Vitamin D is one of the fat-soluble vitamins, obtained from your diet and produced by your body after exposure to sunlight. Excess vitamin D, or hypervitaminosis D, almost always occurs due to overuse of supplements rather than from your diet or sun exposure. Increased blood calcium levels resulting from too much vitamin D can cause both short- and long-term health effects.
Vitamin D
Vitamin D helps your body absorb dietary calcium, maintain balanced calcium and phosphate levels for healthy bones and regulate cell growth, muscle activity and your immune system. Whether obtained from food, sunlight or supplements, your body must alter the vitamin D molecule before it becomes active. Your liver first converts the vitamin to 25-hydroxyvitamin D, or 25(OH)-D, which your kidneys then turn into the short-lived active form called calcitriol. The intermediate 25(OH)-D form best reflects your overall vitamin D status, and is the form measured if vitamin D excess is suspected.
Sources of Vitamin D
Most foods contain little or no vitamin D naturally, exceptions including fatty fish like salmon and tuna, fish liver oils, and small amounts found in beef liver, cheese and egg yolks. Milk, processed breakfast cereals, baby formula, orange juice and various other foods in the United States commonly have supplemental vitamin D added. Your body synthesizes its own vitamin D chemically when your skin is exposed to ultraviolet light, with as little as 30 minutes of sunshine twice a week enough to maintain normal vitamin D levels for most people, according to the NIH Office of Dietary Supplements. Your liver and fat tissue store surplus vitamin D, helping you maintain adequate levels even over the winter when combined with normal dietary intake. Two functionally equivalent forms of vitamin D supplements, D2 and D3, are widely available both over-the-counter and by prescription.
Hypervitaminosis D
Vitamin D toxicity is almost always due to the overuse of supplements. Your body self-regulates its vitamin D production from sunlight, and except in rare cases the amount of dietary vitamin D you obtain won't result in hypervitaminosis. With the exception of cod liver oil, at over 1,000 IU of vitamin D per serving, even supplemented foods generally supply less than 150 IU of vitamin D in a serving and make reaching toxic levels through diet alone extremely unlikely. The Institute of Medicine has determined the safe upper limit of vitamin D intake for adults to be 4,000 IU daily, and for infants and young children from 1,000 to 3,000 IU a day. Taking a dosage as high as 10,000 IU daily for short periods of time is unlikely to produce obvious symptoms of hypervitaminosis.
Symptoms of Hypervitaminosis D
Hypervitaminosis D causes elevated blood calcium levels, with symptoms including constipation, poor appetite, fatigue, muscle weakness, vomiting and dehydration, according to MedlinePlus. Elevated blood pressure, frequent urination and increased thirst can also occur. Laboratory testing to confirm the diagnosis typically includes blood 25(OH)-D and calcium levels. If not recognized and corrected, hypervitaminosis D can eventually lead to bone damage, kidney stones and calcium deposits in blood vessels and soft tissues with eventual damage to your heart and kidneys. In most cases, simply decreasing or stopping your vitamin D supplements will be the only treatment required.
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