|Vitamin D Revisited|
|Leon H. Rottmann, Ph.D., Professor Emeritus, University of Nebraska-Lincoln
Lifeline, Fall 1996, Vol XIV, No 4, pp 1-2
The isolation of Vitamin D was delayed because of its confusion for a time with Vitamin A. Both of these vitamins are fat-soluble and occur together in nature. As more metabolites have been found through clinical research of the past fifty years, the metabolism of Vitamin D has been further clarified; however, research information continues in the developmental stage. Since the middle ages cod liver oil has been used as a remedy for rickets, but it was not until after World War I that the scientific basis for the cure was established.
Vitamin D can be acquired either as preformed Vitamin D by ingestion or by exposure to sunlight. Ingested Vitamin D is absorbed with the fats from the intestine with the aid of bile. Vitamin D from the skin is absorbed directly into the bloodstream. Both forms are carried in the bloodstream, bound to Vitamin D plasma binding protein (DBP), and move on to the liver where they are transformed into the active form. Storage sites of Vitamin D and its active forms are liver, skin, brain, bones and likely some other tissues which may not yet be known.
The active form of Vitamin D is essential for normal growth and development and is important for the formation of normal bones and teeth. Like other steroid hormones, the active form of Vitamin D is localized in the nuclei through a receptor mechanism. It is thought to induce the formation of proteins that transport calcium or phosphorus. Along with parathyroid hormone and thyrocalcitonin, Vitamin D has an important role in the maintenance of the appropriate serum levels of calcium and phosphorus to support normal mineralization of bone. The appropriate level of serum calcium is also important in the function of the neuromuscular system. In accordance with this general overall purpose, the active form of Vitamin D has the following functions within the body:
Other functions for Vitamin D are being discovered. Receptors for the active form of Vitamin D have been found not only in the intestinal, renal, parathyroid and skeletal tissues, but also in skin, breast, pancreas and connective tissue. It is thought that the active form of Vitamin D, which is present in the beta cells of the pancreas, may have a role in insulin secretion, possibly through maintenance serum calcium, which is important for adequate insulin secretion.
Vitamin D is found only in small and highly variable amounts in butter, cream, egg yolk and liver. The best food sources are fish liver oils. At this time within the U.S., approximately 98 percent of all fluid milk and evaporated milk are fortified with Vitamin D. Most dried whole milk and evaporated milk are fortified as well as some margarines, butter, selected cereals and some infant products. The milk used to make cheese is usually not Vitamin D-fortified. As an ingredient in food, Vitamin D is remarkably stable; preparation or foods containing Vitamin D can be warmed or kept for long periods of time without its deterioration. Packaging is typically marked as "stable to heat and oxidation."
Vitamin D deficiency in adulthood may result in osteomalacia. It is characterized by pronounced softening of the bones, which leads to deformities, especially of the limbs, spine, thorax and pelvis. Radiographic findings in the bones are translucent (Looser's zones), which are diagnostic of osteomalacia. Typical symptoms are a rheumatic type of pain and general weakness. There may also be a waddling gait and tetany manifested by facial twitching. Although osteomalacia is seen occasionally in men, it is most often observed in women of child-bearing age who have become depleted of calcium because of multiple pregnancies and inadequate diet. Osteomalacia is frequently confused with a disease having similar symptoms, osteoporosis.
Prevention of osteomalacia typically relates to an adequate supply of Vitamin D, calcium and phosphorus in the diet. Vitamin D must be assured from either sunshine, ultraviolet lamp, natural food source, fortified food source, or a concentrated supplement. If osteomalacia is already present, a high dosage of Vitamin D along with medication may be administered. Calcium supplements may also be necessary. The pain, lack of energy and weakness, which are typical associations with osteomalacia, will usually disappear within 1 to 2 months after treatment is started.
It is known that hypervitaminosis D can occur and cause pathological changes in the body when Vitamin D is taken in excess. These changes, consequences of hypercalcemia, are excessive calcification of bone, and calcification of soft tissues such as the kidney (including kidney stones), lungs and even the tympanic membrane of the ear, which can result in a high level of hearing loss and/or deafness. Headache and a non-defined nausea may be included in the subjective findings. Infants and small children given excessive amounts of Vitamin D may suffer gastrointestinal upsets, bone fragility, retarded growth and mental retardation. Small children are often diagnosed with critical problems when they are mistakenly administered an adult dosage.
Vitamin D toxicity develops slowly and over time; there is a great deal of individual variation in response to toxicity and in the susceptibility to Vitamin D toxicity. A little may be acceptable to one patient and the same dosage highly toxic to the next patient. Too much or too little are both highly problematic. The celiac who needs Vitamin D therapy should always be monitored and have regular contact with the monitoring physician. In the case of Vitamin D, "what's good for the goose may be highly toxic to the gander."
Because the present intake of Vitamin D seems to be more than adequate for most patients, it has been suggested that perhaps fortification of foods other than those already standardly fortified (milk and margarine) should be reduced or discontinued. This would include mainly cereal products. Thus, the celiac consumer would need to plan around or omit other selections and especially supplements which may move total diet selections into the toxic range. The self-managed celiac may wish to evaluate dietary patterns for foods high in Vitamin D and make the appropriate changes needed in consultation with physician and dietitian consultation.
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