve nutrition, whereas if diet quality is high, increased intake is unlikely to provide substantial benefit and could possibly be causing harm in the long-run.

Cow dairy is preferable to high sugar drink alternatives or poor quality fat alternatives as well as red meat consumption. However, the substitution of nuts, legumes, whole grains and vegetables provide a substantial benefit above and beyond dairy consumption. So telling people to cut out dairy must be considered in a broader context of their overall diet.

Finally, it is important to point out that when consumption of milk is low, the two nutrients of primary concern, calcium and vitamin D can be obtained from other foods or supplements without the potential negative consequences of dairy foods. Alternative dietary sources of calcium include kale, broccoli, tofu, nuts, beans, and fortified orange juice. For vitamin D, supplements can provide adequate intake at far lower cost than fortified milk. Other sources of vitamin D come from cod liver oil, fatty fish and mushrooms or time spent in direct sunlight.

References

Vanga SK, Raghavan V. How well do plant based alternatives fare nutritionally compared to cow's milk?. J Food Sci Technol. 2018;55(1):10–20. doi:10.1007/s13197-017-2915-y
Thorning TK, Raben A, Tholstrup T, Soedamah-Muthu SS, Givens I, Astrup A. Milk and dairy products: good or bad for human health? An assessment of the totality of scientific evidence. Food Nutr Res. 2016;60:32527. Published 2016 Nov 22.
Ding M, Li J, Qi L, et al. Associations of dairy intake with risk of mortality in women and men: three prospective cohort studies. BMJ. 2019;367:l6204.
Willett WC, Ludwig DS. Milk and Health. N Engl J Med. 2020 Feb 13;382(7):644-654.