Generated Summary
This review article from the New England Journal of Medicine examines the role of dairy consumption in human nutrition and disease prevention. The authors review the composition of dairy products, their impact on growth and development, bone health, body weight and obesity, and their relationship to various diseases such as cardiovascular disease, diabetes, and cancer. The article also discusses the environmental effects of dairy production and the implications for public health recommendations. The study utilizes a review of existing literature, including prospective cohort studies, meta-analyses, and randomized controlled trials, to assess the benefits and risks associated with dairy consumption. The authors evaluate the current recommendations for dairy intake and discuss the need for a more nuanced understanding of dairy’s role in a healthy diet, considering factors like overall diet quality and the potential adverse effects of high dairy consumption.
Key Findings & Statistics
- The recommended intake of milk or equivalent dairy products in the United States is three 8-oz (237 ml) servings per day for adults and children 9 years or older, an amount that is substantially higher than the current average intake among adults of 1.6 servings per day.
- Countries with the highest intakes of milk and calcium tend to have the highest rates of hip fractures.
- In randomized trials that used bone mineral density as a surrogate for fracture risk, calcium supplements of 1000 to 2000 mg per day resulted in 1 to 3% greater bone mineral density than placebo.
- Among men, milk intake during adolescence was linearly associated with a 9% greater risk of hip fracture later in life for every additional glass consumed per day.
- In a meta-analysis of 29 randomized trials, no overall effects of milk or other dairy foods on body weight were seen.
- In a 1976 study in the United Kingdom, 581 elementary-school children living in a low-income area were randomly assigned to receive either school lunch with milk (about 7 oz, presumably of whole milk) or lunch without milk. After 21 months, no significant difference in weight was seen.
- In a recent trial designed to examine the effects of dairy calcium, 274 adolescent girls with BMIs ranging from the 50th to the 98th percentile were randomly assigned either to a group that received an intervention that substantially increased their consumption of low-fat dairy products (from 0.6 servings per day to 3.2 servings per day) or to a control group in which consumption was to remain constant (and increased only from 0.6 servings per day to 0.7 servings per day). Contrary to the hypothesis, no effect on amounts of body fat was reported after 1 year.
- In a meta-analysis that included 29 cohort studies, intakes of milk (total, high-fat, and low-fat) or total intakes of dairy foods were not associated with overall mortality.
- Whole milk was associated with higher total mortality, but consumption of low-fat milk and cheese was not.
- Consumption of dairy foods was associated with lower mortality than consumption of processed red meat and eggs, with similar mortality to consumption of unprocessed red meat, poultry, and fish, and with significantly higher mortality than consumption of plant-based sources of protein.
Other Important Findings
- Milk contains all essential nutrients and multiple anabolic hormones.
- Milk processing, including pasteurization and fermentation, has potential health implications.
- Normal growth and development can be obtained throughout childhood without dairy products if attention is given to diet quality.
- Cow’s milk contains substantial amounts of branched-chain amino acids (leucine, isoleucine, and valine), which are key to protein quality.
- High lifelong milk consumption has been linked to meet calcium requirements for bone health.
- Total calcium intakes ranging from less than 555 mg per day to more than 1100 mg per day were unrelated to the risk of hip fracture.
- Milk consumption increases attained height.
- Consumption of low-fat milk was positively associated with gain in body-mass index (BMI).
- In three cohorts of young children, consumption of full-fat or 2%-fat milk was associated with lower BMI or lower risk of obesity than was consumption of low-fat or skim milk.
- Consumption of organic milk may have slightly higher amounts of n-3 polyunsaturated fatty acids and beta carotene than conventional milk.
- Consumption of dairy products is strongly correlated with rates of breast cancer, prostate cancer, and other cancers.
- Milk consumption is most consistently associated with a greater risk of prostate cancer.
- Milk consumption may exacerbate atopic tendencies.
Limitations Noted in the Document
- Balance studies, used to determine calcium requirements, have limitations including short duration and high habitual calcium intakes.
- Trials assessing bone mineral density as a surrogate for fracture risk are limited by transient effects and the need for long-term studies.
- Cross-sectional studies cannot establish causality.
- Studies on milk and body weight are subject to confounding and reverse causation.
- The reported health effects of dairy foods depend strongly on the specific foods or beverages to which they are compared.
- Many studies have been initiated among persons in midlife or later, whereas many cancer risk factors operate in childhood or early adult life.
Conclusion
The review concludes that the current recommendations to greatly increase dairy consumption are not justified. While dairy products contain essential nutrients, these can also be obtained from alternative sources. For adults, there is a lack of evidence supporting high dairy consumption for fracture reduction, and total dairy consumption has not been clearly related to weight control, diabetes, or cardiovascular disease risks. Dairy consumption may increase the risks of certain cancers. The health effects of dairy depend on what it’s compared to. Reduced-fat dairy does not have clear advantages over whole milk. The high nutrient density of milk can be beneficial where diet quality and energy intake are compromised, but in populations with generally adequate nutrition, high milk consumption might increase fracture risk and is linked to greater height and cancer risks. The article suggests that the optimal milk intake depends on overall diet quality, and if diet quality is low, milk can improve nutrition, whereas in a high-quality diet, increased intake offers no benefits and may be harmful. They suggest that guidelines should designate an acceptable intake, deemphasize reduced-fat milk, and discourage the consumption of sugar-sweetened dairy products in populations with high rates of overweight and obesity. In conclusion, the review highlights that the benefits of milk consumption are often overemphasized, and a more cautious approach to dairy intake is warranted, especially given the availability of alternative sources of essential nutrients.