There always seems to be a ginormous pendulum swing every time a food is classified as a "superfood". A relatively obscure food goes from unknown and untouched to eaten 5 times a day with the hope that it will somehow prolong your life or help you lose 15lbs. This pendulum swing is no different when we look at kale. Five years ago no one knew what kale was, now everyone and their mother is eating kale chips, drinking kale smoothies, and eating kale salads at Whole Foods. In some instances, this is not a good thing.
While kale is certainly something that can be part of a healthy diet, we must look at a food from root to tip to determine how big of a part of our diet it should be. There is a lot going for kale from a nutrient standpoint, but there is also a significant drawback, notably that it contains goitrogenic compounds. Goitrogens are substances that can interfere with thyroid function by binding to receptors where iodine should attach. The thyroid turns iodine in to thyroid hormones and a deficiency can lead to thyroid dysfunction. When goitrogens attach to iodine receptors in the thyroid, the thyroid is unable to make thyroid hormones. In theory, if you consume enough goitrogens, you could be getting enough iodine and still be in an iodine deficient state.
I don't believe that this is the typical route that a person consuming kale would be getting themselves in to trouble. In moderation, I don't think kale would have any significant effect on thyroid function. It could interfere with thyroid function when consumed in excess and under the proper conditions, though. First, other cruciferous vegetables such as broccoli, cauliflower, and cabbage also contain goitrogens so you would have to take a look at these other goitrogens in the diet. Second, cooking inactivates some of the goitrogens so people who cook their kale are less likely to have a problem than someone who eats kale smoothies or eats cups of it a day in their salad. Finally, even if you did consume a good amount of kale, it's only likely to become a problem if you aren't taking in sufficient iodine. This is where I think a problem can set in.
Goiter, a swelling of the thyroid due to iodine deficiency was once a significant problem in the United States. It was so big that the landlocked and mountainous areas where goiter was common was referred to as the goiter belt.
To combat this problem, iodine was added to table salt. The result, steep declines of goiter as Americans in the goiter belt were now getting sufficient levels of iodine. I would like to say this is where the story ends, but I don't believe that to be the case. While adding potassium or sodium iodide to table salt helped correct the iodine deficiency, Americans have been turning from table salt to sea salt, which doesn't contain iodine. While I cannot know for sure, I'd imagine this switch is highly prevalent in the kale crowd.
There are other sources of iodine in the American diet. Bread used to have significant amounts of iodine in it until they started using bromine, which happens to also be a goitrogen. Cow dairy also contains significant amounts of iodine. I say this as large swaths of people switch from cow's milk to almond milk while I am still frantically trying to find the teats of an almond. Again, this is a switch I feel is safe to say is quite prevalent in the kale crowd.
So where does this put us with regard to kale? Kale can be a healthy part of your diet provided that the diet is diverse, doesn't focus on kale as an excessive green of choice, and that you get sufficient iodine. Good sources of iodine tend to come from the sea. Kelp and other seaweeds are very good sources as are eggs.
The underlying issue, however, is that people take good foods and call them superfoods to elevate them to the level that they can be consumed endlessly without issue. Kale is a great food, but for someone who is already eating cruciferous vegetables regularly there really isn't any added benefit to eating a lot of kale. It's high in fiber, vitamins A, C and K, and that's about it; these are nutrients that are typically high in vegetables. Kale is also a good source of the omega 3 fatty acid ALA, which would be great except for the fact that humans convert ALA to usable omega-3 fatty acids at less than a 5% rate.
Many boast about the high ORAC score of kale, which is basically a way of measuring how well a food helps the body quash free radicals. Kale is high on the ORAC list...If you don't count berries, about 2 dozen other fruit, pretty much every spice in the world, and at least half a dozen other greens that are freely available at any supermarket such as arugula and beet greens. My point here isn't to prevent you from eating kale, it's to show you that it really isn't appreciably better than most vegetables. So why is it a superfood again?
So where does this put us with regard to kale? Kale can be a healthy part of your diet provided that the diet is diverse, doesn't focus on kale as an excessive green of choice, and that you get sufficient iodine. Good sources of iodine tend to come from the sea. Kelp and other seaweeds are very good sources as are eggs.
The underlying issue, however, is that people take good foods and call them superfoods to elevate them to the level that they can be consumed endlessly without issue. Kale is a great food, but for someone who is already eating cruciferous vegetables regularly there really isn't any added benefit to eating a lot of kale. It's high in fiber, vitamins A, C and K, and that's about it; these are nutrients that are typically high in vegetables. Kale is also a good source of the omega 3 fatty acid ALA, which would be great except for the fact that humans convert ALA to usable omega-3 fatty acids at less than a 5% rate.
Many boast about the high ORAC score of kale, which is basically a way of measuring how well a food helps the body quash free radicals. Kale is high on the ORAC list...If you don't count berries, about 2 dozen other fruit, pretty much every spice in the world, and at least half a dozen other greens that are freely available at any supermarket such as arugula and beet greens. My point here isn't to prevent you from eating kale, it's to show you that it really isn't appreciably better than most vegetables. So why is it a superfood again?