Dear Mark: Iodized Salt, Weather and Health, and Mercury Fillings

IodineIt’s Monday, which means it’s time for another edition of Dear Mark. This week, I’m covering three topics. First up is iodized salt and its place in the Primal eating plan. It’s processed, it’s conventional, and plenty of alternative health purveyors warn against eating it, but how bad is it? Is there actually a place for it in a Primal lifestyle? After that, I discuss the myriad ways the weather can affect our health and physiology, including lowering body temperature, impairing immune function, increasing blood pressure, and triggering joint pain. Finally, I explore whether or not mercury fillings in our teeth are a health hazard, weighing the evidence from both sides of the argument. I also discuss what to do if you want to have them removed.

Let’s get to it!

Dear Mark,

My understanding of salt use is that sea salt is not iodized. I have read that iodized salt is important to the proper function of the thyroid gland. Supposedly restaurants and other places are steering clear of conventional iodized salt because it is viewed as unhealthy, and can discolor food in cooking.

What are your thoughts on this? Am I wise to still use conventional iodized table salt?



Correct. Sea salt generally is not iodized, unless specified on the package. Sea salt does naturally include some iodine, though usually not enough to qualify as “iodized.” If you don’t have another source of iodine in your diet, and you’re not taking iodine or kelp supplements, I’d say stick with using some iodized salt in your cooking. Maybe half iodized salt and half sea salt.

What sorts of foods contain iodine? Seaweeds of all types contain the most iodine, with the aforementioned kelp (or kombu) being the densest and most easily attainable source. Next time you make broth or soup, add a two inch strip of kelp to the pot toward the end of cooking. After seaweed, iodine content falls off. Other seafoods, like cod or shellfish, are going to be good – but wildly varying – sources of iodine. One filet of cod might have the full RDI while the next fillet has a quarter of it. It’s difficult to know for sure (but isn’t that always the case when dealing with food?). After seafood, yogurt appears to be the best source, followed by eggs, milk, and strawberries. Again, the actual iodine content of these foods depends almost entirely on the iodine content of the soil on which they were cultivated or raised, which also varies.

You should also consider the goitrogen content of your diet. Goitrogens interfere with the function of the thyroid, effectively increasing the amount of iodine required to make sufficient thyroid hormone. Examples of goitrogen-rich foods include cruciferous vegetables like broccoli, cabbage, kale, cauliflower, bok choy, turnips, and Brussels sprouts, as well as soy. Exhaustive online lists of goitrogens include things like strawberries and peaches and spinach, although I’m not sure how detrimental those actually are.

As long as you’re eating enough seafood, eggs, with the occasional foray into the sea vegetable realm without overdoing it on the broccoli, you should be okay without iodized salt. A dash or two of iodized salt, or maybe some kelp granules sprinkled on top your food, can be extra insurance and shouldn’t be harmful. Note, though, that the iodine contents of iodized salts don’t usually match up with the claims on the package. It’s best to just eat a wide variety of foods known to be rich (if unreliable) sources of iodine, so you know you’re covered in the long run.

Hi Mark,

Living in Europe, I often hear Europeans complain about how their ailments often come from the weather. If someone mentions circulation problems, headaches, dizziness, joint pain, heart palpitations, low or high blood pressure, or stomach aches, more often than not, someone will chime in: “It must be the weather!” Even doctors here will say this. They attribute it to temperature differentiation and transitions from cold to warm and vice versa. Some also think you’ll get sick if you walk barefoot on a cold floor. And as soon as one starts to get a cold or sore throat, they wrap a scarf around their neck and wear it indoors as well as outdoors.

So my question is, does weather really have an effect on our health as they make it out to have? Thanks!


Well, weather certainly has a measurable effect on our physiology. Whether that translates to an effect on our “health,” I’m not so sure. Let’s go through it a bit, though. Disregarding the obvious ones like heat stroke, dehydration, frostbite, etc, what physiological markers can weather affect?

Body temperature: Being warm-blooded animals, we generally maintain a consistent body temperature regardless of the outside temperature. That doesn’t come freely, though. We have to actively maintain body temperature through subconscious physiological adjustments. When it’s hot, we sweat, and the evaporating water cools us. Our body hairs also lie flat in hot weather, preventing hot air from building up and getting trapped between the hairs. Our arteries also dilate, allowing increased blood flow and improved heat loss. When it’s cold, the opposite happens. We stop sweating, our hairs stand up (goosebumps), and our arteries constrict. Blood flow is directed toward the warmer (and vital) interior and away from the “non-essential” extremities to further reduce cooling. Taken to an extreme, this can manifest as frostbite, or tissue death.

