You've 'got milk.' Whether it's good for you depends on your age, health and sex, studies say

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(CNN) – Perhaps it's a creamy white comfort drink that's reminiscent of childhood, or just another alternative to water, tea, soda and coffee.

Whatever the reason, more and more Americans appear to be turning to cow's milk during the pandemic.

Sales of cow's milk totaled $4.5 billion for the 20-week period that ended on July 18, an increase of 11.7% compared to the same period last year, according to Nielsen data.

A thrilled dairy industry has jumped on that bandwagon with a relaunch of the iconic "Got Milk?" campaign of yesteryear.

But should you "get milk"? What are the health pros and cons of low- and full-fat dairy? The answer is complicated, often depending on your age, sex and health status.

Weight gain and milk

A recent review by Harvard nutritionists Dr. Walter Willett and Dr. David Ludwig, published in the New England Journal of Medicine, looked at the role of milk in bone health, cancer, weight gain and cardiovascular risk.

When it comes to weight gain, results are mixed. "Overall, the findings of prospective cohort studies and randomized trials do not show clear effects of milk intake on body weight in children or adults," the review concluded.

Of course, there would be benefits to drinking milk in countries where malnutrition is an issue, so the overall quality of a person's diet is key in making recommendations.

The 2020 Dietary Guidelines Advisory Committee, a group of experts charged with providing the US Department of Health and Human Services and the US Department of Agriculture with science-based recommendations, continues to recommend lower-fat dairy options except for the first year of life.

"If meat and dairy foods are included in the dietary pattern, choosing lean cuts and lower fat dairy options is preferred," the committee advised.

Milk and disease

For adults, there are some negative associations between milk and health. Despite the fact that milk is recommended for "strong bones," countries with the highest intakes of milk and calcium have the highest rates of hip fractures, according to the New England Journal of Medicine review. Studies also have shown no clear benefit of calcium in reducing bone fractures.

In regards to cancer, the review found high consumption of dairy foods was associated with an increased risk of prostate cancer and may contribute to endometrial cancer. But it appears to reduce the risk of colorectal cancer.

Any connection between diabetes or cardiovascular disease and milk intake is not clear. Experts say it's often a case of what you eat in place of the food. For example, dairy intake may be lower risk for cardiovascular disease compared to red meat but higher risk when compared to plant-based sources of protein.

Overall, the review concluded that "the health benefit of a high intake of milk products has not been established, and concerns exist about the risks of possible adverse health outcomes," Willett and Ludwig wrote. "Therefore, the role of dairy consumption in human nutrition and disease prevention warrants careful assessment."

The final 2020 through 2025 US nutritional guidelines for adults and children will be released at the end of the year. Until then, experts suggested parents discuss their individual family risk with their health care provider to decide on the role of dairy in their family's diet -- whole or not.

"If you have a very strong family history of cardiovascular disease with heart attacks and things like that, you would want to choose a different dietary path and avoid full-fat dairy more than a family without those risk factors," said pediatric endocrinologist Dr. Tamara Hannon, a member of the American Academy of Pediatrics' Committee on Nutrition, in a prior interview.

Children and milk

When it comes to babies ages 12 to 24 months, parents should rely on whole milk and full-fat dairy, according to current guidelines (PDF) from the American Heart Association, the American Academy of Pediatrics and other major US organizations. Australia (PDF) and the UK have similar guidelines.

Then, if the child is growing well, parents should switch to low-fat dairy products at age 2 to protect children from the risk of obesity and cardiovascular disease, according to existing US guidelines. (The UK, however, does not recommend skim milk for children under 5.)

The fear of obesity is real: Last year, a study by the World Obesity Organization predicted that the number of obese children around the world will grow from 150 million currently to 250 million by 2030.

However, the connection between whole-fat dairy products and weight gain, obesity and cardiovascular disease risk in children has recently been challenged.

A study published in March found whole-fat dairy products were not linked to weight gain, obesity or any measures of cardiovascular disease risk.

Yet another by Canadian researchers found that kids between the ages of 1 and 18 years old who consumed whole milk were 39% less likely to be obese than kids who drank reduced-fat milk. But the cause and effect is not clear, as the study was only observational.

Kids who drink full-fat milk could feel more full, or else there could be the chance that parents who drink lower-fat milk have kids who are bigger, a rationale known as "reverse causality," explained study author Jonathon Maguire, an associate professor of pediatrics at the University of Toronto, in a prior CNN interview.

Maguire and his team will now conduct a randomized, controlled trial with 500 children under the age of 5 over a two-year period to see more directly whether the fat content of the milk they drink is a key factor in determining their weight. The researchers will account for the kids' snacking habits, their starting weights and other factors that could affect weight.

It's important we "understand the health effects of what we do with our children," said Maguire, who led the Canadian study on obesity and milk types. "Parents are making these decisions on a day-to-day basis," and they need evidence.

Jury is still out

Another confounding variable about dairy research is the quality of the studies that have been done.

For example, research may be supported by grants from the dairy industry. While that doesn't necessarily negate the findings, the optics aren't good. Studies have found that when industry sponsors research, a good proportion of the results are favorable to the product.

Many of the studies on milk and dairy are "observational" studies, which is a type of research that can only find an association between two outcomes; it cannot establish cause and effect. Randomized controlled trials -- the gold standard of science -- are expensive and difficult to do in nutrition.

In a March review of full-fat milk and children, for example, only two of the studies were more scientifically based, yet even those results were questionable. In one, researchers asked children in Mexico to switch from full- to low-fat dairy and found there was no difference in their weight at the end of the study.

"But that was because they were eating more tortillas along with the reduced-fat milk, so they were getting the same numbers of calories," Hannon, who was not involved in the study, said.

On the other hand, "whole fat milk may lead to higher satiety, and therefore smaller portions and similar calorie intake," said Christopher Gardner, a professor of medicine who directs nutrition studies at Stanford Prevention Research Center.

"I think the bigger questions are: Are either of them -- whole or reduced fat -- 'good' for you? And what would you be drinking instead of milk?" said Gardner, who was not associated with the study. "If it's milk versus soda, I would pick milk. If it's milk versus water, I'd pick water."

This story was first published on CNN.com, "You've 'got milk.' Whether it's good for you depends on your age, health and sex, studies say."