We’ve all heard that milk "does a body good," but is that true, or just a catchy marketing slogan? The U.S. Department of Agriculture (USDA) considers dairy an essential food group because of its high calcium levels and recommends adults eat or drink three cups a day.

People are drinking less milk than they used to. According to an NPR article, the average American drinks 18 gallons a year, down from around 30 gallons in the 1970s. The decrease may be due to the expansion of alternative beverage options such as plant-based milks, sports drinks, and vitamin water. Consumers simply have more choices and may be choosing flashier, trendier drinks.

A 2019 survey focusing on consumers’ attitudes and behaviors when it comes to milk and plant-based alternatives found that 62 percent of respondents consumed only dairy milk in the previous six months, while 26 percent consumed both dairy and plant-based milks. Among dairy milk drinkers, the reasons most often given for drinking milk were its high calcium content (61 percent), good taste (60 percent), and overall nutritiousness (54 percent).

You may experience a lot of digestive problems

According to a 2013 paper published in the European Review for Medical and Pharmacological Sciences, roughly 75 percent of the world’s population will become lactose intolerant at some point in their lives. This means that their bodies will no longer be able to produce enough lactase, the enzyme needed to break down and digest lactose, the sugar found in milk and other dairy products. The undigested lactose can cause a range of unpleasant symptoms, including pain, gas, bloating, and diarrhea. The severity of symptoms can vary significantly from person to person, depending on how much lactase an individual can produce and how much lactose they consumed. Lactose makes up approximately five percent of milk, although it may be more or less concentrated in other dairy products.

But just because you’re lactose intolerant doesn’t mean you have to give up drinking milk. One easy option is lactose-free milk. To create this product, manufacturers add the lactase enzyme to regular milk. The lactase breaks down the lactose, creating a finished product that doesn’t contain any of the difficult-to-digest milk sugar. Another option is taking Lactaid pills, which contain lactase, before drinking milk.

You may lose weight

Although a glass of milk doesn’t exactly scream "diet food," research suggests that regularly drinking milk may promote weight loss. One 2011 study, for instance, published in Nutrition, Metabolism and Cardiovascular Diseases, found that participants consuming a high-milk diet (three servings of low-fat milk daily for eight weeks) lost significantly more weight than those on the soy milk, calcium supplement, or control diets. The milk drinkers lost an average of 4.43 kg (9.77 lbs), while the control group lost only 2.87 kg (6.33 lbs).

One reason that milk may help you lose weight is that it appears to help suppress hunger cues. A Cambridge University Press study published in 2010 tracked obese women for six months as they lost weight on a calorie-restricted diet. Half the participants received a milk supplement; the other half received a placebo. The researchers found that those who consumed milk felt less hunger and experienced "a smaller increase in desire to eat" during their diet. They also had a smaller decrease in feelings of fullness while on the diet than the control group.

It’s unclear why milk may help people lose weight, but it could be linked to its calcium and vitamin D content (via WebMD).

You’ll protect the health of your bones

A lot of people assume milk is important for kids because it helps build strong bones, but milk protects the health of your skeleton at any age. Although we often think of bones as unchanging once we reach adulthood, they’re actually a dynamic tissue that’s constantly being broken down and rebuilt. As you age, however, the rate at which bone is broken down starts to outpace the rate at which new bone is formed.

Milk is a rich source of calcium. The National Institutes of Health (NIH) noted that 99 percent of the body’s calcium is found in bones and teeth. In addition to providing structural support, "the body uses bone tissue as a reservoir for and source of calcium, to maintain constant concentrations of calcium in blood, muscle, and intercellular fluids." The NIH recommends that adults get around 1,000 mg of calcium a day, depending on age, sex, and pregnancy or lactation status. A cup of milk contains 276–299 mg, depending on fat content. Milk is also often fortified with vitamin D, which is needed for proper bone development and remodeling.

You’ll be getting a lot of high-quality protein

Drinking milk every day is a great way to increase your protein intake, especially for individuals who don’t eat meat. According to Healthline, a cup of milk contains 7.7 grams of protein, 80 percent of which is casein, and 20 percent of which is whey. Both are easy to digest and contain all essential amino acids, making them complete proteins. In fact, these proteins are often isolated and used as protein powders and supplements.

In an interview with Harvard Medical School, registered dietitian Nancy Rodriguez explained that protein is essential for growing and maintaining muscle. It’s also needed "to make hair, blood, connective tissue, antibodies, enzymes, and more." In addition, the proteins in milk possess unique characteristics that make them particularly beneficial. As Healthline noted, casein increases the absorption of certain minerals, including the calcium that gives milk its good reputation. Whey is an excellent source of branched-chain amino acids (BCAAs), which can reduce muscle soreness and exercise fatigue.

The recommended dietary allowance (RDA) for protein is expressed as a formula: 0.8 grams per kilogram of body weight. Rodriguez and other health professionals, however, suggest that getting up to twice the RDA is a "safe and good range to aim for."

