What Is Zinc?
Zinc supports a healthy immune system, is needed for wound healing, helps maintain your sense of taste and smell, and is needed for DNA synthesis. Zinc also supports normal growth and development during pregnancy, childhood and adolescence. Zinc is an essential mineral that is found in almost every cell. It stimulates the activity of approximately 100 enzymes, which are substances that promote biochemical reactions in your body. 
Zinc is thought to have antimicrobial, anti-sticking, cell-protective, copper absorbing, enzyme regulating and growth stimulating effects.  There are research indications that adequate supplies of zinc are essential to the development and maintenance of a healthy immune system and that aging is associated with immune impairment that can sometimes be partially repaired with zinc supplementation. 
DR. DAHL'S COLDCHASER®, uses Zinc Monomethionine, a patented zinc formula. Americans are eating more dietary fiber. Unfortunately, fiber inhibits zinc absorption. In fact, a high fiber meal has been shown to reduce zinc absorption to under 3%. 
Vitamin A keeps eyes healthy; is essential for growth and health of cells of organs, skin and hair; works as an antioxidant (protects cells from damage). 
Vitamin A is a family of fat-soluble compounds that play an important role in vision, bone growth, reproduction, cell division, and cell differentiation (in which a cell becomes part of the brain, muscle, lungs, etc.) [10-14]. Vitamin A helps regulate the immune system, which helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses [10,15-19]. Vitamin A also may help lymphocytes, a type of white blood cell, fight infections more effectively.
Vitamin A promotes healthy surface linings of the eyes and the respiratory, urinary, and intestinal tracts . When those linings break down, it becomes easier for bacteria to enter the body and cause infection. Vitamin A also helps maintain the integrity of skin and mucous membranes, which also function as a barrier to bacteria and viruses [18-20].
Retinol is one of the most active, or usable, forms of vitamin A, and is found in animal foods such as liver and whole milk and in some fortified food products. Retinol is also called preformed vitamin A. It can be converted to retinal and retinoic acid, other active forms of the vitamin A family .
Storage of vitamin A is decreased during times of illness, stress and with alcohol use unless intake is increased. About 90% of the storable vitamin A is in the liver. Zinc is needed by the body to help release stores of vitamin A for use. Excess alcohol intake depletes vitamin A stores. Also, diets high in alcohol often do not provide recommended amounts of vitamin A .When vitamin A is deficient, vitamin C seems to be lost more rapidly from the body.
When can vitamin A deficiency occur?
Vitamin A deficiency is common in developing countries but rarely seen in the United States. Approximately 250,000 to 500,000 malnourished children in the developing world go blind each year from a deficiency of vitamin A . In the United States, vitamin A deficiency is most often associated with strict dietary restrictions and excess alcohol intake . Severe zinc deficiency, which is also associated with strict dietary limitations, often accompanies vitamin A deficiency. Zinc is required to synthesize retinol binding protein (RBP) which transports vitamin A. Therefore, a deficiency in zinc limits the body's ability to mobilize vitamin A stores from the liver and transport vitamin A to body tissues .
Night blindness is one of the first signs of vitamin A deficiency. In ancient Egypt, it was known that night blindness could be cured by eating liver, which was later found to be a rich source the vitamin . Vitamin A deficiency contributes to blindness by making the cornea very dry and damaging the retina and cornea .
Vitamin A deficiency diminishes the ability to fight infections. In countries where such deficiency is common and immunization programs are limited, millions of children die each year from complications of infectious diseases such as measles . In vitamin A-deficient individuals, cells lining the lungs lose their ability to remove disease-causing microorganisms. This may contribute to the pneumonia associated with vitamin A deficiency [12,16-17]. About 90% of the storable vitamin A is in the liver. Zinc is needed by the body to help release stores of vitamin A for use. Chronic infections and illnesses can reduce the body's levels of vitamin A, thereby weakening the mucous membranes and making them more susceptible to viral infection.
Dietary Sources of Vitamin A: Liver, fortified milk, eggs; cheeses; and fish liver oil are good sources of vitamin A.
Vitamin C: Vitamin C is a water-soluble vitamin.
The following excerpt is taken from Tabers Medical Dictionary: Vitamin C: Ascorbic acid, a factor necessary for formation of collagen in connective tissues and essential in maintenance of integrity of intercellular cement in many tissues, esp. capillary walls. Vitamin C deficiency leads to scurvy. Note: The recommended daily allowance is 60 mg. Large daily doses of vitamin C have been recommended for prevention and treatment of the common cold. Although the effectiveness of vitamin C for this purpose has not been established, it is felt that the vitamin may at least decrease the severity of cold symptoms. 
