Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

There are several autoimmune diseases that affect the thyroid gland, the most common being Graves’ disease and Hashimoto’s thyroiditis. Graves’ disease results in hyperthyroidism, an overactive thyroid gland. Though slaughterhouses are supposed to remove thyroid glands, should some neck meat slip in, you can suffer a similar syndrome, called hamburger thyrotoxicosis. But that’s not from your body making too much thyroid hormone, but your body eating too much thyroid hormone. Graves’ disease is much more common, and meat-free diets may be able to help with both, 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 antioxidants, which are possible protective factors against autoimmune diseases. Maybe it’s because plants are packed with anti-inflammatory compounds. After all, a whole food plant-based diet is basically synonymous with an anti-inflammatory diet. But you don’t know, until you put it to the test.

It turns out the exclusion of all animal foods was associated with half the prevalence of hyperthyroidism compared with omnivorous diets. Lacto-ovo-vegetarian and fish-only diets were associated with intermediate protection, but a 52-percent lower odds of hyperthyroidism among those eating strictly plant-based diets.

This apparent protection may be due to the exclusion of animal foods, the benefits of plant foods, or both. Animal foods––like meat, eggs, and dairy products––may contain high estrogen concentrations, for example, which have been linked to autoimmunity in preclinical studies. Or, the decrease in animal protein may downregulate IGF-1, which is not just a cancer-promoting growth hormone, but may play a role in autoimmune diseases as well. Or, it could be the good stuff in plants that may protect cells––like the polyphenol phytochemicals, such as flavonoids found in plant foods. Maybe it’s the environmental toxins that build up in the food chain. For example, fish contaminated with industrial pollutants, like PCBs, are associated with an increased frequency of thyroid disorders.

Okay, 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 methionine intake, an amino acid concentrated in animal protein––thought to be one reason why the consumption of whole plant foods is likely to have a favorable influence on longevity, through decreasing the risk of cancer, heart disease, and diabetes. And 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. But put a vegetarian on a diet of meat, eggs, and dairy, and within as few as four days, you can drive down levels. 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.

Vegan diets tended to be associated with lower, not higher, risk of hypothyroid disease. Why the word “tended”? Because the associated protection against hypothyroidism incidence and prevalence studies did not reach statistical significance. It wasn’t just because they were slimmer. The lower risk existed even after controlling for body weight; so, they think maybe it’s because animal products may induce inflammation. The question I have is: If you have someone who already has Hashimoto’s, what happens if you change their diet? That’s exactly what I’ll explore next.

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