An exciting new study shows that lack of vitamin D may change the colon bacteria to an overabundance of harmful bacteria to cause inflammation that increases risk for autoimmune diseases such as inflammatory bowel disease, multiple sclerosis, diabetes, lupus and some types of arthritis (Frontiers in Microbiology, October 24, 2019). The researchers also showed that using ultraviolet light to raise blood levels of vitamin D may increase the concentration of healthful colon bacteria that help to control inflammation.

The study was conducted in the middle of winter with no direct sun exposure. When compared to women who took vitamin D pills, women who did not take vitamin D pills had:
• much lower blood levels of hydroxyvitamin D,
• a significant, harmful reduction in the variety of different colon bacteria,
• reduced levels of healthful colon bacteria, and
• an increase in harmful colon bacteria.

The women were then given three one-minute sessions of full-body UVB light exposure in one week. The women who were not taking vitamin D pills and had low blood levels of hydroxy vitamin D showed:
• a high rise in blood levels of hydroxyvitamin D (while the women taking vitamin D pills had no rise in vitamin D at all)
• a marked increase in diversity of colon bacteria, as a marker for an increase in healthful bacteria (those who were taking the vitamin D pills had no change in their colon bacteria)
• a marked increase in the concentration of healthful Lachnospiraceae bacteria
• an increase of healthful Firmicutes and a decrease in harmful Bacteroidetes bacteria in their colons

This new study suggests that lack of skin exposure to sunlight causing a deficiency of vitamin D may be one of the risk factors for autoimmune diseases. Other studies have shown that:
• Vitamin D helps to protect your intestinal lining from being invaded by harmful bacteria that turn on your immune system to cause inflammation (Autophagy, 2015;12:1057-1058)
• Vitamin D helps to suppress inflammation (Front Immunol, 2016;7:627)
• Vitamin D deficiency promotes inflammation that causes overgrowth of harmful colon bacteria (Metabolism, 2017;69:76-86)
• Vitamin D pills help to suppress chronic inflammatory diseases (J Clin Endocrinol Metab, 2018;103:564-574)
• Raising blood levels of vitamin D in those suffering a deficiency increases healthful colon bacteria (Eur J Nutr, 2018;58:2895-2910)
• Lack of vitamin D increases risk for colitis in mice (J Nutr, 2013;143:1679-1686)
Foods are a very limited source of vitamin D, so you need to get more than 80 percent of your vitamin D from sunlight (Indian J Endocrinol Metab, 2018;22:249) or pills.

Avoid Large Doses of Vitamin D Pills
One of the main functions of vitamin D is to help keep bones strong and control calcium metabolism. Vitamin D increases calcium absorption in the intestines, strengthens bones by increasing bone remodeling, and increases the effects of parathyroid hormones. Other reported functions of vitamin D are controversial, and most people do not benefit from taking higher doses of vitamin D (>1000 IU/day) or having very high blood levels of that vitamin (>20ng/ml). Raising blood levels of hydroxy vitamin D from 20 to 30 ng/ml with high doses of vitamin D pills increases calcium absorption by only one percent and does not increase bone mineral density or physical function, compared with placebo (Curr Opin Endocrinol Diabetes Obes, Dec 2016;23(6):440-444).

Very high doses of vitamin D can cause nausea, constipation, kidney stones, frequent urination, muscle weakness, irregular heartbeats and possibly arteriosclerosis. A study from Denmark showed that very high blood levels of hydroxy vitamin D (above 140 ng/ml) are associated with increased risk of premature death (J Clin Endocrinol Metab, Aug 2012;97(8):2644-52). High-dose vitamin D3 supplements (70,000 IU·wk for 12 weeks) caused a significant increase of a toxic vitamin D metabolite called 24,25[OH] vitamin D, and reduced parathyroid levels and decreased body responses to vitamin D itself (Med Sci Sports Exerc, Feb 2017;49(2):349-356).

Current Guidelines
The Institute of Medicine recommends that adults need only 600-800 IU of vitamin D per day and that blood levels of hydroxy vitamin D do not need to be higher than 20 ng/ml (Curr Opin Endocrinol Diabetes Obes, Dec 2016;23(6):440-444). Higher blood levels of vitamin D (greater than 20 ng/ml) do not make bones stronger than lower blood levels as they do not reduce levels of parathyroid hormone or bone resorption (Curr Rheumatol Rep, June 2011;13(3):257-64). Large doses (4000 IU/day) of vitamin D did not slow declining physical function in sedentary men over 70 (Journal of the American Geriatrics Society, 11/22/2016). For most people, high dose vitamin D pills (greater than 2000 IU/day) will not improve health and may harm you (N Engl J Med, Nov 10, 2016;375:1817-1820).

My Recommendations
You probably do not need to take vitamin D pills if your blood level of hydroxy vitamin D is above 20 ng/ml unless you have a condition that your doctor feels puts you at increased risk for the signs and symptoms of a deficiency. You can take up to 1000 IU/day of vitamin D pills if your blood levels are below 20 ng/ml.

Excess exposure to sunlight can cause skin cancer, but most people can meet their needs for vitamin D by exposing a small area of skin to sunlight for short periods as long as they do not damage their skin by burning it. Get out of the sunlight immediately if you feel your skin heating up, burning or getting red.