The second place is your weight and your age. The institute of medicine and endocrine society recommendations in individuals who are overweight or obese that the adequate intake should be two or three times the daily requirement of vitamin D to improve serum vitamin D levels. Older people have lower levels of the substance in the skin that the UVB light converts into vitamin D precursor called 25 OHD and there is experimental evidence that older people are less efficient vitamin D producers than younger people.

  The third is the color and the temperature of your skin. Melanin is the substance in skin that makes it dark. It "competes" for UVB with the substance in the skin that kick-starts the body's vitamin D production called 25 OHD. As a result, dark-skinned people tend to require more UVB exposure than light-skinned people to generate the same amount of vitamin D. Warm skin is a more efficient producer of vitamin D than cool skin. So, on a sunny, hot summer day, you'll make more vitamin D than on a cool one.

  In fourth place we got gender and genetic factors. Gender differences have been suggested for vitamin D status, with a lower level occurring especially in post-menopausal women, increasing the risk of bone fractures and osteoporosis. Gender significantly affects vitamin D status. The lower 25 OHD levels observed in females, as compared to males, play a relevant role in the vitamin D production. An international consortium of researchers and doctors has identified four gene variants in the vitamin D pathway that may play a significant role in vitamin D deficiency.

  The last but not the least is the health of your gut, liver, and kidney. The vitamin D that you consumed in food or as a supplement is absorbed in the part of the small intestine immediately downstream from the stomach. Stomach juices, pancreatic secretions, bile from the liver, the wall of the intestine, they all have some influence on how much of the vitamin is absorbed. Estimates shows that a healthy person only absorb between 60 to 80% of vitamin D intake. On the other hand, conditions that affect your gut and digestion, like celiac disease, chronic pancreatitis, Crohn's disease, and cystic fibrosis, and kidney disease can reduce vitamin D absorption even more.  

Without vitamin D, calcium cannot function properly in your body. Therefore, most of the symptoms of vitamin D toxicity are related to hypercalcemia, which means excessively high calcium levels in your blood. Hypercalcemia can be life threatening and requires immediate medical attention.

  Symptoms of hypercalcemia include: High blood pressure and cardiovascular disease. Kidney stones, kidney injury, and even kidney failure. Digestive distress, such as loss of appetite, vomiting, nausea, constipation, and stomach pain.

  As we just learned the excessive calcium should be deposited in our bones instead of circulating in our bloodstream. In other words, the hypercalcemia can lead to a loss of bone density. Symptoms of this condition include brittle bones, fractures, bone cysts, abnormal gait, and bone pain.

  All of these factors can contribute to individual variations in how a drug is processed and how effective it is for each person. Therefore, it's important to work closely with a healthcare provider to find the right medication and dosage for your individual needs. This information is not intended to be a substitute for professional medical diagnosis, or treatment.