You may or may not experience congestion

There’s a lot of debate when it comes to whether or not drinking milk causes increased or thickened mucus. One 2005 paper published in the Journal of the American College of Nutrition found that among individuals with the common cold, "milk intake was not associated with increased nasal secretions, symptoms of cough, nose symptoms or congestion." But people who assumed milk would increase mucus production reported more respiratory symptoms after they drank milk. So even if milk doesn’t actually affect mucus production, it may change your subjective experience of a cold if you’re a "believer." The paper’s authors noted, however, that those with a true milk allergy can experience increased mucus production because of activation of their immune system after drinking milk.

A 2018 study found that participants on a dairy-free diet reported significantly less nasal congestion than those consuming dairy. However, it’s important to remark that these were subjective evaluations and may not have corresponded to an objective increase in mucus production. Why would there be a discrepancy between how someone feels and what’s actually happening? According to VeryWell Health, one possibility is that "milk coats the mucus, making it feel thicker."

You may get acne breakouts and other skin problems

If you’re one of the 12 to 22 percent of American women living with adult acne, drinking milk every day may not be doing your skin any favors. The Mayo Clinic cited four underlying causes of acne: excess oil production, clogged hair follicles, bacteria, and too many androgens (male sex hormones). Although milk (or any other food) doesn’t directly cause acne, certain dietary factors may trigger or worsen a breakout.

It isn’t clear why milk and other dairy products encourage acne flare-ups in some individuals. Healthline highlighted several theories, including a possible connection between the hormones given to dairy cows and our own delicate hormonal balance. The growth hormones naturally present in milk might also be to blame. A third possibility is that dairy, combined with refined carbohydrates, may increase insulin levels and "make skin more prone to acne." One 2014 study found that nonfat milk was more closely associated with acne than other dairy products.

Even if milk isn’t aggravating your acne, it may be causing other skin problems. As registered dietitian Trista Best told Health Digest, "Sensitivities to dairy can also express themselves on the skin through rashes, dry patches, and uneven skin tone."

You may reduce your risk for type 2 diabetes

Swapping out soda for a glass of milk could decrease your chances of getting type 2 diabetes. A 2013 paper noted that individuals with metabolic syndrome were five times more likely to have type 2 diabetes. Metabolic syndrome encompasses a collection of interconnected conditions that include high blood sugar, high cholesterol, high blood pressure, and obesity. The paper’s authors found that the proteins in milk can reduce these individual metabolic dysfunctions both directly and indirectly. Casein and whey, for instance, stimulate increased insulin production, which lowers blood sugar levels. The proteins in milk can also reduce the number of triglycerides in the blood after a meal and lower blood pressure. Milk proteins also help control appetite and increase exercise performance, both of which may contribute to losing weight. The researchers concluded that, taken together, these effects reduce the likelihood of an individual getting type 2 diabetes.

According to the American Diabetes Association, approximately 34.2 million Americans have diabetes, with 1.5 million new cases diagnosed each year. Diabetes can lead to a number of serious health complications and is the "seventh leading cause of death in the United States."

Your arthritis may get worse

According to Dr. Frank Hu, a professor of epidemiology and nutrition at the Harvard T. H. Chan School of Public Health, researchers can’t agree on whether or not milk and other dairy products are inflammatory. He noted, for example, that one 2015 study published in The Journal of Nutrition concluded that dairy products cause low-grade inflammation. But a 2017 meta-analysis of 52 studies found that dairy "generally has anti-inflammatory effects, except in people allergic to cow’s milk."

If dairy products are inflammatory, drinking milk every day isn’t a good choice for the nearly one in three Americans between the ages of 18 and 64 living with arthritis. Among individuals with arthritis, the Centers for Disease Control and Prevention (CDC) reported that half have persistent joint pain and one in four have "severe joint pain."

But inflammation isn’t always a bad thing. Acute inflammation occurs immediately after an injury and produces warmth, redness, swelling, and pain. This brings white blood cells to the area, where they can begin the healing process. But if the inflammatory response becomes chronic, the body becomes confused and begins attacking healthy tissue. Chronic, low-grade inflammation is believed to cause or worsen a number of conditions, including arthritis.

Your blood pressure may improve

Hypertension (high blood pressure) is a big problem in the United States. According to the Centers for Disease Control and Prevention (CDC), 108 million Americans have high blood pressure, but only 24 percent of these individuals have the condition under control. High blood pressure is a major risk factor for heart disease and stroke, which claim the lives of hundreds of thousands of Americans each year.

Although discussions about lowering blood pressure through diet tend to focus on salt intake, drinking milk may also be helpful. Milk’s power to decrease blood pressure may come down to its mineral content. Staci Nix McIntosh, a registered dietitian and assistant professor at the University of Utah, explained: "Calcium, magnesium and potassium, in particular, are nutrients that help lower, or maintain, a low blood pressure." She pointed out that although milk is known for its calcium content, it’s also a solid source of the other two minerals.

Milk proteins may also be helpful. According to a 2010 study published in Obesity, individuals who received either casein or whey supplements had significantly lower blood pressure (both systolic and diastolic) at the end of the 12-week study compared to the control group.

