A few months ago, a patient came in the office and dropped off some disturbing information about eating cabbage.
I had prescribed thyroid hormone replacement to her for years. She was feeling great, but this latest piece of information caused her concern.
One of her friends had told her that eating cabbage was dangerous while taking thyroid supplements. So she did a little research on the Internet. What she found out surprised her.
According to one doctor’s site, “Vegetables from the cabbage family should be limited. These include broccoli, cauliflower, cabbage, turnips, mustard greens, kale, spinach, Brussels sprouts, kohlrabi, rutabagas, horseradish, radish and white mustard.”
He warned that eating these vegetables would suppress the thyroid gland. He then referenced several books that have popularized this common misconception. Every website she visited gave the same warning, and referenced the same materials.
It’s true that cruciferous vegetables, such as the ones listed above, contain natural chemicals called glucosinolates. And in high enough doses, glucosinolates can interfere with thyroid hormone synthesis.
Many people call glucosinolates by another name: goitrogens. That’s because anything that interferes with thyroid hormone synthesis could technically result in a swelling of the thyroid gland. When this happens, the medical name for it is a goiter.
But cruciferous vegetables are not the only foods that contain goitrogens. Many other foods also contain these substances. The list includes corn, sweet potatoes, lima beans, turnips, peanuts, cassava, canola oil, and soybeans. That’s the bad news.
The Good News?
In over 35 years of medicine, I have never once seen a case of goiter caused by eating these foods. So where does this myth come from anyway?
In the 1950s and early 1960s, there was a lot of interest in finding the “best” animal feeds for both livestock and pets. In this case, the word “best” refers to the most economical.
Some of these researchers looked at the effects on guinea pigs of diets consisting primarily of cabbage. What they found was that when the guinea pigs ate enough cabbage, they developed a goiter.
That’s when they first learned about how glucosinolates cause a decrease in the synthesis of thyroid hormones.
As a result of lowered thyroid hormones, the part of the guinea pigs’ brains called the pituitary kept releasing another hormone called TSH (thyroid stimulating hormone). TSH is the signal to the thyroid to make more hormone. Too much TSH causes the thyroid to swell up from over activity. The end result is a goiter.
Another Cause of Goiters
By the way, the same thing can happen when there’s a deficiency of iodine in the diet. Iodine is a critical element in the synthesis of thyroid hormones. So when there’s not enough iodine present, thyroid hormone production goes down, TSH levels go up, and a goiter can develop.
One of the other problems that happened from putting the guinea pigs on the cabbage diets was that they were not getting enough iodine. And in these experiments, the combination of getting excessive amounts of glucosinolates with deficient amounts of iodine was just too much for the little guys to handle. This research was how the cabbage/thyroid myth first started.
In 1995, a group of scientists finally began to re-examine the relationship between iodine and goitrogen intake. This time it was rats. Some of the rats ate diets low in iodine.
The researchers gave all of the animals a dose of a glucosinolate called thiocyanate every day. They gave a dose equal to what they would have received if one-third of their food was from raw cabbage. It was high enough to suppress thyroid hormone synthesis.
After two months, all of the animals had developed a goiter. But the significant point was that the animals on the low iodine diet developed a much greater degree of goiter than those with normal iodine levels.
Not only that, but all of the animals taking the thiocyanate lost excessive amounts of iodine in their urine. This created an even greater iodine deficiency.
So, at least in rats, eating a diet for two months in which 30% of all the calories come from raw cabbage will result in a loss of iodine. And the combination between the excessive iodine loss and the direct suppressive effect of the thiocyanate will create a goiter.
So Why Doesn’t the Same Thing Happen to Humans?
I think the answer is obvious. How many people out there eat diets similar to what these rats ate? Have you ever met a person who ate a long-term diet in which one-third of everything he ate consisted of raw cruciferous vegetables? I haven’t.
Remember, the vegetables have to be raw. Heat inactivates the glucosinolates in these foods. Even light steaming will inactivate them. And, if there is such a person, he would also have to be deficient in iodine at the same time.
So although it is theoretically possible, the actual incidence of the problem seems to be non-existent. In fact, for this report, I looked at the world literature. I could not find even one case of low thyroid function in humans from eating cruciferous vegetables.
So don’t worry about eating cabbage or any of the other cruciferous vegetables. They are extremely healthy for you. They are loaded with a wide variety of essential nutrients. They also lower cholesterol, as well as contain substances that prevent breast, colon, liver, ovarian, and lung cancer. And if that’s not enough, they prevent and even treat ulcers and other stomach and intestinal conditions.
Lakshmy, R., P.S. Rao, et al. “Iodine metabolism in response to goitrogen induced altered thyroid status under conditions of moderate and high intake of iodine.” Horm Metab Res. 1995 October;27(10):450-4.
Paynter, O.E., G.J. Burin, R.B. Jaeger, and C.A. Gregorio. “Goitrogens and thyroid follicular cell neoplasia: evidence for a threshold process.” Regul Toxicol Pharmacol. 1988 March;8(1):102-19.
Stolic, V. and P. Langer. “Biosynthesis of thyroid hormone following cabbage feeding in guinea pigs.” Physiol Bohemoslov. 1963;12:251-7.