Blood pressure: Recall that in order to restrict blood flow and maintain body temperature in cold weather, our arteries constrict. This vasoconstriction triggers an increase in blood pressure, which, particularly in susceptible or already-hypertensive people, can precipitate dangerous conditions like stroke or heart attack. In fact, head and neck cooling directly increases blood pressure, heart attack and stroke deaths peak in winter time, and a low body temperature is associated with an elevated risk of stroke. If you’re perpetually cold, or suffer from a low body temperature, it’s possible that this could trigger a pathologically hypertensive state.

Immunity: Can you really catch a cold from being out in the cold without a jacket? That’s what mothers have been claiming for eons, but how true is it? Well, a low body temperature is associated with lowered immune function. There are several lines of evidence showing this:

So, yes, there is something to all those old wives’ tales. If walking barefoot on a cold floor lowers your core body temperature enough to impair your immune function, it may very well leave you open to infection. However, cold weather in and of itself is not the cause. Lowered body temperature is the cause. Keep your body temperature up and you’ll be more resistant.

Joint pain: That old guy with the limp who claims he can predict the weather might be telling the truth; human joints contain sensory nerves called baro-receptors, which are highly attuned to fluctuations in atmospheric pressure. They’re particularly sensitive to the drops in pressure that occur when the weather turns from dry to wet, like when a rainstorm is about to hit. A recent review of rheumatoid arthritis studies concluded that “pain in some individuals is more affected by the weather than others,” suggesting that while the weather/joint pain connection isn’t true in anyone, it does exist in some people.

However, healthy, intact, stable joints shouldn’t hurt because of incoming wet weather. Weather won’t cause pain in something that isn’t already a bit impaired.

I hope that answered your question.

Do you recommend having amalgam mercury-containing fillings taken out and replaced with some other filling substance? Can mercury in your teeth cause havoc in your body after years of being there? Can mercury in your teeth cause hormone imbalance, thyroid problems, hair loss, brain fog, etc. and other symptoms of hormone imbalance?


That’s a tough one. The evidence is pretty clear, though, that mercury-containing fillings do release mercury into your body, and they may increase the levels of mercury in your body, despite what mainstream outlets will tell you. What’s not totally clear is whether the dose provided by the typical mercury filling is enough to negatively impact health. Some reviews have found that only “hypersensitive” people are vulnerable to mercury fillings (even though they do give off mercury). Meanwhile, the presence of mercury amalgams has been fingered as a risk factor for autoimmune disease.

I don’t doubt that it’s hard to find evidence of health problems just from looking at people with fillings. It seems like looking at what happens to people who had fillings removed would be more helpful. Seth Roberts, in whose dedication to the science of self-experimentation I place a lot of trust, found that his brain function (as measured by timing himself doing basic arithmetic problems) markedly improved after removing two mercury fillings. There’s also evidence that mercury filling removal can improve thyroid function in patients with autoimmune thyroiditis and health in patients with autoimmune diseases in general. Evidence of benefits upon removal appears to be evidence of harm, no?

Unfortunately, the very act of eating, of masticating, of chewing, of grinding up food against your mercury filling and then ingesting it increases the amount of mercury released and absorbed, which is why gum addicts with mercury fillings absorb way more mercury than more moderate chewers with fillings (or non-chewers with mercury fillings, for that matter). Man can’t live on smoothies alone, however. He must chew his food, and this makes mercury avoidance tough when it’s sitting in your mouth. Removal seems prudent.

I’ve heard that unless specific precautions are taken, the very act of removal can release a ton of mercury into your body, more than would happen over the lifetime of just keeping the fillings. Depending on how the filling is drilled out, the patient can inhale the mercury fumes or swallow the cloud of mercury particulates. As I mentioned, however, Seth had great results from removal. If you decide to get them removed, heed the advice given by the International Academy of Oral Medicine and Toxicology with regards to safe removal of fillings. Find a dentist who understands and adheres to these guidelines before you even consider going forward with the procedure.

That’s it for this week, guys. As always, keep sending in any questions you have and I’ll do my best to get to them. Thanks for reading!

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

If you'd like to add an avatar to all of your comments click here!