Podcast: The Best Diet for Thyroid Function Part 1
It’s crazy when you think about all
of the different kinds of food we eat. We just swallow and hope it
all works out for the best. Well, it turns out
there are better ways to think about keeping our bodies
humming healthfully along. Welcome to Nutrition Facts. I’m your host Dr. Michael Greger. Did you know that the mighty
thyroid controls your metabolism, produces hormones, and helps
regulate your body temperature? What can we do to support it? Let’s start with a story about
plant-based milk and iodine. Adequate dietary iodine is required
for normal thyroid function. The two thyroid hormones
are named after how many iodine atoms they
contain: T3 and T4. Now, given that iodine is extensively
stored in the thyroid gland itself, it’s not something you
have to get every day, but your overall diet needs
to have some good source. Unfortunately, the common sources
aren’t particularly health-promoting: iodized salt, [and] dairy foods because iodine-based cleansers
like betadine are used to sanitize the udders, which results in some
iodine leaching into the milk.
They also add iodine to cattle feed, and some commercial breads have
iodine-containing food additives. So if you put people on a paleo-type
diet and cut out dairy and table salt, they can develop an iodine deficiency,
even though they double their seafood intake, which
can also be a source. What about those switching to diets
centered around whole plant foods? They’re also cutting down
on ice cream and Wonder Bread, and if they’re not eating anything from
the sea—seaweed, sea vegetables— they can run into the same problem.
Her parents reported striving to
feed her only the healthiest foods. The 3-year-old only got plant-based,
unsalted, unprocessed foods with no vitamin supplementation.
Now that could have been deadly. With no vitamin B12, those on strictly
plant-based diets can develop irreversible nerve damage, but
in this case, a goiter arose first due to inadequate iodine intake. Here’s another case of veganism
as a cause of iodine-deficient hypothyroidism in a toddler after
weaning. Now before weaning, he was fine because his mother
kept taking her prenatal vitamins, which luckily contained iodine. Most vegetarians and vegans are
unaware of the importance of iodine in pregnancy, just as clueless
as their omnivorous counterparts.
The American Thyroid Association and
the American Academy of Pediatrics has recommended that women even
just planning on getting pregnant should ingest a daily supplement that
contains 150 micrograms of iodine, yet only 60% of prenatal vitamins marketed
in the US contain this essential mineral. So, despite the recommendations,
about 40% lack it. Therefore, it’s extremely important that pregnant and
breastfeeding women read the labels to ensure they’re receiving
an adequate amount. Women of reproductive age have
an average iodine level of 110, which is fine for nonpregnant
individuals, but we’d like to see at least
150 in pregnancy. It’s a 24-hour urine test,
in which iodine sufficiency is defined as 100 mcg/liter of pee in nonpregnant
adults, which your average vegan fails to reach in the largest study
done to date, out of Boston.
The recommended average daily
intake is 150 mcg/day for most people, which you can get in a
cup and a half of cow’s milk. Sadly, plant-based milks are
typically not fortified with iodine, averaging only about 3 mcg/cup.
In the largest systematic study to date, although many plant-based milks are
fortified with calcium, they only found just 3 of 47 fortified with iodine. Those that were had as much as cow’s
milk, but those that weren’t fell short. Plant-based milk companies brag about
enriching their milk with calcium, and often vitamins B12, D, and vitamin A,
but only rarely are attempts made to match the iodine content. The only reason cow’s milk has as
much as it does is that they enrich the feed, or it comes
dripping off their udders. So why don’t plant-milk
companies add iodine, too? I was told by a food scientist at
Silk that my carrageenan video played a role in them
switching to another thickener.
