Few foods naturally contain vitamin D, though some foods are fortified with the vitamin. For most people, the best way to get enough vitamin D is taking a supplement because it is hard to eat enough through food. Vitamin D supplements are available in two forms: vitamin D2 (“ergocalciferol” or pre-vitamin D) and vitamin D3 (“cholecalciferol”). Both are also naturally occurring forms that are produced in the presence of the sun’s ultraviolet-B (UVB) rays, hence its nickname, “the sunshine vitamin,” but D2 is produced in plants and fungi and D3 in animals, including humans. Vitamin D production in the skin is the primary natural source of vitamin D, but many people have insufficient levels because they live in places where sunlight is limited in winter, or because they have limited sun exposure due to being inside much of the time. Also, people with darker skin tend to have lower blood levels of vitamin D because the pigment (melanin) acts like a shade, reducing production of vitamin D (and also reducing damaging effects of sunlight on skin, including skin cancer).
The Recommended Dietary Allowance for vitamin D provides the daily amount needed to maintain healthy bones and normal calcium metabolism in healthy people. It assumes minimal sun exposure.
RDA: The Recommended Dietary Allowance for adults 19 years and older is 600 IU daily for men and women, and for adults >70 years it is 800 IU daily.
UL: The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause harmful effects on health. The UL for vitamin D for adults and children ages 9+ is 4,000 IU.
Many people may not be meeting the minimum requirement for the vitamin. NHANES data found that the median intake of vitamin D from food and supplements in women ages 51 to 71 years was 308 IU daily, but only 140 IU from food alone (including fortified products).  Worldwide, an estimated 1 billion people have inadequate levels of vitamin D in their blood, and deficiencies can be found in all ethnicities and age groups. [2-4] In industrialized countries, doctors are seeing the resurgence of rickets, the bone-weakening disease that had been largely eradicated through vitamin D fortification. [5-7] There is scientific debate about how much vitamin D people need each day and what the optimal serum levels should be to prevent disease. The Institute of Medicine (IOM) released in November 2010 recommendations increasing the daily vitamin D intake for children and adults in the U.S. and Canada, to 600 IU per day.  The report also increased the upper limit from 2,000 to 4,000 IU per day. Although some groups such as The Endocrine Society recommend 1,500 to 2,000 IU daily to reach adequate serum levels of vitamin D, the IOM felt there was not enough evidence to establish a cause and effect link with vitamin D and health benefits other than for bone health. Since that time, new evidence has supported other benefits of consuming an adequate amount of vitamin D, although there is still not consensus on the amount considered to be adequate.
Vitamin D and Health
The role of vitamin D in disease prevention is a popular area of research, but clear answers about the benefit of taking amounts beyond the RDA are not conclusive. Although observational studies see a strong connection with lower rates of certain diseases in populations that live in sunnier climates or have higher serum levels of vitamin D, clinical trials that give people vitamin D supplements to affect a particular disease are still inconclusive. This may be due to different study designs, differences in the absorption rates of vitamin D in different populations, and different dosages given to participants. Learn more about the research on vitamin D and specific health conditions and diseases:
Bone health and muscle strength
Type 2 diabetes
Risk of premature death
Few foods are naturally rich in vitamin D3. The best sources are the flesh of fatty fish and fish liver oils. Smaller amounts are found in egg yolks, cheese, and beef liver. Certain mushrooms contain some vitamin D2; in addition some commercially sold mushrooms contain higher amounts of D2 due to intentionally being exposed to high amounts of ultraviolet light. Many foods and supplements are fortified with vitamin D like dairy products and cereals.
- Cod liver oil
- Tuna fish
- Orange juice fortified with vitamin D
- Dairy and plant milks fortified with vitamin D
- Beef liver
- Egg yolk
- Fortified cereals
Is There a Difference Between Vitamin D3 and Vitamin D2 Supplements?
Vitamin D3 can be formed when a chemical reaction occurs in human skin, when a steroid called 7-dehydrocholesterol is broken down by the sun’s UVB light or so-called “tanning” rays. The amount of the vitamin absorbed can vary widely. The following are conditions that decrease exposure to UVB light and therefore lessen vitamin D absorption:
- Use of sunscreen; correctly applied sunscreen can reduce vitamin D absorption by more than 90%. 
- Wearing full clothing that covers the skin.
- Spending limited time outdoors.
- Darker skin tones due to having higher amounts of the pigment melanin, which acts as a type of natural sunscreen. 
- Older ages when there is a decrease in 7-dehydrocholesterol levels and changes in skin, and a population that is likely to spend more time indoors.
- Certain seasons and living in northern latitudes above the equator where UVB light is weaker.  In the northern hemisphere, people who live in Boston (U.S.), Edmonton (Canada), and Bergen (Norway) can’t make enough vitamin D from the sun for 4, 5, and 6 months out of the year, respectively.  In the southern hemisphere, residents of Buenos Aires (Argentina) and Cape Town (South Africa) make far less vitamin D from the sun during their winter months (June through August) than they can during their spring and summer months.  The body stores vitamin D from summer sun exposure, but it must last for many months. By late winter, many people in these higher-latitude locales are deficient. 
Note that because ultraviolet rays can cause skin cancer, it is important to avoid excessive sun exposure and in general, tanning beds should not be used.
Signs of Deficiency and Toxicity
Vitamin D deficiency may occur from a lack in the diet, poor absorption, or having a metabolic need for higher amounts. If one is not eating enough vitamin D and does not receive enough ultraviolet sun exposure over an extended period (see section above), a deficiency may arise. People who cannot tolerate or do not eat milk, eggs, and fish, such as those with a lactose intolerance or who follow a vegan diet, are at higher risk for a deficiency. Other people at high risk of vitamin D deficiency include:
- People with inflammatory bowel disease (ulcerative colitis, Crohn’s disease) or other conditions that disrupt the normal digestion of fat. Vitamin D is a fat-soluble vitamin that depends on the gut’s ability to absorb dietary fat.
- People who are obese tend to have lower blood vitamin D levels. Vitamin D accumulates in excess fat tissues but is not easily available for use by the body when needed. Higher doses of vitamin D supplementation may be needed to achieve a desirable blood level. Conversely, blood levels of vitamin D rise when obese people lose weight.
- People who have undergone gastric bypass surgery, which typically removes the upper part of the small intestine where vitamin D is absorbed.
Conditions resulting from prolonged vitamin D deficiency:
- Rickets: A condition in infants and children of soft bones and skeletal deformities caused by failure of bone tissue to harden.
- Osteomalacia: A condition in adults of weak and softened bones that can be reversed with supplementation. This is different than osteoporosis, in which the bones are porous and brittle and the condition is irreversible.
Vitamin D toxicity most often occurs from taking supplements. The low amounts of the vitamin found in food are unlikely to reach a toxic level, and a high amount of sun exposure does not lead to toxicity because excess heat on the skin prevents D3 from forming. It is advised to not take daily vitamin D supplements containing more than 4,000 IU unless monitored under the supervision of your doctor.
Symptoms of toxicity:
- Weight loss
- Irregular heart beat
- Hardening of blood vessels and tissues due to increased blood levels of calcium, potentially leading to damage of the heart and kidneys
Did You Know?
- Catching the sun’s rays in a sunny office or driving in a car unfortunately won’t help to obtain vitamin D as window glass completely blocks UVB ultraviolet light.
Original Source https://www.hsph.harvard.edu/nutritionsource/vitamin-d/