The addition of vitamin D to rice was at one time mandatory under US Federal regulations for commercial food distribution, beginning ca 1920 or so when it was discovered and its effects demonstrated. It appears not to be required (in rice) now, but does appear at times.

The move to make this addition was based on a dietary deficiency common in people who subsist on diets where rice is a principal component of the diet and other foods are not available or not used in sufficient amounts. Nearly all commercially sold milk in the US is supplemented with this vitamin, although milk products like cheese generally are not.

Medical researchers maintain that for most people, except in the tropics, it is not possible to manufacture sufficient vitamin D from exposure to sunlight to meet normal body requirements for good health, and a significant part of it must come from the diet. Even in the tropics, in dark skinned people the skin melanine may block enough UV to cause deficiency. The amounts required are quite small, but are necessary to avoid rickets in children during the period of bone development and osteoporosis in adults. (Recommended amounts increase significantly for older people.) Deficiencies are implicated in a number of other diseases and illnesses, including some cancers.

Wikipedia indicates that a recent Austrialian campaign to get people to "cover up" to avoid skin cancers from UV exposure were so successful that a majority of Australians now are considered "vitamin D deficient." Dietary supplements, either by modification of diet or by food additives, are recommended for most people who live above 30 degrees latitude due to their lesser exposure to UV in sunlight, and for people at all latitudes who do not spend significant parts of their time outdoors in sunlight.

A number of fairly common foods can supply the necessary amounts for people with insufficent UV exposure; but at higher latitudes consuming enough of these natural foods to supply healthy quantities of vitamin D often leads to displacement of other food types needed for good health - and/or to obesity.

There is some, but limited, excretion of this vitamin by breakdown in the body, but since it is a fat soluble material, it is only slowly broken down and excreted, and it is efficiently stored and accumulated in fatty tissues. Overdose can result from accumulation if extreme amounts are ingested artificially, but the tolerance range is very broad.

Ingestion of both shark and polar bear liver is discouraged, since these animals (that share low exposure to UV for very long periods) accumulate extremely high concentrations of vitamin D in the liver, and ingestion by humans of significant amounts can produce vitamin D toxicity. If you are a shark or a polar bear you may omit adjustments in your diet and the taking of suplements.

If you are not a shark or a polar bear, you should make sure that your diet accomodates an effective intake of foods that are a source for this vitamin, OR you should use products in which supplemental additions can provide it. Long experience has shown that doing both won't harm you.

John