Is Soy Milk Bad for You? 8 Side Effects of Drinking It Every Day

Soy milk and other foods made with soybeans were once hailed as superfoods with no negative effects. But, over the years expert opinions on the safety of soy have seesawed between good for us and "dangerous" for our health.

Experts are now confident that drinking soy milk every day can be a good choice for most people, but could also come with some downsides.

Here are the potential long-term effects of drinking soy milk every day.

"One long-term negative side effect of drinking soy milk every day is that many conventional brands add sugar and other potentially harmful ingredients to improve the taste and consistency," says Anya Rosen, RD, LD, CPT.

Leadings brands of vanilla-flavored soy milk have as much as 14 grams of added sugar per serving. People assigned female at birth (AFAB) should reduce their added sugars to less than 25 grams a day and people assigned male at birth (AMAB) to less than 36 grams per day to avoid a higher risk of chronic diseases like heart disease, kidney disease and certain cancers, according to the American Heart Association.

"Taking in too much added sugar over time can lead to poor blood sugar management, increasing the risk of chronic disease and illness," adds Rosen. When drinking soy milk, read the label carefully to avoid ones with excessive added sugars.

Soy milk has some natural ingredients that slow the digestion of protein and carbs and can decrease the body's ability to absorb other necessary nutrients. These "anti-nutrients" include trypsin inhibitors, lectins and phytic acids, along with indigestible oligosaccharides.

"The phytic acids [in soy milk] can bind to iron, zinc, calcium, magnesium and potassium which then forms a complex that can't be absorbed by the intestine, making them less bioavailable," says Veronica Rouse, RD at The Heart Dietitian.

While soy milk appears to be a good source of calcium with 23% of the daily value in one cup, you won't be absorbing that much. "The calcium found in soy milk is not absorbed as well as the calcium in cow's milk. That being said there's no evidence that eating moderate amounts of soy causes deficiencies," adds Rouse.

The long-term benefits of eating nutrient-rich foods like soy outweigh the negative effects of anti-nutrients found in them, according to the Harvard T.H. Chan School of Public Health.

The thyroid gland regulates metabolism. People AFAB with thyroid problems are often cautioned to avoid foods with soy, but this is debatable.

People AFAB with normal thyroid function should not have any problems drinking soy milk or eating other foods with soy, per an October 2016 study in ​Public Health Nutrition​. People AFAB with borderline abnormal levels of thyroid-stimulating hormone (TSH), on the other hand, have a higher risk of developing thyroid problems from drinking soy milk every day.

If you take a thyroid hormone replacement medication, you should wait 4 hours after you've taken it before drinking soy milk. You should also wait 4 hours after drinking soy milk to take thyroid-supporting supplements like calcium or iron, per the Mayo Clinic.

Isoflavones in soy have been linked to lower levels of inflammation, according to June 2016 research in ​Nutrients​. While inflammation can be short-term and beneficial, such as the healing of a cut or injury, it also has a strong relationship with a number of chronic diseases such as heart disease, cancer and rheumatoid arthritis.

One small study compared inflammatory markers in rheumatoid arthritis patients when drinking soy milk for four weeks versus cow milk for four weeks. Drinking soy milk significantly improved specific markers of inflammation, per the March 2015 study in the ​Nutrition and Dietetics Journal of Dietitians Australia​.

"Incorporating daily soy protein in the form of soy milk can reduce your LDL cholesterol levels," Rouse says. An analysis of 46 studies found that eating 25 grams of soy protein per day was related to a decrease in LDL-cholesterol and total cholesterol, per an April 2019 study in the ​Journal of Nutrition​.

The cholesterol-lowering effect of soy milk is related to a specific part of soy protein (the 7S globulin fraction), which is also found in a variety of other legumes, according to the ​Journal of Nutrition​ study​.​

Soy milk is an excellent source of protein: One cup of has 7 grams, according to the USDA. In fact, it may be a better dairy alternative compared to almond or coconut milk, which can have as little as 1 gram of protein per serving, says Lindsay Martin, RDN, LDN, owner and consultant for Nutribolix.

The Recommended Dietary Allowance (RDA) for protein in adults is 0.8 grams of protein per kilogram of body weight per day. This is the lowest amount estimated to meet the needs of all adults, but depending on age and activity levels, you may need more, per a May 2019 article in ​Nutrients​.

When resistance training, protein needs can be as high as 1.6 grams per kilogram of body weight each day to improve muscle mass, per a March 2018 review in the ​British Journal of Sports Medicine​.

Drinking soy milk every day is a good way to get protein. One to two 8-ounce servings of soy milk per day is considered safe and will give you many of the same nutrients as cow's milk, according to the American Institute for Cancer Research (AICR).

In the past, in-vitro and animal studies have linked soy milk to cancer, so people started to question if drinking soy milk every day is bad for you.

Interestingly enough, human studies have actually shown the opposite effect: that soy was linked to a reduced breast cancer risk, especially in premenopausal people, according to a March 2017 study in ​Cancer​.

Researchers from China analyzed 785 studies involving over 23,000 AFAB and found that those who ate soy foods were 69 percent less likely to develop breast cancer than people AFAB who did not eat soy foods, per a January 2015 meta-analysis in the ​International Journal of Clinical and Experimental Medicine​.

The study also found that diets high in fruits and vegetables and low in fat are significantly associated with a lower risk of breast cancer.

Soybeans are high in tryptophan — an amino acid the body uses to help make melatonin (also known as the "sleep hormone"), according to the USDA. They're also a great source of isoflavones, a phytoestrogen that weakly mimics estrogen in the body.

Estrogen influences sleep duration and quality, and researchers have found that the isoflavones in soy milk may act in the same way. To do this, they surveyed 1076 adults on their soy intake and sleep duration and quality.

Results showed that people who ate the most isoflavones (two or more servings of soy foods daily) had a 64 percent improvement in sleep duration and a 91 percent improvement in sleep quality over the course of a month compared to people who did not eat isoflavones, per the December 2015 study in ​Nutrition Journal.​