Have you heard of Myth Busters? It’s a television show that proves certain myths to be false.
Myths, as you may know, start out as hearsay. And then, after enough people have heard the story enough times, myths often graduate into the category of proven facts.
Today, I’m going to take on the myth of brassica vegetables causing low thyroid function. What’s the truth?
Brassica vegetables include rutabagas, turnips, kohlrabi, cabbage, collard green, kale, cauliflower, broccoli, and Brussels sprouts. Over the past few years, the myth has grown that eating these vegetables can suppress thyroid function.
The reasoning behind the myth is that after they are digested, brassica vegetables produce glucosinolates. These can then degrade to two substances, goitrin and thiocyanate. And when scientists give these substances to animals, the result is a decrease in thyroid hormone production.
But here’s the big question. Does this actually happen in people eating brassica vegetables? Are the goitrin and thiocyanate levels high enough after the eating brassica vegetables to interfere with thyroid hormone production in real people? Recently, scientists set out to answer this question.
The first thing they did was to analyze how much goitrin was needed to decrease thyroid function. To find the answer, they used radioactive iodine measurements in the thyroid gland before and after being exposed to various levels of goitrin. They found that thyroid function was inhibited by 194 μmol of goitrin, but not by 77 μmol of goitrin.
The conclusion of the researchers was that, “Collards, Brussels sprouts, and some Russian kale (Brassica napus) contain sufficient goitrin to potentially decrease iodine uptake by the thyroid.” But they also noted, “However, turnip tops, commercial broccoli, broccoli rabe, and kale belonging to Brassica oleracae contain less than 10 μmol of goitrin per 100 gram serving and can be considered of minimal risk.” But that’s not all they analyzed.
They also looked to see if the thiocyanate levels from brassica veggies were enough to suppress thyroid function. They discovered that eating brassica vegetables produced no more thiocyanate than was normally found in the diet anyway. The amount came to 10 μM, which is significantly lower than the 40-69 μM of thiocyanates that are found in the rest of an average diet. In their words, “Thiocyanate generated from consumption of brassica vegetables can be assumed to have minimal adverse risks for thyroid health.”
So, please eat as many of these vegetables as you like and have no fear for your thyroid. That is unless you are juicing large amounts of raw brassica vegetables, especially kale, Brussels sprouts, and Collard greens.
In most cases, cooking deactivates the formation of goitrin and thiocyanates, so the only real danger is in eating the raw stuff. And don’t forget that there is always more to the story. Goitrin and thiocyanates are not the only results of eating brassica veggies. After digestion, brassica vegetables also produce sulforaphane, phenethyl, and indolylic isothiocyanates. And all of these substances have been shown to have anti-cancer activity.
Yours for better health,
Frank Shallenberger, MD