Apparently, we should blame science for not always proving our hypotheses correct. At least, that's what Scott Adam's argument essentially amounts to.

"How do you make people trust a system that is designed to get wrong answers more often than right answers?"

You give them a real science education and hope they understand that even the "wrong" answers are the right answers. The other half is to stop the media from making every new incremental discovery a "potential cure for cancer", or suggesting that every slightly contradictory piece of research on nutrition overturns everything we know and held dear about [cholesterol, fat, sugar, etc.].

"A lot of people believe there is a discordance between what we eat today and what our ancestors evolved to eat"

"The popularity of these so-called caveman or Stone Age diets is based on the idea that modern humans evolved to eat the way hunter-gatherers did during the Paleolithic..."

The emphasized words sum up the evidence backing up a "Paleo Diet"--it's a belief system, not science. We have a bevy of research to support the health benefits of foods such as legumes and whole grains and barely a scrap which suggests they cause harm. Is there a chance some future research will demonstrate that whole grains and legumes cause health problems that more than offset any potential benefits? Sure, but there's also people holding out for proof that homeopathy works.

I'm not saying you can't eat a Paleo Diet and be perfectly healthy, I'm just saying that it's pseudoscience based on an appeal to wisdom and an appeal to nature. We might also argue that humans haven't had time to evolve for wearing clothing (based off low circulating vitamin D levels) and that therefore we should definitely stop wearing them, and there is a similar paucity of research. Suffice to say: it's not science, it's a pure-and-simple belief system.

The brief is somewhat misleading. It's only found in large amounts in the liver. Most meat has little. Beans and leafy greens are the best sources, but with fortification it's probably not much of a concern for any American.

The research may be new, but it encounters the same old problem: increasing the lifespan of a mouse by 25% (hint: it's measured in months) is much different than increasing human lifespan. The last "anti-aging miracle" I read about, lowering IGF-1 levels, provided just as much misguided hope. Mice with low levels of IGF-1 lived longer--surely the same must be true for humans too, right? Not quite... low levels of IGF-1 are associated with higher risk of ischemic heart disease, and may also be associated with greater risk for sarcopenia.

Do more people die from reaching the natural limit to their life, or from heart disease and complications due to fractures? Until a research team can demonstrate that altering these pathways provides tangible human benefit (without a hidden consequence), we're just learning how to increase our favorite pet rodent's life.

References:

Laughlin GA, Barrett-Connor E, Criqui MH, Kritz-Silverstein D. The prospective association of serum insulin-like growth factor I (IGF-I) and IGF-binding protein-1 levels with all cause and cardiovascular disease mortality in older adults: the Rancho Bernardo Study. J Clin Endocrinol Metab. January 2004;89:114-20.

Giovannini S, Marzetti E, Borst SE, Leeuwenburgh C. Modulation of GH/IGF-1 axis: potential strategies to counteract sarcopenia in older adults. Mech Ageing Dev. October 2008;129:593-601. doi: 10.1016/j.mad.2008.08.001.

Here's a citation:

Garland CF, French CB, Baggerly LL, Heaney RP. Vitamin D supplement doses and serum 25-hydroxyvitamin D in the range associated with cancer prevention. Anticancer Res. February 2011;31:607-11.

Cancer prevention is correlated most closely with serum levels of 40 ng/mL or above (as is alleviation of depression), and to reach this level in 97.5% of the population, 9,600 IU/day was necessary. This is almost double the current UL. Of course, current recommendations for daily dosage is based off 20 ng/mL being 'sufficient', while most experts in the field now believe that 30 ng/mL should be the baseline for 'sufficient' and that most positive effects will be found at serum levels of 40 - 50 ng/mL.

Vitamin D toxicity is rare, and only occurs when serum 25(OH)D levels exceed 150 ng/mL. It has never been demonstrated at doses of less than 20,000 IU/d and generally requires greater than 40,000 IU/d. Most incidents have been due to accidental ingestion, such as from a milk supply that was accidentally fortified with vast amounts of D3.

In the end, supplementation needs to be based off serum 25(OH)D levels, which can be measured by a doctor. You may need more or less to reach 'ideal' levels, and it's impossible to say exactly how much without testing. The test is cheap and hopefully will become a standard part of a routine examination, considering that vitamin D affects at least 35 different systems in the body. Without the test, 2,000 IU/d will keep you under the UL (even though it should really be changed to reflect the science behind the toxicity), and will likely keep your levels above 30 ng/mL. Remember that most dairy products are fortified with D3 which should be considered a part of the total.

Vitamin K and biotin are two other vitamins that we produce ourselves, don't need from food to survive, yet are still considered vitamins. Just because they're not essential (or are conditionally essential) doesn't make them not a vitamin. Considering the word vitamin itself is a misnomer for all but a couple vitamins today (it comes from "vital amine", yet only a couple vitamins have amine groups), it's somewhat moot anyhow. Is vitamin D actually a vitamin? Doesn't really matter, we all know what "vitamin D" is, and if you want to get technical, you can use terms like cholecalciferol or ergocalciferol instead.