Dosage

The dosage underneath is the Recommended Dietary Allowance (RDA), but be aware that this dosage is the minimum that you require per day, to ward off serious deficiency of this particular nutrient. In the therapeutic use of this nutrient, the dosage is usually increased considerably, but the toxicity level must be kept in mind.

Male 5,000 IU per day (1,000 µg equivalent), female 4,000 IU per day (800 µg retinol equivalent), although 10,000 IU per day is normally used in supplementation.
Toxicity and symptoms of high intake

Dosages exceeding 15,000 IU per day must be taken under medical supervision. Toxicity can appear in some individuals at relatively low dosages and the symptoms may include nausea, dizziness, menstrual problems, skin changes and dryness, itchiness, irritability, vomiting, headaches and long term use can cause hair loss, bone and muscle pain, headache, liver damage, and an increase in blood lipid concentrations.

Pregnant women must be careful as a high intake of this vitamin can cause birth defects.

Pro-vitamin A - beta-carotene does not cause toxicity.

Be careful if you in the unlikely event run across polar bear on a menu - 500 gram (about ½ a pound) of polar bear liver will deliver about 9,000,000 IU to your diet - a very lethal dose. Headaches, blurred vision, loss of hair, drowsiness and diarrhea, enlargement of the spleen and liver can all be indications when your intake is too high.
Best used with

Take vitamin A with B group vitamins, vitamins C, D and E, choline, essential fatty acids together with calcium, phosphorus and zinc for the best results.
When more may be required

More of this vitamin is required when you consume alcohol, on a low-fat diet, or a diet high in polyunsaturated fatty acids, if you smoke or live in a polluted area. It may also be indicated if you suffer from diabetes or have an under-active thyroid gland. Be careful of vitamin A in pregnancy.
Enemy of vitamin A

Retinol is destroyed by light, high temperatures as well as when using copper or iron cooking utensils. Beta-carotene rich vegetables and fruit must not be soaked in water for long periods, since the nutrients can be lost like that.
Other interesting points

There seems to be no toxicity when ingesting large amounts of beta-carotene - you might however have a slightly orange colored skin, as the carotene gets stored in your skin.
Food sources of vitamin A

Liver, milk, egg-yolk, carrots, dark green leafy vegetables and yellow fruits are high in vitamin A or beta-carotene.
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