Green smoothies: Not necessarily a “health food”

In the health community, the green smoothie has become the poster child for healthy eating. If you are a green smoothie fan, your typical recipe probably looks something like this:

  • 1 apple, peeled, cored, and chopped
  • 8 ounces unsweetened almond milk
  • 2 cups stemmed and chopped spinach or kale
  • 1 cup broccoli

This green smoothie is full of veggies—spinach, kale, and broccoli—so it must be healthy, right? Well, not necessarily. While cruciferous vegetables and leafy greens certainly have health benefits, consuming large amounts of them in green smoothies may not be healthy in the long run, for several significant reasons.

  1. Cruciferous vegetables, such as kale, broccoli, cauliflower, and cabbage, have been found to contain high levels of a toxic heavy metal, thallium.
  2. Cruciferous vegetables contain goitrogens, which are naturally occurring plant chemicals that inhibit the uptake of iodine by the thyroid gland and reduce the production of thyroid hormone, thus lowering thyroid function.
  3. Many leafy greens, such as spinach and collard greens, are high in oxalates. Oxalates are plant-based compounds that may promote kidney stone formation and inflammation when consumed in large amounts.

As you can see, it may be time to reconsider whether regularly drinking green smoothies is a wise choice for our health.

Cruciferous veggies and leafy greens undoubtedly have many health benefits, but consuming large amounts of them in green smoothies may not be healthy in the long run

Are there heavy metals in your kale?

The soil in which vegetables grow has a significant impact on their micronutrient content. However, just as beneficial minerals are transferred from soil into plants, so too are toxic metals. Unfortunately, research shows that the toxic heavy metal thallium, which occurs in soil as a byproduct of smelting and coal-burning, preferentially accumulates in cruciferous vegetables such as kale, broccoli, cauliflower, cabbage, brussels sprouts, and bok choy. (1) This means that people who eat a lot of cruciferous vegetables, as green smoothie aficionados are wont to do, may be exposing themselves to high levels of thallium. In fact, an astute molecular biologist and alternative health practitioner named Ernie Hubbard found this to be just the case among his health-conscious clients, who were experiencing many bizarre symptoms that did not fit any standard disease pattern. After much investigation, he was eventually able to trace their symptoms back to thallium toxicity resulting from their cruciferous vegetable-heavy green smoothie habits. (2)

Unfortunately, when it comes to thallium, even low-level exposures can cause symptoms such as nausea, diarrhea, stomach pain, hair loss, and peripheral neuropathy. (3) While some medical authorities have argued that toxic metals are present throughout our environment and that we shouldn’t worry about trace levels in otherwise-healthy vegetables, the problem remains that heavy metal exposures are additive and synergistic. Many heavy metals have similar adverse effects on the body, so continuous consumption of thallium in vegetables, plus daily exposure to other heavy metal sources, exponentially increases the body’s toxic burden. Based on this evidence, if you are regularly drinking green smoothies and experiencing any unusual health problems, it may be time to investigate your thallium level and reconsider your green smoothie habit.

If you aren’t ready to get rid of your green smoothie just yet, at least consider using only organic cruciferous vegetables in your drink. Soils that are rich in carbon-based matter, such as soil on organic farms, have been found to impede the transfer of thallium into plants; this effect is less likely to occur in the carbon-depleted soils used on conventional farms. This means that organic produce may contain lower levels of heavy metals, such as thallium, compared to conventionally raised produce, potentially resulting in a healthier green smoothie. (4, 5, 6)

Green smoothies and the thyroid gland

Concerningly, green smoothies may also have adverse effects on the thyroid gland. The thyroid gland requires the mineral iodine to produce thyroid hormones. Cruciferous vegetables—common ingredients in green smoothies—contain compounds called glucosinolates, which inhibit iodine uptake by the thyroid. This may result in a reduced ability of the thyroid gland to produce hormones, leading to reduced thyroid function. (7) Furthermore, the risk of high cruciferous vegetable intake impairing thyroid function is greater in people with iodine deficiency. Iodine deficiency is not uncommon in folks on a Paleo or otherwise “healthy” diet, since the main dietary sources of iodine available are sea vegetables, iodized salt, dairy, and fortified foods, which are often excluded on a Paleo or unprocessed diet.

While large quantities of raw cruciferous vegetables pose a problem to the thyroid gland, cooked cruciferous vegetables appear to be much safer. Cooking cruciferous vegetables stimulates the production of an enzyme called myrosinase that helps to deactivate goitrogenic glucosinolates. (8)

Opting to eat cruciferous vegetables in their whole-food form is another way to reap the health benefits of these vegetables without getting a heavy hit of goitrogens; it is much harder to overeat vegetables when they are in their whole form versus juiced or blended in a green smoothie.

Oxalates in green smoothies

Another significant problem with green smoothies is that they are often high in oxalate, a type of naturally occurring plant compound that promotes kidney stone formation and inflammation in certain people when eaten in excess. (9) Oxalate occurs in high amounts in spinach, chard, dandelion greens, beets, collard greens, berries, broccoli, cabbage, and carrots. In some people, oxalates accumulate in body tissues and provoke inflammation. In fact, oxalate accumulation has been associated with chronic pain, nephrolithiasis (aka kidney stones), neurological symptoms, vulvar pain, and fibromyalgia pain.

Currently, a high oxalate intake is defined as the consumption of 250 mg oxalate per day. For reference, one cup of raw spinach contains around 656 mg oxalate. (10) As you can see, it would be quite easy to overdo it on oxalates if you are regularly drinking green smoothies, which often contain a cup or more of spinach! Research has confirmed that green juices made using common vegetables contain high levels of soluble oxalates and that the consumption of these drinks may precipitate oxalate kidney stone formation. (11, 12) Some green smoothie proponents advocate “rotating” greens to include low-oxalate options such as mustard greens, watercress, and lettuce; this may help prevent green smoothie-induced oxalate overload.

Certain genetic variants, such as the SLC26A1 variant, and pre-existing gut issues, such as Crohn’s disease and dysbiosis, may increase an individual’s susceptibility to the harmful effects of dietary oxalates. For these individuals, reducing dietary oxalate intake may significantly relieve symptoms. (13, 14) Replenishing the gut with beneficial bacteria from probiotics and fermented foods may also assist in the degradation of oxalate, since several probiotic species, including Lactobacillus rhamnosus, Bifidobacterium animalis subsp. lactis BI07, and Oxalobacter formigenes, have been found to degrade dietary oxalate. (15, 16, 17)

Final thoughts on green smoothies

Unfortunately, green smoothies are not quite as healthy as we have been led to believe, due to their thallium, goitrogen, and oxalate content. Rather than relying on green smoothies as a primary source of vegetables, I recommend eating veggies in their whole-food form and cooking cruciferous vegetables to lower their goitrogen content. If you are not ready to forgo green smoothies entirely, choose organic vegetables, which may be lower in heavy metals; rotate your greens so that you are regularly including low-oxalate options such as mustard greens and watercress; and consider supplementing with oxalate-degrading probiotics such as Lactobacilli and Bifidobacteria.

Now I want to hear from you. Are you a fan of green smoothies? Have you noticed any adverse health effects as the result of your green smoothie habit? Let me know in the comments below.