Is the apparent protection of plant-based diets for thyroid health due to the exclusion of animal foods, the benefits of plant foods, or both?
Several autoimmune diseases affect the thyroid gland, and Graves’ disease and Hashimoto’s thyroiditis are the most common. Graves’ disease results in hyperthyroidism, an overactive thyroid gland. Though slaughter plants are supposed to remove animals’ thyroid glands as they “shall not be used for human food,” should some neck meat slip in, you can suffer a similar syndrome called Hamburger thyrotoxicosis. That isn’t from your body making too much thyroid hormone, though. Rather, it’s from your body eating too much thyroid hormone. Graves’ disease is much more common, and meat-free diets may be able to help with both diseases, as plant-based diets may be associated with a low prevalence of autoimmune disease in general, as observed, for example, in rural sub-Saharan Africa. Maybe it’s because plants are packed with “high amounts of antioxidants, possible protective factors against autoimmune disease,” or because they’re packed with anti-inflammatory compounds. After all, “consuming whole, plant-based foods is synonymous with an anti-inflammatory diet.” But you don’t know until you put it to the test.
It turns out that the “exclusion of all animal foods was associated with half the prevalence of hyperthyroidism compared with omnivorous diets. Lacto-ovo [dairy-and-egg] and pesco [fish] vegetarian diets were associated with intermediate protection.” But, for those eating strictly plant-based, there is a 52 percent lower odds of hyperthyroidism.
As I discuss in my video The Best Diet for Hypothyroidism and Hyperthyroidism, this apparent protection “may be due to the exclusion of animal foods, the [beneficial] effects of plant foods, or both. Animal foods like meat, eggs, and dairy products may contain high oestrogen concentrations, which have been linked to autoimmunity in cell and animal studies.” Or it could be because the decrease in animal protein by excluding animal foods may downregulate IGF-1, which is not just a cancer-promoting growth hormone, but may play a role in autoimmune diseases. The protection could also come from the goodness in plants that may “protect cells against autoimmune processes,” like the polyphenol phytochemicals, such as flavonoids found in plant foods. Maybe it’s because environmental toxins build up in the food chain. For example, fish contaminated with industrial pollutants, like PCBs, are associated with an increased frequency of thyroid disorders.
But what about the other autoimmune thyroid disease, Hashimoto’s thyroiditis, which, assuming you’re getting enough iodine, is the primary cause of hypothyroidism, an underactive thyroid gland? Graves’ disease wasn’t the only autoimmune disorder that was rare or virtually unknown among those living in rural sub-Saharan Africa, eating near-vegan diets. They also appeared to have less Hashimoto’s.
There is evidence that those with Hashimoto’s have compromised antioxidant status, but we don’t know if it’s cause or effect. But if you look at the dietary factors associated with blood levels of autoimmune anti-thyroid antibodies, animal fats seem to be associated with higher levels, whereas vegetables and other plant foods are associated with lower levels. So, again, anti-inflammatory diets may be useful. It’s no surprise, as Hashimoto’s is an inflammatory disease—that’s what thyroiditis means: inflammation of the thyroid gland.
Another possibility is the reduction in intake of methionine, an amino acid concentrated in animal protein, thought to be one reason why “regular consumption of whole-food vegan diets is likely to have a favourable influence on longevity through decreasing the risk of cancer, coronary [heart] disease, and diabetes.” Methionine restriction improves thyroid function in mice, but it has yet to be put to the test for Hashimoto’s in humans.
If you compare the poop of patients with Hashimoto’s to controls, the condition appears to be related to a clear reduction in the concentration of Prevotella species. Prevotella are good fiber-eating bugs known to enhance anti-inflammatory activities. Decreased Prevotella levels are also something you see in other autoimmune conditions, such as multiple sclerosis and type 1 diabetes. How do you get more Prevotella? Eat more plants. If a vegetarian goes on a diet of meat, eggs, and dairy, within as few as four days, their levels can drop. So, one would expect those eating plant-based diets to have less Hashimoto’s, but in a previous video, I expressed concern about insufficient iodine intake, which could also lead to hypothyroidism. So, which is it? Let’s find out.
“In conclusion, a vegan diet tended to be associated with lower, not higher, risk of hypothyroid disease.” Why “tended”? The associated protection against hypothyroidism incidence and prevalence studies did not reach statistical significance. It wasn’t just because they were slimmer either. The lower risk existed even after controlling for body weight. So, researchers think it might be because animal products may induce inflammation. The question I have is: If someone who already has Hashimoto’s, what happens if they change their diet? That’s exactly what I’ll explore next.
Doctor’s Note:
This is the third in a four-video series on thyroid function. The first two were Are Vegans at Risk for Iodine Deficiency? and Friday Favorites: The Healthiest Natural Source of Iodine.
Stay tuned for the final video: Diet for Hypothyroidism: A Natural Treatment for Hashimoto’s Disease.
