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I read that we gradually lose the ability to synthesize Vitamin D as we age and after 40 years old or so we are down to only 5% of our original capacity. Sorry I can't find the study again.

Anyone heard of this? If true, maybe I'm wasting a lot of time in the sun and tanning booth and should be just supplementing.

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My level of Vitamin D is fine without supplementing at 82nMol/L . I am in my 60's and have fair skin/freckles. I live in northern BC but travel to Mexico for the winter and therefore get a reasonably constant amount of sun. I never use sunscreen but I am careful not to burn. – henny Aug 16 2010 at 16:58

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The ability to synthesize Vitamin D in the skin from sunlight does decrease with age. I found This Study by Michael Holick comparing synthesis in young adults and older people. Unfortunately it will be behind a paywall for most people. I couldn't find any other studies measuring this directly.

Six healthy white subjects aged 20-30 and six older white subjects aged 62-80 with skin type III received whole body exposure to "sunlight" (32 mJ/em2) in a National Biologic light box emitting energy between 260 and 360 nm.2

Serum concentrations of vitamin D were measured for 7 days. In the young adults circulating vitamin D concentrations increased from 2-6 ng/ml to 30 ng/ml within 24 hours and slowly declined towards baseline by 7 days.

In the older volunteers, with the same skin type and exposed to the same amount of simulated solar radiation, circulating vitamin D rose from 1.5 to only 7.6 ng/ml.

The average production of Vitamin D from the same dose of UV light was quite abit lower in the older subjects compared to the young adults. However it is more like 30% than 5% even for 60-80 year olds so you will still be making Vitamin D if you are out in the sun. It would be cheaper and safer taking a supplement in addition to sunlight rather than spending money on tanning booths.

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Might be that health has something to do with it. If you are more healthy and eating more healthy, you may be doing a better job of manufacturing vit D. Also, the sun creates other reactions in the body resulting in other substances whose function are little understand but probably useful in some way. So IMO, the sun is better when you can get it than relying on supplementation. IMO, we don't understand enough yet to be as good with supplementation as we can with real foods and the real sun. – Eva Aug 16 2010 at 15:23
Matthew, thank you for the link. I'm going to try and obtain the study. Eva, this is a great angle. I couldn't get the study that Matthew quoted yet but I seriously doubt they checked nutrition profiles. Since I went fully primal/paleo it is impossible to get a sunburn anymore. I used to burn easily. So I know my skin is reacting completely differently to the sun. Doesn't confirm or refute the study but it does question one of the variables. Thank you and I'll keep searching with this in mind. -- Tom – Tom Aug 16 2010 at 16:43
There was no background of the subjects, diet etc, it was only a small study. Of course there will be individual variation. The study measured the response to the same dose of UV light. A longer sun exposure could produce the same level of Vitamin D as a younger person. – Matt Aug 16 2010 at 17:23
I am fair skinned, light haired, with hazel eyes. The other day, I went hiking for 3 hours under the full hot noonday sun. (no sunscreen of course) When I got back, I was pink, but turned to tan by the next morning. I wonder if I had stayed out the entire day, if I would have gotten burned or not. Seems definitely harder to burn these days. I have not yet succeeded but I have not yet gone out on one of late spring all day long rockhunting excursions so a full testing has not yet been done! – Eva Aug 17 2010 at 2:26
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Not everyone loses the ability to synthesize and activate vitamin D, in other words it is not always age-related.

The kidneys are a prime site of activation ergo if significant kidney dysfunction is going on, vitamin D activation will also be compromised. The liver is another site -- the P450 enzyme system and other enzymes are involved. GLUTEN and high fiber diets KNOCK THIS OUT. Ergo happy wholediseasegrains are at the center of vitamin D deficiency combined with blatant sun-phobia with the overuse of sunscreen.

Drugs -- many drugs (birth control, statins, any sulfur-derived pharmaceutical which is A LOT OF PHARMACEUTICALS, diuretics/'water pills', etc) cause photo-hypersensitivity.

Lack of vitamin B3 -- induced by phytic acid by oats, grains, whole brown rice, etc (also known as the vampire condition -- pellagra) causes photosensitivity which again will affect and increase sun avoidance, further worsening vitamin D deficiency.

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