I just wish we could see more directly a sense of how little we know about good nutrition.

My favorite example is Vitamin B12, mostly because I learned the story when I reduced my meat consumption to about once a month. So there's this family of massive molecules containing the metal cobalt which we can't make on our own because we're not adapted to eat cobalt, and they're all called "Vitamin B12". It helps in the last step of making the protein building-block called methionine; if you get malnourished in this way your body just basically fills up with "almost finished" methionine which is useless and might even be poisonous. There can also be a slight risk of nerve damage although as I understand it we don't actually understand why the hell that would be.

The biggest cause of malnutrition in this sense is simply being old -- you can't absorb Vitamin B12 so well in the first place because it is huge and weird, and old people have even more trouble either because their gut is slowly dying or because they're infected with H. pylori, which is a safe bacteria more than 80% of the time and might even be important for a well-functioning stomach, but might sometimes cause a chronic tummy-bug.

Okay, so that's the problem, but what do we know about how much of the B12s we need and get? We know very little. The US Recommended Daily Allowance of B12s is 2.4 μg, but the US Daily Value for B12 is 6.0 μg. How much do you get in your food? You get almost none from vegetables, and vegans are basically all ipso facto suffering from B12 deficiency unless they take supplements or eat enriched cereal -- but most of them are quite healthy and see no problems from it. As for vegetarians like (almost-) me, the exact range for milk and eggs to get to this limit is imprecise because the limit itself varies by a factor of 3, so you might need to eat three eggs per day or else ten; so you might need to drink two cups of milk per day or else five, to live up to those numbers. So in theory there are lots of vegetarians also who come in at half or one-third their daily requirement, and never notice anything. Meanwhile there are people over 60 who eat plenty of meat but are starting to feel the effects anyway.

We actually don't even know how well you absorb it. A USDA researcher named Lindsay Allen did a 2011 study where she marked Vitamin B12 in eggs so that she could detect it, and then fed those eggs to 10 people. She found that the body absorbed 50% of a 1.5 μg dosage but only 20% of a 2.6 μg dosage. Let me repeat that. She found that, in absolute value terms, eating more B12 led to less of it being absorbed, 0.75 μg versus 0.5 μg. It's probably not statistically significant with 10 people, but still. We don't even know that eating more leads to you absorbing more of it. (I also wonder whether RDAs are "amount absorbed" while DVs are "amount eaten", as this is pretty much never reported, but I will assume that they aren't crazy and that it's always "amount eaten.")

I wouldn't use the scare-quotes around "science" when talking about nutrition science, as I think there is potential for real research here and I think people do indeed do it -- but it's a lot of work before we have a firm understanding, and most of that work hasn't been done yet, and yet you hear these confident conclusions. I would just like to hear people publish standards in the form, "you might need this much, or that much, we don't really know."

The engineer in me craves error bars. Where are the error bars?