Hopefully, they’ll see this video
and consider adding iodine, too, or some company will snatch at
the market advantage opportunity. The researchers conclude that individuals
who consume plant-based milk not fortified with iodine may be
at risk for iodine deficiency unless they consume alternative
dietary iodine sources, the healthiest of which is sea
vegetables, which we’ll cover next. Dairy milk supplies between a quarter and
a half of the daily iodine requirement in the United States, though milk
itself has little native iodine. The milk iodine content is mainly
determined by factors like the application of iodine-
containing teat disinfectants. The iodine residues in milk
appear to originate mainly from the contamination
of the teat surface. The teats are sprayed or dipped with
betadine-type disinfectants, and the iodine just kind
of leaches into the milk. Too bad most of the plants-
based milks on the market aren’t enriched
with iodine, too. Fortified soy milk is probably
the healthiest of the plant milk, but even if it was enriched with iodine,
what about the effects of soy on thyroid function? It’s funny when I searched the
medical literature on soy and thyroid, this study popped up: a cost-effective way to train
residents to do thyroid biopsies.
Just stick the ultrasound probe
right on top and go to town. It turns out on ultrasound your
thyroid gland looks a lot like tofu. Anyway, the idea that soy may
influence thyroid function originated over eight decades ago when
marked thyroid enlargement was seen in rats fed raw soybeans, though the
observation that people living in Asian countries have consumed
soy foods for centuries with no perceptible thyrotoxic effects
certainly suggests their safety. The bottom line is there does
not seem to be a problem with people who have
normal thyroid function. However, soy foods may inhibit
the oral absorption of Synthroid and thyroid hormone replacement
drugs, but so do all foods. That’s why we tell patients to take it
on an empty stomach. But you also have to be getting enough iodine,
so it may be particularly important for soy food consumers to make sure
their intake of iodine is adequate.
What’s the best way to get iodine?
For those who use table salt, make sure it’s iodized. Currently, only half of table
salt sold contains iodine, and the salt used in processed
foods is typically not iodized. Of course, ideally, we
shouldn’t add salt at all. Dietary salt is a public health hazard.
Think this title is a little over the top? Dietary salt is the #1 dietary risk
factor for death on the planet Earth, wiping out more than
three million people a year twice as bad as not
eating your vegetables. What’s the best source of iodine, then? Sea vegetables! You can get
a little iodine here and there from a whole variety of foods, but the most concentrated source
by far, with up to nearly 2,000% of your daily allowance
in just a single gram— which is like the weight
of a paperclip: seaweed. Given that iodine is extensively stored
in the thyroid, it can be safely consumed intermittently, meaning you
don’t have to get it every day, which makes seaweed use
in a range of foods attractive, and occasional seaweed intake
enough to ensure iodine sufficiency. However, some seaweed
should be used with caution due to its overly high iodine
content, like kelp.
Too much iodine can
cause hyperthyroidism, a hyperactive thyroid gland.
A woman presented with a racing heartbeat, insomnia,
anxiety, and weight loss thanks to taking just two
tablets a day containing kelp. In my last video, I noted how the
average urinary iodine level of vegans was less than the ideal levels, but
there was one kelp-eating vegan with a urinary concentration of over 9,000. Adequate intake is when
you’re peeing out 100 to 199.
Excessive iodine intake is when you
break 300; 9,437: way too much. The recommended average daily intake
is 150 mcg/day for non-pregnant, non-breastfeeding adults, and we may want
to stay below 600 on a day-to-day basis, whereas a tablespoon of
kelp may contain 2,000. I’d stay away from kelp
because it has too much, and stay away from hijiki because
it contains too much arsenic. Here’s how much common seaweed
preparations should give you an approximate daily allowance:
two nori sheets, you can just nibble on them
as snacks as I do; one teaspoon of dulse flakes, which
you can just sprinkle on anything; one teaspoon of dried arame, which
is great for adding to soups; or one tablespoon of seaweed salad. If iodine is concentrated in marine
foods, this raises the question of how early hominins living in
continental areas could have met their iodine requirements. Well, here’s what bonobos do,
perhaps our closest relatives.
During swamp visits, they
all forage aquatic herbs. We would love it if you could
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