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Secret Side Effects of Drinking Smoothies, Says Science

You may think your smoothies are always healthy, but research shows there are some unwanted side effects.
FACT CHECKED BY Faye Brennan

Sometimes drinking a smoothie is all you need for motivation before a morning workout. It's a reliable way to pack in some fruits, veggies, and your favorite nut butter, plus they're quick and easy to make. While smoothies can help you in many ways, like adding more vitamins and protein to your daily life, they also come with some potential side effects as well. 

Although it's not fun being the bearer of bad news, it's time we look at some of the potential negative side effects of drinking smoothies. Read on to find out how smoothies may affect our health, and if you're curious about trying other types of food before your workout, check out our list of the 6 Best Pre-Workout Foods, According to Experts

1

You might weaken your teeth

young man making smoothie in modern kitchen
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Fruit is a healthy part of any balanced diet. But the acidity found in fruit, especially in a smoothie that is consumed as a meal replacement, can actually weaken our teeth over time. 

Contact with too many naturally occurring acids, like the ones found in fruit, has been proven to cause dental erosion, which can lead to things like an increase in tooth sensitivity and pain, as well as loss of surface area. A 2013 study published in the British Dental Journal found that smoothies containing fruits like strawberries, bananas, cranberries, blueberries, and cherries led to an increase in the erosion of surface hardness on the tooth samples.

This research also suggests that smoothies containing dairy in the form of milk or yogurt are healthier for our dental health because of the higher levels of calcium, phosphates, and gut-healthy probiotics. 

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2

You might increase your risk of kidney stones and inflammation

woman pouring smoothie from blender into cup
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Another possible side effect of drinking smoothies is the formation of oxalate crystals in your body, which can lead to kidney problems. Oxalic acid is a compound found in many different foods, but is especially high in certain leafy green vegetables like spinach. Although this is a natural compound, it can lead to the formation of kidney stones if consumed too frequently. Not only is oxalic acid known to cause kidney stones, but research shows that it can also lead to inflammation, chronic pain, increased fibromyalgia pain, and gut hormone imbalance.

If you want to keep enjoying your smoothies but want to avoid too many oxalates, try using less spinach or swapping it out entirely. A 2015 study in the Nutrition and Food Science Journal created two green juice combinations that were similar in everything except the amount of spinach. They found that the green juice with less spinach came back with significantly fewer oxalates.

Related: 12 Vegetables That Become Healthier When They're Cooked

3

You might decrease the effectiveness of  your medication

chia flax seeds on top of green smoothie
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Certain leafy greens that are popular choices for smoothies, like kale or spinach, are often high in vitamin K. This vitamin is crucial for our body's production of blood clots that allow our wounds to heal. However, for people who have an issue with too many blood clots or need to take blood thinners for other reasons, too much vitamin K can be an issue. 

According to a report published in Nutrition Reviews, consuming between 700-1500 micrograms of vitamin K from one serving of vegetables can pose an issue for those taking blood thinners. To put this in perspective, just 1/2 cup of kale is already around 531 micrograms. If you're using kale as your main vegetable in your green smoothie, you may want to monitor the amount you're using. 

The Cleveland Clinic also points out that while you may want to watch your intake of vitamin K, you don't want to cut it out of your diet completely. It's still a necessary part of a healthy diet and is a nutrient our bodies need in order to heal from injury properly.

Related: What Happens to Your Body When You Eat Kale

4

You might slow the function of your thyroid

smoothies with juice
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Our thyroid gland controls more than we may realize. It produces the hormones that regulate our energy, which, in turn, affects our brain development, digestion, heart health, and many other important functions. In order for our thyroid to function properly and produce these hormones, it needs adequate levels of iodine. 

Studies have shown that cruciferous vegetables (think kale, broccoli, cabbage, etc.) can actually block the production of thyroid hormones with a natural compound called glucosinolate. These are known to lower the intake of iodine in the thyroid and therefore potentially lead to thyroid disorders. By drinking smoothies packed with kale, we could run the risk of interrupting the function of our thyroid gland.

If you're a green smoothie lover, it may be important to monitor your consumption of kale and note how it may be affecting the way your body feels over time.

RELATED: Signs Your Thyroid is Out of Whack, According to a Doctor

5

You might ingest some toxic metals

Green breakfast smoothie for weight loss avocado pear brazilnut spinach
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Before you go stuffing your blender to the brim with kale, you may want to read about a toxic metal found in soil called thallium. This heavy metal is commonly found in soil where there's been nearby industrial activity like power plants or coal-burning. Thallium prefers to latch on to growing cruciferous veggies like broccoli, cauliflower, bok choy, and most importantly for green smoothie lovers: kale. 

According to the Journal of Environmental Geochemistry and Health, consuming thallium on a regular basis, even in trace amounts from a daily green smoothie, can potentially pose a threat to our health over time, especially in our nervous systems. This doesn't mean we need to throw out our beloved kale, but it may encourage us to shop organic when we can. There's a lot of current research that supports sticking to organic vegetables because organic farms often use soil rich in carbon, which can stop the transfer of thallium to our favorite green veggies. 

If you want to learn how to eat healthy on our budget, read 5 Easy Hacks to Make Healthy Eating More Affordable.