(NaturalNews) Now there's evidence that stress, fatigue, and depression are connected to low serum levels of vitamin D, in addition to known immunity, illness and disease issues.
The more up-to-date integrative or holistic MDs and naturopaths recognize 25-hydroxy-vitamin D or 25(OH)D3 test serum level should be in the 50 to 80 ng/ml (nanograms per milliliter) for optimum health. 
The 25(OH)D3 test is the only test that will accurately assess your vitamin D levels. Yet, sometimes doctors order the 1,25-dihydroxy-vitamin D test. Often, people whose vitamin D serum levels are actually low will read high on that test and be led to believe that all is well and be misdiagnosed.
Serum levels of D3 need to be higher than previously thought
Mainstream medicine pegs normal levels at under 50 ng/ml down to 30 ng/ml and even lower. According to Dr. Frank Lipman, those readings are sufficient "to prevent rickets or osteomalacia (soft bones), but not for optimal health." Dr. Lipman is the founder of the Eleven Eleven Wellness Center in NYC.
He lists fatigue, restless sleep and poor concentration among other symptoms of vitamin D deficiency. Over 80% of his clients are vitamin D deficient, which correlates with the general population epidemic. 
Dr. Anna Dorothea Hoeck, MD, from Cologne, Germany, has treated fatigue with vitamin D therapy, using the 25(OH)D3 test first, then boosting those readings accordingly.
She has discovered with actual clinical treatments that those with lower levels of chronic fatigue recovered quickly, while more severe cases required adding minerals with the vitamin, especially calcium and magnesium.
The academic paper based on her clinical work with vitamin D can be obtained from source .
Both doctors urge inexpensive and accessible vitamin D3 cholicalciferol (not pharmaceutical D2) supplementation. Some who are exposed to sunlight get low readings. Dr. Lipman presents some D3 dosage guidelines in his article. 
Research findings on D3 deficiency and fatigue and depression
Neurology researchers at the Isfahan University of Medical Sciences in Isfahan, Iran, studied 200 multiple sclerosis (MS) patients, half of whom had low vitamin D levels by European standards, which are typically low compared to American Holistic or Integrative standards.
Converting their 25(OH)D3 readings from nmol/l (nanomols to liter) to ng/ml revealed typically low European standards compared to what Dr. Lipman and others consider normal.
They considered above 30 ng/ml normal and below that insufficient. Deficient readings began at 10 ng/ml. Neither are close to the 50 to 80 ng/ml D3 serum readings considered optimum.
Nevertheless, despite these low standards, the researchers made an association of low D3 significant for depression but not as significant with fatigue. Maybe a fatigue association would have been made with higher 25(OH)D3 readings.  [3a]
Another study with a smaller number of MS patients, 59, was conducted in the Netherlands. They too use those same lower standards, and they came up with similar conclusions: that depression was associated with vitamin D insufficiency and deficiency, while fatigue was not as significant. 
Again, could fatigue have been a factor within what they considered to be normal ranges but are really not? It seems the NYC and German doctors think so, as they both consider fatigue a symptom of low vitamin D serum readings within higher standards.
The online magazine, BodyEcology, mentions the low vitamin D levels are often sub-clinical, which means you may be functioning but not in optimum health, or what Dr. Lipman calls "the walking wounded."
That article also mentions how stress produced cortisol may inhibit vitamin D receptor functioning. So even if you're getting sufficient D3 from sunlight or supplements, you may need to get more high quality sleep to use it.
The magazine article also recommends eating fatty fish, high quality cod liver oil, eggs from pastured hens and liver from grass fed cows (liver again?!). 
Vitamin D3, the pre-hormone that kick starts other hormonal activities but is called a vitamin, helps avoid depression and fatigue.