In an earlier post I wrote, “Hypothyroidism is so common I suspect an environmental cause.” In fact, I suspect that all common diseases are caused (= made much more likely by) differences between modern life and Stone-Age life. Since then, thanks to comments and email, I have learned more about hypothyroidism. According to Dennis Mangan, it has become a lot more common during the last 100 years, which implies an environmental cause. The most common type of hypothroidism is called Hashimoto’s thyroiditis. It is an autoimmune disorder where the immune system attacks the thyroid, damaging it. A reader with hypothyroidism wrote me:

When I was first tested for thyroid levels, part of the test (which I think is standard protocol) was to test the level of antibodies to thyroid. My levels were off the charts.

This supports what I said. I’m sure that autoimmune diseases are caused by one particular difference from Stone Age life: lack of bacteria in our food. The immune stimulation the harmless bacteria provided can be provided in other ways — bee stings, for example. But I don’t think Stone-Age people got a lot more bee stings than we do.

The usual answer, of course, is: To clean themselves. There are other answers:

Some people think it is used as a way to control their temperature – it keeps their fur smooth, which in the winter traps heat. In the summer, it spreads saliva on the fur and cools them down and can also loosen fur so that they shed more easily. Others think it is a natural way of reducing parasites like fleas or ticks.

Perhaps cats lick themselves to ingest more foreign bacteria and dirt, which they need to be healthy. Test of this proposal: Feed a cat more fermented food, it should lick itself less. (Just as I became less of a foodie when I ate more fermented food.)

The value of evolutionary explanations.

Your appendix — a kind of cul-de-sac off your large intestine — can be dangerous. A British man was recently rushed to the hospital with a burst appendix three weeks after he’d had an operation to remove it. Surgeons have routinely removed it — seemingly without problems. Is the appendix an evolutionary vestige, as Darwin believed, or does it do something beneficial?

In the last few years, two articles — one in Journal of Theoretical Biology, the other in Journal of Evolutionary Biology — by William Parker, a professor of surgery at Duke, and others have argued that the function of the appendix is to harbor bacteria. If diarrhea washes out your intestines, bacteria safely hiddne in the appendix can repopulate them. (A theory supported by the position of the appendix — roughly in the middle of your intestines.) That makes perfect sense.

The connection with my umami hypothesis is that both assume that the foreign bacteria within us are precious and endangered. (My umami hypothesis says we need to consume plenty of bacteria to be healthy and that our food preferences help us do so.) The precious part is widely accepted; it’s the endangered part that’s new. If we need bacteria so much, why should they be endangered? We need our eyes; they aren’t endangered. My answer is that to protect bacteria carries a cost: The most hospitable the digestive system becomes to bacteria, the less effective it will become at everything else, including digestion. And bacteria were/are cheap. Rather than protect them, the system has been shaped to require them. Just as gas-guzzling cars evolved when gas was cheap. Making cars more gas-efficient will make them less efficient at other functions, such as signaling status.

Thanks to Kathy Tucker, James Andrewartha, and James Lucoff.

In the 1920s and 30s, a Chicago pediatrician named Clara Davis did a remarkable experiment/demonstration: Letting young children choose their own food. About eleven children chose from a list of 30 little-processed foods — including sour milk, the only bacteria-rich food on the list — and could eat as much of each one as they wished. The choices included peaches, beef, carrots, beets, barley, bone marrow, pineapple, cabbage, lettuce, potatoes, and sweet breads. Many of the foods were supplied both raw and cooked. The experiment lasted about 6 years.

The main result was that the children were very healthy:

There were no failures of infants to manage their own diets; all had hearty appetites; all thrived. Constipation was unknown among them and laxatives were never used or needed. Except in the presence of parenteral infection, there was no vomiting or diarrhea. Colds were usually of the mild three-day type without complications of any kind. There were a few case of tonsillitis but no serious illness among the children in the six years.

Some of them were malnourished at the start of the experiment; all recovered. One had rickets and was offered cod liver oil. He drank a little bit of it while sick but after he recovered never drank it again.

Davis’s observations support the idea that we have inborn desires that help us choose what to eat. Davis emphasized that there was great variation from one child to another in what they ate — as Weston Price noted a great variation from one healthy community to the next in what they ate. She didn’t give details, however. The notion that our desires, given Stone-Age surroundings, help us choose a healthy diet is what led me to the umami hypothesis. It started with the idea that in the Stone Age our liking for complex, sour, and umami flavors caused us to eat food with more bacteria than fresh food. High-bacteria food tasted better than low-bacteria food; it was more sour, more umami, and had a more complex flavor. Suggesting that we need to eat bacteria to be healthy.