Is Too Much Sun As Bad As Too Little for Vitamin D Synthesis?
By now most of us have heard that many illnesses (Insulin resistance, high blood pressure,immune function heart disease and cancer) have in common a lower level of vitamin D in the blood serum than the general population. Multiple Sclerosis (MS) is a good example. Sun exposure, and therefore synthesis of vitamin D directly into people’s skin, has been correlated with a lower MS risk . Hence some advice to supplement with vitamin D in the hope to slow down MS. Success in this, however, was very dependent on results interpretation [2,3].
Vitamin D is naturally produced in people’s skin, when exposed to sunlight (Photosynthesis). Yes, photosynthesis is not just for plants. More precisely :
“…7-dehydrocholesterol reacts with ultraviolet light of UVB type at wavelengths between 270 and 300 nm, with peak synthesis occurring between 295 and 297 nm….”
What is 7-dehydrocholesterol? A sterol that can can convert into cholesterol and vice-versa . The implication is that, if cholesterol can convert into a precursor of vitamin D, maybe it is not all bad... But excess cholesterol causes plaque deposit in the blood vessels. As a matter of fact, the familiar statin drugs, that decrease and control cholesterol , have been implicated in vitamin D deficiencies 
The graphics below illustrates the chain of reactions initiated by sunlight’s UVB on your skin, ending in vitamin D3 in liver and kidneys
The 7-dehydrocholesterol is located in the deeper two layers of the skin , and the UVB have to traverse the outer layer of skin shown below
Are there skin conditions that can affect its ability to synthesize vitamin D?
The answer is yes. The Livestrong site lists the main ones  : psoriasis, exzema, scleroderma
Excessive sunning and subsequent sunburn (erythema) do damage the skin that can be substantial enough to cause cancer or immunosuppression 
Therefore, a question should come naturally: Can repeated sunburning decrease the skin’s ability to synthesize vitamin D?
A study by Rhode Island researchers  asserts just that, and we quote:
“…Levels of 7-dehydrocholesterol strongly influence cutaneous vitamin D3 synthesis. Post-burn scar tissue was found to contain only 42.5% of the 7-dehydrocholesterol typically found in normal skin, and in the absence of supplementation burn patients often develop progressive vitamin D deficiency...”
So it is conceivable that people who have experienced the two extremes of UVB exposure, too little and too much, end up with a sub-par production of vitamin D. This in turn could result in a higher than normal risk of contracting illnesses correlated with low vitamin D, such as MS.
Maybe, but an English study found a link between no sunburning and a slower MS progression 
“…no history of childhood sunburning was associated with slow [MS disability] progression…” [for a certain type of skin.]
Granted, a slower progression is a different MS aspect than the risk of onset, we will keep our ear to the ground for future studies on the topic
Food for thought and summer caution!
As always, talk to your Doctor
- Neurology ; “Melanocortin 1 receptor genotype, past environmental sun exposure, and risk of multiple sclerosis.” ; Aug 2008
- medinewsdigest ; “MND BRIEF: Study Shows Multiple Sclerosis Burns Up Vitamin D, But Do Vitamin D supplements fight MS?” July 30, 2012
- medinewsdigest ; “In Brief: High Doses of Vitamin D Not Very Helpful in Treating Multiple Sclerosis” ; May 2012
- Wikipedia_Vitamin D
- Biochemistry ; “THE INTERCONVERSION OF CHOLESTEROL, 7-DEHYDROCHOLESTEROL AND LATHOSTEROL IN THE RAT” ; 1961
- Current Medical Research and Opinion ; Vitamin D deficiency, myositis-myalgia, and reversible statin intolerance.” ; Jul 2011
- Livestrong ; Vitamin D Deficiency and Skin Disorders
- Experimental Dermatology ; “Mechanisms underlying UV-induced immune suppression” ;2002
- Acta Derm Venereol 2011; 91: 115–124. ; “Factors Influencing Vitamin D Status”
- Multiple Sclerosis and Related Disorders ; “”Progression of disability in multiple sclerosis: A study of factors influencing median time to reach an EDSS value” ; Oct 2012