It appears to be possible to return some foods to the normal amount of vitamin D

Without running into food fortification regulations.

"Free Range" lard has 500 IU of vitamin D per teaspoon.

It has been proven that adding vitamin D to hen feed will increase vitamin D to >300 IU per egg

Suspect a similar IU per egg would be possible by adding UV to their lighting

Can triple the vitamin D in carrots - Jan 2011 just 14 seconds of UVB light

Vitamin D from mushrooms: a review on optimisation of the process

UV strobes to beef up mushrooms – May 2010

All items in Food Sources of Vitamin D

What is a Fortified Food? Feb, 2010

Fortified foods contain added vitamins and minerals that are not naturally present in those foods. Fortified foods are different from enriched foods; enrichment is used to replace naturally occurring vitamins, minerals and nutrients that have been stripped from food during processing. Fortification is used to add extra vitamins and minerals to food, in order to help prevent widespread occurrence of the many health problems that can occur as a result of certain nutritional deficiencies.

The Logic Behind Food Fortification
Fortified foods have been used to treat a number of widespread public health problems caused by nutritional deficiency. Today, disease of malnutrition, such as rickets (a disease caused by vitamin D deficiency, in which the bones soften and become vulnerable to fracture an deformation), are very rare in the United States and other developed countries. Fortified foods help keep children, pregnant women and the elderly healthy. While the best way to get your vitamins and minerals is to eat a well balanced diet, fortified foods help keep those with limited diets healthy by preventing nutritional deficiency.

How Foods Are Fortified
The FDA has set standards for the maximum levels of nutrients that may be added to fortified foods. Consuming too much of certain vitamins, minerals and nutrients can be harmful, so food fortification levels are set low for safety’s sake.

Nutrients Used to Fortify Foods
Foods are commonly fortified with essential vitamins and minerals. Milk is often fortified with vitamin D and sometimes vitamin A. Orange juice is fortified with vitamin C. Breakfast cereals are fortified with a range of vitamins.

Commonly Fortified Foods
Many of the foods that we eat every day are fortified, not just milk, orange juice and breakfast cereals. American bread has been fortified with niacin since the late 1930s; bread fortification has significantly reduced the incidence of pellagra, a disease caused by lack of niacin in the diet. Vitamin D is added to margarine and other dairy products as well as milk.

One of the first common fortifications of food was the addition of iodine to salt. Iodine deficiency can lead to an enlarged thyroid gland (also known as a goiter) and can cause hypothryoidism. Iodine deficiency in pregnant women can lead to birth defects and mental retardation in their unborn children.

Fruit juice and rice are often fortified with calcium. Water and tooth pastes are flouridated to prevent tooth decay, though this practice remains controversial. Iron and its salts are added to cereals, milk and table salt to prevent iron deficiency anemia.

Foods That Are Not Fortified
Some foods are never fortified. In the United States, fresh fish, meat, poultry, fruits and vegetables may not be fortified, due to concerns that fortifying such foods might reduce public awareness of their inherent nutritional value. While junk foods and soft drinks may be legally fortified, this behavior is frowned upon, as the FDA wishes to discourage consumers from purchasing unhealthy foods just because they are fortified.
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Increased vitamin D in food without fortification – March 2011