1. Sunlight exposure draws down the body's pool of cholesterol.
Vitamin D production begins in the oil glands of our skin, where the steroid 7-dehydrocholesterol is converted to previtamin D3 after absorbing specific wavelengths of light in the ultraviolet range. Exposure to heat converts previtamin D3 to vitamin D3 or cholecalciferol. All of this occurs on/in skin.
2. Sunlight exposure creates reservoirs of vitamin D3 throughout the body that may be tapped during winter or when needed (that is, when calcidiol or 25(OH)D3 is low). Dietary vitamin D does not get distributed in this way.
Much of the vitamin D3 manufactured in skin is deposited throughout the body in muscle and fat cells before it arrives in the liver. Much of the vitamin D3 absorbed from the digestive tract (food or pill) is delivered directly to the liver.
3. Absorption of vitamin D from diet is poor. Only about 50% of what we eat gets absorbed. In addition:
- Diseases that compromise health of intestinal cells (e.g. irritable bowel syndrome, lactose intolerance, inflammatory bowel disease (Crohn's), infectious agents, cancer) lower absorption.
- Bile is required for absorption, so liver conditions (hepatitis, cirrhosis, hepatic steatosis or fatty liver) lower absorption.
- Fat is required for absorption, so if there is steatorrhea or fat malabsorption (many causes, e.g. anemia, cystic fibrosis, celiac, Whipple), absorption will be lower. (There is usually adequate fat in food to ensure absorption.)
- Some medications, such as bile acid sequestrants which are used to lower cholesterol, lower absorption.
"To offer some perspective here, an adult with white skin [pigmented skin takes longer to manufacture vitamin D], exposed to summer sunshine while wearing a bathing suit, generates about 250 µg (10,000 IUs) of vitamin D3 in 15 to 20 minutes."
- Vitamin D Insufficiency: No Recommended Dietary Allowance Exists For This Nutrient, Reinhold Vieth and Donald Fraser, CMAJ, 2002
"Sunshine alone can bring 25(OH)D concentrations to 210 nmol/L in normal people and vitamin D intakes of 30 mg (1200 IU)/d contribute only a negligible fraction of this."5. Relying upon dietary sources to supply vitamin D necessitates consuming higher amounts of environmental pollutants.
- Vitamin D Supplementation, 25-Hydroxyvitamin D Concentrations, And Safety, Reinhold Vieth, AJCN, 1999
Both vitamin D and many organic pollutants (e.g. pesticides) are fat soluble. As such, they reside in fat tissue. Organic pollutants have been shown to bioaccumulate in animal tissue and are found in greater amounts higher up the food chain.
6. Some oil-based formulations of vitamin D may be harmful to the liver.
There is thinking that fish oil, and some forms of oil-based vitamin D, can be harmful to the liver.1 This is related to number 2 above, in that oral D is sent directly to the liver. Anecdotally - some people experienced elevated liver enzymes while taking fish oil (a common source for vitamin D) only to have them fall back into the normal range when they stopped. More study is needed there.
Enlightening post. Perhaps some people with ailments described in section 3, living in extreme latitudes, may be candidates for the Sperti lamp. Focus rays on body parts where the sun don't shine, avoiding face and arms. As for me, I'll stick with my 2000IU gel-caps and whatever little sunshine I get outdoors.
I was interested to read this post since I've always been afraid that D3 supplementation might be worse than sun in some way, but none of these reasons specified give me cause for fear.
1) The body produces far more cholesterol than any-one could consume per day, in order to meet precisely these sorts of needs. The body produces more/less cholesterol according to exactly what the current levels are, so if you "draw on" these levels, you'll just produce more to fill further needs.
2) Vitamin D is only convered into its active form (calcidiol) in the liver anyway, so it would ultimately just be sent there anyway. Most nutrients that the body stores are stored in the liver anyway, so I'm unsure why there would be an advantage to vitamin D not going to the liver.
3) Certainly if you have substantial digestive disorders then you're going to have difficult absorbing nutrients from food. (Ditto if you're artificially lowering your cholesterol- see point 2). If you eat gluten grains then certainly you'd have far more trouble with vitamin D. That's both through damage to the GI-tract and if you're getting your vitamin D from sunlight, since the body will burn through its vitamin D stores 30% quicker.
4) Only asserts the point that people get far more vitamin D from sun than foods: not surprising, most foods don't contain very much. That doesn't suggest for a second that supplements are worse if you take the right amount.
5) No-one could get sufficient vitamin D from food (not supplements) anyway, there are only so many egg yolks one can eat! One's supplements and cod liver oil and be purified.
IMO the only demonstrated advantage of the sun is that you can't conceivably overdose, but overdosing on supplements is easily avoided (getting insufficient from either sun or supplements is far more likely. The only clear downsides to the sun is skin damage and the fact that it's impractical for many people living away frm the equator or working inside.
Hi Dave, I'm on the lookout for adverse reactions too.
I'm sure you know the advantages to sun - manufacture of other hormones (e.g. melatonin) and several neuropeptides, impacting circadian rhythms, sleep cycles, metabolic cycles, fertility, immunity, inflammation. Taking a vit D supplement may increase levels of 25(OH)D3, but it can't substitute for sun.
The big advantage is that sun is free. Millions can't access supplements. Many poor have higher incidence of diseases like malnutrition (obesity is a form of malnutrition), anemia, and infections that make it difficult to benefit from a supplement anyway.
Oh there are tonnes of additional good things about the sun, certainly. My point was that in respect to vitamin D, there was no advantage to getting 10,000IU from the sun over supplements. As it happens I have to use a light box anyway for sleep cycle regulation, so am well aware of the non-vitamin D benefits of (pseudo)sunlight!
And yeh, not having to pay for the sun when it's about is a big advantage, but unfortunately emigrating to somewhere more equatorial would be even more expensive than my 25p, 5000IU supplements!
(For what it's worth here in the UK we do offer vitamin D supplements free to certain 'at risk' groups, (darkers skinned ethnic minorities, who are typically poorer anyway), I think, but as you say, it will do a limited amount of good if their digestive systems are already compromised and if they're lacking synergeistic vitamins.
About 4 years ago I was living in South Dakota. I woke up one morning and I felt so achy, like curling up in the fetal position was so comforting but I couldn't because I had to go to work. I went to work and I drank plenty of water, I ate and it didn't go away so I asked to leave. I went to a local lake and my thought was...let the sun heal me. I went to the lake and took my shirt off (was already wearing shorts) and within in 20 minutes, I felt 100% better. I "told" myself the sun healed me and I was fine. So, is being sick mental? I don't know, but I do know sitting on that bench with no shirt and feeling the sun on me made me feel so much better. I literally was okay in like 20 minutes, I went out that night and had a great time. I also think that maybe because I was sleeping on an air mattress that is what caused the achy feeling, could be, not sure but I was on that air mattress for six months prior. Your body is yours, control it. I rarely get sick, I've had the flu like once. I just don't get sick and my last cold was 4 years aog. I own that. ;) peace
beautiful post -- thank you for it. i was doing a bit of vitamin D research for my own post and landed here. hope you don't mind if i link you :)
Ragamuffin - Beautiful post back - yours. In fact, beautiful blog:
Hope you find time to write between papers.
Post a Comment