The following is quoted from the Physicians Desk Reference (PDR) for non-prescription drugs and dietary supplements 26th edition:
Description: Vitamin C is an important biological antioxidant and has been a popular nutritional supplement for decades. Vitamin C is often used to prevent or ameliorate a wide variety of infections and to enhance the effectiveness of the immune system. It is popular as a promoter of Connective-tissue health in conditions such as minor trauma or capillary fragility.
Other Names: ascorbic acid, calcium ascorbate
Effects: Vitamin C has antioxidant, atherogenic, anticarcinogenic, antihypertensive, antiviral, antihistamine, immunomodulatory, opthalmoprotective, airway-protective and heavy metal detoxifying properties.
Antioxidant effects have been demonstrated as increased resistance of red blood cells to free radical attack in elderly persons and reduced activated oxygen species in patients receiving chemotherapy and radiation. Antioxidant mechanisms have been shown in the reduction of LDL oxidation as well, though studies on the prevention of heart disease and stroke are conflicting. 
An important function of vitamin C is the formation and maintenance of collagen, which forms the basis of connective tissue found in the skin, in cartilage, vertebral discs, joint linings and capillary walls. Vitamin C is needed to manufacture collagen to give support to the body, help wounds to heal, and to maintain healthy blood vessels.
Because of ascorbic acid's role in immunity and collagen formation, its antioxidant effect and the adrenal support it provides, vitamin C is important for maintaining a healthy immune system. 
Dietary Sources of Vitamin C: Citrus fruits; green vegetables; fortified cereal.
Vitamin C works as an antioxidant (protects cells from damage); necessary for healthy skin; regulates metabolism during stress or illness.
What Is Vitamin D?
Vitamin D is a fat soluble vitamin that is found in food and can also be made in your body after exposure to ultraviolet (UV) rays from the sun. Sunshine is a significant source of vitamin D because UV rays from sunlight trigger vitamin D synthesis in the skin [28-29].
Vitamin D exists in several forms, each with a different level of activity. Calciferol is the most active form of vitamin D. Other forms are relatively inactive in the body. The liver and kidney help convert vitamin D to its active hormone form . Once vitamin D is produced in the skin or consumed in food, it requires chemical conversion in the liver and kidney to form 1,25 dihydroxyvitamin D, the physiologically active form of vitamin D. Active vitamin D functions as a hormone because it sends a message to the intestines to increase the absorption of calcium and phosphorus .
The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus [30-31]. By promoting calcium absorption, vitamin D helps to form and maintain strong bones. Vitamin D also works in concert with a number of other vitamins, minerals, and hormones to promote bone mineralization. Without vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults, two forms of skeletal diseases that weaken bones [32-33].
In 2002, the vitamin D intakes of 1,546 non-Hispanic African American women and 1,426 non-Hispanic white women of reproductive age (15 to 49 years) were estimated by analyzing intake of milk and fortified cereals, two common dietary sources of vitamin D . Blood levels of vitamin D were also examined in these groups. Data examined were from the National Health and Nutrition Examination Survey (NHANES) III survey, which interviewed people from randomly selected households all across the U.S. The prevalence of hypovitaminosis D, a term used to describe low blood levels of vitamin D, was 42.4% among African American women and 4.2% among white women. In both groups, blood levels of vitamin D were higher when milk or fortified cereals were consumed more than 3 times per week. Among African American women, the risk of hypovitaminosis D decreased as milk and fortified cereal intake increased. These numbers suggest that large numbers of African American women may not consume recommended amounts of vitamin D. More frequent intake of vitamin D fortified milk and fortified cereals may help prevent hypovitaminosis D in this group.
When can vitamin D deficiency occur?
Nutrient deficiencies are usually the result of dietary inadequacy, impaired absorption and utilization, increased requirement, or increased excretion (loss). A deficiency of vitamin D can occur when:
Vitamin D deficient diets are associated with milk allergy, lactose intolerance, and strict vegetarianism. Infants fed only breast milk also receive insufficient amounts of vitamin D unless they also receive appropriate levels of vitamin D supplementation .
BIOFLAVONOIDS Vitamin P: Bioflavonoids help in the absorption and retention of vitamin C. Bioflavonoids naturally occur with vitamin C in fruit. The main known function of bioflavonoids is to increase the strength of capillaries. Vitamin P deficiency can lead to an increased tendency to bruise or bleed.