Your heart health may get a boost

In addition to lowering blood pressure, certain types of milk may benefit your heart in other ways. A 2018 study published in the European Journal of Clinical Nutrition found that drinking whole milk significantly increased high-density lipoprotein (HDL, "good") cholesterol compared to nonfat milk. Drinking half a liter a day of either whole or nonfat milk didn’t increase low-density lipoprotein (LDL, "bad") cholesterol or triglycerides.

Milk from grass-fed cows may also be particularly good for your heart because it contains high levels of conjugated linoleic acid (CLA). CLA is a type of omega-6 fat found in all beef and dairy products, but it’s 300-500 percent higher in animals fed grass rather than grain. According to a 2018 study, CLA may lower individuals’ risk for heart failure. The study followed more than 3,800 older men for an average of 13 years and found that those with higher levels of CLA in their blood were less likely to experience heart failure. Interestingly, the study authors noted that "elevated CLA% was associated with reduced HF [heart failure] risk only in those with higher dairy fat intake." So simply taking a CLA supplement may not have the same effect.

You may decrease your risk for certain cancers, but increase your risk for others

Milk may be a mixed bag when it comes to cancer risk. A 2014 meta-analysis published in The American Journal of Clinical Nutrition noted that several studies had shown an inverse relationship between dairy consumption and risk for colorectal cancer among both men and women. When it comes to bladder cancer, the paper’s authors noted that previous research had been mixed, with some studies suggesting consuming dairy decreases risk and others showing that it has no impact. Mixed results have also been found when it comes to prostate cancer. Dairy doesn’t appear to affect the risk for ovarian cancer, while low-fat milk specifically has been associated with an increased risk of breast cancer in premenopausal women.

According to the National Cancer Institute, approximately 39.5 percent of Americans will be diagnosed with some form of cancer during their lifetime. The cancers that may be affected by milk consumption are among the most prevalent. Breast cancer is the most common form of cancer, while prostate cancer is third, colorectal is fourth, bladder is sixth, and ovarian isn’t in the top 10.

Your mental health may be affected

Should milk’s tagline be "it does a brain good" instead? Milk and other dairy products may have a significant effect on our psychological health and cognitive abilities. In an interview with The Seattle Times, registered dietitian Susan Kleiner noted that the whey protein in milk can "reduce physical responses to stress, improve mood, and enhance memory."

But not everyone agrees, and effects may vary based on the type of milk we drink. A 2010 study published in the Asia Pacific Journal of Clinical Nutrition concluded that "low-fat dairy may have beneficial effects on social functioning, stress and memory, while whole-fat dairy may be associated with poorer psychological well-being." The authors found that higher intake of whole-fat dairy products was associated with "increased depression, anxiety, stress, cognitive failures, poorer memory functioning and general health." It’s important to note, however, that this group of dairy products included things like ice cream that are quite different from a glass of straight milk, nutritionally speaking.

A meta-analysis published in 2020 in Critical Reviews in Food Science and Nutrition attempted to determine if there was a link between dairy and depression, but the studies they examined offered inconsistent and conflicting results.

You could get ‘addicted’ to milk

Milk isn’t the first thing that comes to mind when you think about addictive substances, but it contains compounds that stimulate the same reward pathways in the brain that light up during drug use. In a 2017 interview with Forbes, Dr. Neal Barnard discussed the drug-like qualities of casomorphins, protein fragments created when the body digests the casein protein found in milk and other dairy products. Casomorphins can cross the blood-brain barrier and are a type of natural opiate. According to Dr. Barnard, "These opiates attach to the same brain receptors that heroin and morphine attach to. They are not strong enough to get you arrested, but they are just strong enough to keep you coming back for more." About 80 percent of the 7.7 grams of protein in eight ounces of milk is casein, although other dairy products contain even higher levels.

A 2015 study published in PLOS One confirmed the addictive quality of dairy. The experiment explored the addictiveness of different foods based on participants’ rankings and responses on the Yale Food Addiction Scale. Although the researchers didn’t test plain milk, three dairy-heavy foods (ice cream, pizza, and cheeseburgers) scored in the top 10.

You could experience potentially life-threatening symptoms if you have a milk allergy

Although many people mislabel their lactose intolerance as a milk allergy, the two are very different. According to the Mayo Clinic, people with a milk allergy have an abnormal reaction to one or both of the proteins in dairy products, casein and whey. Eating dairy triggers an immune response that can cause hives, swelling in and around the mouth, shortness of breath, gastrointestinal distress, and, in severe cases, anaphylaxis and even death. Lactose intolerance, on the other hand, is caused by the body’s inability to digest the sugar (lactose) in milk and "doesn’t involve the immune system."

The American College of Asthma, Allergy & Immunology reported that "between 2% and 3% of children younger than 3 years old are allergic to milk," and about 20 percent will not outgrow the allergy by adulthood. Some allergic individuals may be able to eat foods that contain a small amount of milk that’s been heated to a high temperature for a long enough period of time (such as a muffin) because the heat partially breaks down the milk proteins. But most people with a milk allergy must entirely avoid milk, dairy products, and foods containing casein or whey derivatives.